Valve is the control part of the fluid conveying system. It has the functions of diversion, closure, regulation, throttling, backflow prevention, diversion or overflow relief. Valve is an important control component of pipeline. Pipeline system selection of the most suitable valve is very important, related to the safety of the pipeline, flow loss, cost, etc., must be familiar with the characteristics of the valve and the selection of valve steps and basis.
At present, China's valve industry has been able to produce 12 types of gate valves, globe valves, check valves, ball valves, butterfly valves, throttle valves, cock valves, diaphragm valves, safety valves, regulating valves, traps and special valves; the highest working pressure is 600 MPa, the maximum nominal diameter is 5350 mm, the highest working temperature is 1200 C, the most. Low working temperature is - 196 C, suitable medium is water, steam, oil, natural gas, strong corrosive medium (such as concentrated nitric acid, medium concentration sulfuric acid, etc.), flammable medium (such as benzol, ethylene, etc.), toxic medium (such as hydrogen sulfide), explosive medium and radioactive medium (sodium metal, - pure water circuit, etc.). Valve bearing parts are made of cast copper, cast iron, ductile iron, high silicon cast iron, cast steel, forged steel, high and low alloy steel, stainless acid-resistant steel, Harrington alloy, Inconel alloy, Monel alloy, duplex stainless steel, titanium alloy, etc. And can support the production of a variety of hydraulic, pneumatic, electric valve drive devices. Faced with so many kinds of valves and so complex working conditions, to choose the most suitable valve products for installation in pipeline system, I think, first of all, we should understand the characteristics of the valve; secondly, we should grasp the steps and basis for selecting the valve; thirdly, we should follow the principle of selecting the valve.
I. Characteristics of valves, service and structural characteristics
Use characteristics determine the main performance and scope of use of the valve, belonging to the use characteristics of the valve: the type of valve (closed-circuit valves, regulating valves, safety valves, etc.); product types (gate valves, globe valves, butterfly valves, ball valves, etc.); the main parts of the valve (valve body, cover, stem, disc, sealing surface) materials; Valve transmission mode, etc.
Structural characteristics determine some structural characteristics of valve installation, maintenance and other methods. They are structural characteristics: the length and overall height of the valve, the connection form with pipeline (flange connection, thread connection, clamp connection, external thread connection, welding end connection, etc.), and the form of sealing surface (ring, screw, etc.). Ring, surfacing, spray welding, body body body; stem structure (rotating rod, lifting rod) and so on.
II. Steps for selecting valves
Define the use of the valve in the equipment or device, determine the working conditions of the valve: suitable medium, working pressure, working temperature, etc. determine the nominal diameter and connection mode of the pipe connecting with the valve: flange, thread, welding, etc. determine the mode of operation of the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electro-hydraulic linkage. According to the medium conveyed by pipeline, working pressure and working temperature, the materials of the shell and inner parts of the selected valve are determined: cast steel, carbon steel, stainless steel, alloy steel, stainless acid-resistant steel, grey iron, malleable iron, nodular iron, copper alloy, etc. Select the type of valves: closed-circuit valves, regulating valves, safety valves and so on to determine the type of valves: gate valves, globe valves, ball valves, butterfly valves, throttle valves, safety valves, pressure relief valves, steam traps and other parameters to determine the valve: for automatic valves, according to different needs, first determine the allowable flow resistance, discharge capacity, back pressure, etc. The nominal diameter of the pipeline and the diameter of the valve seat hole are determined. The geometric parameters of the selected valve are determined as follows: structure length, flange connection form and size, height direction of the valve after opening and closing, bolt hole size and number of connections, and the size of the whole valve shape. The existing information is used as follows: valve product catalogue, valve shape, etc. Selection of appropriate valve products such as door product samples III. Basis for selection of valves
The characteristics of the working medium of the selected valve, such as working pressure, working temperature, corrosion performance, whether solid particles are contained, whether the medium is toxic, whether it is flammable and explosive, and whether the medium is viscous, are the requirements for the fluid characteristics of the valve, such as flow resistance, discharge capacity and flow rate. Installation dimension and shape dimension requirements such as characteristics and seal grade: additional requirements for reliability of valve products, service life and explosion-proof performance of electric devices, such as nominal diameter, connection mode with pipeline and connection dimension, shape dimension or weight limitation, etc. (Attention should be paid when selecting parameters: if valve is to be used for control purposes) The following additional parameters must be determined: operation method, maximum and minimum flow requirements, pressure drop of normal flow, pressure drop at closure, maximum and minimum inlet pressure of valves. ) According to the above-mentioned basis and steps for selecting valves, when selecting valves reasonably and correctly, it is necessary to have a detailed understanding of the internal structure of various types of valves in order to make the right choice for the preferred valves.
The final control of the pipeline is the valve. Valve opening and closing parts control the flow direction of medium in the pipeline. The shape of the valve runner makes the valve have certain flow characteristics. This must be taken into account when selecting the most suitable valve for installation in the pipeline system.
Significance of Correct Selection of Valves
Correct selection of valves is very important to ensure the safe production of the device, improve the service life of the valve and meet the long-term operation of the device. Many valve accidents are mainly due to improper selection of valves, such as the use of cast iron valves in cold areas, because water in the oil accumulates in the body of valves, ice in winter.