Key points of valve selection
1. Clarify the purpose of the valve in the equipment or device
Determine the working conditions of the valve: the nature of the applicable medium, the working pressure, the working temperature and the control method of operation, etc.
2. Correctly choose the type of valve
The correct choice of valve type is based on the designer’s full grasp of the entire production process and operating conditions as a prerequisite. When selecting the valve type, the designer should first grasp the structural characteristics and performance of each valve.
3. Determine the end connection of the valve
Among threaded connections, flange connections, and welded end connections, the first two are the most commonly used. Threaded valves are mainly valves with a nominal diameter below 50mm. If the diameter is too large, it will be very difficult to install and seal the connection.
Flange-connected valves are easier to install and disassemble, but they are heavier and more expensive than screw-connected valves, so they are suitable for pipe connections of various diameters and pressures.
Welding connection is suitable for heavy load conditions and is more reliable than flange connection. However, it is difficult to disassemble and reinstall the valve connected by welding, so its use is limited to the occasions that can usually operate reliably for a long time, or where the use conditions are heavy and the temperature is high.
4. Selection of valve material
When selecting the material of the valve’s shell, internal parts and sealing surface, in addition to considering the physical properties (temperature, pressure) and chemical properties (corrosiveness) of the working medium, the cleanliness of the medium (with or without solid particles) should also be grasped. In addition, it is necessary to refer to the relevant regulations of the country and the user department.
The correct and reasonable selection of the valve material can obtain the most economical service life and the best performance of the valve. The valve body material selection sequence is: cast iron-carbon steel-stainless steel, and the sealing ring material selection sequence is: rubber-copper-alloy steel-F4.
In addition, the flow rate and pressure level of the fluid flowing through the valve should also be determined, and the appropriate valve should be selected using existing information (such as valve product catalogs, valve product samples, etc.).
Commonly used valve selection instructions
1:Selection instructions for gate valve
In general, gate valves should be the first choice. In addition to suitable for steam, oil and other media, gate valves are also suitable for media containing granular solids and high viscosity, and are suitable for valves in venting and low vacuum systems. For media with solid particles, the valve body of the gate valve should have one or two purge holes. For low-temperature media, special low-temperature gate valves should be used.
2:Instruction for selection of globe valve
The stop valve is suitable for pipelines that do not require strict fluid resistance, that is, pipelines or devices with high temperature and high pressure medium that do not consider pressure loss, and are suitable for medium pipelines such as steam with DN<200mm;
Small valves can choose globe valves, such as needle valves, instrument valves, sampling valves, pressure gauge valves, etc.;
The stop valve has flow adjustment or pressure adjustment, but the adjustment accuracy is not high, and the pipe diameter is relatively small, it is better to use a stop valve or a throttle valve;
For highly toxic media, a bellows-sealed globe valve should be used; however, the globe valve should not be used for media with high viscosity and media containing particles that are easy to precipitate, nor should it be used as a vent valve or low vacuum system valve.
3:Ball valve selection instructions
The ball valve is suitable for low-temperature, high-pressure, and high-viscosity media. Most ball valves can be used in media with suspended solid particles, and can also be used in powder and granular media according to the sealing material requirements;
The full-channel ball valve is not suitable for flow adjustment, but it is suitable for occasions that require fast opening and closing, which is convenient for emergency shutdown of accidents; usually in strict sealing performance, wear, necking passage, rapid opening and closing action, high pressure cut-off (large pressure difference), In pipelines with low noise, vaporization, small operating torque, and small fluid resistance, ball valves are recommended.
The ball valve is suitable for light structure, low pressure cut-off, and corrosive media; the ball valve is also the most ideal valve for low temperature and cryogenic media. For the piping system and device of low temperature media, low temperature ball valve with bonnet should be selected;
When selecting a floating ball ball valve, its seat material should bear the load of the ball and the working medium. Large-caliber ball valves require greater force during operation, DN≥
The 200mm ball valve should use the worm gear transmission form; the fixed ball valve is suitable for larger diameter and higher pressure occasions; in addition, the ball valve used for the process of highly toxic materials and flammable medium pipelines should have a fireproof and antistatic structure.
4:throttle valve selection instructions
The throttle valve is suitable for occasions where the medium temperature is low and the pressure is high, and it is suitable for the parts that need to adjust the flow and pressure. It is not suitable for the medium with high viscosity and containing solid particles, and it is not suitable for the isolation valve.
5:Cock valve selection instructions
The plug valve is suitable for occasions that require fast opening and closing. Generally, it is not suitable for steam and higher temperature media, for lower temperature and high viscosity media, and also for media with suspended particles.
6:Butterfly valve selection instructions
Butterfly valve is suitable for large diameter (such as DN﹥600mm) and short structure length, as well as occasions where flow adjustment and fast opening and closing requirements are required. It is generally used for temperature ≤
80℃, pressure ≤ 1.0MPa water, oil, compressed air and other media; due to the relatively large pressure loss of butterfly valves compared to gate valves and ball valves, butterfly valves are suitable for piping systems with less stringent pressure loss requirements.
