What Are Pressure Relief Valves, And How Do They Work?
A compressed air pressure vessel and its operator must be protected from an over-pressure situation—the set pressure of the pressure relief device must not exceed the maximum allowable working pressure (MAWP) marked on the air pressure vessel. A pressure relief valve is used to control and limit the pressure build-up in a system.
With a pressure relief valve, the pressure can be automatically relieved by a spring-loaded valve, actuated by the static pressure upstream of the device. The valve opens in proportion to the increase in pressure over the opening pressure.
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.
Pressure relief valves are designed to protect a system during a condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or MAWP. These valves are normally very durable and long lasting and are important in protecting life and property from overpressurized vessels. It is important to compare the pressure relief device data (set pressure and relieving capacity) with the air pressure vessel nameplate and compressor output data to ensure the pressure relief device is adequate for the installation.
Typically, the method of operation of safety valves for compressed air applications is direct-acting. A disc seal is pushed up by the pressure from the system against a spring, which is holding the valve closed. If the opening compressed air force exceeds the force exerted by the spring, the valve disc is lifted off the valve seat and the valve discharges the compressed air to atmosphere. In accordance with 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 to below the set pressure value, usually set at 90%. A set pressure function test should be carried out at least once per year. The test procedure is performed by the user, and test instructions should be supplied by the air compressor manufacturer.
How to test your pressure relief valve
VMAC recommends the inspection of the pressure relief valve for signs of corrosion or loss of functionality. VMAC’s manuals include test instructions to perform a pressure relieve valve inspection:
“To test the pressure relief valve functionality, turn the system on and bring it up to operating pressure. Pull the ring on the pressure relief valve to depressurize the system. Turn the system off, and ensure the system comes back to operating pressure when the system is restarted. If the pressure relief valve is showing loss of functionality, contact your local, authorized VMAC dealer for a replacement part. Relief valve failure can result in air/oil tank overpressurization leading to system failure or rupture.”
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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