Selection principles, criteria, and specific operating procedures for various types of pumps

When designing equipment and installations, the design institute must determine the pump’s intended application and performance requirements and select an appropriate type. This selection begins with choosing the pump’s category and configuration; so what principles should guide pump selection, and on what basis are these decisions made?

I. Understanding Pump Selection Principles

1. Ensure that the selected pump’s type and performance meet the process parameters of the unit, including flow rate, head, pressure, temperature, cavitation margin, and suction lift.

2. The requirements of the medium’s properties must be met.

For pumps conveying 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 canned motor pumps.

For pumps handling corrosive media, the wetted parts must be made of corrosion-resistant materials, such as AFB stainless steel corrosion-resistant pumps and CQF engineering-plastic magnetic-drive pumps.

For pumps handling media containing solid particles, the wetted parts should be made of wear-resistant materials, and, if necessary, the shaft seal should be flushed with a clean liquid.

3. Mechanically, it offers high reliability, low noise, and minimal vibration.

4. Economically, the total cost—comprising equipment, operating, maintenance, and administrative expenses—should be minimized.

5. Centrifugal pumps feature high rotational speed, compact size, light weight, high efficiency, large flow rate, simple structure, pulsation-free fluid delivery, stable performance, ease of operation, and convenient maintenance.

Therefore, centrifugal pumps should be used whenever possible, except in the following cases. ：

a. When metering requirements exist, select a metering pump.

b. When high head is required, flow rate is very low, and no suitable small-flow, high-head centrifugal pump is available, a reciprocating pump may be selected; if cavitation requirements are not stringent, a vortex pump may also be used.

c. When the head is very low and the flow rate is very high, axial-flow pumps and mixed-flow pumps can be selected.

d. When the fluid viscosity is high (greater than 650–1000 mm²/s), a rotary pump or reciprocating pump (e.g., gear pump, screw pump) may be considered.

e. When the medium contains 75% gas, the flow rate is low, and the viscosity is less than 37.4 mm²/s, a vortex pump can be selected.

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 (or electric) diaphragm pumps.

II. Understanding the Basic Criteria for Pump Selection

The basis for pump selection should consider five aspects according to the process flow and water supply and drainage requirements: liquid flow rate, system head, liquid properties, piping layout, and operating conditions.

1. Flow rate is one of the key performance parameters for pump selection, directly affecting the overall plant’s production and conveying capacity. In process design at engineering institutes, it is possible to calculate three flow rates: normal, minimum, and maximum. When selecting a pump, use the maximum flow rate as the primary basis while also considering the normal flow rate; if the maximum flow rate is unavailable, it is common practice to take 1.1 times the normal flow rate as the maximum.

2. The head required by the system is another important performance parameter for pump selection; typically, the pump should be selected based on a head that includes a 5%–10% safety margin.

3. Liquid properties, including the name of the liquid medium, its physical properties, chemical properties, and other characteristics. Physical properties include temperature (°C), density (ρ), viscosity (μ), diameter of solid particles in the medium, and gas content, which affect system head calculations, effective net positive suction head (NPSH) determination, and selection of an appropriate pump type. Chemical properties primarily refer to the chemical corrosivity and toxicity of the liquid medium, serving as critical criteria for selecting pump materials and the type of shaft seal.

4. The piping layout conditions of the equipment system refer to data such as the liquid delivery height, delivery distance, and delivery direction; the minimum liquid level on the suction side; and the maximum liquid level on the discharge side, along with pipe specifications, lengths, materials, fitting specifications, and quantities, in order to perform system head calculations and cavitation margin verification.

5. Operating conditions encompass many factors, such as the liquid’s operating temperature T, saturated vapor pressure P, suction-side pressure PS (absolute), discharge-side vessel pressure PZ, elevation, ambient temperature, whether operation is intermittent or continuous, and whether the pump is fixed or portable.

III. Specific Procedures for Pump Selection

Based on the pump selection principles and basic selection criteria, the specific procedure is as follows:

1. Based on the equipment layout, terrain conditions, water level conditions, and operating conditions, determine the selection of horizontal, vertical, and other types of pumps (pipeline, submersible, submerged, non-clogging, self-priming, gear-driven, etc.).

2. Based on the properties of the liquid medium, determine whether to use a clean water pump, hot water pump, oil pump, chemical pump, corrosion-resistant pump, or slurry pump, or opt for a non-clogging pump.

Pumps installed in hazardous (explosive) areas shall be equipped with explosion-proof electric motors appropriate to the classification of the hazardous area.

