This is the first in a set of articles introducing the basics of pressure relief valve design from a process designer’s viewpoint. Read Part 2, relief scenarios and the relief rate, here. Part 3 on sizing orifices and pipes is here.
Pressure relief valves (also called Pressure Safety Valves, PRVs, or PSVs) are a critical last line of defense in any high-pressure plant environment. They are designed to pop open when a certain set pressure is reached, and release high pressure fluid to a safe disposal location, like a burning flare stack for hydrocarbons. By opening and releasing, they prevent the pressure getting so high that equipment bursts, breaks, or explodes.
The Relief Valve Symbol for P&IDs
Before we begin, since this is the first safety post, I would like to point you towards the disclaimer. Obviously an introductory article on a website is not enough: read the industry standards, your company/client standards, have approved design tools, and have competent people checking your work. Don’t skimp on this task and don’t leave it all up to the vendor, because you may not find the mistake until somebody’s dead or the company has lost millions in damaged equipment. OK?
API 520, and API 521 are good places to start reading more, and maybe ASME Section VIII Division 1. Cheresources.com has a some good relief articles by Philip Leckner, and Chemical and Process Technology is a blog that gets into more depth on relief valves than I can. I also find that relief valve topics are often discussed on message forums so you may get some help there.
The equipment
First let’s talk about the equipment. A relief valve is a piping element that is designed to open when a certain pressure is reached. You place the relief valve on some piping, facing the pressure vessel or equipment you want to protect. As long as the pressure is below the relief valve’s set pressure, a spring holds the relief valve closed. Once the pressure is exceeded, the relief valve spring will be pushed on, the valve will be forced open, letting some fluid through. You can use a relief valve to protect things like pressure vessels or pipes or both, just make sure you have enough valves that all pressurized equipment is protected even when isolation valves are closed.
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