1. What is the ASME code followed for design of piping systems in Process
pipings (Refineries & Chemical Industries)?
(i) B31.1
(ii) B31.3
(iii) B31.5
(iv) B31.9
Answer (III)
2. What do you mean by following items?
i. )ISLB-400 ii) ISMB-600 iii) ISHB-350 iv) ISMC-300 v) ISJB-150 vi) ISLB-200
vii)ISMB-450 viii)ISWB-400 ix) ISJC-200 x) ISLC-350 xii) ISMC-250
i. Indian STD light weight beam, Web size – 400
ii. Indian STD medium weight beam, Web size – 600
iii. Indian STD ‘H’ beam, Web size – 350
iv. I ndian STD medium weight channel, Web size –300
v. I ndian STD junior beam, Web size – 150
vi. Indian STD light weight beam, Web size – 200
vii. Indian STD medium weight beam, Web size – 450
viii. Indian STD wide flange beam, Web size – 400
ix. Indian STD junior channel, Web size – 200
x. I ndian STD light weight channel, Web size – 350
xi. I ndian STD medium weight channel, Web size – 250
3. What is this item?
i. ISA-100X100X12 ii) ISA-80X50X10 iii)ISLT-100X100
i. Equal angle size 100x12 THK
ii. Unequal angle size 80x50x10 THK
iii. Indian STD light weight tee bar size 100x100
4. What is the difference between stub in and stub on branches? Describe with
Which one is preferred?
For branching of one size lesser of run pipe, Stub On is preferred. For other branching
less than one size of run pipe stub in is preferred. The Design is based on ANSI B 31.3
5. What is the difference between Pipe and Tube?
Ans: Pipe is identified by NB and thickness is defined by Schedule whereas Tube is
identified by OD.
6. From which size onwards NB of pipe is equal to OD of Pipe?
Ans: From the size 14” and onwards NB = OD of pipe.
7. Write down the outside diameter of following pipe?
i. 3 inch ii) 6 inch iii) 10 inch iv) 14 inch
i. 3 inch = 88.9mm ii)6 inch = 168.28mm
iii) 10 inch = 273.06mm iv) 14 inch = 355 mm(OD= Size X 25.4)
8. What is the difference between machine bolt and stud bolt?
Machine bolt has a head on one side and nut on other side but stud bolt have nuts on
both sides.
9. What is soluble dam?
Soluble dam is a water-soluble material used for restricting the purging gas within the
10. While welding of pipe trunion to pipe/reinforcement pad you have to put a hole
or leave some portion of welding why?
For venting of hot gas which may get generated due to welding
11. What do you mean by following type of welding
i. SMAW ii)TIG
12. Find out the elevation of marked point ‘A’
Elevation of marked point ‘A’ is 100.050
13. What should be the radius of long radius elbow?
1.5D (Where “D” is the diameter of the pipe)
14. Normally where do we use the following?
i. Eccentric reducers ii)Concentric reducers
i. Eccentric reducers = Pump suction to avoid Cavitation, To maintain elevation (BOP) in
ii. Concentric reducers = Pump discharge, vertical pipeline etc.
15.Concentric reducer is used in pump suction. (Yes / No). Explain.
No. Air pockets may form if concentric reducer is used at pump suction, which results in
Cavitation, and cause damage to Pump. To avoid this problem, Eccentric Reducer with
Flat Side Up (FSU)is used in Pump Suction.
16. What do you mean by Cavitation in Pump?
A pump is designed to handle liquid, not vapour. Vapour forms if the pressure in the
pump falls below the liquid’s vapour pressure . The vapour pressure occurs right at the
impeller inlet where a sharp pressure drop occurs. The impeller rapidly builds up the
pressure which collapses vapour bubbles causing cavitation and damage . This is
avoided by maintaining sufficient NPSH.
(Cavitation implies cavities or holes in the fluid we are pumping. These holes can also
be described as bubbles, so cavitation is really about the formation of bubbles and their
collapse. Bubbles form when ever liquid boils. It can be avoided by providing sufficient
17. What do you mean by NPSH? How do you calculate it?
W.P EL. “A”
W.P.EL –100.050
3 ∅Pipe
Slope 1:100
NPSH: Net Positive Suction Head. NPSH is the pressure available at the pump suction
after vapour pressure is substarcted.
It is calculated as : Static head + surface pressure head - the vapor pressure of your
product - the friction losses in the piping, valves and fittings.
It thus reflects the amount of head loss that the pump can sustain internally before
vapour pressure is reached.
18. What is the ASTM code for the following?
i. CS pipe ii) CS fittings iii)CS flanges iv)AS pipe P5/P11 v)Cast CS Valves
i. CS pipe = A106 Gr.B
ii. CS fittings = A234 Gr.WPB/WPBW
iii. CS flanges = A105
iv. AS pipe = A335 Gr P1/P11
v. Cast CS Valves = A216 Gr.WCB
19. What is the thumb rule to calculate spanner size for given bolt?
1.5 x diameter of Bolt
20. What is the thumb rule to calculate Current required for Welding?
Current (Amp) = [ Diameter of Electrode (mm) X 40] + 20
21. What is steam tracing? How do we decide the location of SSM & CRM.
Steam Tracing is a process which is used to prevent the fluid passing through a
process line from freezing by keeping the temperature high enough for free flow of fluid
and thus maintaining pumpability.
SSM and CRM are generally located 38M max for open system and 24 M max for
closed system when we use LP Steam up to 3.5 kg/sq cm. as a heating media.
22. Which piping items will you drop down before conducting Flushing and
Ans: Items like Control Valve, Orifice plates, Rotameters, safety valves , Thermowells
are dropped or replaced with temporary spools before hydro test.
23. Why do we provide a Dampner in the Piping of Reciprocating Pump?
Ans: To take care of Pulsation.
24.Why do we provide Full Bore Valve in connecting pipeline of Launcher /
Ans: For Pigging.
25. Which parameters will u check during checking Piping Isometrics?
Ans: Bill of Material, Pipe Routing wrt GAD, Supporting arrangement , details of
insulation, hydrotest pressure, painting specs and provision of Vent and Drains at
appropriate locations.
26. What is the ANSI/ASME dimensional standard for steel flanges & fittings?
(i) B16.3
(ii) B16.5
(iii) B16.9
(iv) B16.10
Answer (II)
27. How can flanges be classified based on facing?
a. Flat Face b. Raised Face c. Tongue and groove d. Ring type joint
28. What do you mean by AARH (Flange Finish)?
Ans: Arithmetic Average Roughness Height.
29. Which are the different types of Gaskets?
Ans: Full Face, Spiral Wound, Octagonal Ring Type, Metal Jacketed and Inside Bolt
30. What should be the relative hardness between the RTJ gasket and flange
Ans: For a RTJ flange , the joint ring should have a 30-40 Vickers hardness less than
that of the mating face of flange.( Brinnel hardness for RTJ groove shall be 20-50 BHN
more than the corresponding gasket hardness)
31. From which side of pipe will you take a branch connection?
Ans: When Fluid is Gas, Air or Steam and Cryogenic Service – Topside.
When Fluid is Liquid – Bottom Side.
32. Why don’t we take a branch for Cryogenic Service from bottom side though the
fluid is in liquid state?
Ans: There is the chance of Ice formation during normal operation and since ice flows
from the bottom of the pipe it will block the branch pipe connection.
33. Why do we provide Drip Leg in Steam Line?
Ans: To remove Condensate when there is a rise in the pipe along the flow direction. If
we do not provide the drip leg in steam line, the condensate which forms inside the pipe
will result in Water Hammer effect causing damage to piping system.
34. How do you support any small size HDPE/PVC (Plastic) pipe?
Ans: It should be supported continuously by using channel or Angle so that line should
not Sag or fall from the sleeper/rack due to uneven expansion because of Hot Temp.
35. Why do we provide High Point Vent (HPV) and Low Point Drain (LPD) in piping?
Ans: HPV – for removing Air during Hydro-test.
LPD – for draining water after conducting Hydro-test.
36. Which standard and codes will you refer while designing the piping?
Ans: Following are the codes and standards –
ASME SEC I : Rules for construction of Power Boilers.
ASME SEC VIII : Rules for construction of Pressure Vessels.
ASME B 31.1 : Power Piping
ASME B 31.3 : Process Piping
ASME B 31.4 : Pipeline Transportation system for liquid hydrocarbon and
other liquids.
API RP 520 : Sizing selection and installation of Pressure Relieving
Devices in refineries
API Std 610 : Centrifugal Pumps for Petroleum, Heavy Duty Chemical and
Gas Industry Services.
ANSI/NEMA SM 23 : Steam Turbines for Mechanical Drive Services.
API Std 617 : Centrifugal Compressor for Petroleum, Chemical and Gas
Industry Service.
EJMA : Expansion Joints Manufacturer’s Association.
OISD – 118 : Layout for Oil and Gas Installations.
IBR : Indian Boiler Regulations.
NACE MR – 0175 : Sulfide Stress Cracking Resistant Metallic Materials for Oilfield
NACE MR – 0284 : Evaluation of Pipeline and Pressure Vessel Steel for
Resistance to Hydrogen Induced Cracking.
NACE TM – 0177 : Laboratory Testing of Metals for Resistance to Sulfide Stress
Cracking in H2S Environment.
37. What do you mean by IBR and Which lines comes under IBR purview?
Ans: IBR: Indian Boiler Regulation Act.
Steam lines with conditions listed bellow comes under IBR purview –
Lines for which design pressure is 3.5 kg/sq cm and above.
Line size above 10” having design pressure 1.0 kg/sq cm and above.
Boiler feed water lines to steam generator, condensate lines to steam generator and
flash drum.
38. What are Weldolet and Sockolet? And where they are used?
Ans: Weldolet and Sockolet are basically self reinforced fittings.
Weldolet is used for Butt weld branch connection where standard tee is not
available due to size restrictions and the piping is of critical / high pressure service.
Sockolet is used for socket welding branch connection, which require reinforcing
39. What is the MOC for Superheated high pressure Steam Lines?
Ans: A 335 Gr P I / P II
Composition : 0.5 Mo(P1) /1.25 % Cr-.5 Mo(P11)
40. What is the normal upstream and downstream straight length of orifice flow
Answer : Upstream - 15D Downstream - 5D
41. What are the essential data required for the preparation of equipment layout?
Ans : 1)PFD and P&ID 2. Project Design data 3. Equipment Sizes & Buildings
42. What are the various statutory requirements to be considered during layout?
State Industrial Development Corporation (SIDC)
Central / State Enviromental Pollution Control Boards (PCBS)
Factory Inspectorate
State Electricity Boards
Chief Controller of Explosives (CCOE)
Static & Pressure Vessel Rules (SMPV)
Tariff Advisory Committee
Aviation Laws
Chief Inspector of Boilers (CIB)
Oil Industry Directorate (OISD)
Food and Drug Administartion (FDA)
Ministry of Environment and Forest (MoEF)
43. What do you mean by Composite Flange?
The flange that is made up of more than one MOC is called a Composite flange.
a. Lap Joint Flanges
Insert Flanges are a specialty in the arena of pipe size flanges and consist of two parts
- the insert and the flange ring. The flange ring is the outer part of the insert flange
assembly, containing the bolt holes.
The two piece construction of the insert flange also offers the economy of matching the
insert material to the process pipe (usually some corrosion resistant alloy) while the
outer flange ring may be manufactured from steel. When the environment req uires the
flange ring to be made of some alloy the rotating feature is still maintained.
b. RF flanges with Raised of one MOC and rest of the flange with different MOC
c. RF blind flange with an overlay of 90/10 Cuni for Sea water service.
44. What do you mean by Insulated Joint?
Ans: Insulating Joints are a prefabricated, non
separable union used to isolate specific sections of
Pipelines to prevent corrosion caused by stray
electrical currents or interference from other
pipelines and power transmission cables.
45. What are Insulating Gasket Kits?
Ans: Insulation gasket kits are designed to combat the effects of corrosion often found
in flanged pipe systems. Galvanic corrosion between dissimilar metal flanges (flow of
currents) , flange insulation associated with cathodic protection of underground piping
are also the places where Insulating gasket kits are used. It consists of
Gasket Neoprene faced Phenolic /Glass Reinforced
Insulation sleeve Reinforced Phenolic/Nylon/Polyethylene/(G10)
Insulation washer Reinforced Phenolic/Nylon/Polyethylene/(G10)
Plated Washer Electro plated steel washer
46. What do you mean by Jacketed Piping?
47. What is the min. distance to be maintained between two welds in a pipe
The rule of thumb is that the minimum distance between adjacent butt welds is 1D. If
not, it is never closer than 1-1/2". This is supposedly to prevent the overlap of HAZ s.
Minimum spacing of circumferential welds between centrelines shall not be less than 4
times the pipe wall thickness or 25 mm whichever is greater.
48. What are the different hardness tests carried out?
Brinell Hardness Test
Rockwell Hardness test
Vicker Hardness Test
49. What is the relation between Brinell Hardness No and Rockwell Hardness
22 HRC (Rockwell Hardness) = 238 BHN (Brinell Hardness No)
Piping which is recognized as providing the most
uniform application of heat to the process, as well
as maintaining the most uniform processing
temperatures where steam tracing is not capable of
maintaining the temperature of fluid constant.
Usually used for molten sulphur, Polymers service.
1.During fabrication you observed that one small crack has appeared on a fresh
plate, what type of measure you will take to obtain desired quality with minimum
First identify the exact length of crack by DP test. Drill on the end point to resist further
crack. Remove the crack portion by cutting the strip.
i. What are the fittings required for fabrication of the isometric.
ii. Find out the length of pipe required.
iii. Do joint numbering and show the following things in the isometric.
a) Shop joint
b) Field joint
c) Spool no
Drilling Point
N 173884
EL +104280
EL +103530
EL +102630
N 1736500
E 3182000
Field Joint
Shop Joint
1 INCH ELBOW 90 DEG – 1 NO
2 INCH PIPE - 4.210 MTRS
1 INCH PIPE – 1.424 MTRS
3. Describe different types of destructive and non-destructive tests?
DESTRUCTIVE TEST: Bend test, Tensile test, Impact test, and Hardness test.
NON-DESTRUCTIVE TEST: DPT, MPT, Radiography and ultrasonic test
4. What is mean by ‘PWHT’? Why it is required?
“POST WELD HEAT TREATMENT” This is done to remove residual stress left in the
joint which may cause brittle fracture.
5. What is the minimum thickness of pipe that requires stress relieving to be
done as per B31.3?
Ans: 19 mm thk.
6. What is the difference between Thermostatic and Thermodynamic Steam Trap?
Ans: Thermostatic Trap is actuated by Temp differential and is economic at steam
pressure less than 6 PSI. It is operated by the movement of liquid filled bellows or by
bimetal element which may get damaged by Water Hammer.
Thermodynamic traps are most suited to applications where the pressure downstream
of trap is always less than about ½ the upstream pressure. These are suitable for
pressure higher than 8 PSI. Water hammer doesn’t affect it.
7. What is the Code for Sour Service?
Ans: Code for Sour Service is NACE (NACE MR – 0175)
NACE: National Association of Corrosion Engineers.
8. How much should be the pressure for Hydro-Test?
Ans: Hydrotest pressure should be calculated as follow excecpt as provided against
point no-4.
1. 1.5 Times of Design Pressure.
E 3180600
E 3181400
1” line
2. For a design temperature above the test temperature, minimum test pressure can
be calculated as:
Pt = ( 1.5 X P X St ) / S
Pt: Minimum Test Pressure.
P : Internal design pressure.
St: Allowable stress at test temperature.
S : Allowable stress as design temperature.
( see SE in table A-1 or S in table B-1/2/3).
3. If a test pressure as per above would produce a stress in excess of the yield
strength at test temp. the test pressure may be reduced to maximum pressure that will
not exceed the yield strength at test temp.
4. If the test pressure of piping exceeds the vessel pressure and it is not considered
practicable to isolate piping from vessel, the piping and vessel may be tested together
at test pressure of the vessel when approved by owner and provided the test pressure
for vessel is not less than 115% of piping design pressure adjusted for temperature as
per point no 2.
9. How do you calculate the pipe spacing?
Ans: Pipe Spacing (mm) = ( Do + Dt ) / 2 + 25mm + Thickness of Insulation (mm).
Where: D0 : OD of Small size Pipe (mm).
Dt : OD of Flange of Large size Pipe (mm).
10. How do you calculate the width of Pipe rack?
Ans: W = ( f X n X s ) + A + B.
Where: s=
f : Safety Factor
= 1.5 if pipes are counted from PFD.
= 1.2 if pipes are counted from P&Id.
n : number of lines in the densest area up to size 450
= 300 mm ( estimated average spacing )
= 225 mm ( if lines are smaller than 250 NB )
A : Additional Width for –
Lines larger than 450 NB.
For instrument cable tray / duct.
For Electrical cable tray.
s : 300 mm (estimated average spacing)
: 225 mm (if lines are smaller than 250 NB)
B : future provision
= 20% of (f X n X s) + A
11. Which fluid is used in Heat Exchanger in shell side and tube side?
Ans: Generally corrosive fluid is used from the tube side (as tube can be easily
replaced) and cleaner fluid is used from shell side. Sometimes Hot fluid is also used
from the shell side.
12. What is Reynold’s number and what is the value of Reynold’s number upto
which the flow is laminar?
Ans: It’s a dimensionless number to classify the nature of flow.
Where: Re : Raynold’s no.
___ass Density of fluid.
d : diameter of Pipe.
V : average velocity of fluid.
__Viscocity of fluid.
Flow is laminar upto Re=2100
13. What are Glandless Piston Valves. Where these are used?
Ans:Glandless piston valves are maintenance free valves used in the steam service.
14. How do you carry out Estimation?
1. Input from Bid:-
P&Id, Line list, Temperature, Pressure.
Overall Plant Layout and Piping corridor plan.
Scope of work and the Specifications for the Job.
Specifications for materials like PMS and VMS.
2. Value Addition:-
Items like Valves, Flanges, Speciality items, Reducers can be estimated from P&Id.
Length of Pipes, Elbows, Width of Pipe Rack can be estimated by referring P&Id
and Overall Plot Plan.
No of Tires (on rack) can be estimated by referring the spacing required for pipes
and also the space available.
MTO for Steam Traps, Valves (for Vent and drain) can be calculated by using
Thumb Rules.
3. Loads:-
Hydro Test Loads: Can be estimated by assuming all the Pipes (on a grid) empty
except some bigger size lines filled with Water.
Actual Operating Loads: Gas lines to be considered as empty and rest of the lines
to be considered as filled with the Fluid (which they are suppose to carry in
operating condition).
The loads which ever is higher from above two cases should be referred for
structural loading.
1. What is the objective of stress analysis?
Answer :
1. To ensure that the stresses in piping components in the system are within
allowable limits
2. To solve dynamic problems developed due to mechanical vibration, fluid
hammer, pulsation, relief valves, etc
3. To solve problems associated due to higher or lower operating temperature such
as a) Displacement stress range b) Nozzle loading on connected equipments c) Pipe
displacements d) Loads & moments on supporting structure
2. What are the steps involved in stress analysis (or any stress package carries
Answer :
1. Identify the potential loads that the piping system would encounter during the life
of the plant
2. Relate each of these loads to the stresses and strains developed
3. Get the cumulative effect of the potential loads in the system
4. Decide the allowable limits the system can withstand without failure as per code
5. After the system is designed to ensure that the stresses are within safe limits
3. What are the different types of stresses that may get generated within pipe
during normal operation?
Ans: Axial Stresses (Tensile / Compressive), Shear Stresses, Radial Stresses, Hoopes
4. How are the loads classified in stress analysis package?
Ans : a. Sustained Loads 2. Occasional Loads 3. Displacement Loads (Self limiting
stresses due to thermal effects)
What are the Inputs for stress analysis of a piping system
i) Pipe Size ii) Fluid Temperature iii) Pipe Material
iv)Design pressure v)Insulation Thickness
vi)Specific gravity vii)Friction coeff. viii) Model
5. What are the sources of sustained loads generated in piping system?
Ans a. Pressure b. Dead weight of Pipe and attachments
Sustained load is calculated as
Weight of Pipe with Fluid + Pressure load + Load due to springs
6. How do you calculate the operating load?
T1 – Load due to thermal expansion
7. Give some Examples for occasional Loads.
Wind, wave & earthquake
8. Mention some of Primary Loads (Have their origin in force)
Dead Weight, Pressure, forces due to relief or blowdown, force due to water hammer
9. Mention some of secondary Loads (Have origin in displacement)
Force on piping due to tank settlement
Vessel nozzle moving up due to expansion of vessel
Pipe expansion or contraction
Vibration due to rotational equipments
10. What is the failure theory subscribed under ASME B31.3?
(i) Maximum principal stress theory (Rankines Theory)
(ii) Maximum Shear Theory
(iii) Tresca Thory
Answer : (I)
11. What are the types of failures encountered in Piping?
Answer : 1. Catastrophic Failure 2. Fatigue Failure
12. Select the failure stress range for fatigue failure due to thermal expansion as
per B31.3
(i) (1.6Sc+1.6Sh)f
(ii) 0.78 Sh
(iii) (1.25 Sc+0.25Sh)f
(iv) Sc+Sh
Answer : (III)
Sc and Sh –Basic Allowable material stress in cold & hot condtions respectively.
f ---- is the stress range reduction factor(1 for 7000 cycles
13. What is desired life cycle for Piping in operation?
Ans: Desired life cycle for Piping in operation is 20 Years (7000 Cycles).
The normal no. of cycles for which the displacement or thermal stresses are
designed is
7000 cycles
14. How do you calculate the stress developed due to thermal expansion?
Stress developed = E x e/L
E – Young’s Modulus
e- Increase in length due to thermal expansion
L – Original Length of the pipe
15. How do you calculate the thermal expansion in a pipe?
e= _ x L x Rise in Temperature
_ – Co.efficeint of expansion
L- Length of pipe
16. What do you mean by Stress Intensity Factor (SIF)? Give some examples.
Stress Intensity Factor (SIF) is the ratio of maximum stress intensity to normal stress. It
is used as safe factor to account for the effect of localised stress on piping under
respective loading. In piping it is applied to welds, fittings, branch connections etc
where stress concentration and possible fatigue failure may occur.
Eg: SIF for Reducer and Weldneck Flange : 1.0
SIF for socket weld flange : 1.3
17. Which is the Criteria for Pipe Supporting?
Ans: Following are the points which should be taken into account for proper supporting

