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                    Difference between centrifugal pump and positive displacement pump
                    release2016-12-09 15:11:30  Views

                          Pumps are the second most commonly used industrial equipment after electric motors. Millions of pumps are currently working around the world, delivering thousands of liquids. Pumps are generally divided into two basic categories: kinetic energy pumps and positive displacement (PD) pumps. According to the US Department of Commerce data, about 70% of all pump sales are kinetic energy pumps, and the remaining 30% are positive displacement pumps. When choosing a pump, the first step is to decide which of the centrifugal and volumetric pumps is best for your needs.

                           Since the mainstream industrial pumps are centrifugal pumps, many people think of centrifugal pumps first. There are many types of centrifugal pumps according to their uses, such as centrifugal self-priming pumps, centrifugal pipeline pumps, etc., and the cost of centrifugal pumps is usually lower than volumetric pumps, and it is also the correct pump type to be used in many cases. Each pump agitates fluids in different ways, and each pump has different operating characteristics and curves. However, it is important that the centrifugal pump affects the flow rate of the liquid, resulting in a certain pressure at the discharge port. In contrast, when a positive displacement pump agitates liquid, it first obtains a specific amount of liquid and sends it from the suction port to the discharge port. For a centrifugal pump, the pressure is formed first, and then the flow is generated. For positive displacement pumps, the flow is formed first, and then the pressure appears.

                           Performance comparison between centrifugal pump and positive displacement pump:

                           In order to be able to pick the right type from various pumps, it is important to understand the differences in the operating characteristics of the two pumps. When you look at its performance table (Figure 1a), you can see how different it works. Centrifugal pumps have a variable flow phenomenon that depends on pressure (or head), while positive displacement pumps have a more or less constant flow phenomenon that is independent of pressure.

                           Comparison of the viscosity of liquids pumped by centrifugal pumps and positive displacement pumps:

                           Viscosity plays an important role in the mechanical efficiency of the pump. Since the centrifugal pump runs at the speed of the motor, its efficiency will decrease with the increase of viscosity caused by the increase of friction loss in the pump. Please note that as the viscosity increases, the efficiency of the centrifugal pump decreases rapidly, as does the centrifugal self-priming pump. Another major difference is the effect of viscosity on pump capacity. You will notice in the flow meter that the centrifugal pump will decrease the flow rate when the viscosity increases, while the positive displacement pump will actually increase the flow rate. This is because higher viscosity liquids fill the voids in the positive displacement pump, resulting in higher volumetric efficiency. Keep in mind that there will also be increased pipeline losses in the system. This means that the flow in the centrifugal pump will further decrease as the pump differential pressure increases.

                           Efficiency comparison between centrifugal pump and positive displacement pump:

                           When considering the effect of differential pressure on the mechanical efficiency of the pump, kinetic energy pumps and positive displacement pumps also exhibit different characteristics. For positive displacement pumps, as pressure increases, efficiency actually improves, while centrifugal pumps have an efficiency point. On both sides of this point, the efficiency of the integrated pump will drop significantly.

                           Centrifugal pump and volumetric pump inlet conditions:

                           The requirements for inlet conditions for these two types of pumps also differ considerably. Centrifugal pumps require a predetermined amount of liquid in the pump in order to create a pressure differential. Liquid-free dry pumps cannot be started by themselves. Once started, the centrifugal pump needs to meet the clear inlet pressure requirements recommended by the manufacturer.

                           Since the positive displacement pump stirs the liquid through the expansion and contraction of the liquid volume, a negative pressure will be formed at the inlet, so the positive displacement pump can start the perfusion by itself. In some cases, this is the deciding factor in choosing a volumetric or centrifugal pump.

                           In general, when the viscosity exceeds 150 cP, the flow rate must be able to be predicted over a wide range, or when the pump is expected to start the perfusion itself, a volumetric pump can be considered. When choosing between centrifugal pumps and positive displacement pumps, energy consumption issues must also be considered, as there may be considerable differences in energy consumption between the two pumps. This is particularly important for situations where the flow rate is less than 100 gallons per minute. In this case, the decrease in the efficiency of the centrifugal pump will be greater, and there are many classifications of centrifugal pumps, self-priming pumps, booster pumps, and sewage Pumps, etc. These detailed classifications can better meet customer needs.


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