How to choose pressure relief device for piping pneumatic leak test

[Sean_Wei]

Hello guys, one question has confused me for a long time.
As the requirement of ASME B31.3 345.5.2, a pressure relief device shall be provided during pneumatic leak test.
1. What cases will cause the test pressure to increase, the misoperation of pressure - raising equipment? Or uncontrolled test temperature rise?
2. How to evaluate these pressure increase cases quantitatively and give the clear information for selection of PSVs?

 

[LittleInch]

Usually the biggest is simply the pressurising device, be it a compressor / bottle supply or similar goes flat out / full open regualtor.

For a pressure test the temperature rise should be fairly low and also a fairly lor percent. Remember the P1V1/T1 equation for temperature uses absolute temperature so even say a 30 C rise is only a 10% increase in pressure starting at sat 10C / 283K

So do the maths and work out which is the biggest flow case and its almost certainly the max flow in from your pressure raising equipment assuming the controls have failed or no one turns it off.

 

[goutam_freelance]

How to evaluate these pressure increase cases quantitatively and give the clear information for selection of PSVs

The flow requirement criterion has been given by @LittleInch

As per B31.1 the set pressure for PSV should be 1.33 times the test pressure provided other limits like stress during test are not exceeded. Please check with B31.3

 

[LittleInch]

The flow requirement criterion has been given by @LittleInch

As per B31.1 the set pressure for PSV should be 1.33 times the test pressure provided other limits like stress during test are not exceeded. Please check with B31.3

Gotam,
Where is this in B31.1? I couldn't find it anywhere, but then latest one I have is 2014 tbf.
Usually good to quote the clause number in any code. I searched for 1.33 and couldn't find any clause with that number in it.

Anyway, not correct in B 31.3. This is the requirement for the PSV.


345.5.2 Pressure Relief Device. A pressure relief
device shall be provided, having a set pressure not
higher than the test pressure plus the lesser of 345
kPa (50 psi) or 10% of the test pressure.

The 1.33 is the MAX test pressure above design pressure, so between 1.1 to 1.33 above DP.

345.5.4 Test Pressure. The test pressure shall be not
less than 1.1 times the design pressure and shall not
exceed the lesser of
(a) 1.33 times the design pressure
(b) the pressurethat would exceed 90% of the pressure
described in para. 345.2.1(a)

 

[goutam_freelance]

Where is this in B31.1?

See '137.2.6 Precautions Against Test Medium Expansion.' I have 2018 Ed.

 

[LittleInch]

See '137.2.6 Precautions Against Test Medium Expansion.' I have 2018 Ed.

That section though has number of caveats associated with it (as you note) and personally I think it is a rather dangerous to potentially take it out of context. Another 1/3 on top of the test pressure which could be 1.5 times the design pressure is verging into the high risk end of things unless the test pressure stress is significantly below the SMYS of the material in question. too many if and buts for me and I can't see the benefit of overstressing the material.

Anyway, for pneumatic testing the temperature rise would need to be huge. Over pressure from keeping pouring in air or nitrogen is what the PSV criteria should be. IMHO. Set it at 10% above test pressure and you're covered.