Load of bare pipe + fluid + insulation ( if any ).
Load of bare pipe + waterfill.
Load of valves and online equipment and instrument.
Thermal loads during operation.
Steam out condition, if applicable.
Wind loads for piping at higher elevation, if required.
Forced vibration due to pulsating flow.
Bare pipe with size above 12” shall be supported with Pad or Shoe
18. What is the basic span of supports for 2”/6”/10”/24” pipe.
Basic Span is 5.5m / 9m / 11.5m / 15m respectively.
19. How do we decide the anchor / cross guide and guide for offsite rack piping
Anchor is provided to restrict all the axial and rotational movements of Pipe, whereas
Cross Guide is provided to restrict displacements of Pipe along with the axis
perpendicular to it’s centreline and Guide is provided to restrict the longitudinal
movements of pipes along with it’s axis.
20. Define a typical 6D loop supporting details (Anchor/Guide)
21. Provision of anchor / cross guide for control valve.
22. What are the things to be taken care of while doing pump piping?
Pipe strain may distort equipment alignment, so welding should be done in such a way
that the tension in the equipment flange is minimised
23. What is the Steam out condition?
Ans: Hydrocarbon lines are usually subjected to Steam Out condition and designed
and anlysed at low pressure steam design temperature (should be minimum 180
degree C) or design temp. whichever is more . Lines having negative design temp. is
analysed for both conditions seperately.
24. Where do you provide Anchor and Slotted Support of Heat Exchanger?
Ans: Anchor support of Heat exchanger is provided on the side from which Tube
Bundle will be pulled out for the purpose of Maintenance work also it is based on the
growth of the connecting piping as exchanger should grow with the piping.
25. What do you mean by Hoop Stresses and how do you calculate it?
Ans: Stresses which are generated circumferancially due to the action of Internal
pressure of pipe are called as Hoop Stress. It is calculated by
Hoop Stress (Sh) = Pdo / 4t
Where P = Force Acting from Inside.
Do = OD of Pipe.
t= Pipe Thickness.
26. How does Hoop Stress affect the system?
Ans: As per membrane theory for pressure design of cylinder, as long as hoop stress
is less than yield stress of Moc, the design is safe. Hoop stress induced by thermal
pressure is twice the axial stress (SL). This is widely used for pressure thickness
calculation for pressure vessel.
27. What is the design standard followed for the calculation of allowable forces /
Moments in nozzles of centrifugal compressor & Steam turbines nozzle?
For strain sensitive equipment piping to be routed and supported to limit nozzle
loadings and moments in equipment within allowable limits furnished by respective
vendors or in absence of vendor data API 560/610/615/621/661 & NEMA SM23.
NEMA – SM 23 (Referred by API 617) is used for compressor & steam turbine nozzle.
28. What is the mill tolerence to be considered for the thickness of pipe during
stress analysis as per ASME B31?
(i) 1%
(ii) 2.5%
(iii) 7.5%
(iv) 12.5%
Answer : iv
29. What is the purpose of providing Graphite Pads in supports below shoes?
Answer : To reduce the friction factor. The co-efficient of friction for Graphite Pads is
30. How is piping to Tank inlet nozzle is supported and why?
Ans: Piping to Tank Nozzle is supported with Spring type support (first support from
Nozzle) in order to make the Nozzle safe from the loads which occurs due to the
displacement of pipe (thermal expansion of pipe / tank material, tank settlement etc).
31. What are the two types of flexible spring hangers?
1. Constant Spring and 2. Variable Spring
32. What is the difference between Variable Spring Hanger and Constant Spring
Ans: Variables use coiled springs to support a load and allow movement. The
resistance of the coil to a load changes during compression, which is why these devices
are called "variables". Constant Spring Hanger provides constant support force for
pipes and equipment subjected to vertical movement due to thermal expansion at
locations where maintaining a constant stress is critical. This constant resistance is
achieved by having two moment arms pivoted about a common point. The load is
suspended from one of these arms, and a spring is attached to the other. With an
appropriate choice of moment arms and spring properties, a resisting force can be
provided that is nearly independent of position.
Constant support hangers are principally used to support pipes and equipment
subjected to vertical movement due to thermal expansion at locations where transfer of
stress to other supports or equipment can be critical. The maximum recommended
variation according to MSS standard from the operating load is 25% for variable spring
hangers. If the variation exceeds 25%, a constant support hanger should be used.
The constant resistance to a load is achieved by combining a spring coil with a cam
which rotates about a main pivot point. The cam is designed such that the distances
from the main pivot changes to compensate for the variable resistance during
compression of the coil. The MSS standard provides for a tolerance of 6% in the
constant load through the travel range. Constant support hangers are designed per
MSS, ANSI, and ASME standards.
The sizing of constants primarily depends on the total travel and load.
33. How much should be the difference between the load which will be taken by
Variable Spring Hanger during Cold and Hot condition of Pipe?
Ans: It should be Maximum 25% of Load for which Spring is designed.
34. Differentiate between static load and dynamic load.
Ans: A piping system may respond far differently to a dynamic load than it would to a
load of the same magnitude. Static loads are those which are applied slowly enough
that the system has time to react and internally distribute the loads, thus remaining in
equilibrium. In equilibrium, all forces and moments are resolved (i.e., the sum of the
forces and moments are zero), and the pipe does not move.
With a dynamic load—a load which changes quickly with time—the piping system may
not have time to internally distribute the loads, so forces and moments are not always
resolved—resulting in unbalanced loads, and therefore pipe movement. Since the sum
of forces and moments are not necessarily equal to zero, the internally induced loads
can be different—either higher or lower—than the applied loads.
35. Give different types of dynamic loads with example
1. Random – Wind, Earthquake
2. Harmonic – Equipment Vibration, Pulsation, Acoustic Vibration
Impulse – Fluid Hammer, relief valve opening, slug flow
76. What is Dynamic Analysis and why it is used?
Ans: Dynamic analysis is performed for all two phase lines in order to ensure that the
line supported is safe from vibrations loads which may occur during normal operation as
well as in start up or any upset condition.(Diesel mixed with hydrogen in DHDT process)
36. What is WRC 107 / WRC 297?
Ans: Localised stresses at Nozzle to Shell is calculated by WRC 107 / 297 and these
computed stress values shall be limited in accordance with ASME Sec VIII for Pressure
37. How to get the Foundation Loads?
Ans: Foundation Loads for pipe rack should include the loads of Pipes, Cable Trays
and Instrumentation duct at that location and also the design load for future tier shall be
full load of the most heavily loaded tier in addition to all other wind/seismic/fraction and
piping thermal loads for future pipes.
Load of pipes filled with water( Largest of 1st case – During hydrotesting dead
weight(wt/m X piperack spacing) of pipes + 2 –3 maximum size pipes filled with water
2nd case – Actual commissioned condition except the gas lines ) + Proportionate wt of
extra space required by client (normal 30%) + Load of 1 heavily loaded tier + Electrical
cables + Instrument duct + Guide load for 50% of lines
Guide Load = 0.3X(Dead wt of pipes at including water)
The maximum induced thermal loads on the Anchor at the battery limit shall be limited
F in kg <= 150 X NB of pipe in inches (It should be <2 tonnes)
M in Kgm <=75 X NB of pipe in inches.
Horizontal Load = 0.3 X (Dead wt of pipes including water)
This load is used for designing of foundation bolts.
Foundation loads for any vessel having agitator mounted on top should contain weight
of tank at operating or design condition (whichever is more) plus 20% of it for dynamic
38. What is the maximum expansion absorbed in loops in normal design?
Ans:10 Inches
39. What is the limiting factor in deciding the length of the spool in Jacketed
Ans: Force exerted by dissimilar expansion of inner pipe = Force exerted by dissimilar
expansion of jacket pipe
The stress developed due to this should be within limits as per ANSI B31.3
(Also fabrication constraints)
40. What is the factor to be checked concerning the expansion of header attached
to air cooler piping?
Ans: Vendor drawing to be checked to see how much movement is permitted to
compensate line expansion. To accommodate the diff. Expansion between inlet and
outlet (The inlet temperature >The outlet temperature) offset can be built in to outlet
piping to compensate for diff.expansion.
Since the tubes are of floating design the nozzle flange is of 150# and loads transferred
are to be kept minimum.
Since the tubes are of floating design, the nozzle flange is 150#. Load of the nozzle to
be kept minimum.
41. What is the maximum no. of cell nozzles connected to a single header of air
cooler piping header in normal practice?
Ans: Six nos.
42. What is fluid hammer and how it is generated?
Ans: When the flow of fluid through a system is suddenly halted at one point, through
valve closure or a pump trip, the fluid in the remainder of the system cannot be stopped
instantaneously as well. As fluid continues to flow into the area of stoppage (upstream
of the valve or pump), the fluid compresses, causing a high pressure situation at that
point. Likewise, on the other side of the restriction, the fluid moves away from the
stoppage point, creating a low pressure (vacuum) situation at that location. Fluid at the
next elbow or closure along the pipeline is still at the original operating pressure,
resulting in an unbalanced pressure force acting on the valve seat or the elbow.
The fluid continues to flow, compressing (or decompressing) fluid further away from
the point of flow stoppage, thus causing the leading edge of the pressure pulse to move
through the line. As the pulse moves past the first elbow, the pressure is now equalized
at each end of the pipe run, leading to a balanced (i.e., zero) pressure load on the first
pipe leg. However the unbalanced pressure, by passing the elbow, has now shifted to
the second leg. The unbalanced pressure load will continue to rise and fall in sequential
legs as the pressure pulse travels back to the source (or forward to the sink). The ramp
up time of the profile roughly coincides with the elapsed time from full flow
to low flow, such as the closing time of the valve or trip time of the pump. Since the
leading edge of the pressure pulse is not expected to change as the pulse travels
through the system, the ramp down time is the same. The duration of the load from
initiation through the beginning of the down ramp is equal to the time required for the
pressure pulse to travel the length of the pipe leg.
43. What is the purpose of expansion bellows?
Ans: Expansion bellows are used absorb axial compression or extension, lateral shear
or angular torsion developed in the pipes (specially near nozzles)
44. You have to connect a 20” pipe to a manhole of existing tank , how will you
go about in carrying out the suitability of the manhole flange.
45. What should be the material of shoes for supporting AS pipes & why?
Ans: If CS shoes are used Pad in contact with the pipe to be of Alloy steel to avoid
dissimilar welding at pipe. To avoid alloy steel welding and dissimilar welding fabricated
clamps either of CS or SS can be used.
46. What is the allowable stress range for CS pipes.
Ans: 2070 kg/cm2
47. What are sway braces?
Ans: Sway Braces are essentially a double-acting spring, housed in a canister. Unlike
variable effort supports, Sway Braces are not intended to carry the weight of pipework;
their purpose is to limit undesirable movement. Sway Braces act like a rigid strut until a
small preload is reached, whereafter the restraining force increases in proportion to the
applied deflection. Fig. 1
Undesirable movement can occur due to many phenomena, such as wind loading,
sympathetic vibration, rapid valve closure, relief valves opening, two phase flow or
earthquake. It may be necessary to limit this type of deflection to prevent the
generation of unacceptable stresses and equipment loadings.
The Sway Brace is a cost-effective means of limiting pipework deflection. It should be
noted however that it does provide some resistance to the thermal movement of the
pipework and care should be taken when specifying to ensure that this is acceptable.
Installation of Sway Braces will have the effect of raising the fundamental frequency of
vibration of a pipework system; this is likely to reduce undesirable deflections. Sway
Braces are often used to solve unforeseen problems of resonant vibration. For
situations where the resistance to thermal movement provided by Sway Braces is
unacceptable, you are referred to Pipe Supports Limited’s range of hydraulic snubbers
and dampers.
48. Give a typical stress report including input and output and what is interpreted
form the output.
49. For offshore structures what analysis is performed by Caesar.
50. In an offsite pipe rack change in direction during analysis it is found two
adjacent pipes are having unequal expansion with the inner pipe having 50 cm
thermal expansion. What can be done to eliminate collision during hot condition.
Ans: Use Cold Pull technique. Calculate the thermal expansion of the inside pipe, cut an
equal length form the elbow joint and then reweld with a shorter length to take care of
expansion in hot condition.
51. What are the Insulation material used for piping systems.
1. Fibrous – Rock & Glass Wool
2. Rigid - Calcium silicate, Polyisocyanurate, cellular Glass

