Principles, criteria, and specific procedures for pump selection

I. Understanding Pump Selection Principles 
1. The selected pump type and its performance meet the process parameters of the unit, including flow rate, head, pressure, temperature, cavitation flow, and suction lift. It must also comply with the requirements specific to the medium being handled: 
a) For pumps handling flammable, explosive, toxic, or valuable media, reliable shaft seals are required, or leak-free pumps should be used, such as magnetic drive pumps, diaphragm pumps, and shielded pumps. 
b) For pumps handling corrosive media, the flow components must be made from corrosion-resistant materials, such as stainless steel corrosion-resistant pumps or engineering-plastic magnetic-drive pumps. 
c) For pumps handling media containing solid particles, flow components must be made from wear-resistant materials, and, if necessary, the shaft seals should be flushed with a clean liquid. 
2. Mechanically reliable, with low noise and minimal vibration. 
3. Economically, the total cost—covering equipment expenses, operating costs, maintenance fees, and administrative costs—must be minimized. 
4. Centrifugal pumps boast high rotational speed, compact size, lightweight design, high efficiency, large flow rates, a simple structure, pulsation-free liquid delivery, stable performance, and ease of operation and maintenance. Therefore, unless otherwise specified, centrifugal pumps should be selected whenever possible. 
a) When measurement is required, select a metering pump. 
b) When high head requirements are involved, flow rates are very low, and there are no suitable small-flow, high-head centrifugal pumps available, a reciprocating pump can be used. Alternatively, if cavitation requirements are not critical, a vortex pump may also be considered. 
c) When the head is very low and the flow rate is high, axial-flow pumps and mixed-flow pumps are recommended. 
d) The medium has a relatively high viscosity (greater than 650–1000 mm²/s). ² At /s), consider using a rotor pump or reciprocating pump (such as a gear pump or screw pump). 
e) The medium contains 75% gas, with a low flow rate and a viscosity below 37.4 mm. ² At /s, a vortex pump can be used. 
f) For applications with frequent starts or where priming is inconvenient, pumps with self-priming capabilities should be selected, such as self-priming centrifugal pumps, self-priming vortex pumps, and pneumatic (electric) diaphragm pumps.

II. Understanding the Basic Criteria for Pump Selection 
The pump selection criteria should be determined based on the process flow and water supply/drainage requirements, taking into account five key factors: liquid flow rate, system head, fluid properties, piping layout, and operating conditions. 
1. Flow rate is one of the critical performance parameters for pump selection, directly influencing the overall production and conveying capacity of the entire system. For instance, during process design at the engineering institute, engineers can calculate three flow rates: normal, minimum, and maximum. When choosing a pump, the maximum flow rate should be the primary consideration, while also accounting for the normal flow rate. If the maximum flow rate isn’t available, it’s common practice to estimate the maximum flow as 1.1 times the normal flow rate. 
2. The head required by the system is another critical performance parameter when selecting a pump; typically, the pump should be chosen with a head that includes a 5%–10% margin for safety. 
3. Liquid properties include the name of the liquid medium, as well as its physical and chemical characteristics and other relevant attributes. Physical properties encompass parameters such as temperature (T), density (ρ), viscosity (μ), the diameter of solid particles present in the medium, and the gas content—factors that directly influence the system’s head, effective net positive suction head calculations, and the selection of an appropriate pump type. Chemical properties, primarily referring to the liquid medium’s chemical corrosiveness and toxicity, serve as critical considerations when choosing pump materials and determining the most suitable mechanical seal design. 
4. The piping layout conditions of the equipment system refer to parameters such as liquid delivery height, delivery distance, flow direction, the lowest liquid level on the suction side, and the highest liquid level on the discharge side—along with details like pipeline specifications, their lengths, materials, and the types and quantities of pipe fittings. These factors are essential for performing system head calculations and cavitation margin verification.

