Positive Displacement Pump vs Centrifugal Pump

Figure 1: Centrifugal pump vs positive displacement pump.

1. What is a Positive Displacement Pump?

Positive displacement pump refers to the pump that uses the internal volume changes of the pump cylinder to transport liquid, and positive displacement pump types include reciprocating pump, rotary pump, etc.

Table 1: Types of pumps.

The working principle of the positive displacement pump: the rotation of the pump shaft causes the volume change of the pump chamber, so that the pump outputs a constant flow of fluid, and the fluid is subjected to resistance to generate pressure. Therefore, its necessary working condition is: the pump chamber has sealed volume changes.

More specifically, for each revolution of the pump shaft, the amount of fluid discharged by the pump is the same, and is not affected by the system pressure. As shown in Figure 2 below, the system pressure on the right is significantly higher than that on the left, but the flow at the pump outlet is the same, 1 GPM (gallons per minute).

Figure 2: Positive displacement pump working in lower pressure system vs higher pressure system.

2. What is a Centrifugal Pump?

Centrifugal pump refers to a pump that transports liquid by the centrifugal force generated by the rotation of the impeller.

The working principle of the dynamic pump: by affecting the flow rate of the liquid, there is a certain pressure at the discharge port, and then the liquid is sent to the system by pressure.

Centrifugal pump is the most widely used dynamic pump. Its disadvantage is that when the system pressure is large enough, the pump can not discharge the liquid outwards, and even the phenomenon of liquid backflow occurs, so it cannot be used in high-pressure systems.
As shown in Figure 3, the pressure of the system on the right is much higher than that on the left, so the liquid in the pump on the right is spinning, but it cannot enter the system (note the small white arrows circled in the pump). That is to say, the output flow rate of the pump is affected by the system pressure.

Figure 3: Centrifugal pump working in a lower pressure system vs a very high pressure system.

3. Difference between Positive Displacement Pump and Centrifugal Pump

3.1 Performance (Pressure vs. Flow Rate)

From the above, it can be seen that the first difference between a positive displacement pump and a centrifugal pump is that the output flow of the positive displacement pump is not affected by the system pressure, while that of the centrifugal pump is affected by the system pressure.

Centrifugal pumps can affect the flow rate of the liquid, resulting in a certain pressure at the discharge port. In contrast, when a positive displacement pump conveys a liquid, it first acquires a specific amount of liquid and sends it from the suction port to the discharge port.
For centrifugal pumps, pressure is created first and then leads to the creation of flow moving. For positive displacement pumps, the flow moving is created first, and then leads to the creation of pressure.

You can see how different they are by looking at the performance figure comparison of positive displacement versus centrifugal pumps (Figure 4). Centrifugal pumps have a variable flow that depends on pressure (or pressure head), while positive displacement pumps have a relatively constant flow that is independent of pressure.

Figure 4: Pressure vs flow rate of positive displacement pump and centrifugal pump.

3.2 Viscosity vs. Mechanical Efficiency

Viscosity plays an important role in the mechanical efficiency of the pump. Since centrifugal pumps operate at motor speed, their efficiency decreases as viscosity increases due to increased frictional losses within the pump. Note that the efficiency of the centrifugal pump decreases rapidly as viscosity increases (Fig. 5).

Figure 5: Viscosity vs efficiency of positive displacement pump and centrifugal pump.

3.3 Viscosity vs. Flow Rate

Another major difference is the effect of viscosity on pump capacity. In the flow figure (Fig. 6) you will notice that centrifugal pumps experience a drop in flow as viscosity increases, whereas positive displacement pumps actually have an increasing flow with it. This is because the higher viscosity liquid fills the interspaces within the positive displacement pump, resulting in higher volumetric efficiency.

Figure 6 shows only the effect of viscosity on pump flow. Keep in mind that there will also be increased pipe losses in the system. This means that the flow in the centrifugal pump will drop further as the differential pressure of the pump increases.

Figure 6: Viscosity vs flow rate of positive displacement pump and centrifugal pump.

3.4 Pressure vs. Mechanical Efficiency

When considering the influence of differential pressure on the mechanical efficiency of the pump, the dynamic pump and the positive displacement pump also show different characteristics. Figure 7 shows how the pump efficiency is affected by the consequent increase in pressure.
For positive displacement pumps, efficiency actually increases as pressure increases, whereas for centrifugal pumps there is a Best Efficiency Point (BEP). On either side of this point, on balance, the pump efficiency drops significantly.

Figure 7: Pressure vs mechanical efficiency of positive displacement pump and centrifugal pump.

3.5 Inlet Condition

The requirements for inlet conditions are also quite different for the two types of pumps. Centrifugal pumps require a certain amount of liquid in the pump in order to form a pressure difference, and a dry pump without liquid cannot start by itself. Once in operation, the centrifugal pump needs to meet specific inlet pressure requirements recommended by the manufacturer.

Since the positive displacement pump squeezes the liquid through the expansion and contraction of the volume, a negative pressure will be formed at the inlet. So the positive displacement pump can start perfusion by itself.

3.6 Internal Leakage

When the system pressure is too high, the fluid will automatically find the channel with the least resistance, that is, it will flow back to the inlet of the pump through the clearance between the various parts of the pump. We call this phenomenon the internal leakage of the pump. Once the pump has a large internal leakage, the volumetric efficiency of the pump will be greatly reduced.

The internal leakage of the positive displacement pump is very small, and a large internal leakage occurs only when there are parts damaged in the pump body, as shown in Figure 8, pay attention to the small white arrows in the figure. The internal leakage of the dynamic pump is very large, so it cannot be used in high pressure conditions, pay attention to the small white arrows in the figure.

Of course, when the load overpressure occurs, the dynamic pump can limit the continuing rise of system pressure, which plays a role in safety protection; while the positive displacement pump must be equipped with a corresponding relief valve at the pump outlet to limit the system pressure (except for variable pumps).

Figure 8: Internal leakage of positive displacement pump and centrifugal pump.

3.7 Application

If we were to design a hydraulic lift table and you used a centrifugal pump as the power unit, the result would be very bad. The main undesirable results are as follows:
The lifting speed of the lifting table is affected by the weight of the object.
The weight of the object directly affects the size of the leakage in the pump chamber.
The larger the internal leakage, the liquid in the pump will overheat according to the energy conservation principle.

This problem does not occur if a positive displacement pump is used. For a positive displacement pump, no matter how the system pressure changes (in the pressure range of the pump), it can provide a constant flow to the system. Therefore, the lifting speed of the lifting table will not be affected by the weight of the object.

So positive displacement pumps are very suitable for occasions with large load changes and high pressures.

Table 2: Comparison of centrifugal, reciprocating and rotary pumps.

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