Controlling Vessel's And Tanks


It would seem that controlling a vessel should be a very simple matter -- They don't really do anything! But then, if they didn't do anything why are there so many of them? And why do they have so many different names? Going through a typical set of Piping and Instrumentation Diagrams (P&IDs) I see the following vessels:
• Degassing Drum • Gas Separator • Storage Tank
• Feed Flash Drum • Reflux Accumulator • Day Tank
• Surge Drum • Suction Scrubber • Slug Catcher
• Lube Oil Separator • Head Tank • Deaerator
Although each of these is essentially a simple vessel or tank without any special internal structure, each serves a different purpose. Once it is clear what the purpose of a piece of equipment is, and how it functions, it will also be clear how to control and protect it. Different purposes require different controls.
SURGE TANKS. The most common function of a vessel or tank is to match two flows that are not identical in time but are expected to average out over the long run. Take a feed surge drum, for example. Flow into the unit is more or less steady but is subject to interruption. The flow to the processing unit should be as constant as possible, avoiding sudden change. Nevertheless, it, too, may be subject to interruption due to downstream conditions.
The purpose of the surge drum is to maintain sufficient inventory to feed the process and to maintain sufficient void capacity to continue receiving feed as it arrives. Clearly the tank must be large enough to accommodate any normal discrepancies between input and output over a reasonable period of time. Between the upper and lower bound, the exact value of the level does not matter.
Two separate control parameters are implied: Level and flow. Level control is no problem. Greg Shinskey1 refers to "The easy element -- capacity". A high gain, level controller connected to a valve at either the inlet or the outlet will maintain the level very accurately at its setpoint. The only problem with this approach is that it absolutely defeats the purpose of the vessel. The same effect would be achieved by blocking in the vessel and bypassing the inlet directly to the outlet.
To control flow alone is also quite simple. A flow controller at the outlet, properly tuned, will maintain a steady flow to the process. Unfortunately, there is nothing to make this flow equal to inflow. It will not even equal the average inflow unless there is something to make it do so.
What we need is an instrument that measures the accumulated error between inflow and outflow. The tank itself is that instrument!
Level = Starting Level + ∫ (Inflow - Outflow) dt / Tank Area
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(To a process controls engineer, every piece of equipment is just a big, non-tuneable instrument!) The level transmitter only transmits the process value to the control system. If we now cascade the output of the level controller to the flow controller, we have a system that has one process variable: Accumulated flow imbalance. It has only one point of control: Outflow to the process.
Surge Drum Control
To start this simple process:
• Fill the tank about half full.
• Give the level controller the current level as its set point. (PV tracking does this automatically.)
• Switch the flow controller to automatic with an estimated average flow as its setpoint.
• Switch the flow controller to cascade.
• Switch the level controller to automatic.
The control system will keep the flow "constant" but that constant varies in response to the imbalance between outflow and inflow. It is not important that the initial estimate of average flow be exact. A low guess will result in the tank level rising a little. A new, higher, estimate will result and the outflow will be adjusted accordingly. In the long term the average flow out is not an independent variable at all. It will be exactly equal to the average flow in. This can be accomplished at any arbitrary tank level. The level setpoint is based on the operator's estimate of the nature of the flow interruptions and whether the most probable upset will require additional flow or void capacity.
Should a pump be necessary to transfer the liquid from the vessel to its destination it should be placed between the vessel and the flow measurement. Further information on the control of pumps is found in Controlling Centrifugal Pumps2. This article also includes a section titled "On/off Control" for less critical level applications.
There is a long discussion on the special requirements for level control of steam heat exchangers and condensate receivers in Controlling Steam Heaters3.
Surge drums are sometimes used for gas. The abrupt flow variations of a Pressure Swing Absorption (PSA) unit, for example, often need to be smoothed out before the tail gas can be introduced into a down-stream process. In these cases, pressure takes the role that level has in a liquid process. That is, a pressure/flow cascade is the appropriate solution.
TUNING SURGE TANK CONTROLLERS. Since the exact level of a surge drum is not important, the controller can be tuned very loosely allowing the level to rise and fall in response to any short term imbalances. This exactly serves the purpose of the surge tank; tight tuning defeats it. There is a non-linear control algorithm which specializes in the type of loose control required by surge tanks. One common name is the "gain on error squared" controller. Figure 6-2 shows its characteristic. The controller responds to small errors with a small gain; it responds to large errors with a large gain. This means that in the vicinity of the setpoint, the controller allows the level to drift freely and
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the flow to remain almost constant. With luck, the level will average out again before the deviation from setpoint is too great. If the level changes far from the setpoint so that the danger of running out of capacity exists, the controller responds with a strong signal and rapidly brings the level back to near setpoint.

Another form of non-linear controller is also available: The notch or gap controller. This algorithm has the gain divided into three segments by two break points. The middle segment, on either side of the setpoint, has a low gain to avoid excessive action while the outer segments have a higher gain for a rapid return. It has the advantage of allowing the user to set the breakpoints and gains below the setpoint differently from those above. Its disadvantage is that it has four tuning constants instead of only the one found in the gain-on-error-squared controller. Some gap controllers have a zero gain in the centre segment. This is totally useless as the controller will never bring the level back to the setpoint. (No gain, no action.) Instead it will tend to use either the upper or lower breakpoint as its effective setpoint and return the level with a high gain. It should be noted that for a controller using a velocity algorithm an abrupt change in gain does not imply an abrupt change in valve position, only a change in the rate of movement. This function is more difficult to implement with controllers using the position algorithm as the controller has to be re-initialized with every gain change.
A simple proportional mode controller is sufficient for many surge drum applications. A slow integral may be used to bring the level back to the setpoint during a prolonged change in flow rate, but it should be turned off if cycling results. Do not use the derivative mode! Besides amplifying noise, derivative provides tight control by cancelling out the integrating capacity of the tank and thus defeating its purpose. A tuning rule I have heard of, but have not tested myself is
K = ΔF/F * ΔL/L
Where K = controller proportional gain
ΔF/F = the proportion of flow variations in the uncontrolled flow
ΔL/L = the proportion of level available for surge. This is the distance between the level setpoint and the nearest alarm.
This formula attempts to put the loosest level control consistent with keeping the level away from the alarms. There is a catch, however: It is necessary to predict the amount of flow variation to be expected in the future. Of course it is also necessary to do this to a certain extent when the vessel is sized.
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SUCTION SCRUBBERS. A compressor suction scrubber is an example of a vessel whose purpose is to separate, collect, and dump relatively small quantities of liquid from a gas stream. The following conditions generally apply:
• Precise level control is of no value.
• The liquid flows to some form of drain.
• Smoothness of liquid flow is of no value.
• The average liquid flow is quite small.
• The pressure differential across the valve is high.
• Relatively large slugs of liquid occur occasionally.
The last three conditions would result in a valve that is usually operating near its seat with a high ΔP. It would experience severe erosion resulting in a short, unhappy life. The solution is to control the valve in on/off or "snap acting" mode. There are several ways to accomplish this. The simplest is to tune the controller to a very high gain. This would cause the valve to spend almost all its time in the full open or closed position. Unfortunately the high-gain controller would also try to maintain accurate level control by rapidly switching the valve between these extreme positions. Any saving in seat erosion would be cancelled by a high rate of stem and packing wear. The same response can be achieved by using a simple level switch connected to the control valve via a solenoid. (Pneumatic level switches tubed directly to the valve actuator diaphragm are also available.) A level switch can be viewed as a controller with an extremely narrow proportional band (0%!) and consequently an extremely high gain (100% / 0% = ∞!).
Selecting a switch with a broad deadband results in a great improvement. The valve now remains fully open until a significant reduction in level is achieved. It then remains fully closed until the level substantially rises. With this arrangement it is possible for the valve to have both long life and peak capacity. Recent experience indicates that transmitters are more reliable instruments than switches and also demand less maintenance4. If a transmitter is used the deadband function is accomplished through logic in the control system. This would have the added advantage of allowing the operator access to the high and low setpoints. In some ways the suction scrubber acts as the exact opposite of a surge drum -- it collects slow dribbles of flow and releases them as intermittent surges.
Sometimes there is a third option -- specialized liquid dump valves. These behave somewhat like steam traps in their ability to pop open in the presence of liquid and snap shut in the presence of vapour. Since they are not general purpose instruments, it is best to use them only when there is an opportunity to test their performance; the vendor should be consulted. These devices might be very cost effective in packaged equipment such as on the discharge receiver of an instrument air compressor.
STEAM DRUMS. The purpose of a boiler steam drum is to provide space in which the water and steam may disengage. Since the drum serves at high pressures and temperatures, perhaps up to 3600 psi and 1000ºF (25 MPa and 540ºC), it is expensive to manufacture and there is considerable economic incentive to keep it as small as possible. The techniques of boiler feed water (BFW) control can be applied whenever extremely tight level control is a requirement.
The level of the feedwater in the steam drum must be kept above the bottom of the drum or a
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catastrophic explosion may result. It must also be kept below the steam outlet or liquid water will be carried over. Water droplets will damage superheater tubes, turbine blades, and other equipment. The diameter of the steam drum, and hence its cost, is determined largely by the ability of the control system to keep the water level within bounds.

