RE: Set pressure for a vacuum relief valve

[EGT01]

I'm in the process of reviewing the design basis for a vacuum relief valve on an ASME Sec VIII vessel that has a MAWP of 25 psi internal and 4 psi external.

The vacuum relief valve is set at 4 psi vacuum, same as the external MAWP rating. This is what I'm questioning.

My experience with API 650 and API 620 storage tanks is that the vacuum relief valve must be set so that the valve can achieve the required relief rate without exceeding the external MAWP rating.

What about for an ASME Sec VIII vessel, can the vacuum relief valve be set at the external MAWP rating?

Also, I would appreciate it if someone could direct me to a section or paragraph in ASME Sec VIII that addresses vacuum relief valve setting and/or requirements.

Just for reference, here's two threads I found that had closely related discussions.
thread794-35462
thread124-29282

 

[ab1234]

I am surprised why a Vacuum Relief Valve is being provided for a ASME designed vessel. As per code the equipment can be designed for External Pressure/Vacuum Ref. Clause UG-28,UG-33. Hence a Vacuum Relief Valve will not be required.

In Tanks the vacuum is generated due to outbreathing and since there is no specific rules for designing of Tanks for Vacuum, the Vacuum relief valves are provided. API650/620 limits that Vacuum shall not exceed 0.25Kpa (Refer Clause3.2.4)

Thanks

 

[EGT01]

Unfortunately, the vessel is existing with MAWP as indicated and vacuum relief valve in place. I agree, that if properly specified, the system could have avoided the need for vacuum relief but that is not the case.

My task for the moment is to evaluate the existing vacuum relief device. At first glance, I'm questioning if the set pressure versus external MAWP is acceptable. It appears that UG-28 and UG-33 address construction design and I don't see mention about external pressure relief.

UG-125 through UG-137 is always referenced for internal pressure relief, does it also apply to external pressure relief or is there another section in the Code that addresses external pressure relief?