MIL-HDBK-1003/7
Electro-Hydraulic Operators. Electro-hydraulic operators are used for high
11.11.3
force or torque applications and in remote locations where lack of compressed air supply
rules out the use of pneumatic operators.
Positioners. Positioners are used on control valves to force the stem and
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valve plug to move to a position as called for by the control signal. Positioners are
used primarily to overcome valve stem friction, to compensate for long pneumatic
transmission lines, or when extreme or variable line pressure can offset the valve plug.
Every control valve exhibits from 2 to 10 percent hysteresis unless it is equipped with
a positioner. The effect of valve position hysteresis is indicated in Figure 42.
A positioner should be used for liquid level, volume (as in blending), and
weight (as in blending) whenever a two mode controller (proportional plus integral) is
used. For temperature control, a positioner will be helpful but not essential. As a
general rule, use a positioner on control systems that have a relatively slow response
such as liquid level, blending, and temperature control loops. Do not use a positioner
on control systems which have a relatively fast response such as liquid pressure, most
flow, and some pressure control loops.
Control Room. The control room should be located in the boiler turbine area
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where visual inspection can still be made, but completely enclosed and air conditioned
with high efficiency filtration. Instruments should be mounted on a panel within the
control room. A positive pressure should be maintained within the control room to keep
dust and other dirt particles from entering. The control room must have:
a)
Clean dry atmosphere.
b)
Relatively constant temperature and humidity.
c)
No vibration.
d)
Adequate light.
e)
Reliable electric power, free of surges in voltage and frequency.
f)
Clean, dry air of adequate pressure and capacity (for pneumatic
instruments).
g) Air conditioning (a necessity for electronic distributive control
systems and computers).
See Tables 22 and 23 for typical lists of analog instruments.
With the new microprocessor-based control systems, the operator
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