Where in UG-125 are Relief Valves mandated as Relief Devices

[mechprocess]

Sorry if this is already covered. The thread I found was closed and now I have this same question.

I need to provide relief for an air receiver. The only credible over pressurization scenario is fire case while vessel is valve closed isolated and heated by fire. This includes the inlet to the vessel. Since I work with a bunch of nit pickers, let me explain what the nth degree they are looking for and the dilemma causing it.

Fire heats the unwetted vessel walls. The air heats up and causes the vessel to pressurize to the point of relief. The valve pops open and lets out some air. Now there is less air mass in the vessel. So the air has to get hotter to maintain the relief pressure. The end result is that the vessel fails at some pressure due to the heat. I don't want to sit here and find out when if fails and at what pressure and what's the total energy from the explosion and how long I have to till the fire man puts out the fire.

All I want to do is stick a non-reclosing relief device on this vessel and be done with it.

My 2000 version of UG-125 states specifically "relief device" it says nothing about this device needing to be a relief valve.

Please no guidance on calculating unwetted fire flow rates. I'm up on all that.

I just want to know if there is any requirement that stipulates that a relief device must be a relief valve for a gas filled pressure vessel.

Thanks.

I hope someone can put an end to this debate.

 

[pleckner]

Absolutely NOT!!! Your other options include rupture disks and rupture pins. You do not need to use a relief valve. If these people are "show me" people, just read a little deeper into ASME Section VIII, Div 1. At the end of section UG-125 you will find your answer.

 

[Ashereng]

Is a HIPPS also considered a relief devise? We have used a HIPPS in place of a PSV, albeit not in air service, on a previous project. If yes, then this is another (very expensive) alternative.

High Integrity Pressure Protection System.

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[pleckner]

Yes, approved instrumented safety systems can be used in place of relief devices.

And there is one more possible alternative that I forgot to mention and that is invoking ASME Code Case 2211. This code case allows one to design for containment. Sounds simple, just design your vessel for a pressure greater than any pressure that the system can ever see. Two problems...Code Case 2211 is not always accepted by the local government and even if they do, they have to approve your use of the Code Case before the vessel can be designed. Fat chance getting this to happen within a respectible design schedule.

 

[zdas04]

For vessels that do not have a credible scenario that results in their being liquid full, then the fire case is not credible because adding enough heat from a pooling fire to get to an elevated pressure will result in the metal flowing before the relief device can go off. Long before the melting point, the metal will get soft enough to no longer be able to contain pressure.

David

 

[pleckner]

"zdas04":

I'm not sure how to interpret your first sentence. Are you talking about liquid full vessels or vessels that are not liquid full?

The original question is about an air receiver and alternative relieving devices one can use. The person asking the question specifically asked that we don't question his scenario as he has already beaten it to death.

Now, to your statement. If the vessel is liquid full then the vessel will go to relief and will not fail immediately in a fire. Remember that old chem lab experiment (maybe you don't) when the Prof took a paper cup with water and held it over a bunsen burner? The water boiled and the cup didn't burn....until the water boiled out.

If you are talking about a gas filled vessel, use the gas laws to calculate the pressure rise as the temperature increases to see if you can reach relieving pressure. I would agree that in many instances, the gaskets on the piping around the vessel will fail due to high temperature way before the vessel reaches relieving pressure. Then the vessel will depressure anyways. That's why I don't believe that relief valves for fire in all cases really makes that much sense.

 

[zdas04]

"Pleckner"
I have to say that either my communication skills have deteriorated or you just don't read what is written. My point and your last sentance are identical. If you can't come up with a scenario that will result in a normally gas filled vessel becoming liquid full then the fire case is not needed.

A liquid-full vessel will change pressure about 100 psi per degree F change in temperature so an external fire is a very credible cause of overpressure and relief devices are required.

A gas-full vessel will fail long before the gas laws can describe an overpressure scenario from an external fire.

It might be a good idea to keep your sarcasm in check until you have an idea to whom you are directing it.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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[pleckner]

Sorry if my response read as being sarcastic as it was never intended to be so, so please accept my apologies if it read that way.

Technically, we are in agreement if the gas-filled vessel was stand-alone. But I think you will agree that some consideration must be given per API RP521, Section 3.15.1.2 if the gas filled vessel is within a fire zone produced by another vessel that is liquid filled.