Operation - Emerson Fisher 667NS2 Instruction Manual

667NS2 Actuator
March 2022
Figure 3. Typical Valve Response to Deadband
UPPER
BENCH SET
15
PRESSURE
9
LOWER
BENCH SET
3
PRESSURE
0
CLOSED
NOTE:
DEADBAND IS CAUSED BY FRICTION.
1
A6588‐1
Deadband Adjustments
1. Monitor actuator loading pressure carefully when making adjustments. Do not exceed the maximum pressure
specifications of either the loading regulator or the actuator casings (refer to table 1 for maximum diaphragm
casing pressure).
2. Each actuator spring has a fixed pressure span. Changing the spring compression shifts the span up or down to
make valve travel coincide with the pressure range.
3. For sizes 45 and 70, turn the spring adjusting screw (key 127, figure 4) into the yoke to shift the span up or turn the
spring adjusting screw out of the yoke to shift the span down. For size 80, rotate the spring adjusting screw (key
127) to shift the span.
For successful operation, the actuator stem and valve plug stem must move freely in response to the loading pressure
change on the diaphragm.

Operation

In a reverse‐acting diaphragm actuator, an increasing loading pressure causes the actuator stem to move upward,
compressing the spring. When the diaphragm pressure is decreased, the spring moves the actuator stem downward.
This is shown graphically in figure 2. In the event of failure of the loading pressure to the diaphragm of the actuator,
the actuator stem moves to the extreme downward position. Push‐down‐to‐close valves will close on failure of the
diaphragm loading pressure.
The nameplate attached to the yoke of the actuator provides information about the specific construction and
operating range. The spring and diaphragm have been selected to meet the requirements of the application, and in
service, the actuator should create full travel of the valve plug when the diaphragm pressure (operate) range indicated
on the nameplate is applied.
The nameplate specifies a bench set pressure range in addition to a diaphragm pressure (operate) range. The bench
set range is that pressure range required to stroke the valve fully without any pressure in the body and no packing
friction, as would be the case if the valve were set on the work bench. However, in service with the specified pressure
drop applied across the valve, it should stroke over that diaphragm pressure (operate) range indicated on the
nameplate.
When the control valve and actuator are installed, the actuator should be checked for correct travel, freedom from
excessive friction, and correct action (air‐to‐open) to match the controlling instrument. For successful operation, the
actuator stem and valve plug stem must move freely in response to the loading pressure change on the diaphragm.
14
OPENING
VALVE
RANGE OF
DEADBAND 1
CLOSING
VALVE
OPEN
MID RANGE
VALVE TRAVEL
1.0
0.6
0.2
Instruction Manual
D103472X012