Pinping test Vs backfilling 3

[gusongjun]

Hi, I have some pipes buried underground. To improve schedule, I am wondering whether I can complete piping test after backfill accomplished.

Thanks for your kind concern.

 

[cvg]

pressure (acceptance) tests should always be conducted after backfilling.

 

[SNORGY]

Depends on Code Of Construction.

If to (for example) CSA Z662 (pipelines), the hydrotest is desired after backfilling. If to B31.3 (some lines to this Code actually do end up traversing a lease site underground) the Code would suggest that all joints need to be exposed for visual examination during pressure testing.

Regards,

SNORGY.

 

[cvg]

regardless of that standard, if the Contractor backfills (an already tested) pipe and in the process of backfill and compaction he loosens a joint or damages the pipe it might begin to leak. Therefor, I would not recommend accepting an underground line that was only tested prior to backfill.

 

[BigInch]

I agree with cvg. Test after backfilling.
B31.3 is primarily concerned with flanged connections. Obviously you would not want to lower and then backfill flanged connections. Such movements would tend to loosen a flange. If welded joints are lowered and backfilled with care, the process won't damage welded connections.

**********************
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[zdas04]

If you are doing a hydrotest (as opposed to a pneumatic test), then the more stable you can make the water temperature the better chance you have of success. I had a 24 hour test (company requirement) that failed in the last hour because the temperature dropped 4F as a cold front came through. It was 6 miles of 8-inch that we had not backfilled yet. We put our heads together and left the pipe full of water with vents open and backfilled. Then we redid the test with no problem. Ever since that horrible morning I've specified backfilling as a prerequisite for the test.

As to finding leaks, it is amazing how quickly a lake forms around a piping leak at test pressure. When the pressure starts dropping you'll have a mud bog within an hour if there is any significant volume of water available to exit (i.e. testing 100 ft of 2 inch, there may not be enough water to get to surface).

David

 

[SNORGY]

cvg / BigInch / zdas04:

While the "Mechanical Engineer" in me cannot completely disagree with your points regarding why you should backfill before pressure testing, the "Project Engineer" in me is compelled to rationalize that if the risk of damage to joints as a result of the backfilling operation is what drives this decision, then the problem isn't the sequence of events so much as it is one of incompetent trench substrate preparation and backfilling practices.

I believe that in a pipeline (Z662 in Canada) installation, it's clear - you backfill prior to pressure testing. Then you do a strength test followed by a leak test - or simultaneous strength plus leak test - in accordance with the Standard. However, for a B31.3 installation, I would hydrostatically test to 1.5 x DP with all joints exposed for visual examination, thereby minimizing the Code required test duration and the requirement for a charted test. I would then depressurize but not dewater the line. I would then backfill following a procedure that specifies maximum uncompacted loose lifts, manual compaction between lifts, and so on. I would make the backfilling operation an "Owner witness" point for the Contractor's construction and QA/QC records. I would then - only if required by Owner - re-pressurize with water to a service test leak pressure for about 15-20 minutes. Then dewater.

There are inherent risks with pressure testing. The way the Pipeline Standard (Z662) is written, you pretty much have your leak detected when you create a puddle; in January, that puddle probably has methanol or glycol in it, and an expensive cleanup ensues. To me, if B31.3 allows (or requires) the joints to be exposed for visual examination, then I'd like to see the little puddle (or spray) in the open trench so I can deal with it before it becomes a more significant contamination issue.

Regards,

SNORGY.

 

[zdas04]

Snorgy,
I don't see the disagreement between what I said and what you said. I say backfill. You say backfill. Sounds like the same. Maybe it's a language thing.

I don't work in plants and couldn't care less what B31.3 says to do. B31.8 doesn't have the language requiring joints to be exposed as far as I can tell, so for a pipeline you backfill prior to testing. I've been project manager on a dozen or so of those jobs and can't really see where mechanical engineering ends and "project engineering" (whatever that is) begins, but maybe I'm missing something.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[SNORGY]

zdas04:

I hope I didn't offend you.

I originally referenced B31.3 because of the wording in the original post by gusongjun:

"...I have some pipes buried underground. To improve schedule, I am wondering whether I can complete piping test after backfill accomplished."

That might be a "pipeline", but it might also be buried pipe in accordance with a facility piping code.

So, what I was trying to point out (probably poorly) is that while design and construction in accordance with a "Pipeline Code" (Z662 or B31.8) requires backfilling prior to pressure testing, the opposite is implied with, for example, B31.3.

As for where "Mechanical Engineering" ends and "Project Engineering" begins, I think I am guilty of not explaining the prevalent culture here. Here, the "Mechanical Engineer" dots the i's and crosses the t's and makes sure that the minutia and details address Code and Specification requirements to the fullest possible extent, whereas the "Project Engineer" is more apt to apply poetic license and modify certain things in order to maintain the "spirit and intent" of Codes and Specifications without necessarily doing exactly what they say in precisely the way they are said. Their mandates are different: the "Mechanical Engineer" is mandated to meet or exceed all Code and Specification requirements; the "Project Engineer" is mandated to meet cost and schedule restraints at minimal risk and compromise. Often, the mandates are in conflict.

I certainly did not mean to offend you. My apologies if I did.

Regards,

SNORGY.

 

[gusongjun]

Thanks all replies.

CSA 662-07 clause 8.1.7

Compressor and pump station piping designed in ASME B31.3 shall be pressure tested in accordance with the pressure-testing requirements of ASME B31.3.

I checked the piping design specification, all pipes were designed based on ASME B31.1 and ASME B31.3. Now the contractor is requesting piping testing shall follow CSA Z662 instead of ASME B31.3. Personally, I agree with them. But I inquired some other engineers, QA/Piping stress, all of them told me that piping test shall follow the design code.

Do anybody experience this kind of issue?

Many thanks!

Best regards!

 

[gusongjun]

to SNORGY and zdas04:


Thanks for your replies.

It is my fault. These pipes are buried in plant site. I am civil guy not piping/mechanical. Your responses are very helpful.

 

[SNORGY]

Piping designed to one Code shouldn't be constructed and tested to another Code. All that does is create a system that doesn't fully comply with either Code.

Check, for example, whether the service is sweet or sour, and what level and acceptance criteria have been applied to any weld examinations that have been performed for the lines in question. Z662 would require minimum 15% x-ray in sweet service, whereas B31.3 would specify minimum 10% with different acceptance criteria applied towards the evaluation of inclusions, undercut, and other defects. Check also, for example, if the service is sour and the lines designed to B31.3 withstand scrutiny when evaluated against criteria set out in Z662 Section 5 Tables 5.1-5.3 (or as augmented by the Regulator) with respect to notch toughness.

If you design and build to one Code, you should inspect and test to that Code.


Regards,

SNORGY.

 

[gusongjun]

to SNORGY

Thanks for your reply.