fail‐down mode.
In normal operation, supply pressure loads the upper diaphragm of the trip valve. The valve plug spring keeps the
exhaust port closed. Supply pressure also loads the lower diaphragm through the restriction, causing the plug
assemblies to move down and isolate ports C and F while connecting port A to B and port D to E. Normal actuator
control pressure from the control device is applied to the top of the cylinder through ports A and B and to the bottom
of the cylinder through ports D and E. A volume tank is charged to the maximum supply pressure through a check
valve. The check valve maintains maximum supply pressure in the volume tank if the supply pressure drops.
When the supply pressure falls below the trip point, the exhaust port opens venting the supply pressure that is loading
the lower diaphragm. This causes the upper ports of the plug assemblies to close and disconnect normal pressure from
the control device to the actuator.
Volume tank pressure is then applied through ports C and B to the top of the actuator cylinder, while pressure in the
bottom of the actuator cylinder is vented through ports E and F. The pressure imbalance created forces the actuator
piston down.
When supply pressure is restored, it again loads the upper and lower diaphragms causing the trip valve to reset. The
exhaust port closes, the upper ports of the plug assemblies open, and the lower ports close. Normal actuator control
pressure from the control device is restored through ports A and B and ports D and E. The check valve opens and
recharges the volume tank to the maximum supply pressure.