LTLVRiser Tubes~~~~BFW In~~~Steam Drum1Steam Out1V1VLIC
Fig. 6-3. Single-Element BFW Control
Thus level control of a steam drum has exactly the opposite purpose of that of a surge drum: The water level must be kept within an extremely narrow band and tight control is of essence. It is a simple matter to maintain tight level control... use both the proportional and integral modes and turn up the gain! Figure 6-3, Single-Element BFW Control, shows this very simple arrangement. As always, there are problems. Firstly, high gain means extremely rapid swings in flow rate. The BFW pumps suffer under that type of abuse. There is a second problem, peculiar to boilers, called "swell". Swell is the phenomenon in which a rise in steam demand causes a drop in pressure. This in turn results in a rapid boilup within the tubes which causes the water level to rise. Paradoxically, an increased steam removal rate causes a rise in level due to the swelling of the steam bubbles. The level controller responds by reducing BFW flow at the very moment it is needed most. The swelling water soon collapses as the steam rises to the surface. Now the controller reverses its response and adds a large amount of essentially cold BFW into the system. This causes the water temperature to fall. The cooler water shrinks, lowering the level further. The use of single-element control is not very highly recommended for boilers!

http://www.drLTLVRiser Tubes~~~~~~~Steam Drum1LYBFW In1VV1TTSteam OutFT1VLICV++2VVFY2VPT22
Controlling Vessels and Tanks Fig. 6-4. Two-Element BFW Control
The disturbance to the level is caused by a change in steam withdrawal rate. Since this is a measurable quantity, feed forward can be applied to the level controller output. Figure 6-4 shows how this is accomplished. The compensated steam flow is added to the output of the level controller. Thus a rise in steam withdrawal and the swelling of the water is accompanied simultaneously with a surge of cold
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BFW. Ideally the two cancel out exactly and the controller sees no change in level at all. They will not cancel out exactly for two reasons: Firstly, there is no reason why they should. One effect or the other will predominate. They won't even be simultaneous. Secondly, the BFW flow can only equal the steam withdrawal if the range of the valve is exactly equal to the range of the compensated steam flow. Since these two functions must be exactly equal over the entire operating range, it means that the valve must be perfectly linear and that its ΔP is absolutely constant. Not likely! So the level controller still has some work to do to keep the accumulated error at zero.
The rather farcical suggestion in the previous section, piping the inlet to outlet and bypassing the vessel, suggests a solution to the valve linearity problem: Use the measurement of the steam leaving the boiler as the setpoint to a BFW flow control loop. The level should remain constant once the shrinking and swelling have reached the new equilibrium. This simplistic solution overlooks a basic principle of process control: No two measured quantities are ever identical. In other words, the two flows will never be the same and the level will rise or fall at a rate proportional to the difference. Since level is a measure of the accumulated difference, a level controller is used to correct the BFW flow. What I have just described is the classic three-element boiler level control arrangement as shown in Figure 6-5.
The diagram also illustrates a few other features. Compensation has been applied to account for the effect of pressure on the steam density and its effect on the level transmitter. BFW flow is sometimes temperature compensated since it is most probably preheated and its temperature may vary. For a temperature change from 0ºC to 300ºC (32ºF to 572ºF) the specific gravity changes from 1.000 to 0.712 and a measurement error of 15% will result.
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This detailed exposition of boiler level control is presented only to provide an example of how extremely tight level control can be accomplished when necessary. Boiler control is a rather broad subject and many articles and textbooks have already been published concerning it.
CONTROLLING LIQUID INTERFACES. It is generally assumed that level control refers to the control of a gas/liquid or vapour/liquid interface. It ain't necessarily so. An interface can occur between any two immiscible fluids. Since all gases are miscible with each other in all proportions, interface level control is always taken to mean the interface between two liquids such as oil and water.

Figure 6-6 shows an example of a boot on a crude oil separator. This vessel serves three purposes: It is a gas/oil separator, a feed surge drum and a water separator. A real vessel in this service probably contains inlet baffles and demister pads. Each of the three phases must be individually controlled. But it is possible for the gas phase to discharge to an externally pressure controlled system or even to atmosphere. (Possible yes, acceptable no.) The key to understanding the function of any separator is to think in terms of a constant inventory of each component. To repeat: each component must be controlled individually. The amount of gas flowing in must be balanced by gas flowing out. Changing pressure is a measure of the gas imbalance, therefore pressure control is the appropriate way of controlling the gas outlet. Similarly, oil level is an indication of oil imbalance and water level indicates water imbalance. None of the three streams may be controlled on flow, although a level / flow cascade is often used to smooth out flow variations to the downstream equipment. Pressure / flow cascade is unlikely to be used unless the volume of the vessel is large enough to serve as a gas surge drum. Level / flow cascade on the water is unlikely since the water probably drains to a collection system that itself serves as a surge drum to a number of separators.
Sometimes the ratio of water to oil is too great for a boot separator. In such cases a weir may be used to divide the vessel as shown in Figure 6-7.
Certain precautions must be taken to make sure that the level transmitter actually gives a true indication of the interface. There are basically two varieties of level indicating devices: The first measures the distance of an actual interface from some fixed point. Ultrasonic and radar devices belong to this group. These would be ideal for the purpose except that they are often not suited for installation in pressurized vessels. Furthermore they have difficulty "seeing" anything other than the
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very top interface. Even surface foam and condensation on the instrument “window” can confuse them.

The second, and more traditional, variety integrates some particular property, such as density or dielectric constant, over a span. Displacers, differential pressure transmitters, bubbler tubes, nuclear densitometers, capacitance probes, and even gauge glasses all belong to this variety. The key to successful measurement is that the level sensing device must sense only the two fluids bounding the interface. For a gauge glass this means that the lower tap must be in the lower of the two fluids and the upper tap must be in the fluid immediately above it. There may be NO intervening phases.
Figure 6-8 shows what happens when a gauge glass is connected to a vessel containing a vapour and two liquid phases. Assume that equal amounts of a liquid with Sg = 1.0, e.g. water, and a liquid with Sg = 0.5, perhaps propane, gradually flow into the vessel. Assume further that the span of the gauge glass is four feet, beginning one foot from the bottom of the vessel.

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As the level of the propane rises, it flows into the glass. As both liquids rise further, water begins to enter the bottom of the glass. This is the state shown in vessel A. Up to this point, the glass shows a true indication of the level of propane in the vessel. Once water enters the glass, the propane is cut off. A constant plug, one foot thick, floats on top of the water. Its level no longer bears any obvious relationship to the actual level in the vessel. This is state shown in vessel B. The only relationship between the vessel and the glass is that the hydrostatic pressure is the same for both at the point where the glass taps into the vessel. A gauge glass is really nothing more than a manometer.
Once the level of the propane rises above the upper tap, it flows into the glass and the two interface levels adjust to the same elevation, as shown in vessel C. The gauge will continue to read correctly as long as its lower tap is in the water and the upper tap is in propane. If either fluid is withdrawn so that the upper tap is in the vapour space, the glass will once again read falsely.
This same analysis applies to any type of level indication based on density. Remember that a ΔP transmitter only gives a single reading, i.e. differential pressure. Therefore only a single quantity can be inferred. If the instrument is affected by only two fluids, it can yield the correct interface level between the two. If there are more than two distinct phases within the span of the two taps, it will give a reading based on the average densities of all the fluids within its span.
Capacitance or nuclear level transmitters will give similar results in multiphase situations, based on the average dielectric or nuclear absorption constants, respectively.
So... how can the process controls engineer be assured that the level readings are meaningful if even a gauge glass can't be trusted? Plan "A": Make the entire vessel out of glass. This isn't usually practical so we must fall back upon Plan "B": Every section of a gauge glass must have separate taps into the vessel so that each pair of taps has no "hidden" phase floating in between. Either that, or accept the fact that until the interface reaches its "normal" range, gauge glasses and transmitters will read falsely.
SLUG CATCHERS. Slug catchers are a special instance of three-phase separators frequently found in oilfield service. In addition to the usual separation functions, they are required to serve as surge tanks that can smooth out intermittent flow and also handle occasional very large surges in inlet flow. This is done by having two controllers connected to the oil-side transmitter. The oil overflow controller has its setpoint slightly below the top of the weir. In this manner, any surges can be accommodated by the large volume above the weir. This is in fact a non-linear, adaptive gain transmitter since transmitter gain = Δ output / Δ volume.
The inlet controller responds to the same level but has its setpoint just below the top of the vessel. It takes action only when the level rises to its set-point. This would happen if an unusually large slug of liquid arrived or if an upset in the downstream process caused the system to back up into the slug catcher. The facility would then be operating under "capacity control". Facilities lacking the capacity control feature are likely to experience a high level shutdown precisely when they are attempting to operate at maximum throughput. Not a very desirable occurrence.
It is common for level controllers to be tuned using both the proportional and integral modes. Since the inlet controller is normally functioning with the level well below its setpoint, reset windup will
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occur. This is a phenomena in which the controller attempts to raise the level to the setpoint by forcing an ever higher signal to the valve. This does not work, of course, since the valve is already wide open. If a sudden surge arrives that abruptly raises the level to the setpoint and beyond, the controller will be slow to close the valve since it has "wound up" in the opposite direction. Some form of anti-reset windup is required to prevent an unwarranted high level shutdown under these circumstances. It is probably a bad idea to use an equal percent valve in this application since it, also, is likely to respond slowly to a sudden demand.

It is possible to control the outlet and inlet valves with a single, split-range controller. This method accomplishes the required function of preventing high level shutdowns but has a serious disadvantage. If the setpoint of the combined inlet/outlet controller is set below the top of the weir, it will not take full advantage of the surge capacity of the vessel since the inlet will begin to close well before the top of the vessel is reached. If the setpoint is above the weir, it defeats its purpose by allowing mixed feed to flow directly to the oil outlet before it has time to separate. Thus a split-range controller will sacrifice either separation quality or surge capacity.
PRESSURIZATION SYSTEMS. A tank, vessel, or drum may require a pressurization system for any of a variety of reasons:
• The surface of the liquid in a reflux drum consists of a liquid at equilibrium with its vapour. There may not be sufficient gravity head to provide the net-positive-suction-head required to operate the reflux pump without cavitation. Raising the vessel high in the air above the pump is one way of providing this. Unfortunately the condenser providing the liquid, drains by gravity so it must be raised even higher. The entire arrangement can become extremely expensive. An-other method is to use a canned pump which is sunk deep into the ground. This can also get
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pricey. A blanket gas pad may be a relatively inexpensive way of providing the necessary pressure.
• The liquid in a storage tank is subject to oxidation, e.g. the surge tank of a glycol-based heat exchange system. A blanket of fuel gas will prevent the tank from breathing air as it cycles from empty to full and back again.
• The liquid in a storage tank forms an explosive mixture with air. (A rather extreme form of oxidation!) A continuous gas purge may be required to prevent this.
• The storage tank vents to a flare or other vapour collection system. A gas supply must be provided to make up any volume withdrawn when the withdrawal rate exceeds the fill rate. In other words, the pressurization system serves as a vacuum breaker.
A simple way of providing pressurization is to have a regulator connected to a source of pad gas and a second, back pressure regulator, connected to the vent. Care must be taken to set the back pressure regulator setpoint slightly higher than that of the inlet regulator. If there is no gap between the two settings, the pad gas will blow straight through to the vent. Remember that setting them to the "same" pressure is meaningless.

Often it is necessary to install a complete control loop including a pressure transmitter, a controller, and two valves. This has the advantage of allowing the panel operator to monitor and adjust a single setpoint. It also allows over- or under-pressure alarms to be easily provided. Figure 6-10 shows how the complete pressure control loop is arranged. For the most part it is pretty simple but there are two things to watch for: Firstly, there must be a gap between the action of the two valves. That is the reason for the split range values not meeting at 50%. Secondly, the failure mode of the valves must be taken into account. Since the two valves have the opposite effect, they must have opposite failure modes if they are to be operated by the same control signal. A DCS allows the output of the controller to drive two separate output modules, each characterized in its own way. This means that it is possible for the first 45% of the controller output to produce a 100 to 0% signal for the fill valve, and the last 45% of the output to produce a 0 to 100% signal for the vent valve. In this way both failure modes are accommodated and overlap of valve openings is impossible. The gap in the middle does not cause a problem for the controller. Integral windup will move the output quickly through the gap whenever there is a deviation from the setpoint. The reader should note that the split range control described in this paragraph is not at all the same as that described in the section on Slug Catchers.
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It is possible to achieve the same effect by using a specialized, three-way valve that provides a gap in the middle. Most three-way valves are designed to have full overlap as they are intended for use in diverging/converging service. (If anyone knows of a centre-gap, non-overlapping valve, let me know.)

A number of vendors sell specialized gas blanketing systems capable of self-contained action. They consist of regulators with the very large diaphragms required to drive valves with pressures as low as 0.5" WC (125 Pa, 0.3 oz/in2). Such systems are especially useful now that ever more stringent regulations concerning the emission of volatile organic compounds (VOCs) are being enforced. Figure 6-11 shows one typical arrangement.
Many factors enter into the correct specification of the setpoints and sizes for the various regulator and relief valves. These include:
• The maximum allowable pressure of the tank.
• The maximum allowable vacuum of the tank.
• The vapour pressure of the stored liquid.
• Inbreathing rate dependent on pump-out rate.
• Outbreathing rate dependent on pump-in rate.
• Vapour thermal expansion and contraction rate.
• Tank surface area and insulation.
Table 6-1 provides setpoints applied in a specific case. It must be remembered that actual values differ widely. API 2000, Venting Atmospheric and Low-Pressure Storage Tanks5 and tank vendors provide much information, however it may be advisable to consult a specialist in the field.
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oz/in2 "WC kPa
Maximum Allowable Pressure 4.0 6.9 1.7
Manway Setting 3.5 6.1 1.5
Relief Valve Pressure Setting 3.0 5.2 1.3
Vent Regulator Setting 2.0 3.5 0.9
Fill Regulator Setting 0.5 0.9 0.2
Relief Valve Vacuum Setting -.05 -.09 -.02
Maximum Allowable Vacuum -1.0 -1.7 -0.4

Table 6-1. Typical Tank Blanket Pressure Settings
A brief sermon on tagging: According to ISA 5.1, Instrumentation Symbols and Identification6, all forms of relief valves including pressure, vacuum, spring- or weight-loaded, with or without a pilot are tagged "PSV". Common abbreviations such as "PVSV", "PVRV" or "PRV" have absolutely no official status and therefore are not acceptable as tags on P&IDs.
LEVEL MEASUREMENT. Level measurement is deceptively easy, yet it seems that more time is spent specifying level instruments than any other. The reason is that the correct installation of level instrument is an interdisciplinary effort involving Process, who set the basic functional requirements; Mechanical, who have various constraints such distance of taps from seams; Piping, who have accessibility and orientation requirements; and Instrumentation, who must select from a finite catalogue of available instruments.
Actually this task has become considerably easier in recent years due to the increased use of ΔP transmitters and other instruments such as ultrasonic and radar which do not have a predetermined span. There is no longer any significant penalty in either cost or accuracy if the instrument is specified to cover a broad span. For horizontal vessels the top connection should be vertical at the top of the vessel. The bottom connection should be horizontal a few inches from the bottom. This is necessary to prevent the accumulation of sediment. These connections no longer need to be in the same vertical plane nor do they require the same orientation.
Vertical vessels may still require a bit more attention. While a top-to-bottom span would be ideal, there may be trays, packing, or other internals that would cause a differential pressure in response to flow. It is also necessary for the level connections to remain clear of welding seams. This requirement may cause problems if alarms or other setpoints need to be near the bottom or top of the vessel.
The design process begins with the basic information on a P&ID in a form similar to that shown in Figure 6-12. A brief outline of the vessel including the bridle, if any, holding a gauge glass and a transmitter are shown. The desired values for the level alarms and the setpoint of the controller are also shown.
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The Control Systems engineer must first decide if a ΔP transmitter is the appropriate choice. Let us assume it is. He/she must then try to find an appropriate span for the transmitter. A good rule of thumb is that alarms should not be set any lower than 10% or higher than 90% of the transmitter span. (Shutdown trip settings should not be closer than 5% from either end of span.) Since the two alarms are 42" - 6" = 36" apart, the span should be 36" x 1.25 = 45" thus allowing the alarms to be at 10% and 90% of span. This seems fine, but there is a problem. The first thing to determine is whether the vessel measurements are from the tangent line, from the seams or from some other reference point. In this particular case the vessel title block indicates that measurements are tan-to-tan. Since seams are generally 2" inside the tan lines, the lower tap of the transmitter is ½" above the seam. That is not acceptable. Mechanical considerations often dictate that nozzles may not be welded within 6" of a seam. This means that the lowest transmitter tap cannot be lower than 8" above the bottom tan line. The highest tap cannot be higher then 8" below the top tan line. This implies that the maximum transmitter span on a 48" T/T (tan-to-tan) vessel is 32". Alarms at 10% and 90% must be placed at 11.2" and 36.8". At this point, the Instrument Engineer becomes a broker between Process and Mechanical to help them find a compromise. Alarms at 11" and 37" are agreed upon. Don't forget to transfer this new information back to the P&ID!

