Hydrotest Pressure Drop

[racguy]

Experts,

I have gone through the Hydro test @ 201 bar test pressure, on site, for SA 335P91(size - 1/4" X Sch 80) holding time was an 30mts and Spool was having total 50 butt weld joint & some portion of spool is welded with Gate Valve (1/2" Class 2500)

So the thing is the test was started at 201 bar @ 1835hrs. After 30mins hr (at 1905hrs) pressure went down approx 3bar. And am sure there is no leak found visually on the welded joints or on the temporary joints.

The test was repeated twice, again we found the pressure drop and where all other things were remain same which is no visible leakage observed or felt. The Only thing difference between conditions is that the Temperature difference during 1835hrs and 1905hrs.

And here my question, First of all can it be acceptable (what i mean that any small amount of pressure drop, (like in this case) is acceptable as per code? is code specifies some tolerance for this kind of conditions? or perhaps due to Temperature difference??

Normally, this type of conditions can we go/accept the test on the bases of no leakage found during visual inspection. I guess UG-99 and ASME B31.3 chapter VI clause 345 is not exactly specifying the acceptance criteria. can anyone guide me for any interpretation with respect to ASME?? or can anyone suggest me any other code or standard which is specifying the exact acceptance criteria for Hydro test?

 

[XL83NL]

the test on the bases of no leakage found during visual inspection

That's the criterion for the code (which you dont mention, which code do you use, ASME B31.1), normally. See para 137.1.1, and 137.3.1 to be sure if you need to follow the testing requirements for BEP.

/edt: Im not that familiar with B31.1 so hopefully can back me up here.

 

[DekDee]

racguy,
The pipeline guys on here are able to quote all sorts of calculations for pressure drop based on temperature but they are usually looking at 6, 12, 24 hr tests.
Temperature should not come into your reasoning for repeated pressure drop after 30 minutes (unless snow has suddenly fallen on a sunny day).
Are you 100% sure you have bled all the air ?
Are your valves 1/2 open or fully open ?
Cheers,
Shane

 

[moltenmetal]

IF the applicable code is ASME B31.3-

I do not read the code to require that pressure during a hydrotest must remain constant while the pressure source is shut off. That said, I am not the same as a code interpretation- you may search those yourself, or someone here may quote one for you on the topic, should one exist.

Leakage through JOINTS in the piping is clearly not permitted- no weld, NPT thread, compression fitting joint etc. in the permanent part of the piping system may leak visibly during the test. Leakage through valves, valve packings etc.- components which are not JOINTS in the piping, may be permitted, and in fact may be impossible to prevent. Leakage through flanged connections isn't as clear, but we would ourselves consider leakage from a flanged connection to be a failure until we repaired the leak.

Clients/owners may of course stipulate requirements beyond those required by code. But they cannot claim that "the code" requires that a hydrostatic test only be considered to pass if the test pressure is maintained without visible decrease for X minutes.

That said, you must maintain the test pressure for that period. If there is leakage through something other than a joint, you may in fact have to operate the pump or pressure source in order to keep ahead of that leak. Others here have described tests where it was necessary to pump water at several gallons per minute continuously in order to carry out a successful hydrotest- it's all a matter of scale.

 

[LittleInch]

3 bar drop is probably 1 degree C difference. Did I see that this is a 6mm diameter (1/4") manifold??. That will respond very quickly to temperature drop or even two or three drops...

Maybe do it in gas instead. Much less susceptible to temperature than water.

If you really suspect a leak do a helium test and sniffer.

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

 

[moltenmetal]

If you suspect a really slow weeping leak, a pneumatic test (quite a bit safer now that you've done your hydro) with Snoop (bubble soap) would be adequate. Helium would be necessary only if you can't Snoop all the joints for some reason, or the commodity in the piping is extremely hazardous, and a tiny leak is a big concern. If the commodity is hydrogen, do the helium leak test...

 

[GenB]

Code:rise the pressure to the test pressure then lower to the MAWP then check for leaks. A large pc of pipe may yield a little and give you more space check pipe mtr's for elasticity %.then you can make a calculation also based on the mean metal temp. Differential on pipe and water. Hopefully the water was fed at constant temp and either ways the hottest water will rise to the top and possible affect the test pressure.

General Blr. CA,USA