When selecting a pressure relief valve (PRV) for any application, many factors need to be taken into consideration. One of the most important — and least well understood — is back pressure. This article explains what back pressure is and how it affects the performance of PRVs.
What is back pressure?
Back pressure is the pressure on the outlet of the pressure relief valve. There are two types of back pressure:
- Superimposed back pressure. Superimposed pressure is the pressure in the discharge header before the pressure relief valve opens. Depending on the system, superimposed back pressure can be constant or variable.
- Built-up back pressure. Built-up back pressure is the pressure that develops as a result of flow after the pressure relief valve opens.
When a PRV discharges, these two elements exist together as combined back pressure.
How does back pressure affect PRV operation?
Back pressure can significantly affect a valve’s performance by reducing both its set pressure and its capacity. Too much back pressure can result in chatter (rapid opening and closing), which can damage the valve.
Different types of PRVs are affected differently by back pressure:
Conventional relief valves
For conventional relief valves, back pressure reduces set pressure directly on a one-to-one basis. For example, a valve with a set point at 100 psig that is subjected to 10 psig of back pressure will not reach set point until the system pressure reaches 110 psig. In this example, if the set point is not adjusted to compensate for the back pressure, this can mean that valves are operating at a level that is higher than their maximum allowable working pressure (MAWP).
The effect of back pressure on valve capacity is much more significant. Typically, high back pressure can decrease the PRV’s capacity by approximately 50%.
Balanced bellows relief valves
For balanced bellows relief valves, the bellows mitigate the effects of back pressure up to a certain point. These valves are generally not affected unless the back pressure exceeds 30 or 35% of set pressure. The tradeoff is that they can fail at higher pressures.
Pilot-operated relief valves
For pilot-operated relief valves, back pressure is even less of a problem.
How much back pressure is acceptable?
Back pressure needs to be accounted for when sizing a PRV. In general, back pressure should not exceed 10% of the set pressure, especially for conventional relief valves.
How do you compensate for back pressure?
The key to handling back pressure is to take it into consideration when sizing and selecting your valves. If you know that the back pressure in your system will be higher than the recommended limits, you may need to select a larger valve.
Don’t risk damage to your valves or other equipment caused by too much back pressure. Contact us for assistance selecting the right valve for your application.