PSV After Regulator 1

[TangoCleveland]

Could someone please help my aging memory. . ..

We always require a PSV downstream of a pressure regulator because downstream components are not rated for regulator inlet pressure. Failure scenario is wide-open regulator failure, simple sizing calculation.

I've seen "requirements" in Fisher gas regulator literature, and Sarco steam regulator literature. I also think that I saw an API, CGA or ISA requirement in a Code document, but I haven't been able to get my hands on it because I don't remember which Code it was. I do know that B31.3 301.2.2(c) requires consideration of ". . . failure of control devices."

Does anyone know of a Code document that explicitly requires pressure relief downstream of a regulator? The client wants to have a Code citation for this requirement on oxygen and hydrogen systems.

Larry

 

[djack77494]

I don't think you'll find any "requirement" that there be a PSV downstream of a regulator. Rather, you're required to consider any and all possible overpressure scenarios. Frequently, failure (open) of a regulator could cause overpressurizing of the downstream equipment.

 

[JimCasey]

THe most likely failure of a regulator is a blown diaphragm. When that happens, it cannot sense the downstream pressure, and the spring pushes the reg wide open. Hence the requirement for sizing the SRV at full regulator capacity. If you absolutely positively have a known demand on the downstream system at all times, you could subtract that from the required flow through the SRV. Be sure to assume accumulated relief pressure downstream of the regulator (set +10%)-it may decrease the size of the SRV that you need.

To answer your code question directly: it is good practice to do this, and many A/E firms or users will include the requirement in their internal piping standards. I have never seen it in a code.

 

[EGT01]

TangoCleveland,

You really didn't state specifically what equipment downstream of the regulator you are trying to protect and of course you know not to apply the code requirements intended for one type of equipment to a different type, then....

If you are dealing with piping, I would just backup a little in B31.3 and it would seem that B31.3 301.2.2(a) is quite clear that
"(a) Provision shall be made to safely contain or
relieve (see para. 322.6.3) any pressure to which the
piping may be subjected."

In the case of a pressure vessel, refer your client to ASME Sec VIII, Div 1, UG-125.

The list of codes could go on and on...

For piping systems, maybe you don't have a lot of scenarios you have to consider but don't let yourself get hung-up on the short list of examples in B31.3 301.2.2(c), be sure you are looking at anything and everything that can cause you problems.

 

[TangoCleveland]

National Board Inspection Code (NB-23) has a non-mandatory Appendix G that discusses sizing and installation of steam PSVs downstream of steam regulators. It's not a requirement, though. That's where I saw the "requirement".

Larry

 

[RJB32482]

Now would the total requirement through the regulator be the Cv valve times the square root of the inlet pressure to the regulator? Or could you subtract the pressure relief setting off of the pressure drop? Is this true for both liquids and vapors?

Thanks.

 

[sshep]

Not only can you subtract the relief set pressure from inlet pressure to get the regulator pressure drop, you can use the actual relieving pressure as the downstream pressure (i.e. you may include the overpressure). This is true for liquids and vapors, however vapors may be limited by choked flow if dP/P1 is high enough.

best wishes,
sshep

 

[TangoCleveland]

All the sizing details are covered in API RP520 and RP521. For example, the RV has to be set at MAWP or Design Pressure, but flow through the RV is calculated at 10%, 16%, or 21% overpressure, depending on failure scenario. The percentages are for ASME Sec VIII or B31.3; they're different for B31.1 and boiler service.

Larry

 

[JAlton]

TangoCleveland is correct in his explanation. The requirements depend on the downstream equipment or piping ratings. NBIC (National Board Inspecion Code) SCOP (Sub-Committee Overpressure Protection) has an item open to deal with Pressure Relief Valve Scenarios because there are a lot of questions like this that are not easily answered. Some committee members want to simply reference API Standards & ASME Code. Others say we should recommend an Engineering Consulting Firm. The National Board Staff wants a document which will help users understand the requirements and also aid inspectors in analyzing the system they review for adaquate protection. This item is stalled because the volunteers on SCOP do not have the time available to write the guidelines.

J. Alton Cox

http://www.delucatest.com