Fig. 6-12. Level Setpoints
It is a great convenience to the maintenance staff if the span of the transmitter is exactly equal to the span of the gauge glass. This is not always possible with displacers since both the gauge glasses and the transmitters come in fixed spans. However, it can easily be done for ΔP transmitters. The centre line of the sensing taps must be located at the tops and bottoms of the visible glass. Unfortunately, the associated valves bring the centre line of the gauge glass tap 4½" below the bottom of the glass. Fortunately, in extreme cases, it is possible to place the lower gauge glass tap below the lower bridle tap. There can be no meaningful readings below the lower bridle tap of course, since the bottom of the glass can never drain back into the vessel. As long as the glass itself does not go below the lower tap, it's OK. Since gauge glasses come in fixed lengths, it may not be possible for the upper tap of the transmitter to match the top of the visible glass. Remember that transmitter calibration will be more difficult if the upper tap does not fall within the range of the visible glass.
Occasionally it is necessary to connect either the top or the bottom taps to interconnecting pipe instead of to the vessel itself. If the taps are attached to inlet or outlet lines, the level signal will be affected by flow rate. This effect can be seen in coffee percolators: The level in the gauge glass bobs up and down as coffee is drawn into a cup.
SEALS. Diaphragm seals have become a very popular accessory to ΔP-based level transmitters. A very thin metal diaphragm isolates the transmitter from the process. The space between the
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diaphragm and the sensor itself is filled with a fluid such as silicone. The pressure changes are communicated through the diaphragm to the transmitter via an armoured capillary tube. The volume change between minimum and maximum pressure is extremely small in a modern transmitter; the amount of flex in the diaphragm is correspondingly small. The net effect of this is that the introduction of a diaphragm into a measurement system introduces an error of only several centimetres or less. This is seldom significant in level applications. A second effect is that only an extremely small amount of liquid movement actually occurs in the capillary tubes. This, together with modern low temperature fill fluids, means that the instrument response does not slow down too much on cold days. Figure 6-13 is an example from one vendor's catalogue.
Fig. 6-13. A Typical Differential Pressure Transmitter with Diaphragm Seals
Seals should be considered whenever one or more of the following conditions apply:
• Dirty service - Whenever the process fluid is liable to plug the impulse lines, a diaphragm seal may be installed. It should be isolated from the vessel by a full-ported valve, NPS 2 or 3. Note that two NPS ½ taps are provided on the diaphragm housing for calibration and flushing connections. Seals are especially useful in sanitary service where all hardware in contact with process fluid must frequently be thoroughly washed. It is a good idea to use seals and capillaries of equal length on both the upper and the lower leg in order to maintain a balanced response to errors.
• Corrosive service -- Diaphragm seals made of corrosion resistant materials originated in corrosive service where they were referred to as 'chemical seals'. While the use of full-ported
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connections is not required in corrosive service, it is a good practice to maintain even if it might look strange to have 'such a big valve' for 'only' an instrument.
• Freeze protection -- Diaphragm seals may eliminate the problem of freezing impulse lines. However, in extremely cold weather it may still be necessary to heat trace the capillaries to prevent measurement response from being excessively slow. Self-limiting electrical heat trace is the only way to go! Any heat trace system involving a thermostat will introduce spikes into the measurement system as the heat is switched on and off.
• Uncertain phase -- This is the most frequent of all seal justifications. A warm vapour in equilibrium with its liquid will undergo condensation in the upper impulse line. Cold equilibrium liquid may experience boiling in the lower impulse line. Thus the measurement will slowly drift as the tube fills. Depending upon service and ambient temperatures, condensation and boiling may even alternate throughout the day. If this situation exists, the measurement becomes worthless. The traditional solution is to fill the upper line with a non-volatile, process compatible fluid. Depending on process and ambient conditions this might be water, glycol, oil or something else. The use of fill fluids introduces maintenance problems because any attempt to 'null' the transmitter by opening the equalization valve will drain the upper fluid into the process. It can only be replaced by climbing to the top of the vessel and filling the tube again. Bubbles are also a source of error. Seals provide a captive fill fluid that cannot be lost, does not form bubbles and cannot contaminate the process. (Did I say foolproof?)
UNDERGROUND TANKS. A special requirement concerns underground (UG) tanks. Modern steel UG tanks have a double wall construction. Requirements are outlined in CAN/ULC-S603, Standard for Steel Underground Tanks for Flammable and Combustible Liquids7. The two walls of the tank are approximately an inch (2 cm) apart. A vacuum of 51 kPa (7.5 psi) is drawn on the interstitial space so that the two surfaces are, in many places, actually in contact with each other. A vacuum gauge is connected to the interstice. It must read at least 42 kPa (6.1 psi) of vacuum before the tank may installed. If the reading is ever less than 34 kPa (4.9 psi) the tank should be removed from service and steps taken to determine the cause of the leak. These values are summarized in Table 6-2, below. If a facility has many UG tanks, it may be desirable to connect the tanks to the central control system by means of vacuum transmitters. Low vacuum alarms can then be configured to alert the operators of any cases of leakage.
Interstitial Vacuum psi kPa
Required at manufacturing 7.5 51
Minimum acceptable for delivery 6.1 42
Minimum allowable in service 4.9 34

Table 6-2 Interstitial Vacuum Requirements for Underground Tanks
VOLUME MEASUREMENT. Most vessel and tank content measurements are made in the form of level. When true volume is required for such purposes as custody transfer, the tank volumes are calculated taking into account all details of their geometry as well as dimensional changes resulting
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from the pressure exerted by the density of the liquid. The results of these calculations and calibrations are tabulated by the manufacturer in a form known as “strapping tables”.
True volume measurement is seldom relevant for control purposes since setpoints for controllers and alarms are usually related to specific geometric features of the vessels. The level must often be kept below the vapour outlet or a weir. A frequent requirement is that a specific head be maintained to prevent pump cavitation. Sometimes the requirement is simply to maintain the level near midpoint in order to provide surge capacity. None of these applications benefit from true volume compensation. Figure 6-14, Volume vs. Height for Cylinders and Spheres, provides the correct mathematical relationship between level and volume for these two vessel styles if true volume measurement is actually required. It can be seen that between 10% and 90% little is gained by applying the rather complex calculations required for volume control.

0% Actual Volume 100%0% Transmitter Reading 100%0% Actual Volume 100%CylinderSphereRR-hohLh0% Transmitter Reading 100%RR-hhR
Volume = (R2L/2)(2θ - sin 2θ) Volume = -(π/3)h2 (3R - h)
Where h = height of liquid in vessel
R = radius of vessel
L = length of cylinder
θ is radians and cos θ = (R-h)/R
Note: The volume contained by elliptical vessel heads is ½ that of a sphere of equal radius.
Fig. 6-14. Volume vs. Height for Cylinders and Spheres
SAFETY. Vessels and tanks are probably the most hazardous pieces of equipment in any plant. Duguid’s database20 shows that 22% of all safety incidents are related to storage and blending. This
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may seem a little surprising until one considers that they store energy as well as material. For example:
• A vessel holding a compressed gas can cause a tremendous explosion if it ruptures. That is why "hydrotesting" with air or nitrogen is far more dangerous than with water.

Torrent Kills Nine
The Associated Press
MELILLA, Spain November 19, 1997
A water tank burst and sent a river of water surging through a busy market area in a Spanish enclave on the northwestern African coast, a city official said Tuesday.
Nine people, including two children, died and 30 were injured.
The accident unleashed 24,600 litres of water which dragged cars and market talls through Melilla's streets.
• Storage tanks can hold a considerable amount of gravitational energy. The most notorious example of this energy being released is the infamous "Boston Molasses Disaster" which occurred January 15, 1919. A tank located at the top of a hill ruptured and released two million gallons of molasses down a narrow street. Twenty-one people were killed and 150 were injured.
• The contents of the tank or vessel can be flammable. While a line rupture external to the tank may be the cause of a fire, it is the reservoir of flammable fluid inside a tank that turns a minor fire into a major one. API RP 750, Management of Process Hazards, specifically addresses this point, however, it does not offer much in the way of solutions.
• The contents of a tank can be lethal. The February 1984 release of methyl isocyanate in Bhopal, India was the worst non-nuclear industrial accident in human history. Over 2000 people were killed by the toxic vapour.
The single, most comprehensive guide to the design of vessels is the ASME (American Society of Mechanical Engineers) Boiler and Pressure Vessel Code8. This rather large document deals with all aspects of vessel design, construction and operation. Section VIII, Parts UG-125 to 136, in particular, deal with the requirements for pressure relief.
The pressure relieving requirements for non-pressure vessels, i.e. tanks, are covered in detail in API Standard 2000, Venting Atmospheric and Low-Pressure Tanks5.
Most safety related design practices applying to vessels and tanks are beyond the scope of the instrumentation and controls engineer; relief valves are an exception. Their correct sizing and selection is too broad a subject to be covered in this article especially since there is already a lot of material in print concerning them. Items 9 through 14 of the references below deal extensively with this subject. The earlier section, Pressurization Systems, gives a typical example of pressure protection for an atmospheric tank.

Manufacturing Process Encyclopaedia

Water jet cutting
Other names / variants: Hydrodynamic machining, Abrasive jet cutting

• Jet cutting (or hydrodynamic machining) has been widely used in the food industry for a long time, but more recently it being taken up by general engineering manufacturers.
• The basic process is very simple – a concentrated jet of fluid at almost supersonic velocity is directed at the workpiece which literally ‘blasts’ the material out of the cut.
• In the food industry, oil is used which provides a hygienic means of cutting.
• In metal cutting, fine abrasives in water are used – steel up to 100mm thick can be cut
this way.
• All though the process is quite slow, sheets can often be stacked up and several cut at once.
• The primary advantage for metal cutting is that the process generates very little heat, so the material is not affected in any way.

Process details

Abrasive jet machining
An abrasive jet uses water that is pressurized up to 40,000 pounds per square inch (psi) and then forced through a small sapphire orifice at 2500 feet per second, or about two and half times the speed of sound. Abrasive (often garnet) is then pulled into this high-speed stream of water and mixed with the water in a long ceramic mixing tube. A stream of abrasive laden water moving at 1000 feet per second exits the ceramic tube. This jet of water and abrasive is then directed at the material to be machined. The jet drags the abrasive through the material in a curved path and the resulting centrifugal forces on the
particles press them against the work piece. The cutting action is a grinding process where the forces and motions are provided by water, rather than a solid grinding wheel.

Materials and shapes
• Abrasive waterjets can machine a wide range of thicknesses and materials, including metals, plastics, glass, and ceramics.
• Materials cut by the abrasive jet have a smooth, satin-like cut edge, similar to a fine sandblasted finish.
• Little heat in machining process.

Vacuum forming
Other names / variants: Thermoforming

Almost the opposite of blow moulding - with sucking instead of blowing! As a result, the two processes are useful for different types of shape, although both can only produce parts with thin walls. This process is more properly called thermoforming and relies on the sudden drop in strength and stiffness of thermoplastics above a certain temperature.

Materials and shapes
Only suitable for thermoplastics and some polymer foams. Shapes should have constant section thickness and not 'curve-back' on themselves. Parts cannot have holes or openings. Surface texture good, but fine detail in mould cannot be copied. Suction holes in mould need to be small to avoid leaving a mark on the product. Near-net-shape, but often leaves some waste material that needs trimming (and is difficult to recycle).

Cycle time is limited by heating and cooling of the sheet. Normally cycle times of 5+ units a minute can be achieved. Production rate can be increased by multi-part moulds, although extra trimming will be required. Manual equipment is cheap enough to use in a school workshop. Fully automated equipment can cost over £250,000. Moulds are usually aluminium (although wood can be used for small-scale production) and so relatively inexpensive. Manual systems viable from 1 - 1000 parts. With fully automated systems, only becomes economically viable for batches over 10,000.

Typical products
Advertising signs, bath panels, washing-up bowls, packaging.
Turning is unusual amongst the machining processes in that it is usually the workpiece that moves, whilst the cutting tool remains stationary. Lathes in metalwork shops usually have single point cutting tools. Lathes in woodwork shops often have tools with simple shapes to make turning of complicated shapes more simple. Lathes in industrial woodworking have large, intricate cutting tools, capable of shaping a complicated piece with only a few inserts of the tool.

