Friction Discussion - Emerson Fisher 657C Instruction Manual

657C Actuator (40i, 46i, and 60i)
November 2019
5. Screw the desuperheater stem locknuts up until the indicator disk contacts the bottom of the stem connector. Do
not overtighten the locknuts.
6. Slowly stroke the desuperheater from fully closed to fully open and verify full rated travel is achieved.
Be sure the desuperheater is in the closed position. Loosen the screws (key 17) on the travel indicator scale (key 18),
and align it with the travel indicator disk (key 14). Stroke the desuperheater full travel to ensure that the travel
matches the rated travel on the nameplate. If desuperheater travel is not correct, repeat the stem connector
procedure.

Friction Discussion

If you are attempting to adjust the bench set after the actuator is connected to the desuperheater and the packing
tightened, you must take friction into account. Make the spring adjustment so full actuator travel occurs at the bench
set values
a. Add the friction force divided by the effective diaphragm area with increasing diaphragm pressure, or,
b. Subtract the friction force divided by the effective diaphragm area with decreasing diaphragm pressure.
If the stem connector assembly has been installed, the desuperheater friction may be determined by the following
procedure:
1. Install a pressure gauge in the actuator loading pressure line that connects to the actuator diaphragm casing.
Note
Steps 2 and 4 require that you read and record the pressure shown on the pressure gauge.
2. Increase the actuator diaphragm pressure and read the diaphragm pressure as the actuator reaches a travel
position in the rated travel of the desuperheater that does not contact a travel stop. Make a reference mark on the
travel indicator scale using tape or some other method at this point.
3. Increase the actuator diaphragm pressure until the actuator is at a travel position greater than the position
referenced in step 2 using the reference point to identify first movement.
4. Decrease the actuator diaphragm pressure and read the diaphragm pressure as the actuator returns to the position
referenced in step 2.
The difference between the two diaphragm pressure readings is the change in the diaphragm pressure required to
overcome the friction forces in the two directions of travel.
5. Calculate the actual friction force:
Refer to table 1 for the effective diaphragm area.
It is difficult to rotate the spring adjustor (key 12, figure 7) when the full actuator loading pressure is applied to the
actuator. Release the actuator loading pressure before adjusting. Then re-apply loading pressure to check the
adjustment.
Note
The actuator down stop is the limit for downward movement, and the desuperheater seat is the limit for upward (away from the
desuperheater) movement.
10
Friction
Difference
Force, = 0.5
in pressure
pounds
readings, psig
Effective

diaphragm area,
2
inches
Instruction Manual
D104496X012