10.1 Pump control methods

Pump control methods

Pump control methods varies widely depending on the application, process demands and expected pump performance. Generally speaking, flow can be regulated in many ways, see list below. Throttle regulation, shunt regulation and overflow control all mean that the control takes place outside the actual pump itself. Other methods change the actual performance of the pump including power consumption, either within the pump unit or by means of variable filling, e.g. Archimedian pumps and displacement regulation.

As can be seen from the list below, the only universally applicable method is speed regulation, i.e. it can be used for all kinds of pumps. from the smallest dosing pump with power requirements of 10 to 100 W to the largest possible pump units in power stations with power requirements of 20,000 to 50,000 kW.

There is nothing against the combination of various methods, e.g. the parallel operation of several units with continuous regulation by means of throttling or speed regulation. The choice of optimum methods requires great experience and often extensive economic and process technical analyses. Sometimes there are also special problems to be considered, such as noise levels from control valves when throttling.

Example of common pump control methods

  1. Discontinuous methods: On-off regulation, series and parallel operation, Running with a pole changing motor
    • Limitation and risk with discontinuous methods: Uneven flow, Water hammer with resultant fatigue
  2. Continuous methods – limitations:
    • Throttle regulation – not for contaminated liquids
    • Shunt regulation – not for contaminated liquids
    • Overflow control – feed back of surplus liquids
    • Adjustable blades – only for pump types with axial impeller (propeller pumps)
    • Pre-rotation – only for pumps with axial or semi-axial impeller
    • Speed regulation – no control limitations (except too high/low speed)
    • Displacement regulation – displacement pumps only

Series >>> and parallel>>> pump operation are dealt with in Section 9.5 and 9.6 respectively, Examples of pump performance for pumps with adjustable impeller blades, or with an adjustable guide-vane ring on the inlet side (pre-rotation) are shown in figures 3.110a and 3.110b respectively.

Archimedian (rigid screw) pumps and displacement regulation of metering pumps are further dealt with in Chapter 3, Section 3.4 Positive displacement pump >>>.

Where changes in flow requirements are needed over longer periods, more than about six months say, it is economic to change or turn down a centrifugal pump impeller or modify the blade angles on a propeller.

Control parameters, particularly in continuous regulation, may be flow, pressure, or some quantity derived from these such as level, temperature, concentration and so on. Some examples of the principles of regulation (quantities to be controlled) are reproduced in table 10.1.

Principle regulation 

Sensor type

Example of application

Proportional level regulation
Level guage

Sewage and raw water handling
Process control

Constant volume flow

Flowmeter

Pumping from flow-smoothening 
storage. Process control

Constant pressure at pump

Pressure sensor at pump

Small pure water system.
Simpler process control
for a number of consumers
and short pipelines

Constant pressure at end of pipeline

Pressure sensor at end of pipeline

Pure water systems.
District heating systems.
Sophisticated process control.
Constant pressure at end of pipeline

Pressure sensor and flowmeter 
situated at pump.
Analogical unit for signal conversion. 

Pure water systems.
Control heating systems.
Sophisticated process control.

Ratio control or quota regulation
of two or more flows e.g. mixing
of two or more liquids.

1. Flowmeter in each circuit.
2. Pressure or level sensor in each circuit. Analogical unit for signal conversion.

Dosing and mixing liquids

Table 10.1 Some of the principles of regulation of pumps

The principles of regulation can also be shown by the Q-H diagram, figure 10.1, for the simplest cases.

Pump control at pressure, flow and level regulation
Figure 10.1a

 

Figure 10.1a Pump control and regulation at (a) constant pressure (b) constant flow and (c) level regulation.