Materials and shapes
• Woods and metals are the most commonly turned materials, although difficulties arise with the high-strength metals.
• It is possible, but unusual, to turn polymers. Rigid polymer foams are sometimes turned when producing models for prototypes.
• Turning is usually used to produce parts with radial symmetry (i.e. based on a cylinder).
• It is possible to produce other shapes, e.g. a helix or screw thread, by turning the part slowly and moving the cutting head at a constant rate.
• Wood is the most commonly turned material, as it is easy to produce a wide variety of aesthetic
• The use of dedicated lathes for metal turning is rare on an industrial scale, except for
• Where metal turning is required industrially, it is usually done as part of the function of a
machining centre.
• Wood turning for mass production uses dedicated tooling to dramatically increase production
rates and hence reduce costs.
Typical products
• Chair legs
• bowls
• candlesticks
• large threaded shafts
Transformation hardening
Other names / variants: Laser hardening, Induction hardening, Flame hardening
Transformation hardening is often used in addition to carburising or nitriding, and is primarily used to
improve the mechanical properties of the surfaces of steel components. There are many ways to
"transform" the surface microstructure, but all of them involve heating of the surface followed by a
rapid quench (either in oil or water, or by a "self-quench" because the bulk of the component will still
be cold).
• Flame hardening uses a flame gun to provide the heating. It is inexpensive and flexible;
however it is quite slow, difficult to control accurately and not easily automated. Only external
surfaces can be treated.
• Induction hardening works by placing the component in a high-frequency magnetic field. This
"induces" a current in the surface and so heats it rapidly. It can be used to uniformly treat large
components such as the rolls for a rolling mill. Although this process is expensive and requires
some dedicated tooling, it is easily automated and can be applied accurately – e.g. to just the
teeth on a gear cog.
• Laser hardening works by focusing a laser beam on to the surface to provide very rapid
heating. As a result, a self-quench is usually sufficient. The equipment is very expensive and not
economic for large surfaces, but automation is straightforward and very precise control can be
Surface treatment (generic)
Related processes in this database include: Case hardening, Transformation hardening, Surface
coating Peening
Surface treatment processes apply primarily, but not exclusively to metals. After a component has been
formed and finished (e.g. by grinding), it may still not have acceptable surface properties. There are 4
main reasons why the surface properties may need altering:
1. Improve wear resistance.
2. Improve corrosion resistance.
3. Improve fatigue resistance.
4. Change the aesthetic appearance.
There are various ways these aims can be achieved:
• Coating the surface in a new material - e.g. painting, electroplating
• Altering the surface chemistry/microstructure – e.g. carburising, transformation hardening
• Changing the mechanical properties of the surface – e.g. shot peening, planishing.
Surface coating (generic)
Other names / variants: Chemical vapour deposition (CVD), Physical vapour deposition (PVD),
Painting, Varnishing, Electroplating
By applying a surface coat of a different material, dramatic changes in the surface properties are
possible. Normally the materials used for the coats are too expensive, or have the wrong bulk properties
to use for the whole components. There are several ways to coat the surface of a component:
• Painting / varnishing. Commonly used to provide corrosion resistance for woods, but also
widely used for metals. Relatively inexpensive and flexible.
• Electroplating is a relatively inexpensive way of providing a surface coat, although it relies on
the component being a good conductor and only certain coats are possible.
• Physical vapour deposition (PVD) or sputtering works by "shooting" a fine spray of droplets
at the component. It is mainly used for metals and ceramics. Although very expensive, it can
provide excellent surface properties for high-performance drill bits etc.
• Chemical vapour deposition (CVD) is similar to PVD, but the surface is formed by a chemical
reaction with a special gas rather than using a spray.
Soldering and brazing
Soldering and brazing differ from welding because only the filler melts, not the materials that are
joined. Soldering differs from brazing by the melting temperature of the filler alloy - this is usually
below 450oC for soldering and above 450oC for brazing. Soldering using lead-tin alloys was the first
hot joining process, used as far back as 4000BC.
Materials and shapes
Brazing is usually used for joining metals, and especially where the parts are not of the same material.
Most geometries are possible; however, good join alignment is essential to achieving a strong joint.
Mechanical cleaning or the use of flux is needed to give good joint strength. The strength of the joint is
also dependent on good design. Because of the low melting point of the filler, soldered joints have
limited use at high temperatures. Also, the joints are usually not strong and therefore not used in loadbearing
situations. Soldering aluminium and stainless steel is difficult because of their strong oxide
Equipment is generally low cost, except where automation is used. The need for good joint alignment
usually means fixtures are required, adding to the cost. Wave soldering is the most economic means of
soldering large batches of printed circuit boards.
Typical products
Plumbing, electrical circuits
Sintering and HIPing
There are 2 main types of sintering: with pressure (hot pressing or pressure sintering) and without
pressure (pressureless sintering). A variant used for 3D shapes is called hot isostatic pressing (HIPing)
Much of the research in powder processing is to obtain good quality powder, as this helps to achieve a
good quality component.
Materials and shapes
Mostly used for small (<2kg) components. Dominant method of producing ceramic components. For
non-HIPing, sides must be parallel to allow ejection of part. HIPing can work with complex 3D shapes.
Very good dimensional accuracy (near net-shape process) with 100% material utilisation.
The machinery is expensive, and can cost well over £100,000 for HIPing. The dies are dedicated,
expensive (£5,000+) and need to be replaced after about ten thousand uses. They can take several
weeks to manufacture, so prototype testing is slow. The production rate is dominated by the sintering
stage and is therefore quite slow (2-20 per hour). Because there is little competition, can be economic
for small batches (1,000+) - although still not cheap!
Typical products
cutting tool tips, spark plugs, electrical insulators
Sheet forming
Sheet metal forming (also called pressworking) is among the most important metalworking processes.
It is used in the manufacture of a wide range products as there are many different forming operations
including blanking, drawing, pressing and bending. Sheet metal is produced by rolling and is generally
coiled prior to forming. Parts made this way and subsequently mechanically fastened are said to be
Materials and shapes
Sheets are usually less than 6 mm thick. Dominant material used is mild steel. Blanking (shearing) is
used to cut parts for subsequent processing, sheet is shaped with bending (1-D) and drawing (2-D),
pressing contains elements of all three. Surface finish is usually good, but this is dependent on good die
design and quality. A wide variety of shapes can be made, but die design must account for the elastic
'springback' of the sheet after forming. Some scrap is always produced and cannot be directly recycled.
Primarily used when near-net-shape processes are impractical in terms of time or materials e.g. for car
body panels. Simple manual equipment can cost only a few thousand pounds, but is only used for
prototyping and small batches as the production rates are low. Automated tooling (which can be
expensive) is usually dedicated to individual components, so is normally only used for long production
runs in order to be cost-effective. Production rates with automated equipment can be very high (drinks
cans can be produced at almost a 1000 a minute).
Typical products
Cans, washing machine cases, car body panels, kitchen utensils, hubcaps, metal desks.
Sand casting
Other names / variants: green sand casting
• Sand casting is the oldest form of casting and has been used for millennia.
• It is still widely used today and in the US alone about 15,000,000 tonnes of metal are cast every
• Although almost any sand can be used, a mixture of synthetic sand, clay and water, called green
sand, is preferred by most foundries.
Materials and shapes
• Most metals can be cast, the limit is the melting temperature - the higher it is, the greater the
• There is almost no limit to the size of a sand casting - casings over 5m wide are routinely made
(e.g. ship propellers).
• Most shapes can be made, but the surface often has a characteristic rough finish which may
need machining.
• Removing the extra material left from risers/gates etc. can also greatly add to the cost of the
finished product.
• Porosity can be a problem leaving parts that are prone to cracking.
• The basic equipment cost is low - from £500 to £3,000; automation and higher temperature
furnaces can increase this a lot. Dies can be cheap, but take some time to make.
• The limit on the production rate is usually the cooling. Small parts can be produced at several an
hour - large parts can take hours or even days to cool fully.
• The labour intensive nature of the process mean it is usually only economic for small batches,
although dedicated automation can increase this to 10,000+.
Typical products
• Engine blocks
• cylinder heads
• pump housings
• machine tool bases
• ship propellers
Rotational moulding
Other names / variants: Rotomolding
• Think of a large polymer product, and the chances are it is made by rotational moulding.
• This versatile process is surprisingly inexpensive and is used to make a wide range of everyday
• The main disadvantage is the low production rate which usually limits it to smaller batches.
Process details
Stage 1: Plastic is introduced to a mould in powder form up to the mass required for the required
Stage 2: The mould is then closed and passed into an oven chamber. The mould is then heated
externally to a temperature typically between 220°C and 400°C and is rotated around both vertical and
horizontal axes.
Stage 3: As the mould rotates, the inner surface passes through the mass of powder at the bottom of the
mould. As the mould heats up, the powder begins to melt and adhere to the inner surface of the mould.
The mould continues to rotate in the presence of heat and more plastic melts and builds up to produce
an even layer over the surface of the mould. The mould is then withdrawn from the oven whilst still
rotating and moved into a cooling chamber.
Stage 4: Cool air is directed at the mould and in some cases water jets are used to cool the mould.
When the plastic inside the mould has become solid, the mould can be removed from the cooling
chamber. The plastic component is then removed from the mould and allowed to finish the cooling
process unrestricted by the mould.
Materials and shapes
• Mainly for thermoplastics (especially polyethylene), but some thermosets can be used.
• Used to produce containers and similar hollow products with uniform thin sections.
• Tanks up to 4m across can be made this way; wall thicknesses as low as 0.4 mm are possible.
• Products are near-net-shape and rarely need further finishing.
• Parts do not have to have circular cross-section.
• The surface finish depends on the quality of the die surface; it is possible to include surface
detail such as logos.
• Metal or polymer inserts can be moulded-in during processing.
• All material is used in the product so there is no scrap.
• Parts with large openings may be produced in pairs in a single mould and separated after
removal, or through use of insulation in mould.
• The plastic is formed without pressure or centrifugal force and as such has no moulded in
• Cycle time is limited by heat conduction out of the mould, so increases dramatically for larger
wall thicknesses.
• Thin walled products can be produced at almost 1 a minute, whereas thick walled products
might be as few as 3 per hour.
• Although the tooling is dedicated, the moulds are usually quite cheap.
• Equipment is relatively cheap - between 1 & 10 thousand pounds.
• The long cycle times usually limit economic batch sizes to between 500 and 10,000.
Typical products
• buckets
• plastic footballs
• dustbins
• oil drums
• storage tanks
• traffic cones
Other names / variants: tandem mill, reversing mill
Rolling was first used in the 1500s. The basic operation is a bit like flattening dough with a rolling pin.
Rolling is unusual in that it is primarily used for making stock items rather than making finished
components. Over 90% of worked metals are processed at some point by rolling.
Process details
Reversing mill
• In a reversing mill, a hot ingot in moved back and forth through a set of connected die rolls.
• Each roll gets closer the final shape, the last pass will finish the rolled shape.
• Reversing mills are used for making thick sections such as slabs or large I-beams. In practice,
there do not need to be many separate ‘dies’ (as is shown here) if the operator can move the
rolls closer together between passes.
Tandem mill
• In a tandem mill, a hot slab is passed through a series of flat rolls.
• Each of the rolls reduces the thickness slightly, until the desired thickness is reached. If the final
sheet is not too thick it can be ‘coiled-up’ while it is still hot.
• Tandem mills are mainly used for producing plate and sheet. In practice, 5 or more rolls in
series can be used – in which case the material coming out the end can be going very fast!
Materials and shapes
• For flat sections, ingots over 1m wide are reduced to plates (usually 6mm-300mm), sheet
(0.1mm - 6mm) or foil (about 0.008mm).
• Shaped sections (such as rails and I beams) up to 300 mm across are made using a series of
shaped rolls.
• Specialised forms of rolling can be used to make large rings.
• Hot rolling has poor dimensional tolerance and leaves a poor surface finish.
• Cold rolling can improve these and also improve mechanical properties, but only for small
reductions in thickness.
• For making stock items, rolling has few competitors.For this reason, it is usually performed by
the foundries before passing on to customers for further processing.
• For long shaped sections, rolling is the only viable option for larger cross sections - for smaller
cross section extrusion may be more economic.
• Machines can cost millions of pounds.
Typical products
I-beams, rails, sheets, plates, foil
Rapid prototyping (generic)
Other names / variants: Stereo-lithography, Selective laser sintering (SLS)
Prototyping is the making of a test component before full manufacture begins. These prototypes
provide an important means of assessing a design in a "hands-on" way. Conventionally, prototyping
was performed by machining the component from a solid block. With the advent of CAD/CAM and
CNC machining, this approach has greatly speeded up – but "rapid" prototyping techniques are even
faster. They all work by building-up thin layers in sequence to produce the whole component.
Recent trends in rapid prototyping include:
• The techniques are now being used with scanning techniques to produce exact replicas of
delicate objects such as antique carvings.
• Rapid mould development (rapid tooling), where prototypes produced by one of these
techniques is coated and can be used directly for injection moulding dies etc.
• Making shapes not possible any other way – e.g. custom jewellery, ‘sculptures’ etc.
Powder metal forming
Other names / variants: Sintering, HIPping, Reaction bonding
• One of the first uses for powder metal forming was the manufacture of tungsten filaments for
light bulbs. Advances in the technology mean even structural parts for aircraft (e.g. landing
gear) can be made this way.
• Much of the research in powder forming is to obtain good quality powder, as this helps to
achieve a good quality component.
• Pressureless sintering involves only heat. It can be used for any shape.
• Pressure sintering involves heat and axial pressure, but can only be used for 2D components.
• HIPping (hot isostatic pressing) is a variant used for 3D shapes; it uses a foil bag and a
hydrostatic pressure chamber.
• Reaction bonding involves using a binder (so it can be moulded like plasticine) which is later
burnt off; it can used for most shapes.
Materials and shapes
• Possible sizes range from balls in ball point pens up to 25kg.
• Mostly used for small (<2kg) complex components that are difficult to make from solid stock or
where uniform properties are desired.
• All metals can be processed this way, though extra care is required for some which burn or
oxidise easily.
• Sides must be parallel to allow ejection of part.
• Very good dimensional accuracy (near net-shape process) with 100% material utilisation.
• There can be problems with porosity – although sometimes this can be beneficial (e.g. filter,
• The machinery is expensive, and can cost well over £100,000.
• The dies are dedicated, expensive (£5,000+) and need to be replaced after about ten thousand
uses. They can take several weeks to manufacture, mean prototype testing is slow.
• The production rate is dominated by the sintering stage and is therefore quite slow (2-20 per
• Tends only to be economic for large batches (50,000+) or processing high-strength alloys.
Typical products
• small gears
• magnets
• cutting tool tips
• light bulb filaments
• aircraft landing gear
• bearings (porous)
• filters
Polymer shaping (generic)
Related processes in this database include: Injection moulding, vacuum forming, blow moulding,
rotational moulding,, extrusion (polymers), compression moulding
• The biggest mistake with forming polymers is to design the products the same way as metal
products - it took 20-30 years from the introduction of polymer forming for many manufacturers
