Retaining valve

The position maintenance valve keeps the valve position unchanged in the event of a problem with the instrument air, regardless of whether the air supply is on or off.

The position maintaining valve, also known as FL, is another type of valve besides the air-actuated valve (which closes in the event of FC failure) and the air-shut valve (which opens in the event of FO failure). Its principle involves incorporating a pneumatically controlled valve into the air circuit; when the air supply pressure is normal, the air circuit remains open and the valve is under control； When the air supply pressure drops to the specified value, the air-controlled valve shuts off the air supply, trapping air within the actuator; the valve then stops operating, thus achieving position holding. In foreign designs, FL is further divided into drift on and drift off, because after the valve reaches its set position, slight leaks of instrument air occur, and the direction in which the valve tends to move at this point is the drift direction.