The steps and basis of valve selection before procurement
In a fluid piping system, a valve is a control element whose main role is to isolate equipment and piping systems, regulate flow, prevent backflow, and regulate and discharge pressure. Since it is very important to select the most suitable valve for the piping system, it is also important to understand the characteristics of the valve and the steps and basis for selecting the valve. So far, the valve industry has been able to produce a complete range of gate valves, globe valves, throttle valves, plug valves, ball valves, electric valves, diaphragm valves, check valves, safety valves, pressure reducing valves, steam traps and emergency shut-off valves and other 12 categories, more than 3,000 models, more than 4,000 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 working temperature is -196 °C, and the applicable medium is water, steam, oil, natural gas, strong corrosive media (such as concentrated nitric acid, medium concentration sulfuric acid, etc.), flammable media (such as benz, ethylene, etc.), toxic media (such as hydrogen sulfide), explosive media and radioactive media (metal sodium, -loop pure water, etc.). Valve pressure 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, Hastelloy, Inconel, Monel alloy, duplex stainless steel, titanium alloy, etc. And it is able to produce a variety of electric, pneumatic and hydraulic valve drives. In the face of so many valve varieties and such complex working conditions, to choose the most suitable valve products for the installation of the pipeline system, I think that the first thing to do is to understand the characteristics of the valve; Secondly, we should grasp the steps and basis for selecting valves; Furthermore, the principle of valve selection should be followed. 1
Characteristics of the valve (use characteristics and structural characteristics) use characteristics: it determines the main use performance and use range of the valve, and the characteristics of the valve are: the category of valves (closed circuit valves, regulating valves, safety valves, etc.); Product type (gate valve, globe valve, butterfly valve, ball valve, etc.); The material of the main parts of the valve (valve body, valve cover, valve stem, valve disc, sealing surface); valve transmission mode, etc. Structural characteristics: It determines some structural characteristics of the installation, repair, maintenance and other methods of the valve, which belong to the structural characteristics: the structural length and overall height of the valve, and the connection form with the pipeline (flange connection, threaded connection, clamp connection, external threaded connection, welded end connection, etc.); The form of sealing surface (inlay ring, threaded ring, surfacing welding, spray welding, valve body body); Stem structure form (rotating rod, lifting rod), etc. 2 Steps and basis for selecting valves (1) Select step 1.Clarify the purpose of the valve in the equipment or device, and determine the working conditions of the valve: applicable medium, working pressure, working temperature, etc. 2. Determine the nominal diameter and connection method of the pipeline connected to the valve: flange, thread, welding, etc. 3. Determine the way to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage, etc. 4. According to the medium, working pressure and working temperature of the pipeline, determine the materials of the shell and internal parts of the selected valve: gray cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloy, etc. 5. Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap, etc. 6. Determine the parameters of the valve: for automatic valves, first determine the allowable flow resistance, discharge capacity, back pressure, etc., and then determine the nominal diameter of the pipeline and the diameter of the valve seat hole. 7. Determine the geometric parameters of the selected valve: structural length, flange connection form and size, the size of the valve height direction after opening and closing, the size and number of bolt holes connected, the overall valve appearance, etc. 8. Use the existing information: valve product catalog, valve product sample, etc. to select appropriate valve products. (2) The basis for selecting valves: while understanding and mastering the steps of selecting valves, we should also further understand the basis for selecting valves. 1. The purpose of the selected valve, the working conditions and the operation control mode. 2. 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 and explosive medium, viscosity of the medium, etc. 3. Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing level, etc. 4. Installation size and overall size requirements: nominal diameter, andThe connection method of the pipe and the connection size, external dimensions or weight limits, etc. 5. Additional requirements for the reliability, service life and explosion-proof performance of the valve product. (When selecting parameters, it should be noted that if the valve is to be used for control purposes, the following additional parameters must be determined: method of operation, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop when closed, maximum and minimum inlet pressure for the valve.) According to the above-mentioned basis and steps for selecting valves, the internal structure of various types of valves must be detailed in detail when selecting valves reasonably and correctly, so as to make a correct choice for the preferred valves. The ultimate control of the pipeline is the valve. The valve opening and closing control the flow of the medium in the pipeline, and the shape of the valve flow channel gives the valve a certain flow characteristic, which must be taken into account when selecting the most suitable valve for installation in the piping system. 3
