Air Maintenance Device is a control unit that automatically regulates a continuous pressurized air (or nitrogen) supply to a constant preset value. The standard configuration includes a field adjustable pressure regulator, to and strainer and two spring loaded check valves. Additional options include Pressure Gauges, Pressure Switches / Transmitters, Air Tank and Pressure Safety Valve (PSV). In this session we are going to discuss about such air maintenance device and its working
What is the purpose of Air Maintenance Device?
Utility systems such as air and nitrogen systems are critical parts required to operate and maintain an industrial plant environment. Although air is easily accessible, there are times when nitrogen must be manufactured at the site. The compressors must supply nitrogen and air throughout the plant. Instrumentation is required to monitor and control these systems and must also be monitored by the emergency shutdown system
(1) Pressure Supply Ball Valve
(2) Spring Loaded Check Valves
(3) Pressure Regulator (PCV) Model 2PB
(4) Fixed Restriction Orifice
(5) Spring loaded N.C. Bypass Ball Valve
(6) Pressure switch/transmitter
(7) Pressure gauges
The supply of air or nitrogen enters the air maintenance device through the ball valve of pressure supply  that must be kept open at all times when the AMD is in automatic operation mode. For greater safety and convenience and as protection against incorrect operation, AMD is equipped with a normally closed bypass valve (N.C) operated by spring, ensuring an essential predetermined closed position, at the end of the pressurization of the system.
To quickly fill the system for initial pressurization, the spring loaded bypass valve (N.C)  must be opened. Once the required system pressure has been reached, release the handle of the NC bypass valve, which will automatically return it to the closed position, allowing only restricted air flow to the system through the fixed restriction orifice  . The twin check valves  retain the outlet pressure supply in case the air supply is interrupted. In this case, AMD will maintain the air pressure in the system only for a limited period of time.
The pressure regulator  automatically maintains the system pressure at the preset value. The fixed restriction orifice limits the flow of air in the system of fusible plugs to a value that is significantly lower than that which will be exhausted when a fuse the release device is activated. Ensuring a sufficient pressure drop to activate low pressure equipment such as deluge valves and alarm devices
How to reset process after a service?
1. Open the Supply Isolating valve and introduce air pressure to the inlet of the AMD.
2. Verify that the inlet pressure gauge indicates a high and stable pressure.
3. Open the Bypass valve enough to slowly pressurize the complete system.
4. After the system pressure has stabilized to the required pressure, as shown by the outlet pressure gauge, release the Bypass valve handle which will automatically return it to the closed position. The AMD is now ready for standby service.