The Powergenex LU Lockup Valve Series works by regulating signal pressure to pneumatic actuators. It shuts off the control ports once air supply pressure drops lower than the set pressure, causing the actuator to fail in its last position.
The LUS model is a single-acting type with one supply port and output port and LUD is a double-acting version with two of each. Both types maintain valve position in an emergency until normal air supply is restored.
LUD (Double acting)