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Valve selection procedures and basis

2021-10-08
In a fluid piping system, the valve is a control element whose main function is to isolate equipment and piping systems, regulate flow, prevent backflow, regulate and drain pressure. Because it is important to select the most suitable valve for the piping system, understanding the characteristics of the valve and the steps and the basis for selecting the valve also become crucial.
The valve industry has so far been able to produce a full range of gate, globe, throttle, plug, ball, electric, diaphragm, check, safety, pressure, steam, and emergency shut-off valves. There are 12 categories, more than 3,000 models and more than 4000 specifications of valve products; the maximum working pressure is 600MPa, the maximum nominal diameter is 5350mm, the maximum working temperature is 1200°C, the minimum operating temperature is -196°C, and the applicable medium is water , steam, oil, natural gas, highly corrosive media (such as concentrated nitric acid, medium-concentration sulfuric acid, etc.), flammable media (such as stupid, ethylene, etc.), toxic media (such as hydrogen sulfide), explosive media, and radioactive media ( Sodium metal, - circuit pure water, etc.).
Pressure-bearing parts for valves Cast copper, cast iron, ductile iron, high-silicon cast iron, cast steel, forged steel, high and low alloy steel, stainless acid-resistant steel, Hastelloy, Inconel, Monel, duplex stainless steel , titanium alloys, etc. And can produce a variety of electric, pneumatic, hydraulic valve drive. Faced with so many valve types and such a variety of working conditions, we must choose the most suitable valve system for the installation of the valve product, I think, first of all should understand the characteristics of the valve; Second, we should master the steps and basis for the choice of valve; Follow the principle of selecting the valve.
1. There are two general characteristics of the valve, use characteristics and structural characteristics.
Usage characteristics: It determines the main performance of the valve and the use of the valve, belonging to the valve use characteristics are: the type of valve (closed-circuit valve, regulating valve, safety valve, etc.); product type (gate valve, globe valve, butterfly valve, ball valve, etc.) Valve main parts (valve body, valve cover, stem, valve disc, sealing surface) materials; valve transmission methods. Structural characteristics: It determines some of the structural characteristics of the valve installation, maintenance, maintenance and other methods, belonging to the structural characteristics: the valve structure length and overall height, and the connection of the pipeline (flange connection, threaded connection, clamp connection , male thread connection, welding end connection, etc.); the form of the sealing surface (inset ring, thread ring, build-up welding, spray welding, valve body); valve stem structure (rotating rod, lifting rod) and so on.
2. The steps and basis for selecting the valve are as follows:
(1) Selection step
Identify the use of the valve in the device or device to determine the working conditions of the valve: the applicable media, working pressure, operating temperature, and so on.
Determine the nominal path and connection method for connecting the valve to the valve: flange, thread, welding, etc.
Determine the way to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage.
According to the pipeline conveying media, working pressure, working temperature to determine the selected valve housing and internal parts of the material: gray cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloys. 5 Select the type of valve: closed-circuit valve, regulating valve, safety valve and so on.
Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap, etc.
Determine the valve parameters: For automatic valves, according to different needs to determine the flow resistance, discharge capacity, back pressure, etc., and then determine the nominal diameter of the pipe and the diameter of the valve seat hole.
Determine the geometric parameters of the selected valve: the length of the structure, the form and size of the flange connection, the dimensions of the valve height direction after opening and closing, the size and number of bolt holes connected, and the overall valve size.
Use existing data: Valve product catalogs, valve product samples, etc. Select the appropriate valve product.
(2) The basis for selecting the valve While understanding the steps for selecting the valve, it is also necessary to further understand the basis for selecting the valve.
The use of the selected valve, operating conditions and operating control methods.
The nature of the working medium: working pressure, working temperature, corrosion performance, whether it contains solid particles, whether the medium is toxic, whether it is flammable, explosive medium, viscosity of the medium and so on.
Requirements for valve fluid properties: flow resistance, discharge capacity, flow characteristics, seal rating, and more.
Mounting dimensions and dimensions: Nominal diameter, connection to the pipe and connection dimensions, dimensions or weight restrictions. 5 Additional requirements for the reliability, service life, and explosion-proof performance of electrical devices. (At the time of parameter selection, it should be noted that if the valve is to be used for control purposes, the following additional parameters must be determined: operating method, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop at closing, maximum and minimum inlet of the valve Pressure.) According to the above selection of valves, the rationale and correct choice of valve must be detailed understanding of the internal structure of various types of valves in order to be able to make the right choice for the preferred valve. The final control of the pipeline is the valve. Valve opening and closing parts control the flow pattern of the medium in the pipeline. The shape of the valve flow path allows the valve to have a certain flow characteristic. This must be taken into consideration when selecting the piping system that is most suitable for installing the valve.
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