Piping Pre Commissioning Leak Test and Nitrogen Purging

[king92]

Hello Dear Professionals,

I got promotion as engineer of pre commissioning and commissioning. But after getting into this position only i got lot of doubts that i missed to ask my manager during the preparation of documents in the last project.

1. For the leak test during pre commissioning, how the test pressure value is selected (In my last project we did 90% of design pressure, so i have contacted with him. As per his information it was taken from API standard. But he didn't told me the API number so i couldn't find it.) If possible can you guys help me on this one.

2. Is there any standard is available for inerting or nitrogen purging acceptance criteria.

If you know about these things please let me know. It will be helpful for my career. Thanks in adavance to all dears.

 

[GD2]

Kingsly A,

It looks like you are wading through deep water to catch any straw on your way.

My advise - convince your Boss to hire a Commissioning Engineer for 1 year contract.

Like design, commissioning and start-up is also very structured and needs knowledge, experience and skill.

It will not only allow you to have the plant a safe start-up but you will also acquire tons of knowledge.

GDD
Canada

 

[king92]

Dear GD2, I have experience in site conducted the pre commissioning and commissioning works based upon the procedures created by the engineers. but now i am responsible for creating that procedures. so for conducting safe work and following the previously employed procedures here is not a problem for me. why i am asking is that i need to know the standard where they took the values from. But here nobody understand what i am asking is. everyone saying you need experience. It's not the case every work has been already studied and standatised based on country. I know the indian standard which covers pre commissioning and commissioning works. If you guys want to refer you can refer on (IS 15663_Part 3) so please don't tell me there is no any standard available. there will be but you guys dont know or dont want to share it. American standards also there for pre commissioning and commissioning but i don't know the standard number for reference that's why i am asking.

 

[king92]

API RP 1110 (Section 4.1.7.4 Leak Test)
refer this standard for leak testing of pipelines. it gives the test pressure also duration. but this is for pipeline but for leak testing of piping system in the refinery including equipment's during commissioning. i need to know the standard number.

so if you don't know, please understand and don't tell that there is no any standard available. if you know please share.

 

[PetarK]

Hello king92,

Can you speak with you managers who led this work on your previous jobs? They might be willing to share their knowledge (and maybe their documentation) with you.

Have you marked up the limits of the test boundaries? Have you correctly identified any pressure specification breaks? You might need different test pressures for different sections.

What is the design code that was used?
If it is ASME B31.3 “Process Piping: ASME Code for Process Piping”, you should to refer to that for maximum allowable leak test pressure but be careful to not exceed the design operating pressure or to exceed 90% of the setpoint of any pressure safety relief valve, whichever is lower.

For drying, you need to consult your process engineers to determine what is the required water content to prevent problems (like hydrate formation during/after startup). I often use the Pipeline Rules of Thumb as a guide for drying methods. If you are conservative, determine what is the lowest temperature for any hydrate risk and reduce the water dewpoint 10°C colder than that (e.g. if they hydrate risk is -30°C then dry to a water dewpoint of -40°C). If I am on site after the hydrotests are completed I will witness that the draining of water was done correctly. If there was any water left in low spots you will not be able to dry the pipework and equipment quickly.

For inert gas purging there are multiple requirements:
What is your design requirement for maximum allowable oxygen before introducing hydrocarbons?
If you are working with hydrogen it will be lower than if you are working with natural gas. This needs to be calculated by a process or process safety engineer who knows what hydrocarbon will be introduced into the pipework.
For inerting (reducing oxygen), I use British Standard BSI PD CEN/TR 15281:2006 but I have only used it for smaller sections of plant. It does provide good guidance on calculating the amount or nitrogen required for flow-through purges and for cycle purges.

Do you need to do a gross leak test at lower pressure or a leak test near operating pressure?
I have previously used a gross leak test with nitrogen at 7 barg and let it sit for 24 hours. This saves time at this stage but you will need to monitor for leaks later when you pressurize with hydrocarbon when you exceed 7 barg.

A leak test at higher pressure with nitrogen is typically at 90% of the set pressure of the pressure safety relief valves so that they do not lift.

Do you have multiple line specification breaks?
If you have multiple line specification breaks then you will need to do leak tests in different sections as you might have a section that will be at 90 barg with an adjacent section that is only designed for 18 barg. This will require different section to be valve isolated and tested separately.

Do you have a process engineer who can check the leak/purge test packs?

Have you calculated what pressure drop will be caused by drop in temperature between day and night? This is sometimes confused for a leak when it is due to gas cooling.

Regards,
Petar K

 

[king92]

Dear Petar K,

Thanks for sharing your knowledge, i was confused before but now i am clear for the leak test. Like you said we have did the high pressure leak test in the last project with 90% of PSV set pressure. So now i am clear about that one.

For the pressure drop during leak test, i have referred. How Temperature Affects Pipeline Hydrostatic Testing Pipeline and Gas Journal by J.C. Gray (0.2 % per hour).

But for purging and dry-out, i have find some IN & GOST standards. But these are only for the pipelines. So i have checked the project requirements and it shows 2% Vol of oxygen before introducing hydrocarbons.

Thanks for your support. I really appreciate it.