The minimum pressure valve (MPV) has a secondary purpose, that is, acting as a non-return valve when the compressor stops or off-load.
This is to prevent the line air pressure is not lost by reverse flow from the air system back to atmosphere via the compressor.
When the compressor stops, the intake valve is automatically closed, and the internal pressure within the compressor is gradually reduced or vent out to the air intake.
This is achieves by mechanically, hydraulically or electrical solenoid valve.
This is to prevent hydraulic locking and possible damage when the compressor restarted.
The compressor will start with a light load and low starting current. Safety Devices
HAT = Hight Air Temperature Switch = Trip at 110 degree C
Safety devices are install to protect the compressor against malfunction due to
high air oil temperature,
lack of oil,
lack of cooling air or water or
abnormal electrical supply. 
Monitoring Devices
Monitoring devices are also installed, Certain design have more components and some with the minimum number.
Signals when filters are to be cleaned or replace,
Separator differential pressure,
minimum & maximum pressure,
oil & air end bearing temperature readings,
water separator conditions,
hour meter,
motor conditions to indicate maintenance frequency.
Compressors with constant running control, start-stop control, manual dual control, automatic dual control, lead-lag operation for multiple compressors application.
Run-on timers for keeping the compressors running to minimize condensation during short demand periods, run-on timers to shut down compressors when there is no demand for air, are offered by various manufacturers.
They are compact, free from vibration, and with low noise levels. The air delivered is cool, clean, and free from pressure pulsation, and in some cases air receiver is not necessary.
They can be mounted anywhere including immediate work areas.
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