A plunger actuator is used to allow free movement with contaminated system pressure. The pressure source can be an enlarged pipe section from the protected system or a large direct line from the system. The actuator does not "see" any valve friction because the clutch does not engage until the actuator takes up the clutch slack. At set point, the ball valve closes in milliseconds. ADVANTAGES: The device operates with proven elements. Valve friction does not affect accuracy. A ball valve designed for the application is used. After closing, the valve can be opened in minutes for a pin change by removing the vent plug and applying pressure to open the valve. When the valve is in its original position, change the pin. Remove the auxillary pressure source and replace the vent plug. The valve is now in a ready position to protect the downstream system from excess pressure