Pump selection

Principles for selecting pumps: When designing equipment, the design institute must determine the purpose and performance requirements of the pump and select the appropriate type of pump. This choice starts with selecting the type and design of the pump. So, what principles should be used to choose a pump? What is the basis then? I. Understanding the principles for pump selection 1. Ensure that the type and performance of the selected pump meet the requirements of the process parameters such as flow rate, head, pressure, temperature, net positive suction head, and suction lift of the installation. 2. The requirements of the medium’s properties must be met. For pumps used to transport flammable, explosive, toxic, or valuable fluids, it is necessary to have reliable shaft seals or to use leak-free pumps such as magnetically driven pumps, diaphragm pumps, and shielded pumps. For pumps used to transport corrosive fluids, the moving parts must be made of corrosion-resistant materials, such as AFB stainless steel corrosion-resistant pumps and CQF engineering plastic magnetically driven pumps. For pumps that transport media containing solid particles, it is required that the convective components be made of wear-resistant materials, and where necessary, the shaft seals 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, except in the following cases, centrifugal pumps should be preferred whenever possible: a. When metering is required, a metering pump should be used. b、When a very high head is required, the flow rate is low, and no suitable centrifugal pump with a high head for low flow rates is available, then a reciprocating pump can be used; if the requirement for cavitation is not high, a vortex pump can also be chosen. c. When the head is very low and the flow rate is high, axial flow pumps and mixed-flow pumps can be used. d. When the medium viscosity is high (greater than 650–1000 mm2/s), rotor pumps or reciprocating pumps (gear pumps, screw pumps) can be considered. e. When the gas content in the medium is 75%, the flow rate is low, and the viscosity is less than 37.4 mm2/s, a vortex pump 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. Understand 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, system head, properties of the fluid, pipeline layout, and operating conditions. 1. Flow rate is one of the important performance parameters for selecting a pump; it is directly related to the production capacity and conveying 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 taken as the basis, with consideration also given to the normal flow rate. In the absence of a specified maximum flow rate, 1.1 times the normal flow rate can generally be used 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 safety 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 determining the system’s head pressure, calculating the effective net positive suction head, and selecting 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 choosing 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 needed for calculating the system head and verifying the net positive suction head. 5. There are many aspects related to operating conditions, such as the operating temperature of the liquid, the saturated vapor pressure P, the suction side pressure PS (absolute), the pressure in the discharge side container PZ, altitude, ambient temperature, whether the operation is intermittent or continuous, and whether the pump’s location is fixed or movable. III. Specific procedures for selecting a pump: Based on the principles and basic criteria for pump selection, the specific steps are as follows: 1. 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.), taking into account the layout of the installation, terrain conditions, water level conditions, and operating conditions. 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 according to the explosion area classification. 3. Determine whether to choose a single-suction pump or a double-suction pump based on the flow rate ； Depending on the required head, decide whether to use a single-stage pump or a multi-stage pump, as well as whether to choose a high-speed pump or a low-speed pump (such as an air-conditioning pump). Multi-stage pumps have lower efficiency than single-stage pumps; therefore, when both types of pumps are suitable for a given application, a single-stage pump should be preferred 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 usually be used as the maximum flow rate) and the head with a 5%–10% margin added. These parameters are then used to identify the appropriate model from the type chart or series performance curves. 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 vertical or horizontal lines from these two points respectively; if the intersection point falls exactly on the performance curve, then that pump is the one to be chosen. However, this ideal situation occurs rarely in practice. Usually, one of the following two scenarios arises: In the first scenario, 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 selected. Or try to reduce the pipeline resistance loss. The second case: if the intersection point is 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 according to the required values of Q and H, using the corresponding formulas. 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. After the pump model is determined, for pumps that handle water-like physical and chemical media, it is necessary to consult relevant product catalogs or samples, and use the performance tables or curves associated with that model to make adjustments, in order to determine whether the normal operating point falls within the preferred operating range of the pump Is the effective NPSH greater than (NPSH)? Can the geometric installation height also be adjusted inversely using 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 essential to convert the pump’s performance curve obtained from water-based tests into a curve corresponding to that viscosity (or density). In particular, the suction performance and input power must be carefully calculated or verified. 7. Determine the number of pumps and the redundancy rate: a. 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. b. 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). c. 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 can still handle 70% of the transportation requirements. d. 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, the customer can provide the basic requirements for the pump they are interested in, and our company will carry out a selection process or recommend better pump models. If the design institute has already determined the model of the pump when designing the equipment, it should be configured in accordance with the institute’s requirements.

Dear expert upstairs, may I ask what criteria the design institute uses when developing sealing solutions? Is it the existing production process?