C H A P T E R 7
Pump control in the broadest sense gives the pump user (1) the flow rate, pressure or liq-
uid level desired, (2) protection for the pump and system against damage from the pumped
liquid, and (3) administrative freedom in decisions on operations and maintenance.
Control System Types Pump control systems range in complexity from single hand-
operated valves to highly advanced, automatic flow control or pump speed control systems.
Pump type and drive type are factors in control system choice. For centrifugal pumps,
either change of speed or change of valve setting can control the desired variable. For pos-
itive displacement pumps, whether reciprocating, rotary, screw, or other type, control is by
change in speed, change in setting of bypass valve, or change in displacement. The last-
mentioned method is found in metering and hydraulic drive pumps. Although this chap-
ter considers only control systems having valves as final control elements, the sensing
elements discussed also serve in pump speed control systems.
Pump control systems divide readily into two types: on-off and modulating. The on-off
system provides only two conditions: a given flow (or pressure) value or a zero value. A
valve is therefore either open or closed, and a pump driver is running or not. The modu-
lating system, on the other hand, adjusts valve setting or speed to the needs of the
moment. Either type of system can be automatic or manual.
System Essentials All control systems have
1. A sensing or measuring element
2. A means of comparing the measured value with a desired value
3. A final control element (a valve) to produce the needed change in the measured variable
4. An actuator to move the final control element to its desired position
5. Relaying or force-building means to enable a weak sensing signal to release enough
force to power the actuator
The sensing or measuring element is often physical