Since the majority of industrial pumps are centrifugal pumps, many people consider centrifugal pumps first. Centrifugal pumps are generally less expensive than positive displacement pumps and are the right pump to use in many situations. Each pump mixes fluids in a different way, and each has different operating characteristics and curves. But the important thing is that centrifugal pumps affect the flow rate of the liquid, which results in a certain pressure at the discharge port. In contrast, when a positive displacement pump mixes the liquid, it first takes a certain amount of liquid and sends it from the suction port to the discharge port. For centrifugal pumps, pressure is generated first, and then flow occurs. For positive displacement pumps, flow is generated first, and then pressure occurs. The main differences between positive displacement pumps and centrifugal pumps:
1. The difference between the media of positive displacement pumps and centrifugal pumps:
(1) The speed and flow rate of positive displacement pumps are proportional, such as gear pumps, screw pumps, plunger pumps, etc. For example, their speed is 100 rpm and the flow rate is 1 cubic meter, while at 1000 rpm it can be understood as 10 cubic meters, but in practice there is a little power loss, and the flow rate is about 9.5-9.8 cubic meters.
(2) Centrifugal pumps rely on the high-speed rotation of the motor to throw the medium (generally a medium with low viscosity: such as water, diesel, kerosene, gasoline, etc.) out through the flow channel of the impeller.
(3) The difference between positive displacement pumps and centrifugal pumps is that the medium that a positive displacement pump can transport is relatively high in viscosity, such as heavy oil, viscous oil, asphalt and other high-viscosity media; centrifugal pumps can only transport some low-viscosity media, and its transport viscosity can only be lower than 3Cst (centistokes).
2. Differences in performance between positive displacement pumps and centrifugal pumps:
In order to be able to choose the right type from the various types of pumps, it is important to understand the differences in the operating characteristics of the two pumps. When looking at their operating function table (Figure 1a), you can see how different their operating principles are. Centrifugal pumps have a variable flow rate that depends on the pressure (or pressure head), while positive displacement pumps have a more or less constant flow rate that is independent of the pressure.
3. Differences in viscosity performance between positive displacement pumps and centrifugal pumps:
Viscosity plays an important role in the mechanical power of the pump. Since centrifugal pumps operate at motor speed, their power decreases as the viscosity increases due to increased friction losses in the pump. Please note that the power of centrifugal pumps decreases rapidly as the viscosity increases.
4. Differences in performance between positive displacement pumps and centrifugal pumps:
Another major difference is the effect of viscosity on the pump capacity. In the flow table, you will notice that centrifugal pumps experience a decrease in flow as the viscosity increases, while positive displacement pumps actually experience an increase in flow. This is because the higher viscosity liquid fills the voids in the positive displacement pump, resulting in higher volumetric power. Effect of viscosity on pump flow. Keep in mind that there will also be an increase in pipe loss in the system. This means that the flow in the centrifugal pump will decrease as the pump differential pressure increases.
5. Differences in power between positive displacement pumps and centrifugal pumps:
When considering the effect of differential pressure on the mechanical power of the pump, kinetic pumps and positive displacement pumps also exhibit different characteristics. How the pump power is affected by the pressure that increases accordingly. For positive displacement pumps, power actually increases with the increase in pressure, while centrifugal pumps have an optimal power point (BEP). On either side of this point, the integrated pump power will fluctuate greatly.
6. Differences in inlet conditions between positive displacement pumps and centrifugal pumps:
The requirements for inlet conditions for these two types of pumps are also quite different. Centrifugal pumps require a certain amount of liquid in the pump in advance to form a pressure difference. A dry pump without liquid cannot start on its own. Once started, the centrifugal pump needs to meet the clear inlet pressure requirements advocated by the manufacturer.
Since positive displacement pumps mix liquids by the expansion and contraction of the liquid volume, a negative pressure condition will be formed at the inlet, so positive displacement pumps can start to prime themselves. In some cases, this is the only factor that determines whether to choose a positive displacement pump or a centrifugal pump.