to use them well. For instance, because of the comparatively low strength and stiffness of most
polymers, "ribbing" is often incorporated into the design – this will have an effect on which
processes can be used.
• Generally, polymer products are formed near-net-shape and do not require further finishing.
This "one-stop" processing gives polymer processing a cost advantage over metal processing.
• Polymer processing basically splits into those suitable for thermosets and those suitable for
thermoplastics, although care must be taken over toxic fumes for both.
Polymer extrusion
Unlike metal extrusion, polymer extrusion is a continuous process. A useful variation of the process
called co-extrusion can be used (for example, to coat wires in-line for electrical cables). Polymer
extrusion is sometimes used as a 'melter' for feeding other shaping processes such as injection
moulding or blow moulding.
Materials and shapes
Mainly used for thermoplastics, but can be used with rubbers and some thermosets. Complex shapes
with constant cross-section can be easily formed. Because of shrinkage, die design can be difficult (and
hence expensive) if good dimensional accuracy is required. Near-net-shape process, only the ends of
the extrusion are wasted.
The cost of the machines is high - well over £50,000. Die design can be expensive; the actual dies
usually cost a few thousand pounds to produce and need replacing after 10-100km of extrusion.
Depending on size, parts can be extruded at rates from 1-60m/minute. Because of the high costs, it is
usually only economic to produce lengths over 10km - although there is little competition for many of
the possible shapes.
Typical products
Channels, pipes, sheet, architectural mouldings, cables, coated wires.
Other names / variants: Shot peening
• Peening is only used for metals.
• All metals can have their strength improved by ‘working’ them – i.e. deforming them past the
elastic limit. Peening only does this to the surface, by firing ‘shot’ (like small ball-bearings) at
it. This can result in greatly improved fatigue resistance (useful for components which undergo
cyclic loading such as turbine blades).
• Peening is flexible and relatively inexpensive unless significant automation is used.
• Peening can also be used for shaping thin sheets – but this isn’t a surface treatment!
Milling will be familiar to anyone with experience of a metal workshop. The machines used
industrially can be extremely sophisticated - the cutting head is often able to twist and turn in many
directions! As well as being used for many small products suitable for school workshops, milling has
been used for large scale items such as aeroplane wings and tanks!
Materials and shapes
Almost any material can be milled, although difficulties arise with very brittle materials (e.g. ceramics)
and very hard materials (e.g. tool steel). Milling is used in metals primarily to shape parts by cutting
edges, slots or grooves. It is often used to complete parts that have been formed by a near-net-shape
process (e.g. casting or forging). Milling is unusual for wooden products, although variants such as
routing can be used to form grooves and mouldings.
Milling machines vary in price from £1,000 to £1,000,000. Milling is generally a very slow way to
produce a component - but it can be economic for prototyping or small batches. High speed machining
centres are used where the accuracy of milling is required to finish a component. The cost of milling on
a commercial scale is often a balance between higher speed and longer tool-life.
Typical products
Finishing surfaces (e.g. top of engine block), wooden furniture, architectural mouldings
Metal shaping (generic)
Related processes in this database include: Forging, Die Casting, Lost Wax Casting, Sand Casting,
Extrusion (metal), Rolling, Sheet forming
Bulk metal shaping is generally done near-net-shape by forging or casting although further finishing
work is usually required. However, it does reduce the material wasted by machining and is usually
much faster. For many components, casting and forging are in direct competition and there is often no
easy way to decide which is the better choice; both are usually undertaken by specialist companies
(foundries and forges respectively).
There are a variety of sheet forming processes suitable for metals less than 6mm thick, and in general
all products based on sheet will be made using one of these processes. Sheet is made by rolling, which
is also used to produce most large stock items held by material suppliers and a few final products such
as I-beams.
Extrusion is also used to produce some stock items with constant cross-section, such as tubes, and
some finished items, such as window frames.
Metal extrusion
• Metal extrusion was developed in the late 18th century for making lead pipe. The basic process
of forcing a round billet through a shaped die is still used today.
• Modern variants can produce clad products in one go - e.g. copper clad with silver.
• Wire drawing is related to extrusion but is used for smaller (round) sections and the metal is
pulled through the die rather than pushed.
Materials and shapes
• Mainly used with the softer metals, e.g. aluminium, copper, zinc.
• Generally speaking, the softer the metal, the more intricate the shapes that can be made.
• Useful for long thin parts with constant cross-section.
• Possible cross-sections are usually less than 100mm across.
• Dimensional tolerance and surface finish may be poor with hot extrusion.
• Cold extrusion is possible for some metals giving better properties.
• Although extrusion appears to be a continuous process, it is really a batch process as it needs to
be interrupted to load new billets.
• Typical machine prices are in excess of £50,000.
• Dies can cost upwards of £1000 to make (depending on size), but a lot more to design well.
• More frequent die replacement is needed for higher strength metals.
• Production rates from 5-10metres/minute are possible.
• Usually only economic for several thousand metres +
Typical products
• Tubing
• aluminium window frames
• railings
• trims
• wires
Mechanical fastening
Other names / variants: rivets, snap-fits, screws, bolts, nuts
Related processes in this database include: Joining (generic)
Mechanical joining falls into two distinct groups: fasteners and integral joints. Examples of fasteners
include: nuts and bolts, screws, pins and rivets; examples of integral joints include: seams, crimps,
snap-fits and shrink-fits.
Some form of mechanical joining needs to be used where products need to be taken apart during their
normal life, e.g. where repair or maintenance is likely.
With the move towards efficient recycling, there is likely to be increased use of mechanical fastening.
Materials and shapes
• Virtually any material in any shape can be joined by mechanical fastening - given enough
• Practical limitations come from being able to form holes - this limits the options for ceramics
and composites. Snap-fit joints are especially suitable for low stiffness materials like polymers.
• Especially good for joining different materials (e.g. composite to metal).
• Joint quality is reliable and readily determined, given sufficient operator skill. However,
mechanical joining usually reduces fatigue life.
• Essential where two parts will move relative to each other (e.g. hinges for doors).
• The non-permanence of many fasteners is useful for products that may need repair/maintenance
or need access to the interior.
• Can be economic for any batch size from one-offs to mass production (with or without
• Ease of mechanical joining (especially with snap fits) means low skilled workers can be used.
• For fasteners, there can be a significant stock cost in ordering and keeping track of so many
• By far the dominant means of joining parts.
• Competes with welding for thick metallic sections where a permanent joint is needed.
• Competes with adhesives for polymers and woods where a permanent joint is needed.
Mechanical cutting
Other names / variants: Sawing, hacksaws, bandsaws, circular saws
One way of splitting a workpiece in two is to plastically press a shape out of it, such as with blanking.
This entry deals with splitting a workpiece in two by removing a thin slice of material by mechanical
means (the other main approach is removal by intense heat). These processes are usually called
"sawing" and include: hacksaws, bandsaws, circular saws and friction saws.
Other specialist cutting processes include gear cutters.
Materials and shapes
• Generally speaking, sawing is only for straight line cuts all the way through a workpiece.
• Some sawing processes are capable of producing curves, and some can be used for cutting
• In general, wood and metals are easily cut - although the higher the strength of the metal the
greater the rate of wear on the cutting teeth.
• Polymers can be cut, but care must be taken to avoid any melting.
• Glass can be cut by "score-and-snap" techniques.
• Composites are not usually cut (other than edge trimming) after forming as it can have a serious
impact on the mechanical properties.
• If possible, avoid cutting! A surprising number of designs involve a cutting process followed by
a joining process. Although this can be more economical than making in one-piece, it is not
usually the case.
• The main use for cutting is to reduce stock items (usually from rolling or extrusion) to the
correct length.
• For thin workpieces (up to 6mm), mechanical cutting competes with sheet process such as
• For thicker workpieces, new processes such as plasma-arc, water jet and lasers are becoming
competitive because of their greater flexibility.
Typical products
• I-beams
• window frames
• joists
• architectural mouldings
Machining (generic)
Related processes in this database include: drilling, milling, turning, mechanical cutting, grinding,
Machining is one of the most widely used types of process found in industry, particularly for metals.
There are many variants including milling, grinding and drilling — all share the common feature of
removing material with some form of cutting tool.
As it can be expensive, extensive machining of a product is limited to trials or low volume products. It
should be kept to a minimum for high volume products and so is not used for most consumer items.
Industrially, milling, turning and drilling are often combined in CNC machining centres which can
produce a wide variety of shapes at high speeds. These machines can contain over 200 different cutting
tools, which are automatically replaced as they wear out.
It is possible (but unusual) to machine polymers – care must be taken as they can melt. In addition,
machining polymers usually leaves a rough finish (they are normally smooth after moulding).
Mechanical cutting is a type of "machining" used to separate parts – the most commonly know
processes are saws.
Lost wax casting
Other names / variants: Investment casting
• Some form of lost wax casting has been used since 4000BC.
• It is now mainly used for medium size batches where good quality is required.
• The fine dust and harmful fumes require careful control of the workplace to avoid health
problems for operators.
Materials and shapes
• Suitable for most metals, leaving a good surface finish which usually does not require further
finishing steps.
• Best for small complex-shape parts, but can be used for parts from 5g to 100kg.
• Not much metal scrap, and it can be easily recycled. Wax can be re-used but ceramic coating
must be disposed of carefully.
• The production cycle is slow: usually only 1-5 castings can be made an hour, depending on the
size. Assembling lots of patterns on one tree can help in achieving a reasonable production rate.
• The basic cost of the equipment can be as little as £1,000, although automated kit can be a lot
more. The cost of the patterns is usually only a few hundred pounds, but they can take several
weeks to make.
• Although the setup costs are low, the low manual production rate means that only batch sizes of
up to 50 are economic; this can rise to a few thousand if automated.
Typical products
• Jewellery
• dental implants
• hip replacements
• valves
• wind instrument keys
Laser processing (generic)
Although lasers are often thought of as "sci-fi", they are a surprisingly versatile tool in manufacturing
and can be used for:
• Cutting of most metals (up to 30mm thick) and woods. Over 75% of lasers are currently used
for sheet metal cutting as they can provide accurate cuts at high speeds. Because there is no
contact, it doesn’t matter how hard the material is and there is no tool wear.
• Welding of most metals up to 20mm thick without the need for a filler. They can also be used
for high speed spot welding (used for Gillette razors).
• Drilling of burr-free precision holes with no further finishing required. A common application
is the cooling holes in turbine blades – it can be over 20x faster than competing techniques.
• Surface hardening of steel component - see transformation hardening for further details.
Other applications include paint removal and rapid prototyping. Industrial lasers start at about
£100,000, but because they are very flexible and easily automated they can often prove cost-effective
Joining (generic)
Related processes in this database include: Welding, Brazing, Adhesive bonding, Mechanical
• It is unusual for a product to be made in one-piece – almost all products consist of components
which must be joined in some way.
• The most familiar joining processes are probably mechanical fasteners and adhesives and, as a
result, designers often think they understand these the best. However, mechanical fastenings
such as snap-fits are often over looked and modern adhesives are greatly under-rated because
they are thought of as "just glue".
• In addition to these processes, there are a variety of "hot processes" such as welding and
brazing which can often provide stronger and more economic joints for metal parts.
• The one thing which is key for all the processes is to design the joint for the process, and not to
design the joint before deciding on the process – a good joint for welding can be disastrous for
adhesive bonding, and vice-versa.
• Joints are often a source of weakness in failure – they are very important in design.
Injection moulding
Essentially, injection moulding is die-casting for polymers. It is normally automated and used for high
speed, high volume production where component quality does not need to be high - over 50% of
polymer parts are produced this way.
The mark left by the ejector pin can often be seen on cheap mouldings. To increase the shapes possible,
sophisticated dies with moving and unscrewing inserts are used.
Injection moulding machine
[Pictures courtesy of Withersdale Plastics Ltd.]
Materials and shapes
• Thermoplastics dominate, but can also be used for thermosets, rubbers, polymer foams and
short-fibre composites.
• Can make intricate shapes, though not suitable for thick sections.
• Typical part sizes are 100-600g, although parts up to 25kg can be made at great expense.
• Parts generally do not require finishing, although parts for feeders etc. may require removal.
• A wide variety of surface finishes and embosses can easily be incorporated into the die design.
• Thermoplastic scrap is easily recycled, but other materials must be disposed of carefully.
• To reduce costs, several parts are often moulded together on a "tree-like" structure; parts can
then be separated after moulding.
• The cycle time is limited by solidification time and time to open and close the mould.
Production rates from 1-20 parts/minute are readily achievable.
• Capital cost for machines are from £10,000 - £100,000 and dies can cost between £1,000 and
• Injection moulding is only economic for batches of 10,000 - 100,000 or more and so is usually
Typical products
• toys,
• model-making kits,
• handles,
• food containers,
• cups,
• electrical and plumbing fittings
Grinding / Polishing
The basic principle of grinding is similar to that of using sand paper to smooth wood. Where it is used
it will be the final finishing operation, with the possible exception of painting. Although grinding
wheels (which can be up to 2m!) are commonly found in industry, they are being replaced by abrasive
belts. Unusually for a mechanical process, grinding usually works best with harder materials, rather
than softer materials.
Materials and shapes
Grinding and polishing are finishing operations used where great dimensional accuracy or a good
surface finish are required. Polishing often produces a lustrous surface finish - this is due to softening
and smearing of the surface from the frictional heating. Primarily used with metals and ceramics.
Although grinding does remove material, almost none of this can be recycled.
On an industrial scale, the wear on grinding equipment is significant and this adds greatly to the cost.
The variable wear on a grinding wheel makes control of automated equipment more difficult and hence
expensive. The production rate depends on the level of finish required - the limiting factor is usually
the overall cost. As with the other machining processes, grinding and polishing should be avoided if at
all possible
Glass forming
Sheet glass is produced by drawing, rolling, and floating. Drawing is also used to produce fibres, rods
and tubes.
Discrete glass products (e.g. bottles) are made by blowing, pressing and casting.
All these processes begin with molten glass (which looks like red-hot thick syrup). A further process,
called sagging, is useful for products with shallow curves (e.g. plates) or light embossings.
Materials and shapes
• There are over 750 types of glass, but they can all basically be formed in the same ways.
• Drawing and rolling give a rough finish which normally needs grinding and polishing. Float
glass has a smooth surface.
• A variant of drawing is used to make rods and tubes.
• Blowing is used to produce hollow thin-walled items; it is similar to blow moulding of
thermoplastics. The surface finish is acceptable for most applications.
• Pressing produces parts with greater dimensional accuracy, but cannot be used for items with
thin walls or inward curves.
• Production rates and costs strongly depend on the type of process and the size of component.
• The different processes are generally suited to different shapes, so there tends to be little
