9.8 Pressure maintenance pump

Pressure maintenance pump

Pressure maintenance pump, as the name implies, is intended to uphold a certain pressure in a pump installation (circulation system). The liquid being pumped is often hot and, because of the high temperature, has high vapour pressure. The pressure maintenance prevents cavitation at high points in the pipework and pumps, in other words cavitation is avoided. Pressure maintenance also compensates for unintentional leakages and takes up variations in the volume of liquid and pipework due to changes in temperature and pressure.

Normal cases, where pressure maintenance is necessary, are district heating networks and hot water circulation systems in buildings. Pressure maintenance is carried out at a point in the pipe network where it will be most effective, which will vary from case to case. If the pressure maintenance pump is placed on the suction side of the main pump then a constant inlet pressure is maintained, whilst the outlet pressure varies with change in flow and vice-versa.

The simplest method of providing pressure maintenance is by means of a high-level tank. A higher pressure can be provided if required by the introduction of a pressure maintenance pump.

Figure 9.8a Arrangements for pressure maintenance.

The by-pass line ensures a minimum flow to the pressure maintenance pump, even though no liquid is supplied to the main pipeline, thus preventing unacceptable temperature increases in the pump. The by-pass line can also be used to maintain constant pressure for varying flows to the main pipeline.

Figure 9.8b Pressure maintenance pumps for different control methods.

The main line pressure is equal to the pressure increase due to the pump plus the supply pressure. Constant throttle in the by-pass line causes the pressure level in the main line to vary by an amount ΔH dependent upon the flow in the main line. Varying throttle enables constant pressure to be maintained. Regulation of pressure maintenance can also be carried out in other ways, for example, by throttling the delivery pipe or by regulating pump speed.

Fire pumps

Most stationary fire pump assemblies are installed with devices that automatically sense a pressure loss in a fire protection system, and start running to supply water or boost pressure in that system. From time to time, though, small water leaks, unwanted pressure drops, or even temperature changes may “fool” the fire pump into starting when it isn’t needed.

To prevent these false starts and maintain the fire pump’s life expectancy, small pressure maintenance or “jockey” pumps are installed to maintain a relatively constant pressure on the fire protection system. A jockey pump should be sized to make up the allowable leakage rate within 10 minutes.