Is there an ASME Certified Pressure Relief Valve for two phase fluid? 2

[OzGuy]

Hi,

I have an application for a PSV on gas service where the tube rupture scenario results is two phase fluid (10% liquid & 90% gas). We tend to use a full port pilot operated valve, however vendor has commented that the offered valve is ASME certified for gas service only and not for two phase. Vendor has sized the PSV for dual phase scenario as per ASME Code Section VIII but it will not be ASME certified (i.e. stamped for dual phase.)

Another vendor has confirmed the PSV will be ASME certified and the capacity will be volume of gas which worries us if there is something wrong.

I am not familiar with certifying process but I assume ASME tests actual valve samples with air and certifies them for gas and vapour service or tests them with water and certifies them for liquid service.

The application is for a gas plant and both vendors are very reputable worldwide. Option of using API letter PSV rather than full port/bore is not feasible due to inlet/outlet pipe sizes and no suitable size API PSV has the capacity we require.

I wonder if there is any PSV, ASME certified for two phase fluid.

I appreciate if you could share your thoughts on this.

Thanks,
OzGuy

 

[met11]

Hope this helps:

API 520 Part 1:

5.10.2 Recommended methods for sizing pressure-relief devices in two-phase service are presented in
Annex C. The user should be aware that there are currently no pressure-relief devices with certified
capacities for two-phase flow since there are no testing methods for certification.

C.1.4 Coefficient of Discharge
The value for the effective coefficient of discharge for two-phase flow is a subject of current debate [4] [6] [7]
and is not likely to be resolved without actual testing of PRV behavior with two-phase fluids. As a result, a
conservative recommendation regarding an estimate of the effective coefficient of discharge has been
provided within the guidance on sizing methods, see C.2.1.1, C.2.2.1, and C.2.3.1.

 

[The Obturator]

An ASME VIII SRV qualified for a "U" stamp must have been capacity tested against the code requirements and the capacity verified by the National Board. The valves design is recorded for these purposes. For liquid, ASME VIII has been requiring accreditation since 1985, and capacity must be at 10 % overpressure. Due to the varying degrees of multi phase flow (including flashing, % of gas/liquid, temperature etc., etc.), no agreement has been reached on how this can be certified.

However, some manufacturers have developed a trim (nozzle seat, disc, disc holder or other), initially for full liquid flow, that satisfies the SRV's opening and closing behaviour required by ASME VIII whether liquid or gas. Hence a "multi phase trim valve" can be offered. Certified capacity remains at 100 % water or air flow. Some manufacturers are known to have done their own multi phase flow testing to develop such a trim, but cannot certify to ASME. 2 separate nameplates, 1 each for each phase, is nonsense and would only confuse (or would it if clearly marked "OR" ?).

Personally, I would be more interested in reviewing the vendors approach to multi phase sizing. What method has been used, and what back up they have. You should also look for the SRV having a good high coefficient of flow (for liquid) for stable operation.

With a modulating pilot operated SRV, where trim is not an issue, the valve is good for multi phase, no back pressure effects, close working to set pressure without leakage and much better reseating capabilities. However, same nameplate issues.


Per ISO, only the term Safety Valve is used regardless of application or design.