The following are the principles that should be followed in the selection of valves: (1) The valve flow channel for shut-off and open medium is a straight-through valve, and its flow resistance is small, and the valve for shut-off and open medium is usually selected. Downward closed valves (globe valves, piston valves) are rarely used because of their tortuous flow channels and higher flow resistance than other valves. Where high flow resistance is permitted, a closed valve can be used. (2) The valve for flow control is usually selected as the valve that is easy to adjust the flow rate as the flow control. Downward closing valves, such as globe valves, are suitable for this purpose because the size of the seat is proportional to the stroke of the shut-off. Rotary valves (plugs, butterfly, ball) and flexural body valves (clamp valves, diaphragm valves) can also be used for throttle control, but usually only within a limited range of valve diameters. The gate valve is a disc-shaped gate to the circular valve seat mouth to do a cross-cutting movement, it can only control the flow well when it is close to the closed position, so it is usually not used for flow control. (3) The valve for reversing the diversion can have three or more channels according to the needs of the reversing shunt. Plug and ball valves are more suitable for this purpose, so most valves used for reversing diverters are selected as one of these valves. However, in some cases, other types of valves can also be used for reversing diverts as long as two or more valves are properly connected to each other. (4) Valves for medium with suspended particlesWhen there are suspended particles in the medium, it is most suitable to use the valve with wiping effect along the sliding of the sealing surface of its closing parts. If the back and forth motion of the shut-off to the seat is vertical, it is possible to hold the particles, so the valve is only suitable for basic clean media unless the sealing face material allows for the particles to be embedded. Ball valves and plug valves have a wiping effect on the sealing surface during the opening and closing process, so they are suitable for use in media with suspended particles. At present, whether in the petroleum, chemical industry, or in the pipeline system of other industries, valve applications, operating frequencies and services are ever-changing, to control or eliminate even small leakage, the most important and critical equipment is the valve. The ultimate control of the pipeline is the valve, and the valve’s service and reliable performance in various areas is unique.
Valve inspection and maintenance after purchase
The valve is not repaired and maintained in a timely manner, resulting in valve disrepair leakage or ineffective switching; The valve is not regularly overhauled and tested for pressure testing, and even has not been cleaned, tested and technically identified for many years, resulting in debris being deposited in the valve, not closing tightly, and serious oil seepage and oil channeling; The valve is not closed after overhaul, or the nozzle is not blocked after the valve is removed; The valve dust orchid gasket is made of materials that are not resistant to oil and pressure. Therefore, it is necessary to strengthen the inspection of the valve and strive to prevent problems before they occur.
The main content of the valve inspection
- Whether there is leakage at the dynamic seal of the valve stem and the static seal of the flange gasket 2, whether the opening and closing state is normal 3, whether there is any damage and leakage of the valve body and other abnormal phenomena 4, rotate the normally open or normally closed valve for 1~2 times or do a lifting test 5, lubricate the valve stem of the normally open or normally closed valve 6, check and debug the power head and electrical system of the pneumatic valve
What are the 10 problems that are often encountered in the use of valves? - Why is it easy to oscillate when the double seat valve is working with a small opening?
For a single core, when the medium is a flow-open type, the valve stability is good; When the medium is a flow-closed type, the stability of the valve is poor. The double-seat valve has two spools, the lower spool is in the flow closed, and the upper spool is in the flow open, whichIn the case of small opening work, the flow closed valve core is easy to cause valve vibration, which is why the double-seat valve cannot be used for small opening work. - Why can’t the double seal valve be used as a shut-off valve?
The advantage of the double-seat valve spool is that the force balance structure allows a large pressure difference, and its outstanding disadvantage is that the two sealing surfaces cannot be in good contact at the same time, resulting in large leakage. If it is artificially and forcibly used for shut-off occasions, it is obviously not effective, even if many improvements have been made to it (such as double sealed sleeve valves), it is not desirable. - What kind of linear control valve has poor anti-blocking performance and good anti-blocking performance of quarter-turn valve?
