MAWP and Safety Valve Set Pressure

[AttilaK]

Hi all,
I have a strange question about pressure vessel designing in ASME code.
Is it possible to have a design pressure of 2.5 bar and a safety valve set to 1 bar ?

After the calculation of the components, I will have the MAWP of the completed vessel that must be equal or higher than the design pressure.
Looking at the definition of MAWP i read the following:

"...It is the basis for the pressure setting of the pressure setting of the pressure relieving devices protecting the vessel. The design pressure may be used in all cases in which calculations are not made to determine the value of the MAWP..."

What does it mean exactly? Does it force me to use a safety valve equal or over the design pressure?

Thank you in advance

 

[davefitz]

AS I read your posting , it may be feasible to have all three ( design, mawp, lift P) at 2.5 bar, but there are other requirements vis a vis max permitted overpressure during safety valve lifting events.
In any case the first relief valve must lift at or below MAWP.

"Whom the gods would destroy, they first make mad "

 

[AttilaK]

Thank you for the reply.

I'll try to explain better.
I need to have design pressure at 2.5 bar and so the MOWP will be at least 2.5 bar.
Then I need to set the pressure of the safety valve at not more than 1 bar to restrict the maximum operating pressure.
Is it possible?
My concerning is that the safty valve to 1 bar may force me to reduce the "official" MOWP or design pressure under 1 bar.

 

[LittleInch]

I know it's fairly low pressure but using a "safety valve" to restrict operating pressure doesn't sounds like a good idea to me.

However setting any operational limit is up to you - it should not affect any ratings of the vessel and you could have any pressure you like equal to or less than the MAOP which itself can be set anywhere equal to or less than the design pressure.

As said above, the purpose of the MAOP and pressure setting of the "safety valve" is to protect the vessel itself and any directly connected components, not act as any sort of operational control hence the wording in the code.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[TD2K]

I don't like using a PSV as a 'control valve' but there's no Code restrictions to setting your PSV at 1 barg given a design pressure of 2.5 barg and a MAWP somewhere in excess of that.

 

[Bmechz]

Dear MDARC,

Please note that MAWP of the vessel shall not be less than Design pressure,you can consider MAWP same as design pressure code allows it.

Regarding pressure relief valve , i think you can set a pressure Of relief valve between MAWP of vessel and Operating pressure. In no case set pressure shall be more than MAWP of the vesseL.

Bmechz

 

[CollinsCombustion]

Just curious why the process is not deciding this for you? e.g. 700lb MAWP boiler operating at 475psig with reliefs set at 550psig? all of this is well within the MAWP.. there will be 25psig higher drum pressure than the non return "check valve" for a header that runs 450psig....

 

[moltenmetal]

You can set a relief valve for any pressure LOWER than the MAWP of the devices being protected.

A pressure relief valve should not be used for process control purposes: it's a safety valve, which needs to be actuated infrequently so that its availability in the case of an emergency is high- you don't want it to be worn out or damaged when the emergency happens. And if your desired operating pressure is 1 bar, don't forget that the relief valve will start to "simmer" (prematurely open) at a pressure somewhat lower than 1 bar.