Pneumatic Proof testing 1

[Kellymint]

I have 6000 psi 1/4" S.S Helium supply line for a test lab that was only hydrostat pressure tested to 6750 psi. The facility contractor should have been proof tested this to 1 and 1/2 times, or 9000 psi. The system is already installed in place and it is not feasible to perform another hydro. For a pneumatic proof test, in place of a hydrostat, I have seen test pressures anywhere from 10% to 25% over MEOP. I do not have a latest copy of ASME 31.1 or 31.3. Does anybody know what the test pressure should be and how long I should hold that pressure? And if only 110% is needed for a pneumatic test, do I really need to perform the proof test again?

 

[DLANDISSR]

KELLYMINT:
PER B31.3-1996 SECTION 345 THE TEST PRESSURE FOR A PNEUMATIC
TEST IS 110% OF THE DESIGN PRESSURE. ASSUMING YOUR SYSTEM HAS A DESIGN PRESSURE OF 6000PSIG 100Deg F THE TEST PRESSURE WOULD BE 6600PSIG.

SINCE IT WAS TESTED AT 6750PSIG A NEW TEST WOULD BE UNNECESSARY.

A HIGHER TEST PRESSURE MAY BE SPECIFICE BY THE DESIGN ENGINEER OF THE SYSTEM BUT B31.3 REQUIRES ONLY 110% FOR A PNEUMATIC TEST AND 150% FOR A HYDROSTATIC TEST OF THE DESIGN PRESSURE.

 

[PVRV]

Is there a restriction on PNEUMATIC test pressure ?

 

[davefitz]

Not sure if the code specifies a restriction, but if any of the materials used have an allowable stress based only on yield stress and UTS, then you better not exceed 130% design pressure for pnuematic test, espescially if there are any pressure vessels involved. There are local stress raisers that have a stress concentration of over 3, and if the test temperarture is below the brittle trnasition temperature for any weldment then you would be testing in a not-too-safe manner.

 

[davefitz]

For most components operating below the creep regime ( below 750 F) the new ASME allowable stress is based on 1/3 UTS or 2/3 yield stress, whichever value is lower. If the allowable stress is 2/3 yield and you exceed 150% design pressure during the pneumatic test , you are at yield stress for some alloys. This is not a safe area to operate at ( even for short periods) with a pneumatic test.

 

[DLANDISSR]

There is no restriction on pneumatic test pressures. If the pipe and components have been selected properly for the design pressures a test pressure of 110% of the design pressure would not over stress the system.

For hydrostatic tests there is a restriction per B31.3. For pipe systems at elevated temperatures the allowable stress at test temperature(St) and the allowable stress (S) at design temperature (Per Table A-1) should be ratioed and multiplied by 1.5 and the design pressure to determine the test pressure. St/S must not exceed 6.5.

 

[Duck50]

If you have a 1/4" SS line it is probably seamless tubing with compression tube fittings which means that a hydrotest is useless sense hydro's are for testing weld integrity only. With a harmless fluid like helium a simple leak test at service pressure ought to be adequte.

GTD

 

[krossview]

Duck50 maybe onto something here and assumed it is seamless tubing.

If not specified properly the contractor may have installed seamed tubing, due to lower cost and may had to hydrotest the line at lower pressure due to a lower pressure rating of the seamed tubing.

This may be a wild idea, but experienced the installation of seamed tubing myself, while we expected seamless tubing.
It does not hurt to check the type and pressure rating of the installed tubing.

Krossview/OK

 

[aurrous]

There are seveal problems with the statement that you have made. First is the pressures, If your line was only tested to 6750psi. That means by b31.1 requirements, your maximum allowable working pressure is only 5625. Do the math, 6750/1.2 = 5625. once you tested to this pressure, and the material was certified to this pressure, if you go above that pressure you have violated/ voided the certification of the material. Next you cant violate the test pressure by more than 6% (sec I) or you have effectively "destroyed" the material by certification. It has to be scrapped. Meaning if you certifed the pipe to a MAX of 5625, tested at 6750, if you hit 7155+ you have to throw the material away. SO having said all that. If you want run the material over 5625psi, i would suggest you retest the material. In the pressence of an AI. Its a certification problem not an an engineer function. The engineer can say Yeah it will hold more, but the AI says the paper says this, go above that, and ill take it out of service! hope you understand.
You and i know it will hold more, but its not what we know, its about what we can prove on paper! And as for allowable maxes, sec I for all other section is 90% of yeild strengh ( 2% offset) at test pressures. also 137.1.4 at no time during the test shall and part of the piping system be subjected to a stress greater than that permited by 102.3.3(B) During pressure tests performed in accordance with 137 the circumferential (hoop) stress shall not exceed 90% of the yield strenght (2% offset) at test temp. blah blah. the sum of long stresses due to test pressures and live and dead loads at the time of test, excluding occasional loads, shall not exceed 90% of the yield at test temp.
anyways i hope that helps.

 

[aurrous]

i think i miss read your post. If the test was Hydro testeed, replace 1.2 with 1.5 and do the math from there.
1.2 for pnue.
1.5 for hydro.

 

[aurrous]

for duck 50. hydros pnue are for testing the system integrady, PIPE, WELDS, VALVES, FITTINGS, so on. If you want to test a weld, use NDT Mt, rt, ut, vt, so on.

double check the Material data sheet.

 

[davefitz]

reply to DLANDISSR:

There are serious restrictions and limitations required by code when pnuematic testing . See for example Sect VIII div 2 par AD-151, AD-151.2, At-400, AT-410, AT-411.

Pneumatic testing has a much greater potential for catastrophe than hydrotest if pressure vessels are involved.
Be certain fo code limitations and make darn sure the test temperature is above any ductile limits of the weld zones.