In the process industry, these two terms are used interchangeably although they differ in their operation. Relief valves and safety valves both prevent overpressure in industrial processes. Although they are designed for the same task, they are different in how they achieve this. Read another article on how to select and size safety valves and relief valves.
Safety valves are seen as a last resort safety measure. They are found in power plants, petrochemical systems, boilers, and many more. Safety valves are fail-safe devices that automatically stop the increase of overpressure beyond a determined limit. If the setpoint is reached the safety valve will fully open until it is below the setpoint again.
For more information on safety valves, read our in-depth article on safety valves.
A relief valve, also known as a pressure relief valve, is a device that lowers the pressure to prevent damage to the system. Their function is to protect pressure sensitive equipment from damage caused by overpressure. They are critical in a pressure system to ensure that system failures are avoided. To prevent system failure the pressure must be kept below a predetermined design limit. Each relief valve has a set point at which it starts to open and starts to prevent overpressure.
For more information on pressure relief valves, read our in-depth article on relief valves.
The difference between pressure relief and safety valves lies in 3 main areas: their purpose, their operation, and their setpoint.
The first difference between the two lies in their purpose. The purpose of a relief valve is to keep the pressure in a system within set limits to prevent overpressure. Relief valves are designed to prevent damage due to overpressure conditions. Safety valves have a fail-safe purpose. Their main purpose is to protect property, the environment, and foremost people.
The second difference lies in their operation. The opening of relief valves is directly proportional to the pressure increase after the setpoint. The closing of the valve also happens gradually. Safety valves will pop open immediately when the setpoint is reached. It is designed to relieve the system of dangerous overpressure as fast as possible. Relief valves are commonly used for fluids like oil or water. Safety valves are commonly used for compressible gasses like air or steam.
The third main difference lies in the setpoint. First, note that the setpoint should not be confused with the set pressure. The setpoint (response pressure) is the pressure at which the valve starts opening. The setpoint is usually set below overpressure conditions and above the working pressure level. Safety valves already open a small amount before the setpoint is reached and pop open at the setpoint, or at a specified value after the setpoint. Relief valves only start opening at the setpoint.