Piping Interview Questions

1 . What is NPSH?

Answer: NPSH: Net Positive Suction Head It is the measure of the pressure drop of the liquid as it moves from the inlet of the pump to the eye of the impeller, determined by testing and expressed in feet of water by the pump manufacturer.

##2. What is Cavitation?

Answer: Cavitation is a rapid collapse of vapour 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 vapour pressure occurs right at the impeller inlet where a share pressure drop occurs.

3. What is velocity head?

Answer: The head loss caused by the consumed power to accelerate stationary fluid to suction line velocity during pump startup. It’s about 1 ft.

4. While welding of pipe trunion to pipe/reinforcement pad you have to put a hole or leave some portion of welding. why?

Answer: For venting of hot gas which may get generate due to welding

5. What do you mean by following types of welding?


Answer: SMAW: Shielded Metal Arc Welding TIG: Tungsten Inter Gas welding

6. Where are 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.

7. How is suction line supported?

Answer: Commonly supported under the elbow adjacent to the pump nozzle.

8. How should the suction line reducer be placed?

Answer: It should be placed top flat.

9. 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.,

10. What are suction line requirements?

Answer: To flat reducer, strainer, block valve, no pockets, as short as possible with consideration to stress, min straight length of 3D.

11. 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

12. What is accepted distance between pumps?

Answer: 1200 MM / 4 feet.

13. What is special requirement for discharge line of positive displacement pump?

Answer: Relief valve whose discharge is connected in suction line between the isolating valve and the pump.

14. What should be clearance between flanges of exchangers?

Answer: 1000 MM.

15. 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.

16. Where Double pipe or Fin Tube Exchangers used?

Answer: Used when the liquid has a greater resistance to heat flow than another or when the surface area is small.

17. What are the types of tube arrangments?

Answer: U-tube, Fixed tube, kettle arrangement.

18. 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.

19. How can flanges be classified based on facing?


  1. Flat face
  2. Raised face
  3. Tongue and Groove
  4. Ring type joint

20. What are different types of Gaskets?

Answer: Full face, spiral wound, Octagonal ring type, Metal jacket and inside bolt circle.