Basic questions asked in piping engineer's interview for reference

1. Can you explain in detail three or more major differences between code ANSI B31.1 and code ANSIB31.3?

Answer: There is only one major difference between the two, B31.1 is for Power Piping, and B31.3 is for Refinery/Chemical Plant Piping.

Based on Design Stresses in B31.1 is the lowest of the specified minimum tensile strength divided by 3.5 and in B31.3 is the lowest of the specified minimum tensile strength divided by 3.

In B31.1 requires preheating to 200 Deg F or post weld heat treatment for carbon steel with thickness less than or equal to 19.0mm. In B31.3 requires neither preheating nor post weld heat treatment for the same thickness range.

2. There is a power plant inside a Process refinery. Where exactly the ANSI B31.1 & ANSI B31.3 scope break occurs?

Answer: Based on my experience there were two cases. Case #1, B31.1 stopped at the Power Plant Unit block valves. Thus all piping inside the Power Plant was B31.1.

Case #2, B31.1 stopped at the equipment(Boiler) isolation block valves and then all other piping was B31.3. This is normally the choice of the owner/operator/client.

3. Which of the following piping system is more health hazardous.
a) Fuel oil piping
b) Process piping with Caustic
c) process piping with HF acid
d) Sulphuric acid piping.

Answer: c) process piping with HF acid

4. There is a steam piping with low pocket but without steam trap. What will be worst consequence of this layout?

Answer: There will be a build up of condensate to the point that a slug will be pushed by the steam flow. This slug of condensate will cause “water hammer” and could rip the piping apart

5. In what circumstance, the reducer of a pump suction piping will be in bottom flat position. Explain why the reducer should be so.

Answer: When the source of supply is above the pump, then the eccentric reducers must be placed with the flat side down. 

When the source of supply is below the pump, then the eccentric reducers must be placed with the flat side up. This will avoid an air pocket at the pump suction and allow air to be evacuated.

6. P&ID shows a spec break (at Flange) between carbon steel & stainless steel specification. What additional arrangements you have to make for that dissimilar material flange joint?

Answer: Use the insulating Gasket.

7. A stainless steel piping specification mentions Galvanized carbons steel bolts. What is your first reaction on this and how do you rectify it?

Answer: If that is what the Spec call for then that is what I am supposed to use. But, I would ask the Piping Material Engineer (PME) why he/she specified galvanized bolts.

8. How many types of piping specialty items do you know? Why it is called a piping special? Why not we include them in standard piping specification

Answer: Flame Arrestor, barred Tee, injection quill, y-type strainer, basket strainer, flexible Hose, Hose connection, sampling quill, mechanical interlock, expansion joint, bird screen, air release trap, auto drainer, seal, bleed ring, flow sight glass, Corrosion Coupon,

They are called them SP items because they are NOT written into the normal Piping Material (Line Class) Specifications. They are not included because they are normally of limited use, purchased from a limited product line vendor and are often after thoughts.

9. 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.

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

Answer: The primary function 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.

11. Double block and bleed Valves operate on the principle that isolation can be achieved from both the upstream and downstream process flow / pressures

If you don't want to shut down the entire facility, so you decide to block off and depressurize just the section you're working on. Just upstream is a double block and bleed Valve - a trunnion-mounted Ball Valve with self-relieving seals and a bleed Valve to vent the cavity. You close the Ball Valve and open the bleeder. Now you can de-pressurize the line downstream and open it up to work on it.

12. In a typical tie-in where should the spectacle blind be inserted? a) after block valve and towards existing plant b) before block valve and towards new plant. Explain why.

Answer: The Spec Blind shall be placed on the Unit side of the Unit Block valves. This placement allows for the closing of the Unit isolation block valve, the unit side is depressured and drained. Then the spec blind can be installed for isolation of the unit.

13. Stress intensification factor (SIF)” Where do we use this? Explain this term. 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

14. When all design parameters are same, whose thermal expansion is higher among the following? A) Carbon steel b) Stainless steel c) Duplex steel d) Cast Iron e) Galvanized Carbon steel.

Answer: b) Stainless steel

15. In a hose station the hose couplings used for water, air & steam should be different type. Do you agree? Explain your view.
Answer: I agree. If they are all the same then the hoses can be connected to the wrong services and could result in the injury of an operator (i.e. thinking the hose is connected to water when it is connected to steam).

16. What is your view on the usage of Metallic expansion joints? When they become necessary and when they could be avoid

Answer: Metal Expansion joints are necessary for systems that convey high-temperature substances such as steam or exhaust gases, or to absorb movement and vibration

Metal pipe expansion joints can withstand the design temperatures, pressures, as well as, provide the capacity necessary to absorb thermal growth of the piping system

It can be avoided by providing the flexible pipe routing to equipment. So that all the thermal expansion and movement absorbed by the piping system.

17.What is your view on the usage of Metallic expansion joints? When they become necessary and when they could be avoid

Answer: The largest hottest lines on the outside edge of the pipe rack working in with cooler lines in towards the middle of the rack. This allows the longer loop legs as you lay the loops back over the other lines to the other side of the rack and back. The lower temperature loops would be “nested” inside the larger, hotter loops. Future rack space” is normally at the direction of the Client. It may be anything from 0% to as much as 25%.

18. 18. When a utility line (like condensate or water etc) is connected permanently to a process piping what precaution we have to take to avoid cross contamination?

Option #1, double block valve with a drop-out spool.
Option #2, Double block valve with a spec blind.
Option #3, double block valves with a bleed valve.

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