In an air compressor, an unloader valve works in conjunction with a pressure switch to maintain safe pressure levels. Compressed air trapped within the compressor is released to the atmosphere, so the start-up of the compressor remains unaffected when the pressure switch calls for air.
Unloader valves can be operated manually, electronically or pneumatically (using air pressure). Pneumatic unloader valves can be configured to load or unload if air pressure is applied or removed.
When the compressor is charging the system the inlet valve is open and the outlet valve is closed. If the discharge pressure from the compressor exceeds its safe limit -- 10 bar or 65.8 Kilogram per square inch (psi) -- air pressure acting on a spring lifts a diaphragm or membrane against the downward pressure and opens the unloader valve.
The principal advantage of pneumatic unloader valves is that they prevent air compressors from starting under load. Other advantages include that they can used to control the capacity of a compressor remotely or completely automate the process.