Basic knowledge of pumps

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I. What is a pump? A pump is a machine that transports liquids or increases their pressure. It transfers the mechanical energy of the prime mover or other external energy to the liquid, thereby increasing the energy of the liquid. 　　Pumps are mainly used to transport liquids such as water, oil, acids, alkalis, emulsions, suspensions, and liquid metals; they can also handle mixtures of liquids and gases, as well as liquids containing suspended solids. 　　Pumps can generally be classified into three categories based on their working principle: positive displacement pumps, dynamic pumps, and other types of pumps. In addition to classification by working principle, it can also be classified and named using other methods. For example, based on the driving method, they can be divided into electric pumps and hydraulic pumps, etc.; based on the structure, they can be divided into single-stage pumps and multi-stage pumps; based on their application, they can be divided into boiler feed pumps and metering pumps, etc.; based on the properties of the liquid being transported, they can be divided into water pumps, oil pumps, and slurry pumps, etc. 　　There is a certain interdependent relationship among the various performance parameters of a pump, which can be represented by curves; these are known as the pump’s characteristic curves. Each pump has its own specific characteristic curve. II. Definition and Historical Origin of Pumps: Machines used to transport liquids or increase their pressure. In a broad sense, a pump is a machine that transports fluids or increases their pressure, including certain machines used for transporting gases. The pump transfers the mechanical energy of the prime mover or the energy from other sources to the liquid, thereby increasing the energy of the liquid. 　　The lifting of water is very important for human life and production. In ancient times, various water-lifting devices existed, such as the chain pump in Egypt (17th century BC), the shadoofel in China (17th century BC), the windlass (11th century BC), the water wheel (1st century AD), as well as the screw invented by Archimedes in ancient Greece in the 3rd century BC. Around 200 BC, the ancient Greek artisan Ctesibius invented the earliest piston pump – a fire pump. Records of 4-blade vane pumps date back to as early as 1588, and various other rotary pumps have since been developed. In 1689, D. Papin of France invented the volute centrifugal pump with a 4-blade impeller. In 1818, centrifugal pumps with radially straight blades, semi-open double-suction impellers, and volutes appeared in the United States. Between 1840 and 1850, H.R. Worthington in the United States invented the piston pump with steam acting directly on it, where the pump cylinder and the steam cylinder were opposite to each other, marking the emergence of the modern piston pump. Between 1851 and 1875, multi-stage centrifugal pumps equipped with guide vanes were successively invented, making it possible to develop high-head centrifugal pumps. Subsequently, various pumps were developed one after another. With the application of various advanced technologies, the efficiency of pumps has been gradually improving, and their performance range as well as applications have also expanded. http://www.junmingmeco.com/d/file/p/2023/08-29/754d238aa4dd0aa6819317dba8a4e568.png III. Classification criteria for pumps (I) Working principle 1) The working principles can be further divided into blade-type, positive-displacement type, and other types. 　　①Vane pumps rely on the dynamic action of a rotating impeller on the liquid to continuously transfer energy to it, thereby increasing the liquid’s kinetic energy (primarily) and pressure energy. Subsequently, the kinetic energy is converted into pressure energy in the discharge chamber. Vane pumps can be further classified into centrifugal pumps, axial flow pumps, partial flow pumps, and vortex pumps, among others. 　　②Positive displacement pumps transfer energy to a liquid periodically by means of the periodic changes in the volume of the sealed working space that contains the liquid, thereby increasing the pressure of the liquid enough to force it out. Depending on the type of movement of their working elements, they can be divided into reciprocating pumps and rotary pumps. 　　③Other types of pumps transfer energy in other forms. For example, a jet pump draws in the fluid to be transported into the pump through high-speed ejection of the working fluid, where mixing occurs and momentum exchange takes place to transfer energy; a water hammer pump transfers energy by lifting a portion of the fluid in flow to a certain height during braking; an electromagnetic pump achieves transportation by causing the electrically conductive liquid metal to flow under the action of electromagnetic forces. Additionally, pumps can also be classified according to the properties of the liquid they transport, the driving method, structure, and application. 　　2) Classification by the number of working impellers ① Single-stage pump: that is, there is only one impeller on the pump shaft. 　　② Multi-stage pump: This refers to a pump that has two or more impellers on its shaft; in such a case, the total head of the pump is the sum of the heads generated by each of the impellers. 　　3) Classification by operating pressure: ① Low-pressure pumps: pressure below 100 meters of water column; ② Medium-pressure pumps: pressure between 100 and 650 meters of water column; ③ High-pressure pumps: pressure above 650 meters of water column. (Multi-stage centrifugal pumps can reach 2800 m) 4) Classification by the way water enters the impeller: ① Single-sided inlet pump: also known as a single-suction pump, meaning it has only one inlet on the impeller; ② Double-sided inlet pump: also known as a double-suction pump, meaning it has inlets on both sides of the impeller. Its flow rate is twice that of a single-suction pump; it can be considered as two single-suction pump impellers placed back to back. 　　5) Classification by the type of joint in the pump casing: ① Horizontal split-case pump: That is, a joint is provided on the horizontal plane passing through the axis line. (The most common type of horizontal split-case pump is a double-suction pump.) ② Vertical split-case pump: that is, the split surface is perpendicular to the axis line. 　　6) Classification by pump shaft position ① Horizontal pump: The pump shaft is in a horizontal position. 　　② Vertical pump: The pump shaft is in a vertical position. 　　7) Classification by the way in which the water exiting the impeller is directed to the discharge chamber ① Volute pump: After exiting the impeller, the water enters the pump casing, which has a spiral shape. 　　② Vane pump: After exiting the impeller, the water enters the vanes located outside it, and then proceeds to the next stage or flows into the outlet pipe. (Frequently used in multi-stage pumps and axial flow pumps) II. Operating principle: An impeller composed of several curved blades is placed inside a pump casing that features a volute channel. The impeller is fixed to the pump shaft, which is connected to the motor and can be rotated by the motor. The suction inlet is located at the center of the pump casing and connected to the suction pipeline, with a check valve installed at the bottom of the suction pipe. The side of the pump casing serves as the discharge port, which is connected to the discharge pipeline and equipped with a control valve. 　　Centrifugal pumps are able to transport liquids primarily due to the centrifugal force generated by the rapidly rotating impeller, which is why they are called centrifugal pumps. 　　Operation process of a centrifugal pump: Before starting the pump, fill it with the liquid that is to be transported. 　　After the pump is started, the pump shaft drives the impeller to rotate at high speed, generating centrifugal force. Under this action, the liquid is thrown from the center of the impeller toward its periphery, the pressure increases, and it flows into the pump casing at high speed. Within the pump casing, as the flow channel continues to expand, the flow velocity of the liquid decreases, causing most of its kinetic energy to be converted into pressure energy. Finally, the liquid flows into the discharge pipe from the outlet at a high static pressure. After the liquid inside the pump is expelled, a vacuum is created at the center of the impeller. Due to the pressure difference between the surface pressure (atmospheric pressure) and the pressure inside the pump (negative pressure), liquid enters the pump through the suction pipeline, filling the space left by the liquid that has been removed. 　　When a centrifugal pump is started, if there is air inside the pump casing, the density of air being much lower than that of the liquid results in a very low centrifugal force generated by the rotating impeller. The low pressure created at the center of the impeller is not sufficient to create the vacuum level required to draw in liquid; as a result, the centrifugal pump cannot operate. To ensure the pump is filled with liquid before startup, a check valve is installed at the bottom of the suction pipe. In addition, a control valve is also installed on the outlet pipeline of the centrifugal pump, used for starting and stopping the pump as well as regulating flow rate.