Pressure Safety Valves in Low Pressure ASME Systems

[Capta1nChaos]

Hello Everyone,

Problem: When working with non-ASME stamped vessels, what is the max pressure I can set a pressure safety relief valve to?

Background: My company deals with low pressure pipe work below 15 PSIG and we have always were able to slip through not having to get ASME stamped pressure vessels due to this. We have an internal debate going on about what the max pressure we can set a Pressure Safety Valve to. Can someone help point me toward the relevant code? I’m an electrical guy by trade and am no stranger to standards.

Our vessels are pressure tested to 25 PSIG but they aren’t stamped for service at that pressure. My co workers seem to think that means we can set a safety valve to 16 PSIG. However, my limited knowledge of ASME standards leads me to believe that would require us to get the vessel stamped for a higher pressure and the max pressure that vessel could see is 15 PSIG and below.

 

[The Obturator]

ASME VIII Rules that a Pressure-relief Valve (and vessel) must be NB/UV Code Stamped for pressure 15 PSIG and above.

ASME VIII Division is forpressures up to 10,000 PSIG but there are alternative rues available in ASME VIII Division III

Pressure-relief designs are available as high 15,000 PSIG, though most are API-526 standard types limited to a Cl. 2500 inlet flange rating.

Your 16 PSIG Vessel would require the vessel and PRV to be code stamped.

Please note there is a separate forum for Safety Relief Valves (alternative name for PRV).


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[LittleInch]

The maximum pressure you can set your relief valve to is the MAWP of your pressure vessel or piping whichever is lower.

I don't know why a pressure vessel designed for a MAWP of <15psi is being tested at 25psi. 22 would seem more like it at 1.5 times the MAWP.

So if you set the relief valve to 16psi, then this brings you into the ASME VIII system.

Different places have different regulations and sometimes volume is also used to decide if the item is a PV or not.



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

 

[MJCronin]

I agree with LittleInch .... Regarding safety, the volume of the protected tank/vessel also comes into play !!!.

If necessary is there a problem with the usage of cheaper and easier to maintain rupture disks ?

Can you give us dimensions/ pictures, descriptions of materials ?

MJCronin
Sr. Process Engineer

 

[Capta1nChaos]

We are working with all stainless 304 piping and vessels. The vessel in question is a glycol exchanger that cools down gas while moving from point A to B. This glycol exchanger is does not carry an ASME stamp at all, it is intended for 15 PSIG and below systems. Historically, we have set our Pressure Safety Valves to 15 PSIG as the MAWP of the glycol exchanger is 15 PSIG.

To me, it seems we would have to set our Pressure Safety Valve to 14.5 PSIG to ensure we do not exceed the 15 PSIG MAWP of the Glycol Exchanger. Because if our system did ever see more than 15 PSIG we would fall under SME VIII systems. Is that correct or am I overthinking this?

 

[LittleInch]

Would be interesting to know the actual wording of ASME VIII whether it is 15psi and above or not relevant if 15 psi or below??

If the former and your vessel has a MAWP of 15 psi then it might already fall into ASME VIII.

I don't know if anyone is going to split hairs like that but if I was you I would set your relief systems at 15 psig. Bearing in mind most simple spring reliefs have a -8% tolerance you should be fine.

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

 

[The Obturator]

Pressure-relief Valve Set Pressure tolerance at pressures up to 70 PSIG is +/- 2 PSIG per ASME.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***