Valves are devices used to control fluid flow, pressure and direction. The fluid to be controlled can be a liquid, a gas, a gas-liquid mixture, or a solid-liquid mixture. The valve is usually composed of valve body, valve cover, valve seat, opening and closing parts, driving mechanism, sealing parts and fasteners. The control function of the valve is realized by changing the size of the flow channel area by relying on the driving mechanism or fluid to drive the opening and closing parts to lift, slide, swing or rotate. Valves have a wide range of uses and are closely related to people's daily life. For example, taps for water pipes and pressure relief valves for LPG stoves are all valves. Valves are also indispensable components in various mechanical equipment such as internal combustion engines, steam engines, compressors, pumps, pneumatic transmissions, hydraulic transmissions, vehicles, ships and aircrafts. Before two thousand years BC, the Chinese used bamboo pipes and wooden plug valves on water pipelines, and later used sluices on irrigation channels, plate check valves on smelting bellows, and bamboo for well salt mining. Pipe and plate check valves for extracting brine. With the development of smelting technology and hydraulic machinery, copper and lead cock valves have appeared in Europe. With the use of boilers, a lever-weight safety valve appeared in 1681. Before the advent of the Watt steam engine in 1769, the stopcock and check valve had always been the most important valves. The invention of the steam engine made the valve enter the field of machinery industry. In addition to the use of plug valves, safety valves and check valves in Watt's steam engines, butterfly valves are also used to adjust the flow. With the increase of steam flow and pressure, the use of plug valves to control the steam intake and exhaust steam of the steam engine can no longer meet the needs, so slide valves appear.
Around 1840, globe valves with threaded stems and wedge gate valves with trapezoidal threaded stems appeared one after another. This was a major breakthrough in valve development. The appearance of these two types of valves not only met the requirements of various industries for increasing pressure and temperature at that time, but also initially met the requirements for flow regulation. Since then, with the development of the power industry, petroleum industry, chemical industry and shipbuilding industry, various high and medium pressure valves have developed rapidly.
After the Second World War, due to the development of polymer materials, lubricating materials, stainless steel and cobalt-based cemented carbide, ancient plug valves and butterfly valves have obtained new applications, and ball valves and diaphragm valves have been rapidly developed. The variety of stop valves, gate valves and other valves has been increased, and the quality has been improved. The valve manufacturing industry has gradually become an important sector of the machinery industry. Valves can be divided into six categories according to their functions: cut-off valve, regulating valve, check valve, diverter valve, safety valve, and multi-purpose valve.
The shut-off valve is mainly used to cut off the fluid passage, including shut-off valves, gate valves, plug valves, ball valves, butterfly valves, diaphragm valves, pinch valves, etc.; regulating valves are mainly used to regulate the pressure and flow of fluids, including regulating valves, throttle valves, Pressure reducing valve and floating ball regulating valve, etc.; check valve is used to prevent the reverse flow of fluid; diverter valve is used to distribute the path of fluid, or to separate two-phase fluid, including slide valve, multi-way valve, trap and drain Air valves, etc.; safety valves are mainly used for safety protection to prevent boilers, pressure vessels or pipelines from being damaged due to overpressure; multi-purpose valves are valves with more than one function, such as a stop check valve that can both stop and stop the flow Back to the role.
Industrial pipeline valves can be divided into vacuum valves, low pressure valves, medium pressure valves, high pressure valves, and ultra high pressure valves according to their nominal pressure; valves can be divided into normal temperature valves, medium temperature valves, high temperature valves, and low temperature valves according to their working temperature; the valves can also It is classified according to the type of driving device, the connection method with the pipeline, and the material used for the valve body. Valves can be named individually or in combination according to various classification methods, and can also be named according to the structural characteristics or specific purposes of the opening and closing parts.
The basic parameters of the valve are working pressure, working temperature and caliber. For a large number of various valves used in industrial pipelines, nominal pressure and nominal diameter are commonly used as basic parameters. Nominal pressure refers to the maximum working pressure that a valve of a certain material can withstand at a specified temperature. The nominal diameter refers to the nominal inner diameter of the connecting end of the valve body and the pipe.
Valves have different requirements according to their types and uses, mainly including sealing, strength, adjustment, circulation, opening and closing performance. When designing and selecting a valve, in addition to the basic parameters and performance, the performance of the fluid must also be considered, including the phase state of the fluid (gas, liquid or solid particles), corrosiveness, viscosity, toxicity, flammability and explosiveness , Preciousness, rarity, radioactivity, etc.
Sealing performance and strength performance are the most basic and important performance of all valves. The valve seal is divided into two parts: inner seal and outer seal. The inner seal is the seal between the valve disc and the valve seat; the outer seal is the seal between the valve stem and the valve cover, between the valve body and the valve cover, and between the valve body and the pipe connection. When the valve is in use, not only requires good sealing performance, but also must ensure safety.
If leakage occurs due to poor sealing or parts are damaged due to insufficient strength, it will cause varying degrees of economic losses, such as transporting toxic, flammable, explosive or highly corrosive fluids, and may also lead to serious safety accidents. In order to ensure the sealing and strength of the valve, in addition to complying with the relevant standards and regulations to carry out a reasonable structural design and ensure the quality of the workmanship, the materials must also be selected correctly.
Generally, valves for low-pressure non-corrosive fluids use cast iron or cast copper; high- and medium-pressure valves use cast steel or forged steel; high-temperature or high-pressure valves use alloy steel; valves for corrosive fluids use stainless steel, plastic, and corrosion-resistant Alloys (such as copper-nickel-molybdenum alloys, titanium alloys and lead alloys, etc.) or made of cast iron or cast steel with corrosion-resistant materials.
Generally, the sealing surfaces of low-pressure valves are mostly made of brass or bronze, high- and medium-pressure valves are mostly made of stainless steel, and cobalt-based hard alloys are used for high- and medium-pressure valves or high-temperature valves with higher requirements. Polymeric materials have been widely used in valves. For example, the seat of the ball valve is mainly made of polytetrafluoroethylene plastic, and the sealing ring of the butterfly valve and the diaphragm of the diaphragm valve are made of various rubber materials. These materials have better sealing properties than metals in the usable temperature range.
With the development of modern nuclear industry, petrochemical industry, electronics industry and aerospace industry, as well as the development of process automatic control and long-distance fluid transportation, it has promoted the development of modern cryogenic valves, vacuum valves, nuclear industry valves and various regulating valves. develop. There are more and more applications of valve actuation devices for remote control and program control.
In the future, the development of valves will expand product parameters, develop energy-saving, labor-saving and self-control valves, improve structures, adopt new materials and new processes, increase the service life of valves, and develop special valve series, such as liquid oxygen, Cryogenic valves, vacuum valves, valves for the nuclear industry, safety valves, regulating valves, traps and valve actuation devices for liquid hydrogen and liquefied natural gas, etc.