Basic knowledge of pumps

1. Definition of a pump
A pump is a machine that transports liquid or pressurizes liquid. It transfers the mechanical energy of the prime mover or other external energy to the liquid, increasing the energy of the liquid.

2. Main uses of pumps
Pumps are mainly used to transport liquids including water, oil, acid and alkali liquids, emulsions, suspensions and liquid metals, etc. They can also transport liquids, gas mixtures and liquids containing suspended solids.

3. Brief history of the development of pumps
Water lifting is very important for human life and production. Various water-lifting devices have existed in ancient times, such as the chain pump in Egypt (17th century BC), the Chinese tangerine (17th century BC), the windlass (11th century BC) and the waterwheel (1st century AD). More famous is the screw invented by Archimedes in the third century BC, which can smoothly and continuously lift water to a
height of several meters. Its principle is still used by modern screw pumps.
Around 200 BC, the fire-extinguishing pump invented by the ancient Greek craftsman Ctesibius was a primitive piston pump that already had the main components of a typical piston pump, but the piston pump only developed rapidly after the emergence of the steam engine.
From 1840 to 1850, Worthington of the United States invented a piston pump with a pump cylinder and a steam cylinder facing each other and steam acting directly, marking the formation of modern piston pumps. The 19th century was the climax of the development of piston pumps, and they were used in various machines such as hydraulic presses. However, with the sharp increase in water demand, since the 1920s, low-speed piston pumps with greatly limited flow have been gradually replaced by high-speed centrifugal pumps and rotary pumps. However, reciprocating pumps still occupy a dominant position in the field of high pressure and small flow, especially diaphragm pumps and plunger pumps have unique advantages and are increasingly used.
The emergence of rotary pumps is related to the increasingly diversified requirements for liquid transportation in industry. As early as 1588, there were records of four-blade sliding vane pumps, and various other rotary pumps appeared one after another. However, until the 19th century, rotary pumps still had shortcomings such as large leakage, large wear and low efficiency. At the beginning of the 20th century, people solved the problems of rotor lubrication and sealing, and adopted high-speed motor drive, and rotary pumps suitable for high pressure, small and medium flow and various viscous liquids were rapidly developed. The types of rotary pumps and the types of liquids suitable for conveying are more numerous than those of other types of pumps.
The idea of ​​using centrifugal force to convey water first appeared in the sketches made by Leonardo da Vinci. In 1689, French physicist Papin invented the volute centrifugal pump with a four-blade impeller. But what is closer to the modern centrifugal pump is the so-called Massachusetts pump with radial straight blades, semi-open double-suction impeller and volute that appeared in the United States in 1818. From 1851 to 1875, multi-stage centrifugal pumps with guide vanes were invented one after another, making it possible to develop high-lift centrifugal pumps.
Although as early as 1754, Swiss mathematician Euler proposed the basic equations of impeller hydraulic machinery, laying the theoretical foundation for the design of centrifugal pumps, it was not until the end of the 19th century that the invention of high-speed electric motors made centrifugal pumps have an ideal power source, and its superiority was fully utilized. Based on the theoretical research and practice of many scholars such as Renault in the UK and Pfleiderer in Germany, the efficiency of centrifugal pumps has been greatly improved, and its performance range and application field have also been expanding. It has become the most widely used and largest-output pump in modern times.
4. Classification of pumps
Pumps are usually divided into positive displacement pumps, dynamic pumps and other types of pumps according to their working principles, such as jet pumps, water hammer pumps, electromagnetic pumps, and gas lift pumps.
In addition to being classified by working principles, pumps can also be classified and named by other methods. For example, according to the driving method, they can be divided into electric pumps and water turbine pumps;
according to the structure, they can be divided into single-stage pumps and multi-stage pumps; according to the purpose, they can be divided into boiler feed pumps and metering pumps;
according to the nature of the transported liquid, they can be divided into water pumps, oil pumps and mud pumps.
5. Working principle of pumps
Positive displacement pumps rely on the working element to make reciprocating or rotary motion in the pump cylinder, so that the working volume is alternately increased and reduced to achieve the suction and discharge of liquid. The positive displacement pump with reciprocating motion of the working element is called a reciprocating pump, and the one with rotary motion is called a rotary pump. The suction and discharge processes of the former are carried out alternately in the same pump cylinder and are controlled by the suction valve and the discharge valve; the latter forces the liquid to transfer from the suction side to the discharge side through the rotation of working elements such as gears, screws, blade rotors or vanes.
