In water treatment projects, how should pumps be selected?

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1. The principle and classification of pumps: By professional definition, a pump is a power device that converts the mechanical energy of a prime mover into pressure energy and kinetic energy of a fluid, thereby enabling the directed transport of the fluid. When in use, they are usually named according to their purpose, such as submersible sewage pumps, sludge pumps, metering pumps, etc., each with a different working principle. Based on their working principle, they can be classified as follows: 1.1 Vane pumps. Vane pumps include centrifugal pumps (single-stage, multi-stage), axial flow pumps, mixed-flow pumps, vortex pumps, etc. Centrifugal pump – It uses a rotating impeller to drive the fluid to rotate as well; through the effect of centrifugal force, the pressure energy and kinetic energy of the fluid are increased. Axial flow pump – utilizes the airfoil blades on the impeller to increase the energy of the fluid through the lift generated by the rotation of the fluid. Mixed-flow pump – lies between centrifugal pumps and axial flow pumps, utilizing both centrifugal force and lift. 1.2 Positive Displacement Pumps Positive displacement pumps include reciprocating pumps (pistons, plungers, diaphragms) and rotary pumps (gears, screws, vanes, etc.). Reciprocating pumps transfer fluid and increase its pressure by periodically changing the working volume; they are classified into three types: piston-type, plunger-type, and diaphragm-type. Rotary pump – Uses a pair or several rotors with special shapes, such as gears, screws, or other types of rotors, that rotate within a casing to transport fluid and increase its pressure. 1.3 Other types of pumps: Vane pumps and positive displacement pumps cover almost all the common types of pumps. There are also other types of pumps, such as water ring vacuum pumps – these pumps require a certain amount of water to be injected as the working fluid before starting up. The rotation of the radial impeller creates a closed water ring, and the space between the impeller and this water ring expands and contracts periodically, thereby generating negative pressure that draws in gases and expels them, achieving the purpose of vacuum creation. Jet pump – uses the suction effect of high-speed jets to draw in and transport liquids, and can be used for creating a vacuum. 2. Main performance parameters of the pump 2.1 Flow rate and head The amount of fluid that a pump transports per unit of time is called the flow rate; the flow rate of a pump is generally expressed as a volume flow rate, denoted by q. The increase in energy gained by a liquid under unit gravity after passing through a pump is called head, denoted by H, and is measured in meters. During selection, flow rate and head are the key parameters determined by designers based on process calculation results, and they are fixed values. Regarding the performance of the water pump itself, flow rate and head are often related to each other within a certain range. During the selection process, the two factors influence each other; the ideal choice is one in which the fixed values required by the process parameters fall at the point of highest efficiency on the pump’s performance curve. When there are deviations, it is necessary to comprehensively assess the potential negative effects of such deviations and make a decision after careful consideration. 2.2 Shaft Power and Efficiency: The power transmitted by the prime mover to the pump’s shaft during operation is referred to as shaft power, denoted by P, with the unit being kW. The power obtained by the fluid passing through the pump per unit time is called the effective power, denoted as Pe. The efficiency of a pump is the ratio of the useful power to the shaft power, that is, η=Pe/P. The efficiency of a pump is related to factors such as its design quality and the level of mechanical processing; the efficiency level has a direct impact on energy consumption. Additionally, each pump has its own optimal efficiency range, and it is necessary to choose a pump that falls within this range in order to reduce energy consumption. 2.3 The number of revolutions per minute of the pump shaft’s rotation speed is called the rotational speed, denoted by n, and is measured in r/min. The rotation speed generally corresponds to the motor’s standard speed; the motor’s speed can be adjusted using an inverter, and the performance curve of the water pump changes accordingly. When the actual flow rate and head change, adjusting them through frequency conversion can achieve excellent energy-saving results. 