The centrifugal pump functions by first setting the liquid in motion and then converting the velocity into pressure. This procedure is achieved with a pump as shown in Fig. 8.14 where a rotating impeller is contained within a progressively expanding housing that receives the liquid delivered by the impeller. The pressure differential that can be developed by the pump increases with an increase in rotative speed and with an increase in outside diameter of the impeller.
Typical pressure/flow characteristics of a centrifugal pump are shown in Fig. 8.15 for three pumps of different impeller diameters, all operating at the same rotative speed. The maximum pressure rise that the pump can deliver is at zero flow rate, as might occur when the discharge valve is closed. As valves in the system progressively open, the flow rate increases and eventually the pressure differential that the pump can develop drops. Also shown on Fig. 8.15 is the power requirement. A favorable characteristic of the centrifugal pump is the non-overloading characteristic. For example, the maximum power that the pump with the 230-mm (9-in) impeller diameter can draw is approximately 2.5 hp at a flow rate of 11 L/s (170 gpm). The power required by the centrifugal pump decreases as the flow rate is throttled off, in contrast to the characteristic of the positive-displacement pump.
The pressure-flow rate curves of Figure 8.15 apply to ammonia and do not apply to a fluid having a different density. Some manaufacturers present their performance curves in terms of head versus flow rate, because such curves can be used for any fluid. The head may be thought of as the elevation that the fluid would rise in a long vertical tube attached to the discharge pipe of the pump. The conversion of head to pressure is dependent upon the liquid density as expressed by the equations:
In the open-type pump the shaft driving the impeller passes through the casing of the pump. At this penetration, a seal retards the leakage of refrigerant out of the pump and inhibits air leakage into the pump if the pressure within the pump is below atmospheric. The seals are a potential source of leak of the volatile refrigerant, and pump manufacturers have continued to improve the quality of seals. To make the seal more effective, some pumps are equipped with a double seal, and the cavity between the two seals is filled with oil under pressure, as illustrated in Fig. 8.16. The high pressure is provided by the refrigerant, and the oil eventually leaks through the seals to the outside and/or into the system. When the oil level drops low, the reservoir is refilled by means of a manual oil pump.
An important precaution in the design, installation and operation of centrifugal pumps is to prevent cavitation. Cavitation is the conversion of liquid into bubbles of vapor because of an abrupt drop in pressure as the liquid flows into the pump housing or impeller. When these vapor bubbles move into a region of high pressure they collapse violently. Continued cavitation can result in erosion of the metal surfaces of the impeller, and reduction in flow rate. Because the centrifugal pump relies on the high density of liquid in the conversion of velocity to pressure, the presence of vapor can cause marked reductions in pumping rate. In the worst case the pump can become vapor bound and not pump any liquid. This problem is not so critical in positive-displacement pumps where the volume flow rate will remain somewhat constant regardless of whether it is liquid or vapor. But even in the positive-displacement pump the mass flow rate will be degraded by cavitation.
Many of the installation practices surrounding the use of centrifugal pumps are directed toward avoiding cavitation. The next section, Section 8.9, concentrates on one of the most effective means of preventing cavitation providing adequate net-positive-suction head. Section 8.11 deals with the recommended practices for piping the liquid from the low-pressure receiver to the pump.
At this point still another feature will be explained that is a necessary provision for virtually all centrifugal-pump installations. This requirement is to equip the system with continuous bypass of liquid, as Figure 8.17 shows. The purpose of this bypass is to always permit a flow of liquid through the pump, even if all the control valves out in the liquid system have closed off. The purpose of the bypass flow is to avoid trapping liquid in the pump which would be churned by the impeller and vaporized from the heat of friction. It would be possible to develop a control that opens the bypass when the system flow drops low, but this refinement is usually not warranted, and instead the hand control valve of Figure 8.17 is set slightly open.
Some pumps are equipped with what is called a volute vent which allows a line to be connected from the top of the pump housing to the low-pressure receiver. This vent may be opened when the pump starts to clear the pump of vapor and thus permit initiation of liquid pumping.