On failure of control air supply, the pneumatic actuator may be arranged to move to a position that allows the plant to continue to operate safely, in other words, fully opened or fully closed. This is known as fail safe. For example, in the case of a jacket cooling water system, on failure of control air, the actuator opens fully to allow jacket water to the cooler without bypass. On the other hand a fuel oil control valve for boiler closes completely on such a failure. This ensures safety of the plant.
In some other systems, control air supply failure locks the position of valve at that time of failure. This is called fail set. The advantage of this system is that the plant gets stable and have time for normal shutdown or can wait for reestablishment of control air supply for some time. Example for such a system is boiler water level control.