The spool of the linear valve is vertically throttled, and the medium is horizontally flowing in and out, and the flow channel in the valve cavity must turn backwards, making the flow path of the valve quite complex (shaped like an inverted “S” shape). In this way, there are many dead zones that provide space for the media to settle and in the long run, causing clogging. The throttling direction of the quarter-turn valve is horizontal, the medium flows in horizontally and flows out horizontally, which is easy to take away the unclean medium, and the flow path is simple, and the space for medium precipitation is also very small, so the quarter-turn valve has good anti-blocking performance. - Why is the stem of the linear control valve thinner?
It involves a simple mechanical principle: high sliding friction and low rolling friction. The stem of the linear valve moves up and down, and the packing is slightly compressed, which will wrap the stem very tightly, resulting in a large return difference. For this reason, the valve stem is designed to be very small, and the packing is often used with a small coefficient of friction PTFE packing to reduce the backlash, but the problem is that the valve stem is small, it is easy to bend, and the packing life is also short. To solve this problem, the best way is to use the stem of the travel valve, that is, the quarter-stroke type of control valve, its stem is 2~3 times thicker than the straight-stroke valve stem, and the graphite packing with long life is selected, the stem stiffness is good, the packing life is long, and the friction torque is small and the return difference is small. - Why is the shut-off pressure difference of quarter-turn valves larger?
The shut-off pressure difference of quarter-turn valves is large because the resultant force generated by the medium on the spool or valve plate on the rotating shaft is very small, so it can withstand a large pressure difference. - Why do demineralized water media use rubber-lined butterfly valves and fluorine-lined diaphragm valves with short service life?
Demineralized water media contain low concentrations of acids or alkalis, which are highly corrosive to rubber. The corrosion of rubber is manifested as expansion, aging, and low strength, and the butterfly valve and diaphragm valve with rubber lining are poorly used, which is essentially caused by the lack of corrosion resistance of rubber. The back-lined diaphragm valve is improved to a fluorine-lined diaphragm valve with good corrosion resistance, but the diaphragm of the fluorine-lined diaphragm valve cannot withstand folding up and down and is broken, resulting in mechanical damage and shortening the life of the valve. Now the best way is a special ball valve for water treatment, which can be used for 5~8 years. - Why should the shut-off valve be hard sealed as much as possible?
The shut-off valve requires the lower the leakage, the better, the leakage of the soft seal valve is the lowest, and the shut-off effect is of course good, but it is not wear-resistant and has poor reliability. From the double standard of small leakage and reliable sealing, soft seal cutting is not as good as hard seal cutting. For example, the full-function ultra-light control valve, sealed and stacked with wear-resistant alloy protection, high reliability, leakage rate of 10-7, has been able to meet the requirements of the shut-off valve. - Why did the sleeve valve replace the single and double seat valves but not get the wish?
The sleeve valve that came out in the 60s, was widely used at home and abroad in the 70s, and the sleeve valve accounted for a large proportion of the petrochemical plant introduced in the 80s. Today, this is not the case, and single-seat valves, double-seat valves, and sleeve valves are all used equally. This is because the sleeve valve only improves the throttling form, stability and maintenance is better than the single-seat valve, but it is consistent with the single-seat and double-seat valves in terms of weight, anti-blocking and leakage indicators, how can it replace the single-seat and double-seat valves? Therefore, it can only be used together. - Why is selection more important than calculation?
Calculation is much more important and complex than sizing. Because the calculation is just a simple formula calculation, it itself does not depend on the accuracy of the formula, but on the accuracy of the given process parameters. There is a lot of content involved in the selection, and if you are not careful, it will lead to improper selection, which will not only cause a waste of manpower, material and financial resources, but also use efficiencyIf the result is not ideal, it will bring several use problems, such as reliability, life, operation quality, etc. - Why is the piston actuator used more and more in pneumatic valves?
For pneumatic valves, the piston actuator can make full use of the air supply pressure, making the actuator smaller in size and thrust than the diaphragm type, and the O-ring in the piston is also more reliable than the diaphragm, so it will be used more and more.
