Discussion Of Bench Set - Fisher 667 Instruction Manual

SPRING
ADJUSTER
ACTUATOR STEM
UPPER BENCH
SET LOADING
PRESSURE 3
LOWER BENCH
SET PRESSURE
MARK 4
VALVE STEM
50A8379-C
B2429-1 / IL
b. For all other size actuators:
D Slowly lower the actuator down onto the valve. As
the yoke passes over the end of the valve stem, place
the yoke locknut over the valve stem. (Note: On small
size actuators, it may be necessary to remove the indi-
cator disk and re-install it while lowering the actuator
onto the valve because the disk will not go through the
actuator yoke opening).
D Continue to lower the actuator while guiding the
valve stem into the opening in the end of the actuator
stem until the actuator is in place (see figure 4).
D Screw the yoke locknut onto the valve bonnet
and tighten the locknut.
4. Do not connect the actuator stem to the valve stem
at this time. Whenever the actuator is installed on the
valve, it is recommended that you perform the Bench
Set Spring Adjustment procedures below, to verify that
the actuator is still adjusted correctly.

Discussion of Bench Set

The bench set pressure range is used to adjust the
initial compression of the actuator spring with the
valve-actuator assembly "on the bench." The correct
initial compression ensures that the valve-actuator as-
sembly will function properly when it is put into service
and the proper actuator diaphragm operating pressure
is applied.
The bench set range is established with the assump-
tion that there is no packing friction. When attempting
RATED VALVE
TRAVEL MEASURE
4
MARK VALVE
STEM HERE 2
Figure 4. Bench Set Adjustment
Type 667 Size 30-76 & 87
NOTES:
1
THE LOWER PSIG LOADING PRESSURE (MARKED
ON NAMEPLATE) WHERE THE FIRST MOVEMENT OF
ACTUATOR STEM IS DETECTED.
2
MARK THIS POINT WITH TAPE OR A MARKER.
3
THE UPPER PSIG LOADING PRESSURE RETRACTS
ACTUATOR STEM.
MEASURE DISTANCE OF TRAVEL. IT SHOULD
4
EQUAL THE TRAVEL SPAN SHOWN ON THE TRAVEL
INDICATOR SCALE.
1 LOWER BENCH SET
LOADING PRESSURE
to adjust the spring in the field, it is very difficult to en-
sure that there is no friction being applied by "loose"
packing.
Accurate adjustment to the bench set range can be
made during the actuator mounting process by making
the adjustment before the actuator is connected to the
valve (see the Bench Set Spring Adjustment Proce-
dure).
If you are attempting to adjust the bench set range af-
ter the actuator is connected to the valve and the
packing tightened, you must take friction into account.
Make the spring adjustment so full actuator travel oc-
curs at the bench set range (a) plus the friction force
divided by the effective diaphragm area with increas-
ing diaphragm pressure or (b) minus the friction force
divided by the effective diaphragm area with decreas-
ing diaphragm pressure.
For an assembled valve-actuator assembly, the valve
friction may be determined by following the procedure
described below:
1. Install a pressure gauge in the actuator loading
pressure line that connects to the actuator diaphragm
casing.
Steps 2 and 4 require that you read and
record the pressure shown on the pres-
sure gauge.
2. Increase the actuator diaphragm pressure and read
the diaphragm pressure as the actuator reaches its
mid-travel position.
Note
5