3. Based on the flow rate, determine whether to select a single-suction pump or a double-suction pump; based on the head, choose a single-stage pump or a multi-stage pump, and decide between a high-speed pump and a low-speed pump (for air-conditioning applications). Multi-stage pumps have lower efficiency than single-stage pumps; if both single-stage and multi-stage pumps are suitable, prioritize the single-stage pump.

4. Determine the specific pump model

Once the pump series has been selected, determine the specific model from the product catalog or series characteristic curves using the two primary performance parameters: the maximum flow rate (if unavailable, typically take 1.1 times the normal flow rate as the maximum) and the head, with a 5%–10% margin added to the head. The procedure is as follows:

Using the pump performance curve, locate the required flow rate on the horizontal axis and the required head on the vertical axis. Draw perpendicular or horizontal lines upward from the flow‑rate value and to the right from the head value; if the intersection of these two lines falls exactly on the curve, then that pump is the one to select. However, this ideal situation is rare, and in practice you will typically encounter one of the following two scenarios:

Case 1: The intersection point lies above the pump performance curve, indicating that the flow rate meets the requirements but the head is insufficient. If the heads are similar or differ by about 5%, the pump can still be selected. However, if the heads differ significantly, choose a pump with a higher head, or find ways to reduce the pipeline friction losses.

Second case: If the intersection point lies below the characteristic curve, within the fan-shaped trapezoidal region of the pump’s performance curve, this model is tentatively selected. Then, depending on the head difference, it is determined whether to adjust the impeller diameter by trimming.

If the head difference is small, do not perform impeller trimming; if the head difference is large, determine the required flow rate Q and head H, then calculate the impeller diameter using the specific speed ns and the trimming formula. If the intersection point falls outside the fan-shaped trapezoidal region, select the pump with the lower head. When selecting a pump, it is sometimes necessary to consider process requirements and choose pumps with different Q–H characteristic curves.

5. Once the pump model is selected, for pumps whose performance closely approximates that of water—whether for the pump itself or for the conveyed fluid—it is necessary to consult the relevant product catalogs or data sheets and use the performance tables or curves for that model to make adjustments. Verify that the normal operating point falls within the pump’s preferred operating range and that the available NPSH (NPSH) exceeds the required NPSH. Alternatively, you can reverse this process and adjust the geometric installation height based on the NPSH requirement.

6. For pumps handling liquids with a viscosity greater than 20 mm²/s (or a density greater than 1000 kg/m³), the pump performance curve obtained from water tests must be converted to the corresponding performance curve at that viscosity (or density). In particular, the suction performance and input power should be carefully calculated or verified.

7. Determine the number of pumps and the spare capacity ratio:

For pumps operating under normal conditions, a single unit is generally preferred because a single large pump is equivalent to two smaller pumps operating in parallel (assuming equal head and flow). Moreover, the efficiency of a large pump exceeds that of smaller pumps; therefore, from an energy-saving perspective, it is more advantageous to select one large pump rather than two smaller ones. However, when any of the following situations arise, consideration may be given to operating two pumps in parallel:

The flow rate is very high; a single pump cannot achieve it.

For certain large pumps, pumps sized to handle 70% of the required flow can be operated in parallel without a standby pump; when one pump is under maintenance, the other continues to carry 70% of the plant’s delivery requirements.

For pumps that require 24-hour continuous operation, three pumps should be provided: one in service, one on standby, and one under maintenance.

8. In general, customers may submit their “basic pump selection criteria,” and our company will provide a selection or recommend superior pump products. If the design institute has already specified the pump model when designing the equipment, the configuration shall follow the institute’s requirements.

9. Determine the number of pumps and the spare capacity ratio:

For pumps operating under normal conditions, a single unit is generally preferred because a single large pump is equivalent to two smaller pumps operating in parallel (assuming equal head and flow). Moreover, the efficiency of a large pump exceeds that of smaller pumps; therefore, from an energy-saving perspective, it is more advantageous to select one large pump rather than two smaller ones. However, when any of the following situations arise, consideration may be given to operating two pumps in parallel:

The flow rate is very high; a single pump cannot achieve it.

For large pumps requiring a 50% spare capacity, two smaller pumps can be operated while two remain on standby (three pumps in total).

For certain large pumps, pumps sized to handle 70% of the required flow can be operated in parallel without a standby pump; when one pump is under maintenance, the other continues to carry 70% of the plant’s delivery demand.

For pumps that require 24-hour continuous operation, three pumps should be provided: two in service, one on standby, and one for maintenance.

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Yel@zicionpumps.com

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