377L Trip Valve - Emerson Fisher 377 Instruction Manual

377 Trip Valve
June 2017
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.
Figure 7. Fisher 377D Trip Valve Shown in Tripped Condition
SPRING
VALVE PLUG
UPPER
DIAPHRAGM
EXHAUST
PORT
SUPPLY
PRESSURE
LOWER
DIAPHRAGM
PORT D
PORT E
LOWER
PORTS
PORT F
SUPPLY PRESSURE
CONTROL PRESSURE TO TOP OF CYLINDER (BLOCKED)
CONTROL PRESSURE TO TOP BOTTOM OF CYLINDER (BLOCKED)
GE08412‐A
A6905‐1

377L Trip Valve

Figure 8 illustrates trip valve operation in the lock‐in‐last‐position mode.
When supply pressure falls below the trip point, the exhaust port opens venting supply pressure from the lower
diaphragm. This causes the upper ports of the plug assemblies to close and the lower ports to open. Because ports C
and F are plugged, no pressure change occurs on either side of the actuator piston and the piston is pressure‐locked in
position. No volume tank is necessary in this mode.
When supply pressure is restored, the plug assemblies move back into the normal operating position. Supply pressure
from the control device is applied to the actuator through ports A and B and ports D and E.
10
MAIN SPRING
VENT
PORT A
PORT B
UPPER PORTS
PLUG
ASSEMBLIES
PORT C
ACTUATOR
DVC6200
PRESSURE TO TOP OF CYLINDER (FROM VOLUME TANK)
PRESSURE FROM BOTTOM OF CYLINDER (VENTING)
LOWER DIAPHRAGM LOADING PRESSURE (BEING VENTED)
Instruction Manual
D200319X012
CONTROL
DEVICE
CHECK
VALVE
VOLUME
TANK