Emerson Process Management FISHER 657 - Manual (Page 9)

Instruction Manual
D100306X012
657 Actuator (30-70 and 87)
December 2010
9
matches the valve travel on the travel indicator plate. If valve travel is not correct, repeat the stem connector
procedure.
Figure 5. Typical Valve Response to Deadband
UPPER
BENCH SET
PRESSURE
LOWER
BENCH SET
PRESSURE
9
3
0
CLOSING
VALVE
15 1.0
0.6
0.2
OPENING
VALVE
OPEN CLOSED MID RANGE
VALVE TRAVEL
DIAPHRAGM PRESSURE, PSIG DIAPHRAGM PRESSURE, BAR
RANGE OF
DEADBAND
1
UPPER
BENCH SET
PRESSURE
LOWER
BENCH SET
PRESSURE
9
3
0
CLOSING
VALVE
15 1.0
0.6
0.2
OPENING
VALVE
OPEN CLOSED MID RANGE
VALVE TRAVEL
DIAPHRAGM PRESSURE, PSIG DIAPHRAGM PRESSURE, BAR
1
NOTE:
DEADBAND IS CAUSED BY FRICTION.
A6763‐2
RANGE OF
DEADBAND
1
DIRECT ACTING VALVE REVERSE ACTING VALVE
Note
For push‐down‐to‐close valves, the valve plug seat is the limit for downward travel and the actuator up‐stop is the limit for upward
(away from the valve) movement. For push‐down‐to‐open valves, the actuator down stop is the limit for downward movement,
and the valve seat is the limit for upward (away from the valve) movement.
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). You have verified that the right spring was installed in the actuator by completing
the Bench Set Spring Adjustment 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
deadband being too wide are no movement, a “jump” movement, or oscillating movements of the actuator during
automatic loop control. The following steps are provided to determine the span of deadband. The percent of
deadband is helpful in troubleshooting problems with the process control loop.
1. Start at a pressure near the lower bench set pressure, slowly increase pressure until the valve is approximately at
mid‐travel. Note this pressure reading.
2. Slowly decrease pressure until movement of the valve stem is detected, and note this pressure.
3. The difference between these two pressures is deadband, in psi.
4. Calculate the percent of deadband by:
Deadband, psi
Deadband = —————————————— = nn%
Bench Set Span, psi