Tuesday, August 10, 2010

Valves

Valve is used to control fluid flow, pressure and flow devices. Controlled fluid can be liquid, gas, gas-liquid mixture or a solid-liquid mixture.



Valve is usually the valve body, valve cover, valve seat, hoist parts, drive mechanism, seals and fasteners so on. The valve control function is to rely on drive mechanism or fluid driven hoist pieces of lifting, sliding, rotating or turning motion put to change the size of the flow channel area is achieved.



Valve uses a very broad, with people's daily lives are closely related to, for example, useful water taps, liquefied petroleum gas stove valves are valves used. Valve is also a variety of mechanical equipment such as internal combustion engines, steam engines, compressors, pumps, pneumatic actuator, hydraulic gear, vehicles, ships and aircraft an indispensable components.



BC two years ago, the Chinese people in the pipeline on the use of bamboo and cork valve, and later use in irrigation channels, sluices, in refining the use of bellows plate check valve, the use of bamboo in the well salt mining Extraction of salt water pipe and plate check valve.



With the smelting technology and the development of hydraulic machinery in Europe and the emergence of copper Lead plug valves. With the use of boilers, in 1681 has emerged Hammer lever-type safety valve. Watt steam engine in 1769 before the advent of plug valves and check valves have been the main valve.



The invention of the steam engine so that the valve into the machinery industries. Watt's steam engine in addition to using the plug valves, safety valves and check valves, but also used the butterfly valve to regulate flow. As the steam flow and pressure, the use of plug valves control the steam engine and the exhaust steam into the steam can not meet the needs, so there slide valve.



Before and after 1840, appeared in succession with a threaded stem of the cut-off valves, and with a trapezoidal wedge threads of the valve stem, which is the valve in the development of a major breakthrough. The emergence of these two types of valves, not only to meet a variety of industries at the time of the pressure, temperature rising demand, but also initially satisfied the requirements of the traffic conditioning requirements. Since then with the electric power industry, petroleum industry, chemical industry and shipbuilding industries, all kinds of high and medium pressure valves have developed rapidly.



After World War II, due to polymer materials, lubricating materials, stainless steel and cobalt-based alloy development, the old plug valves and butterfly valves to obtain a new application, ball valves and diaphragm valves have developed rapidly. Cut-off valves, gate valves, and other varieties of increasing quality. The valve manufacturing sector has gradually become an important sector of machine-building industry.



Valve-per-use features can be divided into cut off valve, regulating valve, check valve, diversion valve, safety valve, multi-purpose valve six categories. Stainless steel electrode



Cut off valve is mainly used for cut off fluid channels, including the cut-off valves, gate valves, plug valves, ball valve, butterfly valve diaphragm valves, control valves and other folders; regulating valve primarily for regulating the fluid pressure, flow, etc., including the regulating valve, section flow valve, reducing valve and the float valve, etc.; valve used to prevent reverse flow of fluid; divider valve for the distribution of fluid pathways whereabouts, or two-phase flow separation, including the slide valve, multi-pass valve, hydrophobic valves and emptying valves, etc.; safety valve is mainly used for security protection to prevent the boiler, pressure vessel or pipeline overpressure due to the damage; multi-purpose valve is a valve with more than one function, such as the cut-off valve from drying both role of the beginning and end can return to the role.



Industrial pipe valve according to nominal pressure can be divided into the vacuum valve, low pressure valves, medium pressure valve, high pressure valves, high pressure valve; valve according to the working temperature can be divided into normal temperature valves, medium temperature valve, high temperature valve, cryogenic valve; valve Doors can also be according to the type of drive, and pipeline connection methods and the use of body materials classified. Valve can be a variety of classifications to name individually or in combination, but also can open and close parts of the structural characteristics or for specific purposes named.



The basic parameters of the valve is working pressure, working temperature and caliber. For large-scale use in a variety of valves in industrial piping, common nominal pressure and nominal diameter as the basic parameters. PN refers to a material of the valve, in the specified temperature, allowing the maximum working pressure to bear. DN refers to the valve body and pipe end connections on behalf of diameter.



Valve according to their types and uses have different requirements, mainly seal, strength, conditioning, circulation, open and close properties. In the design and selection of valves, in addition to consider the basic parameters and performance, but also take into account the performance of fluid, including fluid-phase state (gas, liquid or solid particles containing), corrosion, viscosity, toxic, flammable, easy to explosive, and radioactive and other precious rarity.



Sealing performance and strength of performance is the most fundamental of all valves, the most important performance. Fall within the ambit of the sealing valve seal and the outer sealing two parts. Sealed inside is a valve between the valve and the valve seat seal; outer seal is a sports site and the stem between the valve cover, valve and the valve cover and the valve body and pipeline connection between the seal parts. Valve when in use requires not only a good sealing performance, and must ensure safety.



If the leakage due to seal well and strong enough Ershi parts or because of damage, will result in different levels of economic losses, such as the transportation of toxic, inflammable, explosive or strong corrosive fluids, can also lead to serious security incidents. In order to ensure the valve seal and intensity, in addition to the provisions of the relevant standards must comply with a reasonable structure design to ensure the process quality, but also must be properly selected materials.



Typically, low-pressure non-corrosive fluid valve used in the use of cast iron or bronze; high and medium pressure valve with cast steel or forged; high temperature or high pressure valve to use alloy steel; for the corrosive fluid valve of stainless steel, plastic, corrosion resistant alloys (eg copper-nickel-molybdenum alloys, titanium alloys and lead alloys, etc.) or cast iron, cast steel lined with corrosion resistant materials.



Typically, low-pressure valve sealing surface most of the use of brass or bronze, high, medium pressure valve most of the use of stainless steel, require a higher high, medium pressure valve or the use of cobalt-base high-temperature valve carbide. Polymer material in the valve has been widely used, such as the valve's seat used mainly PTFE plastic, butterfly and diaphragm valves of the diaphragm seal with a variety of rubber materials. These materials can be used in the temperature range with a better seal than metal.



With modern nuclear industry, petrochemical industry, electronic industry and aerospace industries, as well as process automation and long-distance fluid transport process of development and promote a modern cryogenic valves, vacuum valves, nuclear valves and a variety of industrial control valves开展. Used for remote control and process control valve drives the application of more and more.



The future development of the valve will extend the product parameters, the development of energy-saving, labor-saving, and automatic control valves, improving the structure, and use of new materials and new techniques to improve the life of the valve, and the development of special valves series leavened dough development, such as using in the liquid oxygen, liquid hydrogen and liquefied natural gas, cryogenic valves, vacuum valves, nuclear industrial valves, safety valves, regulating valves, steam trap and valve-driven devices. 

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