Test Section Length for HDPE Pipeline Pressure Testing

[mgreen0302]

My team is working on a pressure test procedure for a 15-mile 6" HDPE natural gas gathering line in the US that has a MAWP of 125 psig, and we've run into a few roadblocks that I wanted to get some input on. My team has a background in carbon steel interstate transmission lines, not "low" pressure plastic gathering systems, so this is new territory for us. We have been going back and forth between a hydrostatic test and a pneumatic test, and a few questions came up with air testing:
[ol 1]
[li]What would be the allowable maximum test section length for a pneumatic test?[/li]
[li]Is there an industry rule of thumb for maximum number of untested tie-in welds on plastic pipeline systems?[/li]
[/ol]

 

[1503-44]

1. There is no defined maximum pipeline length for testing.
2. Yes. Zero.

If not pressure tested, joints must be inspected by other suitable means.

 

[LittleInch]

It comes down to practicality.

For pneumatic testing you normally need to make sure no one is present over the pipeline during the test. Depending where you are 15km could be difficult.

All joints need testing in some form or another.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mgreen0302]

I understand that all welds need testing, but in most cases it is impossible to have zero untested welds/connections unless you are testing barrel to barrel. In an ideal world there would be no untested welds or flange connections, but that is rarely the reality. The interstate transmission company that I used to work for allowed a maximum of 4 untested tie-in welds (with justification) and of course all welds were x-rayed regardless of inclusion in the pressure test. Most NDE methods (x-ray, dye penetrant, etc.) are useless on HDPE so the only other option is visual inspection, which has its faults. I was asking for a rule of thumb, not the gold standard.

 

[1503-44]

A radiographed weld is indeed tested. Not hydrostatically tested of course, but it is tested, hence all joints are tested.

There is no rule of thumb, other than they must be kept to a minimum, meaning impossible to do for one reason or another.

For steel high pressure pipelines, a hydrotest is required. Exception: if a hydrotest is IMPOSSIBLE, a pneumatic test may be conducted. Not sure what the CFRs say about plastic.