III. Specific Procedures for Pump Selection 
According to the pump selection principles and basic selection criteria, the specific steps are as follows: 
1. Based on the equipment layout, terrain conditions, water level requirements, and operating conditions, determine whether to select horizontal, vertical, or other pump types (such as pipeline, submersible, submerged, non-clog, self-priming, gear-driven, etc.). 
2. Based on the properties of the liquid medium, determine whether to use a clear water pump, hot water pump, oil pump, chemical pump, corrosion-resistant pump, slurry pump, or a non-clog pump. For pumps installed in explosive areas, select the appropriate explosion-proof motor according to the specific classification of the hazardous zone. 
3. Determine whether to select a single-suction pump or a double-suction pump based on the flow rate; choose between a single-stage pump and a multi-stage pump depending on the head requirements—specifically, opt for a high-speed pump or a low-speed pump (such as an air-conditioning pump). Note that multi-stage pumps generally have lower efficiency compared to single-stage pumps. If both a single-stage and a multi-stage pump can adequately meet the application needs, prioritize selecting the single-stage pump. 
4. Determine the specific pump model: After deciding which pump series to use, select the main performance parameters—namely, the head and flow rate—by taking into account a 5%–10% margin added to the maximum flow rate (if no maximum flow rate is available, typically use 1.1 times the normal flow rate as the max). Then, locate the exact model on the pump selection chart or the series performance curve. Here’s how to proceed:

Using the pump performance curve, locate the required flow rate on the horizontal axis and the desired head on the vertical axis. Then, draw vertical or horizontal lines upward from the flow rate value and rightward from the head value, respectively. If these two lines intersect precisely on the performance curve, then this pump is the one you should select. However, this ideal scenario rarely occurs—more often, you’ll encounter one of the following two situations: 
a) First scenario: If the intersection point lies above the pump’s performance curve, it indicates that the flow rate meets the requirements, but the head is insufficient. In this case, if the difference in head is small—say around 5%—the pump can still be selected. However, if the head difference is significant, you should choose the pump with the higher head rating or explore ways to reduce the pipeline resistance losses. 
b) Second scenario: If the intersection point lies below the pump’s characteristic curve but still within the fan-shaped trapezoidal region of the pump’s performance curve, this model is tentatively selected. Then, depending on how much the head differs, you decide whether to trim the impeller diameter. If the head difference is minimal, no trimming is necessary. However, if the head difference is significant, proceed according to the required flow rate (Q), head (H), the trimming formula, and calculate the appropriate impeller diameter. If the intersection point falls outside the fan-shaped trapezoid, opt for the pump with the lower head instead. When selecting a pump, it’s sometimes essential to consider specific production process requirements, which may dictate choosing a pump with a Q-H characteristic curve of a particular shape. 
5. After the pump model is determined, for pumps designed to handle physical and chemical media that are similar to water—whether for water pumps or media-specific pumps—it is necessary to consult the relevant product catalogs or samples. Refer to the performance table or performance curve provided for that specific model to verify whether the normal operating point falls within the pump’s optimal working range. Additionally, check if the available NPSHa exceeds the required NPSHr. Alternatively, you can use the NPSHr value to adjust the geometric installation height accordingly. 
6. For conveying viscosity greater than 20 mm ² Liquid pump rated at /s (or with a density greater than 1000 kg/m³) ³ ), be sure to convert the water test pump characteristic curve into the performance curve corresponding to that viscosity (or density), paying particular attention to carefully calculating or verifying the suction performance and input power. 
7. Determine the number of pumps and the redundancy rate: 
For normally operating pumps, typically only one is used, since a single large pump can deliver the same head and flow rate as two smaller pumps running in parallel (assuming identical performance characteristics). Moreover, large pumps generally have higher efficiency than small ones. Therefore, from an energy-saving perspective, it’s often more advantageous to choose one large pump rather than two smaller ones. However, there are specific situations where using two pumps in parallel might be considered: 
a) For large pumps requiring a 50% redundancy rate, two smaller pumps can be used in operation with two as backups (totaling three pumps). 
b) For certain large pumps, it is possible to operate two pumps in parallel at 70% of their flow capacity, eliminating the need for a standby pump. This way, when one pump is undergoing maintenance, the other can still handle 70% of the required production-level delivery. 
c) For pumps requiring 24-hour continuous operation, three pumps should be available: one in operation, one as a backup, and one for maintenance. 
8. Generally, customers may submit their "basic pump selection criteria," and our company will provide a pump selection or recommend superior pump products. However, if the design institute has already finalized the pump model during the equipment design process, we will configure the pump according to the institute's specifications.