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Development and Principles of Valves in General Equipment. Valves are devices used to control the flow rate, pressure, and direction of fluids. The fluid under control can be a liquid, a gas, a gas-liquid mixture, or a solid-liquid mixture. Valves usually consist of a valve body, valve cover, valve seat, operating element, actuation mechanism, seals, and fasteners. The control function of a valve is achieved by using a driving mechanism or fluid to cause the moving part to move up and down, slide, swing, or rotate, thereby changing the size of the flow passage. Valves have a wide range of uses and are closely related to people’s daily lives; for example, faucets used in water supply pipes and pressure regulators used in liquefied petroleum gas stoves are both types of valves. Valves are also essential components in various mechanical devices such as internal combustion engines, steam engines, compressors, pumps, pneumatic systems, hydraulic systems, vehicles, ships, and aircraft. Two thousand years before Christ, the Chinese already used bamboo tubes and wooden plug valves in water conveyance systems; later, they employed sluices in irrigation channels, plate check valves in bellows used for smelting, and bamboo tubes along with plate check valves to extract brine in salt mining. With the development of smelting technology and hydraulic machinery, copper and lead plug valves appeared in Europe. With the use of boilers, the lever-weight type safety valve appeared in 1681. Before Watt’s steam engine appeared in 1769, plug valves and check valves were the most common types of valves. The invention of the steam engine introduced valves into the mechanical industry. In Watt’s steam engine, in addition to plug valves, safety valves, and check valves, butterfly valves were also used to regulate flow. As the steam flow rate and pressure increased, using a stopcock to control the steam inlet and exhaust of the steam engine became insufficient, which led to the development of slide valves. Around 1840, globe valves with threaded valve stems, as well as wedge gate valves with trapezoidal-threaded valve stems, appeared one after another – representing a major breakthrough in the development of valves. The emergence of these two types of valves not only met the growing demands of various industries at that time for higher pressures and temperatures, but also initially satisfied the need for flow regulation. Thereafter, with the development of the electric power industry, petroleum industry, chemical industry, and shipbuilding industry, various high and medium pressure valves saw rapid growth. After World War II, thanks to the development of polymer materials, lubricants, stainless steel, and cobalt-based cemented carbides, the traditional plug valves and butterfly valves found new applications, while ball valves and diaphragm valves experienced rapid growth. The variety of stop valves, gate valves, and other types of valves has increased, and their quality has improved. The valve manufacturing industry has gradually become an important sector of the machinery industry. Valves can be classified into six categories based on their functional purposes: shut-off valves, control valves, check valves, distribution valves, safety valves, and multi-purpose valves. Cut-off valves are primarily used to shut off fluid flow paths, including globe valves, gate valves, plug valves, ball valves, butterfly valves, diaphragm valves, and clamp valves ； Control valves are primarily used to regulate parameters such as the pressure and flow rate of fluids, including control valves, throttle valves, pressure relief valves, and float control valves ； Check valves are used to prevent the reverse flow of fluids ； Shunt valves are used to direct the flow of fluids or to separate two-phase fluids, including slide valves, multi-way valves, check valves, and air release valves ； Safety valves are primarily used for safety protection, to prevent boilers, pressure vessels, or pipelines from being damaged due to overpressure ； A multi-functional valve is a valve that possesses more than one function; for example, a stop-check valve can both shut off flow and act as a check valve. Industrial pipeline valves can also be classified by nominal pressure into vacuum valves, low-pressure valves, medium-pressure valves, high-pressure valves, and ultra-high-pressure valves ； Valves can also be classified by operating temperature into normal-temperature valves, medium-temperature valves, high-temperature valves, and low-temperature valves ； Valves can also be classified by the type of actuator, the method of connection to the pipeline, and the material used for the valve body. Valves can be named individually or in combination using various classification methods, or they can also be named based on the structural characteristics of their operating elements or their specific applications. The basic parameters of a valve are operating pressure, operating temperature, and diameter. For the various valves widely used in industrial pipelines, the nominal pressure and nominal diameter are commonly used as basic parameters. Nominal pressure refers to the maximum operating pressure that a valve made of a certain material is allowed to withstand at a specified temperature. The nominal diameter refers to the nominal inner diameter at the connection end of the valve body and the pipe. Valves have different requirements depending on their type and purpose, with key performance characteristics including sealing, strength, regulation, flow capacity, and opening/closing functionality. When designing and selecting valves, in addition to considering basic parameters and performance, it is also necessary to take into account the properties of the fluid, including its phase state (gas, liquid, or containing solid particles), corrosivity, viscosity, toxicity, flammability and explosiveness, as well as its value, rarity, and radioactivity. Sealing performance and strength performance are the most fundamental and important properties of all valves. The sealing of a valve consists of internal sealing and external sealing. The internal seal is the seal between the valve disc and the valve seat ； The external seal refers to the seal between the moving parts of the valve stem and the valve cover, between the valve body and the valve cover, and between the valve body and the pipeline connection. When in use, valves not only need to have good sealing performance but also must ensure safety. If leakage occurs due to poor sealing or if the components fail because of insufficient strength, it will result in economic losses of varying degrees; moreover, when transporting toxic, flammable, explosive, or highly corrosive fluids, it can also lead to serious safety accidents. To ensure the sealing and strength of the valve, in addition to following relevant standard specifications for proper structural design and ensuring process quality, it is also necessary to select the appropriate materials. Typically, valves for low-pressure non-corrosive fluids are made of cast iron or cast copper ； Cast steel or forged steel for high and medium pressure valves ； Alloy steel is used for high-temperature or high-pressure valves ； Valves for corrosive fluids are made of stainless steel, plastics, corrosion-resistant alloys (such as copper-nickel-molybdenum alloys, titanium alloys, and lead alloys), or of cast iron or cast steel lined with corrosion-resistant materials. Generally, the sealing surfaces of low-pressure valves are made of brass or bronze, while those of high- and medium-pressure valves are mostly made of stainless steel. High- and medium-pressure valves with higher requirements, as well as high-temperature valves, use cobalt-based cemented carbides. Polymer materials are widely used in valves; for example, the seat of ball valves is mainly made of polytetrafluoroethylene plastic, while the sealing rings of butterfly valves and the diaphragms of diaphragm valves are made from various rubber materials. These materials offer better sealing properties than metals within the usable temperature range. With the development of the modern nuclear industry, petrochemical industry, electronics industry, and aerospace industry, as well as advancements in automated process control and long-distance fluid transport, the development of modern low-temperature valves, vacuum valves, valves for use in the nuclear industry, and various control valves has been promoted. Valve actuation devices used for remote control and program control are being used more and more often. The future development of valves will focus on expanding their performance parameters, developing energy-saving, labor-saving, and automatically controlled valves, improving their structure by using new materials and manufacturing processes to extend their service life, as well as creating specialized valve series. These include low-temperature valves for liquid oxygen, liquid hydrogen, and liquefied natural gas, vacuum valves, valves for use in the nuclear industry, safety valves, control valves, check valves, and valve actuation devices.