APPENDIX D
CONTROL OF AIR SIGNAL ON POWER LOSS
FOR PMC-1 ELECTRO-PNEUMATIC CONTROLLER
The PMC-1 electro-pneumatic controller modulates a control
valve by loading or unloading air to the control valve’s
pneumatic actuator in a series of pulses. These pulses are
controlled inside the PMC-1 by two normally closed solenoid
valves.
If the AC power to the PMC-1 should fail for any reason, both
internal solenoid valves will remain closed, and the control
valve will be locked in its last position. If the avoidance of
sudden process changes on power loss is important, this
lock-up action is a desirable feature.
In certain situations, however, safety considerations may
dictate the use of a normally closed control valve that should
shut down the system on loss of power to the controller. In
such cases, the arrangement shown above is recommended.
A standard three-way solenoid valve is installed in the air line
between the PMC-1 and the control valve. The solenoid is
energized from the same power source used by the PMC-1.
When the solenoid is energized, ports B and C are connected,
and the pneumatic circuit between the PMC-1 and control
valve is complete. On loss of power, the solenoid is no longer
energized, and ports A and B are connected, allowing the
pressure on the actuator diaphragm to vent to atmosphere. If
a normally closed actuator has been selected, the control
valve will close, and the system will shut down.
A simpler solution, however, is the PMC-1/Vent on Power
Failure option which eliminates the need for a separate
three-way solenoid valve. Both the venting solenoid and its
power source are included in the PMC-1 internally mounted.
No extraneous air lines or power sources are needed.
13
PMC-1
Up to 100 psi
Plant air
Airset
Line power
Transmitter
Three-way
solenoid valve
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