Control Loops. A single control loop includes a controlled variable sensor,
controlled variable transmitter, the controller, automatic-manual control station, and
final control element including positioner, if any. Control loops used for power plants
are usually of the pressure, temperature, or liquid level type.
deaerator pressure, auxiliary steam pressure, building heating steam pressure, and fuel
oil pressure. For control of boiler pressure the final control element regulates fuel
flow to the boiler in response to boiler drum steam pressure. For other pressure
applications the final control element is usually a pressure reducing control valve
which regulates in response to downstream pressure. A typical pressure control loop is
shown in Figure 39a.
Temperature. Temperature control loops may be used for control of steam
temperature from boilers or desuperheaters and fuel oil temperature from fuel oil
heaters. A typical temperature control loop is shown in Figure 39b.
Level. Liquid level control loops may be used for control of boiler drum
water level, condenser hotwell water level, feedwater heater drain cooler water level,
and deaerator storage tank water level. A typical liquid level control loop is shown in
Flow meters. Flow meters, particularly differential pressure type, have
remained unchanged over the past decade. There have been several new types of flow
meters added to the flow measurement arena. The new meters such as vortex, ultrasonic,
and Doppler have added improved accuracy, linear signals, and rangeability as high as
16:1. With the improvements in electronic transmitters over the past decade there has
been an improvement in their overall specifications. However, the differential pressure
type flow meters have remained the same; accuracy 2-3 percent and rangeability of 4:1
maximum. Cost of electronic transmitters are now lower than pneumatic counterparts and
should be used except for all pneumatic systems. For types of flow meters see Table 25;
for applications see Table 23; and for a listing of required flow measurements for steam
power plants, refer to MIL-HDBK-1003/6.
mounted. Size and ranges are specified by user. Local-mounted gauges give a "backup"
reading and also help operators in determining if equipment or pressure systems are
working satisfactorily. Accuracy is normally 0.5 to 1 percent of span. For test and
calibration purposes use 0.25 to 0.5 percent gauges. Refer to MIL-HDBK-1003/6 for