Principles, criteria, and specific operation methods for pump selection

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When designing equipment, the design institute must determine the purpose and performance of the pump and select an appropriate model. This selection process begins with choosing the type and design of the pump. So, what principles should be used to select a pump? What are the criteria for making such a choice? I. Understanding the principles for selecting pumps 1. Ensure that the type and performance of the selected pump meet the requirements of process parameters such as flow rate, head, pressure, temperature, cavitation threshold, and suction lift. 　　2. The requirements of the medium’s properties must be met. 　　For pumps used to transport flammable, explosive, toxic, or valuable media, a reliable shaft seal is required, or leak-free pumps should be used, such as magnetically driven pumps, diaphragm pumps, and canned pumps. For pumps handling corrosive media, the wetted parts must be made of corrosion-resistant materials; examples include stainless steel corrosion-resistant pumps and magnetically driven pumps made of engineering plastics. 　　For pumps handling media containing solid particles, wear-resistant materials must be used for the wetted parts; if necessary, the shaft seal should be flushed with a clean liquid. 　　3. High reliability in terms of mechanics, low noise, and minimal vibration. 　　4. Economically, it is necessary to take into comprehensive account the total cost of equipment expenses, operating costs, maintenance costs, and management fees to minimize it. 　　5. Centrifugal pumps feature high speed, small size, light weight, high efficiency, large flow rate, simple structure, pulse-free fluid delivery, stable performance, ease of operation, and convenient maintenance. 　　Therefore, centrifugal pumps should be preferred as much as possible, except in the following cases: a) When metering is required, a metering pump should be used. b) When a very high head is needed along with a low flow rate, and no suitable centrifugal pump with a high head for low flow rates is available, a reciprocating pump can be used; a vortex pump can also be chosen if cavitation requirements are not high. c) When the head is very low but the flow rate is high, axial flow pumps and mixed-flow pumps can be utilized. 　　d. When the viscosity of the medium is high (greater than 650–1000 mm2/s), rotary pumps or reciprocating pumps (gear pumps, screw pumps) can be considered.
e. When the medium contains 75% air, the flow rate is low, and the viscosity is less than 37.4 mm2/s, vortex pumps can be used. 　　f. In situations where starting is frequent or pumping is difficult, pumps with self-priming capabilities should be used, such as self-priming centrifugal pumps, self-priming vortex pumps, and pneumatic (electric) diaphragm pumps. 　　II. Understanding the basic criteria for pump selection The criteria for selecting a pump should be considered from five aspects, based on the process flow and water supply/distribution requirements: fluid flow rate, head required by the system, properties of the fluid, piping layout, and operating conditions. 1. Flow rate is one of the key performance parameters when selecting a pump; it directly affects the production capacity and transfer capacity of the entire system. If the process design in the design institute can determine the normal, minimum, and maximum flow rates of the pump. When selecting a pump, the maximum flow rate should be used as the basis, taking into account the normal flow rate as well. In cases where a maximum flow rate is not available, 1.1 times the normal flow rate can generally be taken as the maximum flow rate. 　　2. The head required by the installation system is another important performance parameter for selecting a pump; generally, the head value should be increased by 5%–10% as a margin when making the selection. 　　3. Properties of the liquid, including the name of the liquid medium, its physical properties, chemical properties, and other characteristics. Physical properties include temperature, density, viscosity, the diameter of solid particles present in the medium, and the gas content; these factors are relevant to calculating the system’s head, the effective net positive suction head, and determining the appropriate type of pump. Chemical properties refer mainly to the chemical corrosiveness and toxicity of the liquid medium, and they serve as important criteria for selecting the material for the pump as well as the type of shaft seal to use. 　　4. The pipeline layout conditions of the plant system refer to data such as the liquid delivery height, distance, and direction; the lowest liquid level on the suction side and the highest liquid level on the discharge side; as well as details regarding pipe specifications, their lengths, materials, fitting specifications, and quantities. These are necessary for calculating the system head and verifying the net positive suction head. 　　5. There are many aspects to operating conditions, such as the operating temperature of the liquid, saturated vapor pressure P, suction-side pressure PS (absolute), discharge-side container pressure PZ, altitude, ambient temperature, whether the operation is intermittent or continuous, and whether the pump’s position is fixed or movable. 　　III. Specific procedures for selecting a pump In accordance with the principles and basic criteria for pump selection, the specific steps are as follows: 1. Based on the layout of the installation, topographical conditions, water level conditions, and operating conditions, determine whether to choose a horizontal, vertical, or other type of pump (such as pipeline-type, submersible, underwater, clog-free, self-priming, gear-driven, etc.). 　　