Product description: ZPB-11/21 pneumatic single-directional position retaining valve is one of the actuator accessories, it is used with pneumatic film actuator or bidirectional piston pneumatic actuator. When the air source system fails, it can automatically cut off the intake and exhaust channels of the pneumatic actuator to keep it in the position of the accident and ensure the normal process. When the air source accident is removed, the position retaining valve can automatically resume normal operation. So the pneumatic holding valve in the important automatic control circuit as a safety protection instrument.