In fluid piping systems, valves are control elements whose primary function is to isolate equipment and piping, regulate flow, and prevent back pressure, regulation, and drain pressure. It is also important to know the characteristics of the valve and the procedure and basis for selecting the valve, since it is important to choose the most suitable valve for the piping system. The valve industry so far has been able to produce a full range of gate, globe, throttle, plug, ball, electric, diaphragm, check, safety, pressure, steam trap and emergency shut-off valves And other 12 categories, more than 3,000 models, more than 4,000 specifications of the valve products; maximum working pressure of 600MPa, the maximum nominal diameter of 5350mm, the maximum operating temperature of 1200 ℃, the minimum operating temperature of -196 ℃, the media for the water , Steam, oil, natural gas, corrosive media such as concentrated nitric acid, medium concentration sulfuric acid, flammable media (such as stupid and ethylene), toxic media (such as hydrogen sulfide), explosive media and radioactive media Metal sodium, - loop pure water, etc.). Valve pressure parts made of cast copper, cast iron, ductile iron, high silicon cast iron, cast steel, forged steel, high and low alloy steel, stainless steel, Hastelloy, Inconel, Monel, duplex stainless steel , Titanium alloy and so on. And can produce all kinds of electric, pneumatic, hydraulic valve drive. In the face of so many valve varieties and so complicated a variety of conditions, to choose the most suitable piping system installed valve products, I think, should first understand the characteristics of the valve; secondly, should grasp the choice of the valve steps and basis; Follow the principle of choosing a valve. 1. Valve characteristics generally have two kinds, the use of features and structural characteristics. Features: It determines the main use of the valve performance and scope of use, the use of the valve characteristics are: the type of valve (closed circuit valves, regulating valves, safety valves, etc.); product type (gate valve, globe valve, butterfly valve, ball valve, etc.) ; Valve main components (body, bonnet, valve stem, valve flap, sealing surface) of the material; valve transmission and so on. Structural characteristics: It determines the valve installation, repair, maintenance and other methods of some of the structural characteristics of the structural characteristics are: the length of the valve structure and the overall height of the connection with the pipeline (flange connection, screw connection, clamp connection , External thread connection, welding end connection, etc.); sealing surface form (set ring, thread ring, surfacing, spray welding, valve body); Stem structure (rotary rod, lifting rod) and so on. 2. Select the valve steps and based on the following general: ⑴ choice steps ①clear the valve in the device or device uses, to determine the working conditions of the valve: the medium, working pressure, temperature and so on. ② determine the connection with the valve nominal diameter and connection: flange, thread, welding and so on. ③ determine the way to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage. ④ According to pipeline transportation medium, working pressure, working temperature to determine the selected valve shell and inner parts of the material: gray iron, malleable iron, ductile iron, carbon steel, alloy steel, stainless steel, copper alloy. ⑤ Choose the type of valve: closed-circuit valves, regulating valves, safety valves and so on. ⑥ determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle, safety valve, pressure reducing valve, steam trap, and so on. ⑦ to determine the parameters of the valve: For automatic valves, according to the different needs of the first to allow flow resistance, emission capacity, back pressure, and then determine the nominal diameter of the pipe and the diameter of the valve seat hole. ⑧ determine the geometry of the selected valve parameters: the length of the structure, flange connection form and size, opening and closing the size of the valve height direction, the size and number of bolt holes connected to the overall dimensions of the valve. ⑨ use of existing information: valve product catalog, valve product samples, etc. choose the appropriate valve products. ⑵ based on the choice of valve In understanding the steps to choose the valve at the same time, but also to learn more about the basis for the selection of the valve. ①the use of the selected valve, the use of conditions and operating control mode. ② the nature of the working medium: working pressure, temperature, corrosion performance, whether it contains solid particles, the media is toxic, whether it is flammable, explosive media, medium viscosity and so on. ③ valve fluid characteristics of the request: flow resistance, discharge capacity, flow characteristics, sealing level and so on. ④ installation size and dimensions requirements: nominal diameter, and pipe connections and connection dimensions, dimensions or weight restrictions. ⑤ on the valve product reliability, service life and explosion-proof performance of electric devices and other additional requirements. (When selecting a parameter, note the following additional parameters if the valve is to be used for control purposes: method of operation, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop when closed, maximum and minimum inlet of valve Pressure.) Based on the basis and procedure for choosing a valve, the proper and correct selection of the valve must also be followed by a detailed understanding of the internal structure of the various types of valve so that the correct choice of the preferred valve can be made. The ultimate control of the pipeline is the valve. Valve opening and closing pieces control the flow of medium in the pipeline, the shape of the valve flow path to the valve with a certain flow characteristics in the choice of the most suitable piping system to install the valve must take into account this point. The following principles should be followed to select the valve: ⑴ cut-off and opening of the media used for the valve flow channel is straight-through valve, the flow resistance is small, usually selected as the cut-off and open media valve. Closed-down valve (globe valve, piston valve) because of its tortuous flow, flow resistance than other valves, it is less choice. In the case of allowing higher flow resistance, the choice of closed valve. ⑵ control flow with the valve usually choose to easily adjust the flow of the valve as a control flow. Closed-down valves, such as stop valves, are suitable for this purpose because their seat size is proportional to the travel of the closure. Rotary valves (plug valves, butterfly valves, ball valves) and flexible valve bodies (pinch valves, diaphragm valves) can also be used for throttling control, but usually only for a limited range of valve diameters. Gate valve is a disc-shaped gate of the circular valve seat to make cross-sectional movement, it only close to the closed position, in order to better control the flow, it is not usually used for flow control. ⑶ divert valve with diverting diversion according to the needs of this valve can have three or more channels. Plug valves and ball valves are more suitable for this purpose, therefore, most of the valves used for diverting shunts select one of these types of valves. However, in some cases, other types of valves, as long as the two or more valves properly connected to each other, but also for diverting diverting. ⑷ media valve with suspended particles When the medium with suspended particles, the most suitable for the use of its closure along the sealing surface sliding with a wiper valve. If the closure is vertical to the seat back and forth, the particles may be clamped, so this valve is only suitable for basic cleaning media unless the seal face material allows for embedment of particles. Ball valve and plug valve in the opening and closing process on the sealing surface are wiping effect, it is suitable for use in the medium with suspended particles. At present, no matter in pipeline system of petroleum, chemical industry or other industries, valve application, operation frequency and service are varied, the most important and most critical equipment is to control or eliminate even the slightest leakage. The ultimate control of the pipeline is the valve, the valve in all fields of service and reliable performance is unique.
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