Pneumatic pipe test

[TangoCleveland]

I'd like to hear any experiences with pneumatically testing a buried pipe. We have 3000' of 24" air piping, 165psig adn 100F. It's about 50 years old, coated & wrapped Carbon Steel. We had a failure of a 4" stub last month, and have repaired & inspected what we can. Safety is requiring us to do a 110% pneumatic proof test per B31.3 before we return the line to service. Has anyone done similar testing? What safeguards were used? Was there anything you'd do better next time?

Larry

 

[davefitz]

ASME sect VIII div 2 has about 4 different paragraphs that deal with the safe conduct of a pnuematic test- you may wish to review those first.

Also, the test should be conducted with adequate margin of temperature above ductile transition temperature. Ground temperature is usually 55 F, but it may be preferrable to test when the above ground ambient temp is above 50F if any above ground elements are also being pressurized. You would want to isolate or rope off or place barriers aorund the above ground elements. Also, if the new welded components did not require PWHT per code, you may wish to include PWHT / stress relief of the welds nonetheless.

 

[1969grad]

Air is a Category C fluid as defined by B31.3 so you only have to test to 100% of the MOP.

Lot of energy in a pneumatic test so you need to pressurize in steps and constantly inspect. Rope off the test area and keep everyone except test personnel away especially if the test pressure is for a system rated higher than a Class 150 system.

I think a pneumatic test instead of a hydro test for air, fuel, lines etc. is best when you are concerned about any residual water left over after the test. Just blowing the line with air after the test will not get all the water out.

One of my problems with the B31.3 test requirements is the short test time. After you go to all the trouble to set up the test I would have a hold period of at least one hour after reaching the final test pressure which is more than required by the B31.3 code.

 

[TangoCleveland]

Another complication is that we have several underground isolation valves for branches off this line that are known to have leaks. We can't do a test by holding pressure, because the leaky valves won't let us hold pressure. We would pressurize, feed the known leaks to hold pressure, and check for leaks along the line. We've done spot inspections & thickness, and internal video, which all looked good. Our suspect areas are two 4" drain stubs off the bottom of the 24" main. We are experienced with conventional Sec VIII and B31.3 pneumatic testing, but the underground aspect is making it a little more trick. Thanks for the comments so far.

Larry

 

[1969grad]

If the branch valves leak and you cannot quantify the leak rate then you are wasting your time testing to confirm the strength of the line.

All you can do is just pressure the line up and check visually for leaks.

Suggest you use this opportunity to replace and/or repair the leaking valves. After all, what good is it to try to isolate a pipe branch section if the valve leaks.

 

[bmoorthy]

I do not know whether this is permitted, but you may check up with your local authorities.

Local Pneumatic test. That is, if you have repaired only in a particular small area. Weld a larger pipe or a box encapsulating the repair area (With Fillet welding between pipe and the capsule)and cap it and apply local area testing. Subsequent to the test remove the capsule by grinding. Then test the line at design pressure with the operating fluid. Although this is long shot you might be able to convince the local authority due to safety reasons.

 

[bvi]

Are your safety people requiring the pneumatic test, to test the repair, or the entire line? A test per B31.3 cannot be performed if the line is buried, because a B31.3 test requires visual examintion of every joint for leaks, during the test (this is what drives the test duration, the ten minutes is a minimum). There is no provision in B31.3 for holding the pressure for a period of time and looking at pressure decay. If the test is only for the repaired section, then is is simply a matter of pressurizing to 110% and inspecting the repaired sections for leaks. The cautions offered by Davefitz are all appropriate. There can be considerable hazard in a pneumatic test, and you do have the possibility, depending upon the fracture toughness of the line, for the entire line to unzip.

Regarding Fluid Service Category, the line cannot be considered to be in Category D Fluid Service unless the owner has designated it as such, and it meets all of the requirements, including the pressure limit of 150 psi. In any case, the new construction code requirements of B31.3 are only applicable as you determine them to be for a repair (post construction codes such as API 570 would be the applicable documents). Alternatives are permissiable in repair situations, such as inspection in leiu of leak testing. However, I understand that your safety folks have decided that a pneumatic test is what they want.