Development and principle of universal equipment valves
Valves are devices used to control fluid flow, pressure, and flow direction. The fluid to be controlled may be a liquid, a gas, a gas-liquid mixture or a solid-liquid mixture.
The valve usually consists of a valve body, a valve cover, a valve seat, an opening and closing member, a drive mechanism, a seal, and a fastener. The control function of the valve is achieved by the drive mechanism or fluid driving the opening and closing member to lift, slide, swing or swivel to change the size of the flow passage area.
Valves are widely used, and they are closely related to people's daily lives. For example, taps for water pipes and pressure reducing valves for LPG stoves are valves. Valves are also an integral part of various mechanical equipment such as internal combustion engines, steam engines, compressors, pumps, pneumatic transmissions, hydraulic transmissions, vehicles, ships and aircraft.
Two thousand years before BC, the Chinese used bamboo pipes and cork valves on the water pipelines, and later used sluice gates on irrigation channels, plate check valves on smelting bellows, and bamboo in well salt mining. Tube and plate check valves extract brine.
With the development of smelting technology and hydraulic machinery, copper and lead plug valves have appeared in Europe. With the use of the boiler, a lever heavy duty safety valve appeared in 1681. Before the 1769 Watt steam engine appeared, plug valves and check valves have been the main valves.
The invention of the steam engine made the valve enter the field of the mechanical industry. In addition to the use of plug valves, safety valves and check valves, the watt steam engine also uses a butterfly valve to regulate the flow. As the steam flow and pressure increase, the use of a plug valve to control the steam inlet and exhaust of the steam engine is no longer sufficient, so a slide valve appears.
Around 1840, there were successive shut-off valves with threaded stems and wedge-type gate valves with trapezoidal threaded stems, a major breakthrough in valve development. The emergence of these two types of valves not only met the requirements of various industries for pressure and temperature improvement 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 been rapidly developed.
After the Second World War, due to the development of polymeric materials, lubricating materials, stainless steel and cobalt-based cemented carbide, ancient plug valves and butterfly valves have gained new applications, and ball valves and diaphragm valves have been rapidly developed. The variety of globe valves, gate valves and other valves has increased and the quality has improved. The valve manufacturing industry has gradually become an important part of the machinery industry.
According to the function of use, the valve can be divided into six types: 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 the shut-off valve, the gate valve, the plug valve, the ball valve, the butterfly valve diaphragm valve, the pinch valve, etc.; the regulating valve is mainly used to adjust the pressure and flow of the fluid, including the regulating valve, the throttle valve, Pressure reducing valves and float regulating valves; check valves are used to prevent reverse flow of fluids; diverting valves are used to distribute fluid passages, or to separate two-phase fluids, including spools, multi-way valves, traps, and rows Air valve, etc.; safety valve is mainly used for safety protection to prevent boiler, pressure vessel or pipeline from being damaged due to overpressure; multi-purpose valve is a valve with more than one function, such as the stop check valve can both start and stop and start and stop Back to action.
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 the nominal pressure; valves can be divided into normal temperature valves, medium temperature valves, high temperature valves and low temperature valves according to working temperature; It is classified according to the type of the drive device, the connection with the pipe, and the materials used for the valve body. The valves may be named individually or in combination according to various classification methods, or may be named according to the structural features or specific uses of the opening and closing members.
The basic parameters of the valve are working pressure, operating temperature and caliber. For a wide variety of valves used in industrial pipelines, nominal pressure and nominal diameter are commonly used as basic parameters. Nominal pressure is the maximum working pressure that a valve of a material is allowed to withstand at a specified temperature. The nominal diameter refers to the nominal inner diameter of the end of the valve body to the pipe.
Valves have different requirements according to their types and uses, mainly including sealing, strength, regulation, circulation, opening and closing. When designing and selecting valves, in addition to considering basic parameters and performance, consider the performance of the fluid, including the phase of the fluid (gas, liquid or solid particles), corrosivity, viscosity, toxicity, flammability and explosiveness. , precious rarity and radioactivity.
Sealing performance and strength performance are the most basic and important performance of all valves. The seal of the valve is divided into two parts: inner seal and outer seal. The inner seal is a seal between the valve flap and the valve seat; the outer seal is a seal between the valve stem moving portion and the valve cover, between the valve body and the valve cover, and between the valve body and the pipe coupling portion. Valves require not only good sealing performance but also safety.
If the leak occurs due to poor sealing or the parts are damaged due to insufficient strength, it will cause different degrees of economic loss, 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 the reasonable design of the structure and the quality of the process must be observed in accordance with the relevant standards, the material must be selected correctly.
Generally, valves for low pressure non-corrosive fluids use cast iron or cast copper; cast steel or forged steel for high and medium pressure valves; alloy steel for high temperature or high pressure valves; stainless steel, plastic for corrosion resistant valves, corrosion resistant Alloys (such as copper-nickel-molybdenum alloys, titanium alloys, lead alloys, etc.) or made of cast iron, cast steel lining corrosion-resistant materials.
Usually, the sealing surface of the low pressure valve is mostly made of brass or bronze, and the high and medium pressure valves are mostly made of stainless steel, and high-grade, medium-pressure valves or high-temperature valves are required to use cobalt-based cemented carbide. Polymer materials have been widely used in valves. For example, the valve seat of the ball valve is mainly made of PTFE 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 than metals in the temperature range that can be used.
With the development of the modern nuclear industry, petrochemical industry, electronics industry and aerospace industry, as well as the development of automatic process control and long-distance fluid transport, modern cryogenic valves, vacuum valves, valves for the nuclear industry and various regulating valves have been promoted. development of. There are more and more applications for valve drives for remote control and program control.
Future valve development will expand product parameters, develop energy-saving, labor-saving and self-control valves, improve structure, adopt new materials and new processes, improve valve life, and develop special valve series, such as for liquid oxygen, Cryogenic valves such as liquid hydrogen and liquefied natural gas, vacuum valves, valves for nuclear industry, safety valves, regulating valves, traps and valve actuators.