Principle Of Operation; Maintenance - Fisher 128-PQC Instruction Manual

128-PQC
1/4-INCH NPT
VENT
CONNECTION
1/4-INCH NPT
INPUT
SIGNAL
CONNECTION
LEAK-OFF VENT
CAGE AND SEAT
VALVE PLUG
W5750-1*/IL
3. Make sure that the valve is oriented so that flow is
in the desired direction. If angle flow is desired, switch
the pipe plug to the left-hand end connection as shown
in figure 3.
4. Install the valve following local and national piping
codes when they apply to the application. Use a good
grade pipe compound on all male threads. If continu-
ous operation is required during inspection and main-
tenance, install a three-valve bypass around the con-
trol valve.
5. Refer to figure 3. Connect the input signal line to
the 1/4-inch NPT connection in the lower diaphragm
casing of a spring-close control valve or in the upper
diaphragm casing of a spring-open control valve.

Principle of Operation

The Type 128-PQC control valve has push-down-to-
close valve plug action, as shown in figure 3. With a
metal seat, the valve uses a caged construction and
the valve plug shuts off against the integral seat ring in
the cage. In the cageless soft-seat construction, the
valve plug shuts off against the seat cut into the web
of the valve body.
A nylon insert in the valve plug thread is used to help
keep the valve plug from rotating.
4
TRAVEL INDICATOR
PACKING BOX
PACKING
Figure 3. Type 128-PQC Control Valve with Spring-Close Action
A leakoff vent passes through the packing box and out
the valve body. This permits any process leakage
through the packing to escape before passing along
the stem into the actuator. It also permits loading pres-
sure leakage from the lower diaphragm case to es-
cape before passing along the stem into the valve
body.

Maintenance

WARNING
Avoid personal injury from sudden re-
lease of process pressure. Before per-
forming any maintenance operations:
Disconnect any operating lines pro-
viding air pressure or a control signal to
the actuator. Be sure the actuator can-
not suddenly open or close the valve.
Use bypass valves or completely
shut off the process to isolate the valve
from process pressure. Relieve process
pressure on both sides of the valve.
Drain the process media from both
sides of the valve.
Vent the actuator loading pressure
and relieve any actuator spring precom-
pression.
14A9142-C/DOC