API 526 Design or Relieving Temperature for sized? 1

[Resegone]

Good Morning Everyone

In these days I got problem with one of my customer about selection of a PSV.

I got these data: T(Relief)=1094°F and T(Design of my component) = 649.4°F, with orifice E as calculated in accordance with API 520 Part. I

In accordance with API 526, using the temperature of relief I had to choice :
• Inlet 1 ½’’ 900# RF
• Outlet 2’’ 300° RF
• Body: Chrome Molybdenum

However my customer supply to me the piping class of the isolation valves, upstream the PSV, and according to that is all CS, SA 216 WCB.

Based on following in case I will use the Design Temperature of 649.4°F i could provide a PSV with body in CS in accordance to the piping class upstream and downstream the PSV.

In API 520, 521 and 526 I didn't find a para. that define to use the Relief Temperature, instead of the Design Temperature. In case could you provide the para.

As good practice i think that using the Relief temperature is the best choice, but according to piping classes of my customer it seems to be stranged that the material and particular the rating of the flange, 300#, are not suitable with Temperature of Relieving and Set Pressure and in case all piping upstream my psv will be collapsed. Instead using the Design temperature there would be no problem

 

[The Obturator]

Hi. Note that there is a separate and dedicated forum for safety/relief valves, but I'll answer you here.

You appear to have a PRV on a fire case - please confirm.

In such a scenario, the PRV is calculated using the relieving temperature (as it should be for any relieving condition)

The material selection is generally governed by the design temperature, and sometimes the ambient temperature (which can be quite low).

The ideology is that the PRV is sized to relieve the calculated capacity of the fire case at the incidental relieving temperature and materials are to be suitable for the design temperature stated. The vessel is seeing a fire. It will be considered damaged beyond repair afterwards. The valve will open, and basically stay open to prevent an explosion from the vapourised liquid in the vessel (called a BLEVE - Boiling Liquid Expanding Vapour Explosion).

I'm sure this is somewhere in API. I'll try and find it for you to refer to it.

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

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

 

[Resegone]

Thanks for your Replay.

Confirmed fire case, but only wetted case, not un-wetted.

I agree that temperature relieving Is the best choice

In case of there a Section about PRV or PSV i lo sai ti moderatore if can nove this thread on proper Section

 

[The Obturator]

Please see attached part extract from API-521 (2020), particularly the highlighted portion. While this does not give the answer, it guides you to the fact that 'additional protective measures should be considered'. I regularly came across fire case PRV's where carbon steel body was used for very high relieving temperatures. Hope that puts you on the right path.

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

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