Pressure relief valves - how do they work

Check out API 520. Also you may want to post this to the Safety Relief Valve forum in the Chemical Engineers area.

There are a number of different types of pressure relief valves out there (and I really should be working so I won't get into them all...I'll leave that to someone else).

A relief valve is a valve that is opened by a certain pressure upstream of the valve (set pressure). It is designed to relieve material at a given rate (capacity).

How about an example:

You have a pressure vessel. It is designed to withstand a certain pressure. Above a certain pressure bad things can happen (boom, snap, crackle, pop, etc.). This is where the relief valve comes in. Let's say in this case its installed at the top of the vessel and its a conventional PSV with a spring in it. Joe Operator gets instructions from Sam Engineer to get the plant up and running ASAP. Unfortunately, Joe Operator forgot to open the discharge valve on the vessel. He starts feeding material into the vessel and then decides its time to eat a sandwich. The pressure rises in the vessel to the set pressure. The relief valve which is designed to relieve at this pressure opens up (the pressure inside the vessel is high enough to work against the spring in the valve). The material vents out (to flare or to atmosphere) at a rate sufficient enough to not overpressure the vessel. Sam is on his way home now and he sees a big flame on the flare stack. He quickly turns back into the parking lot and goes to find Joe. Joe opens up the discharge valve and things start returning to normal. The valve closes and waits to work again next time Sam doesn't give clear instructions. The next day Sam gets heck from Pete Manager.

Sorry about the dumb story....WAY too much coffee today. Anyways, there are lots of different types of valves and they don't all work quite the same way but the principle is basically the same. The valve reacts to upstream pressure (either directly or through a sensing method...ie pilot operated valves). It is designed to relieve material at a rate required to protect the equipment....this is where the engineering really comes in. You may be trying to protect against a blocked valve, a runaway reaction, a fire outside of the vessel...

I hope this helps a bit. There are a lot of people here that know this material a hundred times better than I do so hopefully they can give you some useful advice.

I'm not as good with stories as Zoobie, but your most common type, the section VIII Safety-relief valve operates on force-balance.

One standard orifice is 0.196 square inches. Standard orifice designations are D through T, with D being .110, and T being 26.0 square inches if memory serves.
If you need to set the valve to pop at 500 psi, first the manufacturer or a certified repair facility would select the proper spring for the application. Then they crank the spring adjuster down so that the preload is 0.196 x 500 = 98 lbf. Now, when the pressure under the disc gets to 500 psi, the disc will begin to lift. Outside of the disc there is a ring, which may or may not be adjustable. As the disc begins to lift, the flow starts, and it hits the ring and is deflected downward, increasing the lifting force on the disc assembly. The disc then POPS open to its full allowable travel. As long as the pressure remains above setpoint, the valve remains fully open. The rated flow for the valve is measured at the cracking (set) pressure, +10 percent overpressure. Once the pressure goes below the set pressure, the valve is no longer held open by the inlet pressure and it closes. Code valves must close within 10% of the set pressure. Non-code valves sometimes blow-down considerably more than that. It is a bad idea to operate normally at higher than 90% of set pressure because if you get an upset and burp the valve it will stay open until it blows down to its reseat pressure. It's possible for the valve to stay continuously open if the system can supply enough flow at over 91% of the set pressure to keep it open.
Safety-relief valves are like fuses....they are never really expected to operate, but are there to protect life and property in case of another malfunction.

Amendment: CHeck out page 5 of the Crosby catalog for a good drawing of what I was trying to describe.

Thanks for all your help!

:O)

There were a few items of information in the previous description of a Pressure Relief Valve which should be explained in more detail. In a direct spring loaded PRV, as the disc begins to lift, the flow starts, (expansion takes place if it is a compressible fluid)and it hits the ring and is deflected (upward against the "roof" of the disc holder) the increased area of the disc holder is exposed to the same pressure that previously could only act on the disc area. The increased area multiples pressure, increasing the force acting against the spring (F = P*A). This increased force plus the change of direction when the fluid hits the disc holder resulting in a downward force increases the lifting force on the disc assembly. The disc then POPS open to (typically 60 to 70%its full allowable travel). If the pressure continues to increas, the PRV will open further. However, by ASME COde, it does not have to reach full lift until 110% of Set Pressure. As long as the pressure remains above setpoint, the valve remains open. The rated flow for the valve is measured at the set pressure (popping, opening, cracking, start to discharge are all sub-definitions of set ptressure) +10 percent overpressure + 14.7 psia. Once the pressure goes below the set pressure, the valve is no longer held open by the inlet pressure and it begins to close. Code valves must close within 7% of the set pressure if they are submitted to an ASME Lab for Certification Testing. Oterwise, there is NO Blowdown requirement. Code and Non-code valves typically blow-down considerably more than 10%.

Then there are Pilot Operated Pressure Relief Valves. Again the operating principle is based on the area and force relationship. The pressure is directed from the inlet through a sensing tube to a small spring loaded pilot and on to a chamber above the disc. This chamber is approx 30% larger than the nozzle area pushing up on the under side of the disc. This imbalance of pressure due to equal force, but different areas results in the PORV remaining closed. When the system reaches set pressure, the pilot relieves and dumps the pressure of the back side of the disc. The inlet force then pushes the disc open and the vessel pressure is releived through the main valve.

J. Alton Cox

Pressure Relief Valves installed in Liquid Service act differently than PRVs in compressed gas service. There is no expansion to cause the pop action. Instead, the PRV will open in response to overpressure. Non-Code PRVs are typically fully open by 25% overpressure. However, modern ASME Code Liquid Service PRVs typically use a weaker spring than the corresponding size and pressure for compressible fluids. This results in a "gush" at 10% overpressure or less. The definition of set pressure for liquids is also different. Since there is no distinct "pop", the definition may be "first steady stream", "heavy flow" or "initial discharge" among many. Each PRV Manufacturer defines set pressure for his design(s).

J. Alton Cox