2. Based on the properties of the liquid medium, determine whether to use a water pump, a hot water pump, an oil pump, a chemical pump, a corrosion-resistant pump, or a dirt-pump; or opt for a clog-free pump. Pumps installed in explosive areas should use appropriate explosion-proof motors based on the classification of the explosive area. 　　3. Determine whether to choose a single-suction pump or a double-suction pump based on the flow rate; decide between a single-stage pump and a multi-stage pump, as well as between a high-speed pump and a low-speed pump, considering that multi-stage pumps have lower efficiency than single-stage pumps. If both a single-stage pump and a multi-stage pump are suitable for a given application, the single-stage pump should be chosen first. 　　4. Determine the specific model of the pump. After deciding on which series of pumps to use, it is possible to determine the specific model by using two key parameters: the maximum flow rate (if no maximum flow rate is specified, 1.1 times the normal flow rate can generally be used as the maximum flow rate) and the head with a 5%–10% margin added. This is done by referring to the model chart or the characteristic curves of that series. The procedure is as follows: Using the pump’s performance curve, locate the desired flow rate value on the horizontal axis and the desired head value on the vertical axis. Draw a vertical or horizontal line from each of these values respectively; if the intersection point falls exactly on the performance curve, then that pump is the one to be chosen. However, this ideal situation rarely occurs. Usually, one of the following two situations arises: The first situation is when the intersection point is above the performance curve, which indicates that the flow rate meets the requirements but the head is insufficient. In such cases, if the difference in head is small, around 5%, the pump can still be used; if the difference is significant, then a pump with a higher head should be chosen. Or try to reduce the pipeline friction loss. 　　The second option: if the intersection point lies below the characteristic curve, within the trapezoidal range of the pump’s characteristic curve, then this model can be initially selected. Subsequently, it is determined whether to adjust the diameter of the impeller based on the difference in head pressure. If the difference in head pressure is small, no adjustment is needed; if it is large, then the impeller diameter is adjusted using the required values of Q and H, along with the corresponding ns value and the adjustment formula. If the intersection point does not fall within that trapezoidal range, a pump with a lower head pressure should be chosen. When selecting a pump, it is sometimes necessary to take into account the requirements of the production process and choose Q-H characteristic curves of different shapes. 　　5. Once the pump model has been determined, for pumps that handle liquids with physical and chemical properties similar to those of water, it is necessary to consult relevant product catalogs or samples. Using the performance table or performance curve associated with that model, it is necessary to check whether the normal operating point falls within the pump’s optimal operating range, and whether the effective NPSH is greater than the required NPSH value. Can the geometric installation height also be adjusted based on NPSH? 6. For pumps that transport liquids with a viscosity greater than 20 mm2/s (or a density greater than 1000 kg/m3), it is necessary to convert the pump’s performance curves obtained from water-based tests into curves corresponding to that viscosity (or density). In particular, careful calculations or evaluations must be carried out regarding the suction performance and input power. 　　7. Determining the number of pumps and the standby rate: For pumps operating under normal conditions, generally only one pump is used. This is because one large pump is equivalent to two smaller pumps working in parallel (in terms of head and flow rate). The efficiency of a large pump is higher than that of smaller pumps; therefore, from an energy-saving perspective, it’s better to opt for one large pump rather than two smaller ones. However, in the following situations, it may be advisable to use two pumps in parallel: when the required flow rate is extremely high, and one pump alone cannot meet this demand. 　　For large pumps that require a 50% backup rate, two smaller pumps can be used in operation with two as backups (a total of three). For certain large pumps, pumps with a 70% flow capacity can be operated in parallel without the need for backups; when one pump is under maintenance, the other pump continues to handle 70% of the transportation requirements. 　　For pumps that need to operate continuously for 24 hours, three pumps should be kept in reserve: one is in operation, one is on standby, and one is under maintenance. 　　8. Under normal circumstances, customers can submit their \"basic requirements for selecting a pump\", and our company will carry out model selection or recommend better pump products. If the design institute has already determined the pump model when designing the equipment, it shall be configured in accordance with the institute’s requirements. 　　9. Determine the number of pumps and the redundancy factor: For pumps that are operating normally, usually only one pump is needed, as one large pump is equivalent to two smaller pumps working in parallel (in terms of head and flow rate). The efficiency of a large pump is higher than that of smaller pumps; therefore, from an energy-saving perspective, it is better to use one large pump rather than two smaller ones. However, in the following situations, it may be appropriate to use two pumps in parallel: when the flow rate is very high and one pump is not sufficient to meet it. 　　For large pumps that require a 50% backup rate, two smaller pumps can be used in operation with two as backups (a total of three). For certain large pumps, pumps with a 70% flow capacity can be operated in parallel, without the need for backups; when one pump is under maintenance, the other pump continues to handle 70% of the transportation requirements. 　　For pumps that need to operate continuously for 24 hours, three pumps should be kept in reserve: one is in operation, one is on standby, and one is under maintenance.