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# Piping Design Engineer Interview Questions:- Part-4

51. 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 wastage?
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.
52. What is the minimum thickness of pipe that requires stress relieving to be done as per B31.3?
19 mm thickness.
53. What is the difference between thermostatic and thermodynamic steam trap?
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.
54. What is the code for sour service?
Code for sour service is NACE MR–0175), NACE: National Association of Corrosion Engineers.
55. What are glandless piston valves? Where these are used?
Glandless piston valves are maintenance free valves used in the steam service.
56. What is the objective of stress analysis?
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
c) Pipe displacements
d) Loads & moments on supporting structure
57. How much should be the pressure for hydro-test?
Hydro test pressure should be calculated as follow except as provided against point no-4.
1. 1.5 Times of design pressure.
2. For a design temperature above the test temperature, minimum test pressure can be calculated as:
Pt = (1.5 X P X St) / S
Where:-
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.
58. How do you calculate the pipe spacing?
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).
59. How do you calculate the width of Pipe rack?
W = (f X n X s) + A + B. Where,
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 NB
= 300 mm (estimated average spacing)
= 225 mm (if lines are smaller than 250 NB)
· 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
60. Which fluid is used in heat exchanger in shell side and tube side?
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.
61. What is Reynold’s number and what is the value of this up to which the flow is laminar?
It’s a dimensionless number to classify the nature of flow – Re= avd / f
Where: Re = Raynold’s no. a = density of fluid. D = diameter of pipe.
V = average velocity of fluid. F = viscosity of fluid.
Flow is laminar up to Re = 2100
62. Give some examples for occasional loads.
Wind, wave & earthquake
63. What are the steps involved in stress analysis (or any stress package carries out)?
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
64. 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.
Items like valves, flanges, specialty 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.
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).
65. What are the different types of stresses that may get generated within pipe during normal operation?
Axial stresses (tensile / compressive), shear stresses, radial stresses, hoopes stresses.
66. How are the loads classified in stress analysis package?
67. What are the inputs for stress analysis of a piping system?
A) Pipe Size B) Fluid Temperature C) Pipe Material D) Model E) Design pressure F) Insulation Thickness G) Specific gravity H) Friction coefficient.
68. What are the sources of sustained loads generated in piping system?
a. Pressure b. Dead weight of pipe and attachments
weight of pipe with fluid + pressure load + load due to springs W+P1
69. How do you calculate the operating load?
W+P1+T1, T1 – load due to thermal expansion
70. Mention some of primary loads (Have their origin in force)
Dead weight, pressure, forces due to relief or blow down, force due to water hammer effects.
71. 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 equipment’s
72. What is desired life cycle for piping in operation?
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.
73. What is the failure theory subscribed under ASME B31.3?
(A) Maximum principal stress theory (rankines theory)
(B) Maximum shear theory
(C) Tresca theory
Ans: (A)
74. What are the types of failures encountered in piping?
A. Catastrophic failure B. Fatigue failure
75. Select the failure stress range for fatigue failure due to thermal expansion as per B31.3.
(A) (1.6Sc+1.6Sh) f (B) 0.78 Sh (C) (1.25 Sc+0.25Sh) f (D) Sc+Sh
Ans: (C)
Sc and Sh = Basic allowable material stress in cold & hot conditions respectively.
f = is the stress range reduction factor(1 for 7000 cycles)
76. 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
77. How do you calculate the thermal expansion in a pipe?
Thermal expansion e= a x L x Rise in temperature, where,
a = Coefficient of expansion L = Length of pipe
78. 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 localized 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 weld neck flange: 1.0
SIF for socket weld flange: 1.3
79. Which are the criteria for pipe supporting?
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 + water fill.
Load of valves and online equipment and instrument.
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
80. What is the basic span of supports for 2”/6”/10”/24” pipe
Basic Span is 5.5m / 9m / 11.5m / 15m respectively.
81. 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 center line and guide is provided to restrict the longitudinal movements of pipes along with it’s axis.
82. 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 minimized.
83. Where do you provide anchor and slotted support of heat exchanger?
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.
84. What do you mean by hoop stresses and how do you calculate it?
Stresses which are generated circumferentially 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.
85. How does hoop stress affect the system?
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.
86. 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 piping00 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.
87. What is the mill tolerance to be considered for the thickness of pipe during stress analysis as per ASME B31?
(A) 1% (B) 2.5%
(C) 7.5% (D) 12.5%
Ans: D
88. What is the purpose of providing graphite pads in supports below shoes?
To reduce the friction factor. The co-efficient of friction for graphite pads is 0.1
89. How is piping to tank inlet nozzle is supported and why?
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).
90. What are the two types of flexible spring hangers?
1. Constant Spring and 2. Variable Spring
A piping system may respond far differently to a dynamic load than it would to a static 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.
92. What is WRC 107 / WRC 297?
Localized 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 vessels.
93. Give different types of dynamic loads with example.
1. Random – Wind, Earthquake 2. Harmonic – Equipment Vibration, Pulsation, Acoustic Vibration 3. Impulse – Fluid Hammer, Relief Valve Opening, Slug Flow
94. What is the maximum expansion absorbed in loops in normal design?
10 Inches
95. What is the limiting factor in deciding the length of the spool in jacketed piping?
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)
96. What is the allowable stress range for CS pipes?
2070 kg/cm 2
97. What is the factor to be checked concerning the expansion of header attached to air cooler piping?
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 difference 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.
98. What is the maximum no. of cell nozzles connected to a single header of air cooler piping header in normal practice?
Six nos.
99. What are the insulation materials used for piping systems?
A. Fibrous – Rock & Glass Wool
B. Rigid – Calcium Silicate, Polyisocyanurate, Cellular Glass
100. What is the purpose of expansion bellows?
Expansion bellows are used absorb axial compression or extension, lateral shear or angular torsion developed in the pipes (specially near nozzles)
101. What is dynamic analysis and why it is used?
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)
102. What should be the material of shoes for supporting as pipes & why?
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.
103. 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?
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.
104. What are sway braces?
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 pipe work; their purpose is to limit undesirable movement. Sway braces act like a rigid strut until a small preload is reached, where after the restraining force increases in proportion to the applied deflection.
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 pipe work deflection. It should be noted however that it does provide some resistance to the thermal movement of the pipe work 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 pipe work 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.
105. What is fluid hammer and how it is generated?
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 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.
106. What is the difference between variable spring hanger and constant spring hanger?
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.
107. How to get the foundation loads?
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 hydro testing dead weight (wt/m X pipe rack 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 to
F in kg <= 150 X NB of pipe in inches (It should be <2 tones)
M in Kgm <=75 X NB of pipe in inches.
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 loading.

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2 Comments on "Piping Design Engineer Interview Questions:- Part-4"

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Guest
Sayed
7 months 11 days ago

which is the best piping design software ?

Guest
SHAIK MOHD. GOUSE
1 year 6 months ago

dear friends how to check pipe wall thickness wise minimum 6 mm to 20 mm and above pipe hydraulic ( I mean water test sting)how to calculate and minimum,maximum pressure pressure ratings kindly find and show to me.

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