Deadband Measurement - Emerson Fisher 667NS2 Instruction Manual

Instruction Manual
D103472X012
5. Attach the travel scale (key 113) with machine screws (key 126). Tighten the screws.
6. Attach the pointer (key 208) with machine screws (key 126). Tighten the screws.
7. Attach the switches and the magnets as shown in figure 6 and as per the TopWorx GO SWITCH Mounting
Instruction Manual.
Note
When positioning the magnet/switch, axial alignment is necessary. Ensure that the switch and magnet axes are not more than 0.1
inches eccentric.
8. While there is no air in the diaphragm, adjust the gap of the "CLOSED" (upper or right-hand) switch/magnet pair to
a small gap approximately 0.1 inches. Tighten the magnet 47 N•m (35 ft•lbf). Adjust the switch so a gap of
0.075±0.015 inches is between the magnet and the switch. Tighten the switch to 47 N•m (35 ft•lbf). Use wire
gages to verify the gap. Repeat the tightening of the switch if necessary to adjust the gap. Use of offset tools with
the torque wrench may be helpful.
Note
Mount the magnets before mounting the switches, as adjusting one magnet can introduce movement of the other magnet
attached to the target arm assembly, whereas the switches are fixed to the yoke. Recommend under-estimating the gap before
final torquing, as torquing may enlarge the gap.
9. Cycle the valve open. Repeat step 8 for the "OPEN" (lower or left-hand) switch/magnet pair. Use wire gauges to
verify the gap. Repeat the tightening of the switch if necessary to adjust the gap.
10. Calibrate switches per the TopWorx GO SWITCH Mounting Instruction Manual.

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 3). Each actuator
spring has a fixed spring rate (force). You have verified that the correct spring was installed in the actuator by
completing the Bench Set 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
mid­travel. 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 = Deadband, psi
Bench Set Span, psi
= nn %
667NS2 Actuator
March 2022
13