Pump working principle

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Pumps are the most widely used process equipment. Depending on their working principles and structures, pumps include centrifugal pumps, high-speed pumps, reciprocating metering pumps, magnetic drive pumps, liquid ring vacuum pumps, gear pumps, and others. In addition to gear pumps, the 530,000 tons/year PTA plant is equipped with a total of 102 pumps of various types, corresponding to 56 tag numbers [including 2 PX transfer pumps (1 tag number) and 3 acetic acid transfer pumps (2 tag numbers)]. 1. Centrifugal pump: Its working principle involves using rotating impeller blades to force the liquid that enters the pump to rotate along with the blades. Under the effect of centrifugal force, the liquid is pushed along the blade channels toward the outlet of the impeller, gaining pressure energy and kinetic energy. It then continues to flow into the diffuser and the expanding passages of the pump casing; as the liquid passes through these expanding channels, its flow velocity gradually decreases while its pressure increases. The structure of a conventional cantilever centrifugal pump is shown in Figure 3. 2. High-speed pump: Its basic working principle is similar to that of a conventional centrifugal pump; the difference lies in the use of a speed increase gearbox (either single-stage or double-stage) to enable the working impeller to reach rotational speeds several times higher than those of a conventional centrifugal pump impeller (usually between 6000 and 17300 RPM), thereby achieving very high discharge pressures. Depending on the properties and parameters of the medium being transported, high-speed pumps come in vertical and horizontal types. 3. Reciprocating metering pump: A reciprocating metering pump usually consists of two basic components. One is the hydraulic end, whose function is to convert mechanical energy into pressure energy and to directly deliver the process fluid. The other part is the power end, whose function is to convert the rotational motion of the motor into the reciprocating motion of the plunger (and diaphragm) in the hydraulic end, and the stroke size of this reciprocating motion is controlled by a stroke adjustment mechanism located on the power end. The working principle of a reciprocating metering pump is similar to that of a reciprocating compressor, but since the medium being transported is a liquid, its working cycle consists only of two stages: suction and discharge. 4. Magnetic pump: A magnetic pump applies the working principle of a permanent magnet coupling to centrifugal pumps, featuring full sealing, no leakage, and corrosion resistance. By using static seals in place of dynamic seals, the flow-through components of the pump are kept in a completely sealed state, thereby eliminating the problems of leakage that are inherent in the mechanical seals of other pumps. 5. Liquid ring vacuum pump: Liquid ring pumps are primarily used for creating a vacuum and transporting gaseous media. Their working principle is shown in Figure 1: the impeller is mounted eccentrically within the cylinder, and a certain amount of liquid is introduced into the cylinder. When the working wheel rotates and reaches a certain speed, due to centrifugal force, the fluid is thrown outward, forming a liquid ring that adheres to the inner surface of the cylinder. The inner surface of the upper liquid ring is tangent to the hub of the working wheel; a crescent-shaped space is formed between the working wheel and the liquid ring, and this space is divided into several small chambers of varying volumes (elementary volumes) by the blades of the working wheel. As the working wheel rotates, the elemental volume of the right half-circle gradually increases, while that of the left half-circle gradually decreases. Accordingly, sickle-shaped intake and exhaust ports are provided on the end caps on both sides of the cylinder block. The large sickle-shaped hole on the right is the intake port, while the small sickle-shaped hole beside it is the exhaust port. In this way, as the working wheel completes one rotation, each elementary volume expands and contracts once, and connects with the intake and exhaust ports once each, thereby enabling the processes of intake, compression, exhaust, and possibly expansion. Therefore, a liquid ring pump is essentially a positive-displacement compressor. As the gas is discharged, a portion of the liquid is also carried away and reduced in volume; therefore, it is necessary to supply a certain amount of liquid at the inlet to maintain a constant volume of the liquid ring. And thereby carries away heat, serving a cooling function. When a liquid ring pump is in operation, the blades stir the liquid, resulting in significant energy losses known as hydraulic losses; these lost energies are almost equal to the work required to compress the gas. Therefore, the efficiency of liquid ring pumps is very low; their isothermal efficiency ranges from 0.30 to 0.45, while that of larger machines can reach 0.48 to 0.62. Since ordinary vacuum pumps consume less power, liquid ring pumps are often used as vacuum pumps. To avoid excessive hydraulic losses, the maximum circumferential velocity at the outer edge of the working wheel is generally limited to between 14 and 16 m/s, and liquids with lower viscosity are preferred as much as possible. To further reduce hydraulic losses, a structure with a rotatable shell can also be employed. Figure 1 shows the working principle diagram of the liquid ring vacuum pump.