7:Check valve selection instructions
Check valves are generally suitable for clean media, not for media containing solid particles and high viscosity. When ≤40mm, lift check valve should be used (only allowed to be installed on horizontal pipeline); when DN=50～400mm, swing check valve should be used (can be installed on both horizontal and vertical pipelines, such as Installed on a vertical pipeline, the flow direction of the medium should be from bottom to top);
When DN≥450mm, buffer check valve should be used; when DN=100～400mm, wafer check valve can also be used; swing check valve can be made into a very high working pressure, PN can reach 42MPa, It can be applied to any working medium and any working temperature range according to the different materials of the shell and the sealing parts.
The medium is water, steam, gas, corrosive medium, oil, medicine, etc. The working temperature range of the medium is between -196～800℃.
8:Diaphragm valve selection instructions
Diaphragm valve is suitable for oil, water, acidic medium and medium containing suspended solids whose working temperature is less than 200℃ and pressure is less than 1.0MPa. It is not suitable for organic solvent and strong oxidant medium;
Weir diaphragm valves should be selected for abrasive granular media, and the flow characteristics table of weir diaphragm valves should be referred to when selecting weir diaphragm valves; straight-through diaphragm valves should be selected for viscous fluids, cement slurry and sedimentary media; diaphragm valves should not be used for vacuum pipes except for specific requirements Road and vacuum equipment.
Valve selection question and answer
1. Which three main factors should be considered when selecting an implementing agency?
The output of the actuator should be greater than the load of the valve and should be reasonably matched.
When checking the standard combination, it is necessary to consider whether the allowable pressure difference specified by the valve meets the process requirements. When the pressure difference is large, the unbalanced force on the spool must be calculated.
It is necessary to consider whether the response speed of the actuator meets the requirements of the process operation, especially the electric actuator.
2. Compared with pneumatic actuators, what are the characteristics of electric actuators, and what output types are there?
The electric drive source is electric power, which is simple and convenient, with high thrust, torque and rigidity. But the structure is complicated and the reliability is poor. It is more expensive than pneumatic in small and medium specifications. It is often used in occasions where there is no gas source or where strict explosion-proof and flame-proof are not required. The electric actuator has three output forms: angular stroke, linear stroke, and multi-turn.
3. Why is the cut-off pressure difference of the quarter-turn valve large?
The cut-off pressure difference of the quarter-turn valve is larger because the resultant force generated by the medium on the valve core or valve plate produces a very small torque on the rotating shaft, so it can withstand a larger pressure difference. Butterfly valves and ball valves are the most common quarter-turn valves.
4. Which valves need to be selected for flow direction? how to choose?
Single-seal control valves such as single-seat valves, high-pressure valves, and single-seal sleeve valves without balance holes need to be flowed. There are pros and cons to flow open and flow closed. The flow-open type valve works relatively stable, but the self-cleaning performance and sealing performance are poor, and the life is short; the flow-close type valve has a long life, self-cleaning performance and good sealing performance, but the stability is poor when the stem diameter is smaller than the valve core diameter .
Single-seat valves, small flow valves, and single-seal sleeve valves are usually selected to flow open, and flow closed when there is severe flushing or self-cleaning requirements. The two-position type quick opening characteristic control valve selects the flow closed type.
5. In addition to single-seat and double-seat valves and sleeve valves, what other valves have regulating functions?
Diaphragm valves, butterfly valves, O-shaped ball valves (mainly cut-off), V-shaped ball valves (large adjustment ratio and shearing effect), and eccentric rotary valves are all valves with adjustment functions.
6. Why is model selection more important than calculation?
Comparing calculation and selection, selection is much more important and more complicated. Because the calculation is just a simple formula calculation, it does not itself lie in the accuracy of the formula, but in the accuracy of the given process parameters.
The selection involves a lot of content, and a little carelessness will lead to improper selection, which not only causes waste of manpower, material and financial resources, but also unsatisfactory use effect, which brings about several use problems, such as reliability, lifespan, and operation. Quality etc.
7. Why can’t the double-sealed valve be used as a shut-off valve?
The advantage of the double-seat valve core is the force balance structure, which allows a large pressure difference, but 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 compulsorily used for cutting off occasions, the effect is obviously not good. Even if many improvements (such as double-sealed sleeve valve) are made for it, it is not advisable.
8. Why is the double seat valve easy to oscillate when working with a small opening?
For single core, when the medium is flow open type, the valve stability is good; when the medium is flow closed type, the valve stability is poor. The double seat valve has two spools, the lower spool is in flow closed, and the upper spool is in flow open.
In this way, when working with a small opening, the flow-closed valve core is likely to cause valve vibration, which is why the double-seat valve cannot be used for working with a small opening.
