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.
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.
You will hook up the outlet of the relief valve to discharge the contents somewhere safe. Where is "somewhere safe?" To handle flammable releases, refineries often install a flare stack, which contains a flame that is constantly burning. Also there will be a flare header system of pipes to bring any relief valve releases to the flare stack. The flare headers will be designed to have a very small pressure drop so they do not provide any barriers to a fluid release, and the headers will safely carry any fluid releases to the flare stack to be fully combusted. (You don’t want to release flammable gases onto your workers or equipment, or shoot it into the atmosphere, so you burn the gases). In some cases relief can be to safe location that is not a flare stack. In the case of benign gases like steam, air, hydrogen, etc. some companies allow you release the gases high into the atmosphere at a safe vent location. After release, the spring ensures that the relief valve is going to close. Any plant that installed relief valves needs a careful maintenance and testing plan to ensure they are working properly at all times.
Some similar pieces of equipment you may want to learn about:
Rupture disks are disks that are designed to burst open at a certain pressure. They are an alternative technology to a relief valve. The nice thing about rupture disks is that they do not get clogged up like relief valves, so often they are good in corrosive or solids-heavy environments. But the bad side is that they do not close after their release, so your process does not automatically close itself up after the release. Sometimes people will put a rupture disk upstream of a relief valve: the rupture disk faces the corrosive environment, while the relief vale will close after the release. This can work, as long as you use careful maintenance to watch for tiny leaks in the rupture disk. Another feature about rupture disks is that they burst very quickly: this is good for sudden pressure rises, like if a heat exchanger’s tube were to suddenly break and spill high pressure liquid into the shell.
Balanced relief valves and pilot operated relief valves: These are special relief valves that overcome some of the limitations of normal relief valves: they can be helpful if you are worried about chattering, or high pressures downstream of the relief valve. (These concerns are discussed below). Look into these special relief valves up if you have trouble meeting all the rules of relief valve design.
Vacuum Relief Valves / Vacuum Breakers: They protect vessels from a vacuum (lower-than-atmospheric pressure) instead of from high pressures. This is especially an issue for storage tanks, which are surprisingly easy to suck in and destroy with a vacuum. Storage tanks can look like a crushed soda can if subjected to a vacuum.
Trick: For thicker pressure vessels, you can specify that vessels be designed and rated for full vacuum conditions. Specifying full vacuum will usually cost a little more, but may be worth it to avoid the costs of a vacuum relief valve. I often specify full vacuum if I know steam will be used to clean the drum, because condensing steam can cause a vacuum if care is not taken.
The set pressure
Now you have decided to use a relief valve or rupture disk. And there is a system containing some combination of pressure vessel(s) and/or piping that you want to protect from overpressure with the relief device. What set point will you give to determine when the relief valve opens or the rupture disk bursts?
Normally, when designing a pressure vessel, the designers (process and/or mechanical engineers) will set a design pressure. This is the minimum pressure the vessel has to withstand. Sometimes this is set at a certain level for a safety reason, like setting it higher than the maximum pressure the upstream pump can produce. Other times it is determined with a rule of thumb, like “design pressure (psig) = operating pressure (psig) + 10% or + 25 psi, whichever is higher.” The design pressure will be quoted at a certain design temperature, because metal strength varies with temperature. When all a designer knows is the design pressure, you can set the relief valve set pressure equal to the design pressure. This will protect the vessel.
However, once the design of the pressure vessel gets sent to a vessel fabricator, the fabricator is not going to design the vessel to perfectly match the design pressure. Instead, they are going to pick certain standard thicknesses of metal off the shelf, and use these standard pieces to construct the vessel. Then they will determine the maximum allowable working pressure (MAWP) of the vessel, and if it’s an API vessel, stamp that MAWP value right onto the vessel's nameplate. The fabricator will ensure that the MAWP is greater than or equal to the design pressure. If you know the MAWP, you can have the relief valve set pressure = MAWP. This is better than setting the relief valve pressure to the design pressure. Having a higher relief valve set pressure is normally good, because it makes it less likely your relief valve will open, and also for vapour there will be less flow at high pressures.
In general, if you know the MAWP use it, if not use the design pressure. Check out http://www.cheresources.com/asiseeit1.shtml for more discussion on design pressure vs. MAWP.
Sometimes it makes sense to choose a set pressure below the design pressure or MAWP. For example, you may have a reaction that has to be carefully controlled, but becomes a runaway reaction at a certain point. Choosing the right set pressure may let your relief valve act as an extra safety feature, dumping the contents of the reactor before things get dangerous.
Another trick in this area: often a relief valve will have to handle several different kinds of problems, called “scenarios”, which can make the pressure too high. Sometimes if you just set the design pressure of a vessel higher, you make the vessel so strong that you can avoid having to design the relief valve for a difficult scenario, and end up with a much cheaper relief valve. In some cases, the savings on the relief valve side can out-weigh the extra money spent on stronger pressure vessel.
(e.g. I have a vessel at 500 psig design pressure. Relief Scenario 1 generates 10,000 lb/hr but Scenario 2 only generates 1,000 lb/hr. It turns out that Scenario 1 will never get higher than 600 psig. So it may make sense to bump the vessel up to 600 psig design pressure, and only have to design the relief valve for Scenario 2).
If pipes are attached, you need to know their design pressures and protect them too: check out ASME B16.5. Think also about protection of any instruments involved in the protected equipment/piping system.
Edit 2010-04-22: Minor rewrite for clarity.