20 Toughest Piping Engineering Questions asked in Interviews

1. Why Block valve is used at the Root of an instrument like pressure indicator?

Answer: - They are needed to isolate gauges for maintenance during plant operation and during hydrostatic testing of piping system. They are called root valve.

2. Explain what is a “Double block & Bleed” valve? Why we need a bleed valve? When do we use this?

Answer: - The primary functions of a double block and bleed system is for isolation and the secondary function is for intervention. The Double Block and Bleed Valve or a DBBV can perform the tasks of 3 separate Valves (2 separate isolations and 1 drain Valve) which apart from being hugely space saving can also save on weight and time due to installation and maintenance practices requiring much less work and the operator being able to locate and operate all 3 Valves in one location.

3. How control valve block valves should be located?

Answer: - They should be as close to control valve as possible, considering drain requirement and hand wheel clearance. At least one of the block valve should be placed in vertical so that spool can be removed allowing the control valve to be removed.

4. Normally where do we use: i) Eccentric reducer, ii) Concentric reducer?

Answer: -

i) Eccentric reducer: Pump suction to avoid cavitation, to maintain (BOP) in rack

ii) Concentric reducer: Pump discharge, vertical pipeline etc.

5. Draw a typical steam trap station layout and explain why the existence of a by-pass line around the trap is not a good idea, when the condensate is returning to a condensate header?

Answer: - (No drawing) It is not advisable to have a bypass around a steam trap because the block valve could be left open and defeat the purpose of the trap.

6. Why and where drain should be located in control valve assembly?

Answer: - Drain is used to drain the piping between two block valves on two sides of control valve before removing control valve for maintenance. It must be located at lowest point of bottom run. One drain is used if control valve is fails open, and two drain used if control valve fails close, as both sides are blocked in this case. For one drain, it should be upstream side of control valve.

7. Concentric reducer is used in pump suction. YES/NO? Explain?

Answer: - NO. Air pocket may form if concentric reducer is used at pump suction, which results in cavitation and causes damage to pump. To avoid this eccentric with Flat Side Up (FSU) is used in pump suction.

8. What is Cavitation?

Answer: - Cavitation is a rapid collapse of vapor pressure that can produce noise, result in loss of head and capacity and create a severe erosion of the impeller and casing surface in the adjacent inlet areas. The vapor pressure occurs right at the impeller inlet where a share pressure drop occurs.

9. Why and where drain should be located in control valve assembly?

10. What are spandrels?

Answer: - Horizontal structural members located along the longitudinal centerline that is used for structural stability, pipe support or intermediate pipe bents.

11. Where is positive displacement pumps used?

Answer: - Used to move viscous liquids, injects chemicals or additives into a system, or pump quantities too small for centrifugal pumps.

12. What is Stress intensification factor (SIF)” Where do we use this? How many types of these SIF’s exist?

Answer: - 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.

Example: - SIF for Reducer and Weld neck Flange is 1.0 , SIF for socket weld flange is 1.3

13. Why do we provide Drip leg in steam line?

Answer: - To remove condensate when there is a rise in the pipe along the flow direction. If we do not provide the drip leg in streamline, the condensate forms inside the pipe will result in water hammer effect causing damage to the piping system.

14. What is mean by ‘PWHT’? Why is it needed?

Answer: - ‘Post Weld Heat Treatment’ this is done to remove residual stress left in the joint which may cause a brittle fracture.

15. What Are The Main Differences Between ASME B 31.1 And ASME B 31.3?

Answer: - The main differences are listed below:

· Material allowable stresses are different in both codes.

· Stress increases due to occasional loads are different in each code.

· B 31.3 neglects torsion while calculating sustained stresses, but B 31.1 includes it.

· Sustained stress calculation is specific in B 31.1 but undefined for B 31.3.· B 31.1 intensifies torsion but B31.3 does not intensify it.

16. What Materials Are Used For Construction Of Valves?

Answer: - Cast iron, bronze, gun metal, carbon steel, stainless steel, alloy carbon steel, polypropylene and other plastics, special alloys.

17. How to Calculate the Height of a Variable Spring Hanger?

Answer: - Select the height from vendor catalog based on spring size and stiffness class.

For base mounted variable spring hanger the height is mentioned directly. It is the spring height.

For top mounted variable spring hanger’s ass spring height with turnbuckle length, clamp/lug length and rod length.

18. How control station should be supported?

Answer: - It should be supported such that control station piping is self-supporting even if the control valve is removed. Normally, on exit side elbows of bottom run of control valve.

19. What is pipe bent?

Answer: - Pipe bent consists of a vertical column or Columns and a horizontal structural member or Members that carry piping systems, usually above the headroom.

20. What is preferred location for level instrument nozzles?

Answer: - Away from the turbulence at the liquid outlet nozzle, although the vessel is provided with a vortex breaker, instrument should be set in the quiet zone of the vessel for example on the opposite side of the weir or baffle or near the vapor outlet end.

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