Beginner questions about backpressure

[igidy]

Hello everyone, I am new to this forum and to relief devices. I have been given a project regarding balanced relief valves. I understand that the idea is to balance the valve from the effects backpressure. My question is about which surfaces of the valve the "balancing" occurs on. For instance, in a bellows-style balanced valve the top surface of the disc is still exposed to the backpressure. The bellows prevent the backpressure from encountering the top of the stem and spring set-up. Is that all it takes to balance it? In this case do the areas on the top of the disc have to equal that to the exposed areas on the bottom of the disc? What if all the components have equal top surface and bottom surface areas?

I know that these are basic questions, so any comments would be appreciated. Thanks in advance!

 

[Korichnevijgigant]

Check out 4.2.1.3 of API 520 part 1

Should answer your questions

short answer, the inside of the bellows should be at or close to atmospheric pressure