Electro-Hydraulic Operators. Electro-hydraulic operators are used for high
rules out the use of pneumatic operators.
Positioners. Positioners are used on control valves to force the stem and
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
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
Control Room. The control room should be located in the boiler turbine area
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:
Clean dry atmosphere.
Relatively constant temperature and humidity.
Reliable electric power, free of surges in voltage and frequency.
Clean, dry air of adequate pressure and capacity (for pneumatic
systems and computers).
See Tables 22 and 23 for typical lists of analog instruments.