In pneumatic instrumentation systems, instrument air is required to power valve actuators and other instruments - transmitters, controllers, control valves etc. A key component of the instrument air supply system is an air pressure regulator. The air pressure regulator is a simple device. It is used to lower the main instrument air supply of a plant to a pressure suitable for an air-operated instruments; eg, a transmitter, control valve, etc.
Normally, each air operated instrument has its own regulator. So an air regulator is one of the most common devices in the plant. There are various manufacturers of air regulators, eg Masoneilan and Fisher. However, they all work in much the same way. The schematic of a Fisher air pressure regulator is shown below:
Principle of Operation of the Air Pressure Regulator
- The main air supply is connected to the AIR INLET PORT. Air passes into the filtering chamber at the bottom of the regulator.
- Air passes through the filter which removes dirt particles in the incoming air which may block nozzles etc. It then goes into the valve assembly.
- The valve assembly is moved by the range spring pressing on the diaphragm.
- The range spring will hold the valve assembly down until the output pressure is high enough to lift the diaphragm (via the air passage shown). At this point the small spring in the valve assembly closes the valve.
- Air is allowed to pass through a hole at the center of the diaphragm and out of the vent. This maintains balanced pressure across the diaphragm.
- If the outlet pressure is above the pressure set by the range spring, the air will go out through the vent above the diaphragm. When the outlet pressure is correct, the valve assembly opens to set the correct pressure. This pressure exits the regulator through the OUTLET AIR PORT
- If the outlet pressure is below the pressure set by the range spring the valve assembly will stay open until the set pressure is reached.