9.7 Booster pump
Booster pump
Pressure booster pump or booster pumps are primarily employed in long pipelines and large circulation systems. A common case is that pressure in a distribution network is insufficient for certain higher points, for example, the water supply to high-rise apartment blocks, or when an additional tapping point is connected to an existing network.
Pressure boosting is a form of series connection where the pumps are located at considerable distances from each other. The advantages of pressure boosting is a better distribution of the specified pressure in the pump installation is achieved and that the pump and energy costs can be reduced. The latter advantage applying to branched pipework systems.
Figure 9.7a Schematic pressure distribution in a pipeline.
A higher pressure rating may be required for the upstream section of the pipeline and pump for alternative l, or for the second pump in the event that the pressure boosting for alternative I is also divided into two pumps.
Branched pipework systems are usually dimensioned by calculating for the pressure requirements of the furthest tapping point. The required pump specification is therefore based according to this pressure requirement and the total flow consumption. The application of pressure boosting principles can provide an alternative, as illustrated by the following example.
Figure 9.7b Example of conditions for pump sizing in a branched pipe system
Pump data for alternative I becomes:
Q = 1, H = 0,5 + O,5 = 1, P = 1
Pump data for alternative II becomes:
Q1 = 1, H1 = 0,5 + 0,1 = 0,6, P1 = 0,6
Q2 = 0,5, H2 = 0,5 – 0,1 = 0,4, P2 = 0,2
The total installed pump power P = 0,8.
The smaller power requirement for alternative II is explained by the fact that the surplus pressure in pipe BC is eliminated. In alternative I this excess pressure must be throttled to prevent tapping point C from “stealing” flow from D.
A better illustration of the pump’s operating situation is obtained from drawing up the pump and system curves. In figure 9.7c the respective pump and system curves are shown both separately and in relation to each other. The curves are compiled in accordance to Section 9.2 Branched pipe system>>>. The marked points indicate the situation at the specified flow condition.
Figure 9.7c Operating situation in the pressure boosting example (same data as in figure 9.7b).
The Q-H diagram shows, for example, that pressure boosting is not required at all for total flows Q < 0,46, even if the complete flow is tapped off at D. The largest flow which can pass through the pressure boosting pump is Q ≅ 0,67. This occurs when C is completely closed and is dimensioned for the pressure boosting pump motor. The largest flow at C (D completely closed) is ≅ 0,87. For Q > 0,8 (HB < 0,25) then D is completely without flow unless the pressure boosting pump is started.
The pressure boosting point can be controlled in the following way. In order to guarantee full flow at D (QD = 0,5) independent of consumption at C, the pressure boosting pump is started by a signal from a pressure sensor at the branch point when HB falls below 0,46 and is stopped when HB exceeds 0,46 by a certain margin. Within the range HB =0,46 to 0,19 (the total flow Q = 0,46 to 0,87) a situation where flow QD = 0 can occur.
The supplied shaft power is transmitted to the liquid, the temperature rises and the risk of seal leakage and damage due to cavitation increases. Excessive temperatures can be avoided by means of a temperature sensor which transmits a signal to a valve which opens a by-pass line. To protect the pressure boosting pump against cavitation in the event of low pressure on the inlet side, which could occur if pump 1 became damaged for example, a pressure sensor is installed which stops the pressure boosting pump when HB falls below the required cavitation limit (HB min).
Figure 9.7d Installation of pressure booster pump.
The most energy conserving method of regulation would be to control the speed of the pressure boosting pump according to the constant pressure criteria in D.