The flow rate of a positive displacement pump at a certain speed or number of reciprocating times is constant and hardly changes with pressure; the flow rate and pressure of a reciprocating pump have large pulsations, and corresponding pulsation reduction measures need to be taken; a rotary pump generally has no pulsation or only small pulsations; it has self-priming ability, and the air in the pipeline can be removed and the liquid can be sucked in after the pump is started; the discharge pipeline valve must be fully opened when starting the pump; a reciprocating pump is suitable for high pressure and small flow; a rotary pump is suitable for small and medium flow and higher pressure; a reciprocating pump is suitable for conveying clean liquids or gas-liquid mixtures. In general, the efficiency of a positive displacement pump is higher than that of a dynamic pump. A dynamic pump relies on the force of a rapidly rotating impeller on the liquid to transfer mechanical energy to the liquid, increase its kinetic energy and pressure energy, and then convert most of the kinetic energy into pressure energy through the pump cylinder to achieve delivery. Power pumps are also called impeller pumps or vane pumps. Centrifugal pumps are the most common power pumps.
The head generated by a power pump at a certain speed has a limited value, and the head changes with the flow rate; it works stably, delivers continuously, and has no pulsation in flow and pressure; it generally has no self-priming ability, and the pump needs to be filled with liquid or the pipeline needs to be evacuated before it can start working; it has a wide range of applicable performance; it is suitable for conveying clean liquids with very low viscosity, and specially designed pumps can convey mud, sewage, etc. or water-transported solids. Power pumps are mainly used for water supply, drainage, irrigation, process liquid transportation, power station energy storage, hydraulic transmission and ship jet propulsion.
Other types of pumps refer to a type of pump that transfers energy in another way. For example, a jet pump relies on a high-speed jet of working fluid to suck the fluid to be transported into the pump, and transfers energy by exchanging momentum through the mixing of two fluids; a water hammer pump uses the energy generated when the flowing water is suddenly braked to raise the water pressure of part of it to a certain height; an electromagnetic pump causes the electrified liquid metal to flow under the action of electromagnetic force to achieve transportation; a gas lift pump sends compressed air or other compressed gas to the bottom layer of the liquid through a conduit to form a gas-liquid mixed fluid that is lighter than the liquid, and then uses the pressure of the liquid outside the tube to press the mixed fluid up.
6. Main performance parameters of the pump
The performance parameters of the pump mainly include flow rate and head, in addition to shaft power, speed and required cavitation margin.
Flow rate refers to the amount of liquid output through the pump outlet per unit time, generally using volume flow rate;
head is the energy increment of transporting liquid from the pump inlet to the outlet per unit weight. For a positive displacement pump, the energy increment is mainly reflected in the increase in pressure energy, so it is usually expressed in terms of pressure increment instead of head.
The efficiency of a pump is not an independent performance parameter, it can be calculated by formulas from other performance parameters such as flow rate, head and shaft power. Conversely, if the flow rate, head and efficiency are known, the shaft power can also be calculated.
There is a certain interdependent change relationship between the various performance parameters of the pump. The pump can be tested, the parameter values ​​can be measured and calculated respectively, and then plotted into curves to represent them. These curves are called the pump characteristic curves. Each pump has a specific characteristic curve provided by the pump manufacturer. Usually, the recommended performance section is also indicated on the characteristic curve given by the factory, which is called the working range of the pump.
The actual working point of the pump is determined by the intersection of the pump curve and the pump device characteristic curve. When selecting and using the pump, the operating point of the pump should fall within the working range to ensure economic and safe operation. In addition, when the same pump transports liquids with different viscosities, its characteristic curve will also change. Usually, the characteristic curves given by the pump manufacturer mostly refer to the characteristic curves when transporting clean cold water. For dynamic pumps, as the viscosity of the liquid increases, the head and efficiency decrease, and the shaft power increases. Therefore, in industry, liquids with high viscosity are sometimes heated to reduce the viscosity to improve the transportation efficiency.