• Blowing of light bulbs takes place on expensive fully automated equipment, but over 1000
bulbs per minute can be formed.
• Fibre optics can be drawn at speeds of up to 500m/s.
Typical products
• table tops
• bottles
• vases
• television tubes
• windows
• headlights
• light bulbs
• mirrors
• dishes
• optical fibres
Friction welding
Welding is commonly thought of as a process where material is melted - this type of process is more
properly called fusion welding. However, there is another type of welding process, called hot welding,
where the material is heated until it softens but does not melt. Friction welding falls into the latter
category - the heating is provided by the rubbing of the parts to be joined (at speeds which can be up to
Materials and shapes
• Usually, at least one of the parts to be joined must be circular - this can be solid or hollow.
• One of the materials to be joined must soften before melting.
• Used to join different materials to each other (e.g. polymers to metals).
• Solid bars up to 100mm can be joined and pipes up to 250mm.
• Good joint quality depends on good alignment of parts and timing of the final forging together.
• Basic equipment costs around £10,000, but automation can increase this significantly.
• Most suited economically to joining pipes and attaching studs.
• For similar metals, competitive with arc welding for the geometries it can do. But because of the
capital cost, it is not competitive where only a small number of joints are required.
• Competitive with adhesives for polymers for the geometries it can do, especially for a large
number of joints.
• Removal of flash (if required) adds to the cost.
Typical products
• pipes
• studs
Other names / variants: ring-rolling, open-die forging, closed-die forging, drop forging
Related processes in this database include: metal extrusion
• Forging is probably the oldest metalworking process - dating back to at least 5000BC.
• It has advanced a long way from its "blacksmith" image and today there are many hi-tech
variants that compete mainly with the casting processes.
• Although forging can take place "cold", the component is usually heated to reduce the forces
• The forging action can be extremely noisy!
• Impression Die Forging - also called closed die forging, presses metal between 2 dies that
contain a precut profile of the desired part.
• Cold Forging - includes bending, cold drawing, cold heading, coining, extrusions and more, to
yield a diverse range of part shapes. The temperature of metals being cold forged may range
from room temperature to several hundred degrees.
• Open Die Forging is performed between flat dies with no precut profiles is the dies. Movement
of the work piece is the key to this method. Larger parts over 20 tonnes and 10 metres in length
can be hammered or pressed into shape this way.
• Seamless Rolled Ring Forging is typically performed by punching a hole in a thick, round
piece of metal (creating a donut shape), and then rolling and squeezing (or in some cases,
pounding) the donut into a thin ring. Ring diameters can be anywhere from a few inches to 30
Process details
Closed-die forging
A heated blank is placed between 2 halves of a die
A single compressive stroke squeezes the blank into the die to form the part. In hammer or drop forging this happens
by dropping the top of the mould from a height. An alternative is to squeeze the moulds together using hydraulic
Once the die halves have separated, the part can be ejected immediately using an ejector pin.
The waste material, flash, is removed later.
Materials and shapes
• Any metal can be forged, provided the blank is hot enough (( 60% of the melting temperature).
• Typical possible sizes for closed dies range from 10g to 10kg, depending on complexity.
• The part is left with good surface and mechanical properties, although cold-forging can perform
even better.
• Complex parts can be formed using a series of forging dies with increasing levels of detail.
• A draft (taper) angle has to be incorporated to allow easy removal of the part.
• Any waste material squeezed between the die halves, called flash, is readily recycled.
• Production rate is limited by the insertion and removal of the blank, so some form of
automation is often used.
• As a result, machines can cost £100,000+, but can produce many parts a minute (if small).
• As both the machines and the dedicated dies are costly, production runs in excess of 50,000 are
often needed to produce small parts economically.
• Large parts can be produced economically at smaller batch sizes, because there is less
Typical products
• Spanners
• pedal cranks
• gear blanks
• valve bodies
• hand tools
• crankshafts
• coins
Other names / variants: Trepanning
One of the most common of the machining processes - as there are few other ways to produce a deep
circular hole. One of the biggest challenges to the drill designer is how to remove the waste material
out of the hole at the same time as getting the cutting fluid into the hole. Large shallow holes are made
by trepanning, where a disc is removed rather than all the material.
Materials and shapes
• Almost any material can be drilled, although difficulties arise with very brittle materials (e.g.
ceramics) and very hard materials (e.g. tool steel).
• Drilling is used for making circular holes, dimensional accuracy can be improved by subsequent
reaming or boring.
• Holes from 0.5 mm to 50mm are commonly drilled - although the design of the drill bit will
vary quite a lot!
• Drilling is often used to complete parts that have been formed by a near-net-shape process (e.g.
casting or forging) as precision holes are difficult to form with these processes.
• Threaded holes are made by first drilling a cylindrical hole and then "tapping" with a threaded
cutting tool.
• It is normal to try to reduce the amount of drilling required in a component by careful design -
but when an accurate hole is required, drilling has little competition.
• Where drilling is required industrially, it is usually done either as part of the function of a
machining centre, or in a dedicated drill set with multiple heads so that all the holes can be
made simultaneously
Die casting
Other names / variants: ferro-die casting
• Developed in the early 1900s, this is the most common of the casting processes that use a
permanent mould.
• It is used for high volume products, of which small zinc die-cast toys (e.g. "Matchbox" cars) are
probably the most widely known.
• Very small components like zipper teeth can be made at over 20,000 an hour!
Ferro-die is used for high melting point materials such as steels. It uses higher melting point ferrous
alloys for the die materials and is more expensive.
Materials and shapes
• Mostly used for low melting point alloys such as aluminium, zinc and copper. In general only
small parts are made, but it can be used for components up to 25kg.
• Complex parts can be made with good dimensional accuracy and surface detail.
• A draft (taper) angle has to be incorporated to alloy easy ejection of the part.
• Parts are left with good mechanical surface properties.
• Ejector pin marks are often visible.
• The machinery is expensive, and can cost well over £100,000.
• Dies cost many thousand pounds and need to be replaced after a few hundred thousand uses.
They can take several weeks to manufacture, mean prototype testing is slow.
• The production rate depends on how long the part takes to cool before it can be ejected. This
can give rates of 500+ parts per hour in normal conditions.
• Because of the high capital cost, the process is only economic for batches of 100,000+
Typical products
• Small toys e.g. cars/soldiers
• hand tools
• disc drive chassis
• motor casings
• carburettors
Compression moulding
Essentially, this process is forging for polymers - although only one 'hit' is possible. Mainly used for
thermosets and rubbers in mid-size batches as injection moulding is cheaper for thermoplastics. With
thermosets, the chemical reaction provides most of the heat, so little extra energy is required.
Materials and shapes
Mainly used for thermosets, although rubbers, some thermoplastics and chopped-fibre composites can
be formed this way. Limited to simple shapes, although a wider variety is possible with rubbers as they
can be more easily removed from the mould. Possible part size range from 10mm up to 1m. Waste
material, called flash, needs to be removed after moulding and is not readily recycled.
Cycle time is limited by heat transfer, or curing time and is usually over 1 minute. Production rate can
be increased by using multiple cavity moulds. Equipment cost is low compared to similar processes -
about £10,000 - £50,000. Die cost a few thousand pounds, and need replacing after 10-50,000 uses. The
low production rate means that it is only usually economic for batch sizes in the tens of thousands.
Typical products
Dishes, handles, caps, electrical components.
Composite shaping (generic)
The unique structure of reinforced plastics requires special processes to shape them into useful
products. Although some of the polymer forming processes can be used (when the fibres are chopped
and mixed in a polymer), there are special processes which are specific to composites containing long,
continuous fibres (such as CFRP) – it is these that are discussed here. Many of the polymer resins used
can give off toxic fumes, so precautions have to be taken to protect operators from the adverse effects.
Design issues include:
• Avoiding sharp changes in section
• Orienting fibres where possible to improve mechanical properties
• Forming as close as possible to finished shape; drilling holes can dramatically reduce strength
A few years ago, fibreglass Formula 1 car bodies needed to be replaced after every race. New carbon
fibre and precision forming techniques mean that these bodies can now last all season.
Composite forming
Other names / variants: Hand lay-up, Resin transfer moulding (RTM), Spray-up, Pultrusion
The basic aim of all composite forming techniques is to mix a resin with a reinforcement (which may
be as woven mat, long fibres or chopped fibres) to produce the desired shape. This may be done by
using prepregs or performed in-situ. A variety of processes exist for various shapes and scales of
Materials and shapes
• Hand lay-up is perhaps the most familiar process. It can be used for components of virtually
any size, but usually simple shapes. Similar shapes can be made by spray-up, which is faster
but more expensive. Both process can suffer from quality problems - these can be reduced by
using vacuum bagging.
• A variant of compression moulding, called resin transfer moulding (RTM), can be used to
make complex parts or where greater dimensional accuracy is required.
• Hollow parts can be made by filament winding which can produce parts with optimised
mechanical properties.
• Fibres, tapes and mats are produced by processes called pultrusion and continuous laminating;
these can also be used to produce prepregs (composite tapes and mats with resin that has not yet
• There are few composite forming processes, and the decision of which to use is normally
determined first by shape and type of fibre (chopped or continuous), and then by volume of
• In general, composite forming is more expensive than in other material classes. This is primarily
because of the slow production rate due to the curing time of the resins.
• Hand lay-up equipment can cost under £100, but good quality moulds can cost significantly
more and have a lead time of several weeks. It is useful for prototyping or where only a few
parts are required.
• Spray-up, RTM and filament winding can be automated, so are usually used for mass
• The wear on the dies from the fibres is significant in RTM, and they may need to be replaced
after every few thousand injections.
Typical products
Boat hulls, propeller blades, baths, water tanks, structural cables, rocket noses, turbine blades, golf
clubs, tennis racquets, bicycle frames
Ceramic shaping (generic)
Related processes in this database include: glass moulding, sintering, HIPping
There are several ceramic forming processes, although most of them are specific to individual materials
such as throwing for pottery, casting for concrete and slip casting for porcelain.
Because ceramics only melt at very high temperatures, most forming of “engineering ceramics” (like
alumina) is based on using dry powder or "bound" powder which can be moulded; at the dominant
method of forming engineering ceramics is sintering. An exception to this general rule is glass
forming, since glass softens sufficiently for it to be moulded.
Case hardening
Other names / variants: Carburising, Nitriding
Carburising and nitriding are both forms of case hardening and are primarily used to improve the
mechanical properties of the surfaces of steel components. The component to be treated is put into a
special gas atmosphere (gas carburising) at a high temperature. The process works by altering the
surface chemistry because of the diffusion of gas into the solid.
The process is quite slow because it depends on diffusion, so it is normally automated by using a
conveyor belt. It is also possible to use certain liquids (liquid carburising) which speed up the diffusion
so cycle times are shorter.
The main advantages of these processes are:
• only simple equipment is required and no dedicated tooling,
• and any shape can be treated, as long as the gas has a passage to the surface,
• large components can be treated in one go
the main disadvantages are:
• relatively slow,
• not easy to transform only parts of the surface
Blow moulding
Blow moulding is most commonly a batch process used to produce simple drinks bottles. Clever design
of the blank allows the screw top and base of bottles to be thicker than the walls.
Materials and shapes
• Used for simple, thin-walled, hollow products - mainly bottles
• Used with thermoplastics, mainly PET.
• Good, smooth surface finish can be readily achieved.
• Depending on how the hollow blank (parison) is made, scrap can be negligible.
• There is a variant which is continuous and used to produce thin-walled tubes which can be slit
to make cling-film or plastic bags.
• The production speed is limited by opening and closing the mould, so automation is normally
• Production rates from a few hundred to a few thousand per hour can be achieved.
• The tooling and machines are moderately expensive (£10,000 - £100,000).
• Moulds may need to be replaced after about 100,000 uses.
• Only used for high volume products with batch sizes of 100,000+.
Typical products
Bottles and containers up to 0.5 litre
Arc welding
Other names / variants: MMA, TIG, MIG, spot welding, seam welding
• There are several types of arc welding - MMA (Manual Metal Arc) is probably the most well
known. Automated arc processes include TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas).
All arc processes use a filler to join the two pieces - in MMA and MIG the filler also serves as
the electrode which makes the electric arc.
• There are other more specialist arc welding processes such as spot welding or seam welding
which work without a filler.
• Safety precautions must be taken to protect the welder from the bright arc and the noxious
• Good welding requires a lot of skill, and in industry a welder must have special qualifications.
Materials and shapes
• Although many metals can be joined with MMA, it is most commonly used for steel. Other
materials, such as aluminium, are usually joined by more sophisticated arc welding processes
(e.g. MIG, TIG).
• MMA is portable and so suitable for repair or on-site work.
• Thin plates may require only one pass for a successful join. For thicker plates, multiple passes
may be required to fill the gap.
• For thin plates, the edges may be square. For greater thicknesses, the edges need to be bevelled
to allow the gap to be filled more easily.
• In the area that has been affected by heat, the properties of the material may change greatly.
• The cost of MMA equipment can be less than £100. However, the production rate is slow so it
is only economic for one-off jobs, repair work and difficult access situations.
• MIG and TIG are available as manual processes, but they are often automated to improve
quality and production rate.
• For joining thick metals, arc welding has few serious competitors.
• Where reliable joints are essential (e.g. aeroplane wings) mechanical fasteners such as rivets are
used instead of welding.
• Joining of sheet (e.g. car body panels) is usually more economic by other welding processes
such as spot welding.
Typical products
• Car bodies
• ships
• oil rigs
• pipelines
• pressure vessels
Adhesive bonding
Adhesive bonding was first used for load-bearing joints for aircraft in World War II. Significant
advances have been made in the technology since then, but it has still to be widely used industrially for
metals. Adhesives are available in many forms including: liquids, pastes, powders, tapes and films.
Adhesive bonding is often combined with mechanical joining - 'super glue' was first used to prevent
nuts on machinery shaking loose.
Materials and shapes
Any materials can be joined, although some may require special surface preparation. Especially useful
for joining different materials or very thin materials. The mechanical properties of adhesive joints can
be very good, but they usually have poor resistance to 'peeling'. The strength also deteriorates with
temperature and is rarely useful above 100-2500C. Adhesive joints can provide additional benefits as
well as joining, including: sealing, insulation, corrosion protection and vibration damping. Correct
design of the joint is essential for it to be strong. One method is to increase the area, so lap joints are
better than butt joints; another solution is to design interlocking joints and combine with another form
of mechanical joining.
Equipment costs (unless automation is required) can be low, although the cost of the adhesives
themselves can be significant. Where good joint quality is essential, special equipment such as fixtures,
presses and ovens are required which can significantly add to the cost. The production rate is often
limited by the curing time, which can range from a few seconds to many hours (think of 'super glue'
and 'araldite' as common household examples).
Typical products
car mirrors, brake linings, helicopter blades, laminated glass, packaging.