Balanced Bellow Relief Valve Vent Plug 1

[DoraeS]

Hi,
Can anyone explain why for Balanced Bellow PSV the bonnet vent plug MUST be removed?
Thanks.

 

[sshep]

Hey DoraemonS,

In order to get the balanced action and lift at the proper setting, the bonnet pressure must stay at atmospheric pressure. This is the reason given in API RP 520 paragraph 2.2.1.3.2. If the bonnet is closed, the pressure will not be atmospheric due to ambient temperature changes, compression during lift, or a cracked bellows (which is why the bonnet must be routed to a safe location).

The bonnet pressure in a conventional valve is the downstream pressure. In Consolidated Relief Valves, this is achieved by an eductor tube to the discharge side of the valve.

best wishes,
sshep

 

[BenThayer]

just curious why does someone think it should not be removed?

if you have the PSV inside a building and the fluid is toxic, flammable, etc., you might want to run some tubing and vent it outside, etc.