Centrifugal pumps are classified as either radial flow or axial flow. Figure 10-1 shows a radial flow pump. Flow enters the center of the rotating wheel (impeller) and is propelled radially to the outside by centrifugal force. Within the impeller the velocity of the liquid is increased, and this is converted to pressure by the case.
A typical axial flow pump is shown in Figure 10-2. Flow is parallel to the axis of the shaft. A velocity is imparted by the impeller vanes, which are shaped like airfoils.
Most pumps are neither radial flow nor completely axial flow but have a flow path somewhere in between the two extremes. Radial flow pumps develop a higher head per stage and operate at slower speeds than axial flow pumps. Therefore, axial flow designs are used in very high flow rate, very low head applications.
Figure 10-3 shows a typical head-capacity curve for a centrifugal pump. At a constant speed (i.e., rotational velocity), as the head required to be furnished by the pump efficiency curve. For a given impeller shape, the efficiency is a maximum at a design throughput rate. As the rate varies upward and downward from this point the efficiency decreases.
By varying the pump speed the throughput at a given head or the head for a given throughput can be changed. In Figure 10-4 as the speed decreases from N to N2 to N3, the flow rate decreases if the head
required is constant, or the head decreases if the flow rate is constant.
In most piping systems both the head and the flow rate vary because the system has its own required pump head for a given flow rate. This can be seen by the example in Figure 10-5 where the head required by the system for the pump to provided is merely the friction drop in the pipeline between points A and B. This is a function of flow rate and can therefore be plotted as a “system curve” on the pump-head-flow-rate curve. For this system, as the pump is speeded up or slowed down a new
equilibrium of head and flow rate is established by the intersection of the system curve with the pump curve.
Figure 10-6 shows how the throughput can be changed by imposing an artificial backpressure on the pump. By adjusting its orifice, the control valve can shift the system curve, establishing new head-flow-rate equilibria. As the pressure drop across the control valve increases from APj to
AP2 to AP3 the flow rate through the system decreases from Qi to Q2 to Q3.
The advantages of centrifugal pumps are:
1. They are relatively inexpensive.
2. They have few moving parts and therefore tend to have greater onstream availability and lower maintenance costs than positive displacement pumps.
3. They have relatively small space and weight requirements in relation to throughput.
4. There are no close clearances in the fluid stream and therefore theycan handle liquids containing dirt, abrasives, large solids, etc.
5. Because there is very little pressure drop and no small clearances between the suction flange and the impeller, they can operate at low suction pressures (we will define a term, “net positive suction
head,” shortly).
6. Due to the shape of the head-capacity curve, centrifugal pumps automatically adjust to changes in head. Thus, capacity can be controlled over a wide range at constant speed.
Although several impellers can be installed in series to create large heads, centrifugal pumps are only practical for achieving high pressure when there are large flow rates. In addition centrifugal pumps have low maximum efficiencies when compared to reciprocating pumps. Since the efficiency also declines as the flow rate varies from the design point, in actual operation, the pump will operate at still lower efficiencies. Efficiencies between 55% and 75% are common.
