Pressure relief valves (PRVs) or safety valves are a simple but critical part of any air compressor system. Pressure relief valves control and limit the pressure build-up in a system. If the pressure exceeds the amount allowed by the pressure relief valve, the pressure relief valve will automatically open and release air until the pressure reduces.
The rated pressure of the pressure relief valve must not exceed the maximum allowable working pressure (MAWP) marked on the air pressure vessel. This requirement ensures the pressure relief valve always opens before the vessel reaches its maximum pressure tolerance.
In addition, the minimum relieving capacity of the pressure relief device must meet the requirements of the American Society of Mechanical Engineers (ASME) Code. ASME is the international leader in developing and revising codes associated with mechanical engineering.
Disc: Silicone disc seals the valve opening and prevents air from escaping.
Compression Spring: Stainless steel compression spring exerts pressure on the disc to keep the valve closed under normal conditions.
Adjusting Screw: Brass adjusting screw allows for pressure adjustment of pressure relief.
Pull Ring: Stainless steel pull ring allows for manual testing of PRV to ensure proper function.
Stem: Brass stem Push-in after testing PRV to ensure disc is set.
Pressure relief valves for air compressor systems are simple, spring-loaded mechanisms. When the inlet pressure force exceeds the spring load, the safety valve opens proportionately to the pressure increase and allows air to “leak out” as needed.
Pressure relief valves for compressed air applications are direct-acting, automatically reacting if the pressure gets too high. If excessive pressurization occurs, a disc seal moves up due to system pressure against a spring, which closes the valve. If the compressed air force exceeds the force exerted by the spring, the valve disc lifts off the valve seat, and the valve discharges the compressed air to the atmosphere.
Under the requirements of the ASME relief valve standard, the full discharge capacity of the valve typically will be achieved when the system pressure climbs to no more than 10% above the set pressure of the valve. Full shutoff must be achieved if the system pressure falls below the set pressure value, usually 90%.
A set pressure function test should be carried out at least once per year to maintain pressure relief valve effectiveness over time. At VMAC, we recommend inspecting pressure relief valves annually for signs of corrosion or loss of functionality.
The pressure relief valve tests are the responsibility of the owner or operator, while the air compressor manufacturer should supply test instructions. For example, VMAC’s manuals include test instructions to perform a pressure relief valve inspection:
“The Pressure Relief Valve is a regular service item, and must be replaced every 400 hours or 1 year, whichever interval occurs first. Visually inspect the valve and ensure it is not corroded and that the vent holes are not plugged.
Turn the system on and allow it reach full system pressure (factory setting 150 psi). Ensure that air does not leak out of the valve. Air leaking from the pressure relief valve when system pressure is at or below 150 psi indicates that the spring or seat in the valve is worn and the valve must be replaced.
If the pressure relief valve is showing loss of functionality, contact a local VMAC dealer for a replacement.”
If you are operating a system that is not a VMAC air compressor, check your manual or contact the manufacturer for detailed instructions on how to test your pressure relief valve.
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