2.4 Cavitation and NPSH Cavitation occurs when, during the operation of a water pump, the local pressure inside the pump drops to the vaporization pressure of water for some reason; as a result, water vaporizes and forms a gas-liquid mixture. When this mixture reaches areas of higher pressure, the bubbles are compressed and burst, then re-condense back into water. This process generates very high water hammer pressures that can cause erosion and damage to the materials used in the pump. Cavitation mainly applies to vane pumps, and it is a phenomenon that must be avoided in the selection and design of pumps. Cavitation occurs in relation to the performance of the pump, as well as the design of the pump’s suction mechanism. NPSH refers to the difference between the total head of the liquid at the pump inlet and the pressure head at the point where the liquid vaporizes; it is expressed in meters (of water column) and denoted as NPSH (Net Positive Suction Head). Among the performance parameters of vane pumps, manufacturers generally provide the allowable net positive suction head to assist designers during selection and design processes. The parameters related to net positive suction head are as follows: NPSHc – critical net positive suction head, which is determined through cavitation tests ； —The allowable NPSH is the value of NPSH that ensures the pump does not experience cavitation; it is usually taken as (1.1~1.5)NPSHc or NPSHc+K (with K generally set at 0.3). The net positive suction head mainly affects the installation conditions; the lower the allowable net positive suction head, the greater the negative pressure that can be tolerated in front of the pump, and the better the performance of the water pump. 3. Pump selection process 3.1 Determining operating conditions When selecting a pump, it is first necessary to identify the basic operating conditions, including: a. The physicochemical properties of the medium to be transported, as these affect the pump’s performance, materials, and design. These include: medium properties (such as corrosivity, abrasiveness, toxicity, etc.), solid particle content and particle size, density, viscosity, vaporization pressure, etc. b. Selection parameters: flow rate, head, temperature, net positive suction head available of the unit, and operating condition. c. Site conditions such as the installation location of the pump, ambient temperature, relative humidity, atmospheric pressure, degree of atmospheric corrosion, and hazard zone classification. 3.2 Selection of pump type: In water treatment, the appropriate type of pump must be chosen depending on the application. b. Intermediate lifting and tailwater discharge: During intermediate lifting and tailwater discharge, there are very few impurities in the wastewater, especially those that tend to entangle; as a result, there is a wider range of options when selecting a pump, and basically any conventional pump used for transporting sewage can be utilized. When the quality of the tailwater is good, clean water pumps can be considered for use in discharge and recirculation purposes; these pumps generally have higher efficiency than sewage pumps and are also less expensive. c. When self-priming is required, self-priming pumps or rotor pumps can be used. A vacuum pump can be provided as needed. d. Sludge transfer – Primary sedimentation sludge: It has a high concentration of suspended solids and many impurities; screw pumps are generally used, with a crusher provided when necessary. Sludge from the secondary sedimentation tank and sludge recirculation: Sludge has a high water content and its properties are similar to those of water; submersible pumps, mixed-flow pumps, or axial-flow pumps are generally used. For internal recirculation, where a low head is required, wall-penetrating pumps can be employed. Chemical sludge: Depending on the characteristics of the sludge, non-clogging sewage pumps or screw pumps can be selected. Concentrated sludge: A screw pump is generally chosen, but a diaphragm pump can also be used. Dewatered sludge: High head is required for pipeline transportation of dewatered sludge, and a plunger pump can be used. e. For the addition of chemicals at low flow rates, metering pumps are generally used; for the addition of PAM, screw pumps can be employed. For the addition of large volumes of chemicals, corrosion-resistant pumps can be used. 3.3 Determining the pump model: After the type is determined, select an appropriate model based on the pump’s performance curve: a. Consult the series specification chart to ensure that the pump’s rated operating point falls within its efficient operating range, and verify that the pump’s net positive suction head meets the required standards. b. Verify whether the pump’s material, seals, etc., can meet the requirements of the medium to be transported. c. Verify whether the installation and maintenance conditions meet the requirements of the unit design. When two or more specifications meet the requirements, select the equipment with the highest overall score: efficiency (high), weight (low), and price (low).