9. What are the characteristics of the straight-through single-seat control valve? Where is it used?
The leakage flow is small, because there is only one valve core, it is easy to ensure the sealing. The standard discharge flow rate is 0.01%KV, and further design can be used as a shut-off valve.
The allowable pressure difference is small, and the thrust is large due to unbalanced force. The valve △P of DN100 is only 120KPa.
The circulation capacity is small. The KV of DN100 is only 120. It is often used in occasions where the leakage is small and the pressure difference is not large.
10. What are the characteristics of the straight-through double-seat control valve? Where is it used?
The allowable pressure difference is large, because it can offset many unbalanced forces. DN100 valve △P is 280KPa.
Large circulation capacity. The KV of DN100 is 160.
The leakage is large because the two spools cannot be sealed at the same time. The standard discharge flow rate is 0.1%KV, which is 10 times that of a single seat valve. The straight-through double-seat control valve is mainly used in occasions with high pressure difference and low leakage requirements.
11. Why is the anti-blocking performance of the straight-stroke regulating valve poor, and the angle-stroke valve has good anti-blocking performance?
The spool of the straight-stroke valve is a vertical throttling, and the medium flows in and out horizontally. The flow path in the valve cavity will inevitably turn and reverse, which makes the flow path of the valve quite complicated (the shape is like an inverted “S” shape). In this way, there are many dead zones, which provide space for the precipitation of the medium, and if things go on like this, it will cause blockage.
The direction of throttling of the quarter-turn valve is the horizontal direction. The medium flows in and out horizontally, which is easy to take away the dirty medium. At the same time, the flow path is simple, and the space for medium precipitation is small, so the quarter-turn valve has good anti-blocking performance.
12. Under what circumstances do I need to use a valve positioner?
Where the friction is large and precise positioning is required. For example, high temperature and low temperature control valves or control valves with flexible graphite packing;
The slow process needs to increase the response speed of the regulating valve. For example, the adjustment system of temperature, liquid level, analysis and other parameters.
It is necessary to increase the output force and cutting force of the actuator. For example, single seat valve with DN≥25, double seat valve with DN>100. When the pressure drop at both ends of the valve △P>1MPa or the inlet pressure P1>10MPa.
In the operation of split-range regulating system and regulating valve, it is sometimes necessary to change the air-opening and air-closing modes.
It is necessary to change the flow characteristics of the regulating valve.
13. What are the seven steps to determine the size of the regulating valve?
Determine the calculated flow-Qmax, Qmin
Determine the calculated pressure difference-select the resistance ratio S value according to the characteristics of the system, and then determine the calculated pressure difference (when the valve is fully opened);
Calculate the flow coefficient-select the appropriate calculation formula chart or software to find the max and min of KV;
KV value selection——According to the KV max value in the selected product series, the KV closest to the first gear is used to obtain the primary selection caliber;
Opening degree check calculation-when Qmax is required, ≯90% valve opening; when Qmin is ≮10% valve opening;
Actual adjustable ratio checking calculation——general requirement should be ≮10; Ractual＞R requirement
The caliber is determined-if it is unqualified, reselect the KV value and verify again.
14. Why does the sleeve valve replace the single-seat and double-seat valves but not get what you want?
The sleeve valve that came out in the 1960s was widely used at home and abroad in the 1970s. In the petrochemical plants introduced in the 1980s, sleeve valves accounted for a larger proportion. At that time, many people believed that sleeve valves could replace single and double valves. The seat valve became the second generation product.
Until now, this is not the case. 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 better than the single seat valve, but its weight, anti-blocking and leakage indicators are consistent with the single and double seat valves, how can it replace the single and double seat valves Woolen cloth? Therefore, they can only be used together.
15. Why should hard seal be used as far as possible for shut-off valves?
The leakage of the shut-off valve is as low as possible. The leakage of the soft-sealed valve is the lowest. Of course, the shut-off effect is good, but it is not wear-resistant and has poor reliability. Judging from the double standards of small leakage and reliable sealing, soft sealing is not as good as hard sealing.
For example, a full-function ultra-light regulating valve, sealed and stacked with wear-resistant alloy protection, has high reliability, and has a leakage rate of 10-7, which can already meet the requirements of a shut-off valve.
16. Why is the stem of the straight-stroke control valve thinner?
It involves a simple mechanical principle: high sliding friction and low rolling friction. The valve stem of the straight-stroke valve moves up and down, and the packing is slightly compressed, it will pack the valve stem very tightly, resulting in a larger return difference.
For this reason, the valve stem is designed to be very small, and the packing uses PTFE packing with a small friction coefficient to reduce the backlash, but the problem is that the valve stem is thin, which is easy to bend, and the packing life is short.
The best way to solve this problem is to use a travel valve stem, that is, a quarter-turn valve. Its stem is 2 to 3 times thicker than a straight-stroke valve stem. It also uses long-life graphite packing and stem stiffness. Good, the packing life is long, but the friction torque is small and the backlash is small.
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Post time: Nov-27-2021