Should I choose a centrifugal pump or a positive displacement pump? How should one choose a model?

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Pumps are the second most commonly used industrial equipment, after motors. Currently, millions of pumps are in use around the world, transporting thousands of different types of liquids. Choosing the right pump from the many available types is a very complex task. The pump selection process is, to a large extent, a process of matching the capabilities of a particular pump with the system requirements as well as the properties of the liquid to be pumped. In this article, we will start by examining the properties of the liquid to be pumped, beginning from the perspective of user requirements, and then move on to discuss the specific selection of pumps. 1. Basic requirements: In any application, it is first necessary to understand the user’s basic requirements for the pump. For example: inlet conditions, the required flow rate, pressure difference, temperature, and fluid properties such as viscosity, abrasiveness, shear sensitivity, and corrosivity. All of these conditions must be determined before a pump can be selected. The pump can operate properly only under the correct suction conditions. In fact, the biggest problem among the difficulties faced by pumps can be traced back to poor suction conditions. Since the pump’s ability to push liquid is much greater than its ability to draw it in, the inlet conditions should be kept within the pump’s capacity limits. Pressure difference is also a crucial factor; particularly from the perspective of energy savings and the pump’s service life, using smaller pipe diameters and longer pipeline lengths can reduce the initial costs of the system, but it may also result in a higher pressure difference across the pump. This higher pressure difference will be converted into energy consumption, and it may shorten the pump’s service life, which in turn means higher operating costs and lower efficiency. The required properties of the liquid are usually known, and what is important is to understand how a given pump affects these properties. The vast majority of users prefer that the liquid discharged by the pump remain in the same condition as when it entered the pump. When choosing a pump properly, material compatibility, viscosity, sensitivity to shear, and the presence of specific substances or solids are all of great importance. 2. Centrifugal pumps and positive displacement pumps: Once the basic requirements are met and the properties of the liquid are known, the selection of a pump can begin. Pumps are generally divided into two basic categories: kinetic pumps (of which centrifugal pumps are the most common type) and positive displacement (PD) pumps. According to data from the U.S. Department of Commerce, about 70% of total pump sales are kinetic pumps, while the remaining 30% are positive displacement pumps. When selecting a pump, the first step is to determine whether a centrifugal pump or a positive displacement pump is more suitable for your needs. Since the dominant type of industrial pump is the centrifugal pump, many people consider centrifugal pumps first. Centrifugal pumps are usually cheaper than positive displacement pumps, and they are also the appropriate type of pump to use in many situations. Each pump stirs the fluid in its own unique way, and each pump has its own distinct operating characteristics and curves. But it is important to note that centrifugal pumps affect the flow rate of the liquid, which in turn results in a certain pressure at the discharge outlet. On the contrary, when a positive displacement pump stirs a liquid, it first draws in a specific amount of liquid and transports this liquid from the inlet to the outlet. In the case of centrifugal pumps, pressure is generated first, which then leads to flow. In the case of positive displacement pumps, flow is generated first, which then leads to the emergence of pressure. (1) Performance: To select the most suitable type among various pumps, it is important to understand the differences in the operating characteristics of these two types of pumps. By looking at its performance chart (Figure 1a), you can see how greatly its working principle differs. Centrifugal pumps exhibit variable flow rates that depend on pressure (or head), whereas positive displacement pumps have flow rates that are more or less constant and independent of pressure. (2) Viscosity Viscosity plays an important role in the mechanical efficiency of pumps. Since the centrifugal pump operates at the speed set by the motor, its efficiency decreases as the viscosity increases due to increased internal frictional losses within the pump. Please note that as viscosity increases, the efficiency of centrifugal pumps decreases rapidly (Figure 1b). Another major difference lies in the effect of viscosity on pump capacity. In the flow meter (Figure 1c), you will notice that the centrifugal pump experiences a decrease in flow rate as viscosity increases, whereas the positive displacement pump actually sees an increase in flow rate as a result. This is because the higher-viscosity liquid fills the voids within the positive displacement pump, thereby resulting in a higher volumetric efficiency. Figure 1c shows only the effect of viscosity on pump flow rate. Please keep in mind that there will also be an increase in pipeline losses within the system. This means that the flow rate inside the centrifugal pump will further decrease as the pump differential pressure increases. (3) Efficiency When considering the effect of differential pressure on the mechanical efficiency of pumps, kinetic pumps and positive-displacement pumps also exhibit distinct characteristics. Figure 1d shows how the pump efficiency is affected by the increasing pressure. For positive displacement pumps, efficiency actually increases as pressure rises, whereas centrifugal pumps have an optimal efficiency point (BEP). On either side of this point, the overall pump efficiency drops significantly. (4) Inlet conditions: These two types of pumps also have considerable differences in their requirements regarding inlet conditions. A centrifugal pump requires a certain amount of liquid inside it in order to create a pressure difference. A dry pump without liquid cannot start on its own. Once the operation is started, the centrifugal pump must meet the specific inlet pressure requirements recommended by the manufacturer. Since a positive displacement pump stirs the liquid by the expansion and contraction of its volume, a negative pressure is created at the inlet, which allows the pump to start pumping on its own. In some cases, this is the sole deciding factor in choosing between a positive displacement pump and a centrifugal pump. (5) Summary Generally speaking, when the viscosity exceeds 150 cP, it is necessary to be able to predict flow rates over a wide range; or, if it is desired for the pump to start pumping on its own, a positive displacement pump can be considered. When choosing between centrifugal pumps and positive displacement pumps, energy consumption must also be taken into account, as there can be significant differences in terms of energy use between these two types of pumps. This is particularly important when the flow rate is below 100 gallons per minute, as in such cases the decrease in the efficiency of centrifugal pumps is even greater. 　 3. Positive displacement pumps: Even if it has been decided to use positive displacement pumps, there are still many options to consider. Before detailing each type of pumping operation, let’s first review some common operating characteristics of positive displacement pumps. As mentioned above, a rotary positive displacement pump is capable of discharging the same volume of fluid with each rotation of the shaft. This means that the flow rate of the liquid discharged is proportional to the rotational speed. In other words, the flow rate can be controlled simply by changing the speed of the pump. For liquids with higher viscosity, the pump can be metered by simply measuring the rotation speed of the shaft.