Hydrotesting and Leak testing piping systems

[chrisHigg]

We are currently completing the final stages of a build programe for a separator unit. The vessels have been designed to PD5500 and are PED compliant (CE marked)and fully hydrotested. All interconnecting pipework has been individually hydrotested (ASME B31.3)1.5x design.

The complete unit is now assembled (vessels and pipework) and we want to carry out a leak test to demonstrate the joint integrity before sending offshore. Is there a definitive answer on which pressure the system should be 'leak tested' to? I hear 1.1xworking pressure, 1.1 x design pressure, 25% of the design pressure etc and not really sure which approach to take on this.

Any advice would be appreciated.

 

[zdas04]

There is not a lot of guidance out there as you've discovered. I generally use either twice normal operating pressure or design pressure whichever is lower (e.g., a 600 psig MAWP vessel expected to operate at 150 psig, I'd pump it up to 300 psig) and hold it for long enough to inspect all the joints (usually about a half hour). There is nothing magical about any of those numbers, but using that technique I've avoided unpleasant surprises on installation.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[LittleInch]

I've had a look through a variety of codes and oil company procedures and nothing really stands out. The pipeline code ISO 13623 states a requirment for a leak test at 1.1 X MAOP and personnally I'd go for that as MAOP cannot be > design but at least it is realted to the pressure the system will or could see before safety devices intervene. Leak testing is after hydro of course.

I've seen nitrogen testing at 90% of design which was initially a bit scary until you realise that it's going to be working at that pressure normally. The real thing to do is that apart from the really strong stud bolts, any leaks are identified, system de-pressurised and then re-made and tightened up. Small 1/2" screwed NPT instrument taps are notororios for being screwed in wrong and then shooting off killing people when being tightened, but I think you are right, much better to check those joints not hydotested before it goes offshore, but a little surprised this wasn't listed in the company specs or scope of work.

31.3 345.5 refers to pneumatic testing which limits to 1.1. x design and 345.5.5 states reducing to design pressure before "eaxming for leakage". Given your pipework is 31.3, this may be the best to go for.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[chrisHigg]

Thanks for all the feedback.

In the end I went for the 1.1 x operating pressure for 30 mins and seemed to be fine.

Cheers agaian.

 

[AQUE]

I always thought you have to do the leak test at the design pressure to ensure pipe will not leak in the worst case scenario.