Emerson FISHER 657, Инструкция по эксплуатации

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Instruction Manual
D100306X012
657 Actuator (30/30i-70/70i and 87)
June 2018
10
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
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
NOTE:
DEADBAND IS CAUSED BY FRICTION.
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
A6763‐2
RANGE OF
DEADBAND
1
DIRECT ACTING VALVE REVERSE ACTING VALVE
BENCH SET BENCH SET
Loading Connection
The loading pressure connections are made at the factory if the valve, actuator, and positioner come as a unit. Keep
the length of tubing or piping as short as possible to avoid transmission lag in the control signal. If a volume booster,
valve positioner or other accessory is used, be sure that it is properly connected to the actuator. Refer to the positioner
instruction manual or other manuals as necessary.
For actuators shipped separately or whenever the actuator pressure connections are installed, use the following steps: