Deadband Measurement - Emerson Fisher 667 30 Instruction Manual

Instruction Manual
D100310X012
2. Increase the actuator diaphragm pressure and read the diaphragm pressure as the actuator reaches a travel
position in the rated travel of the valve 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 74, figures 6, 7, 8, 9, and 10) 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.
Figure 5. Typical Valve Response to Deadband
UPPER
15
BENCH SET
PRESSURE
BENCH SET
9
LOWER
BENCH SET 3
PRESSURE
0
OPEN
NOTE:
DEADBAND IS CAUSED BY FRICTION.
1
A6763‐2

Deadband Measurement

Deadband is caused by packing friction, unbalanced forces, and other factors in the control valve assembly. Deadband
is the range a measured signal can vary without initiating a response from the actuator (see figure 5). Each actuator
spring has a fixed spring rate (force divided by compression). You have verified that the right spring was installed in the
actuator by completing the Spring Verification steps.
Deadband is one factor that affects the control valve assembly operation during automatic loop control. The control
loop tolerance for deadband varies widely depending on the loop response. Some common symptoms of the
Friction
Difference
Force, = 0.5
in pressure
pounds
readings, psig
CLOSING
VALVE
RANGE OF
DEADBAND 1
OPENING
VALVE
CLOSED
MID RANGE
VALVE TRAVEL
DIRECT ACTING VALVE
667 Actuator (Size 30/30i - 76/76i and 87)
Effective

diaphragm area,
2
inches
UPPER
1.0
15
BENCH SET
PRESSURE
0.6
9
LOWER
0.2
BENCH SET 3
PRESSURE
0
OPENING
VALVE
BENCH SET
RANGE OF
DEADBAND 1
CLOSING
VALVE
CLOSED MID RANGE
OPEN
VALVE TRAVEL
REVERSE ACTING VALVE
May 2018
1.0
0.6
0.2
11