How to Select the Right Valve
Most of the media in chemical production units have the characteristics of high toxicity, flammability, explosion and strong corrosivity. The working conditions are complex and harsh, and the operating temperature and pressure are high. Once the valve fails, the medium leakage will be caused in light cases, the shutdown of the unit will be caused in serious cases, and even malignant accidents will be caused.
How to select the right valve
Scientific and reasonable selection of the right valve can not only reduce the construction cost of the device, but also ensure the safe operation of production.
Key points of valve selection
1. Specify the purpose of the valve in the equipment or device
Determine the working conditions of the valve: the nature of the applicable medium, working pressure, working temperature, operation and control mode, etc.
2. Correctly select the correct valve type
The correct selection of valve type is based on the designer’s full mastery of the whole production process and operating conditions. When selecting the correct valve type, the designer should first master the structural characteristics and performance of each valve.
3. Determine the end connection of the valve
In threaded connection, flange connection and welded end connection, the first two are most commonly used. Threaded valves are mainly valves with nominal diameter less than 50mm. If the diameter is too large, it is very difficult to install and seal the connecting part.
The installation and disassembly of flange connected valves are more convenient, but they are bulkier and more expensive than threaded valves, so they are suitable for pipeline connection of various sizes and pressures.
Welded connection is more reliable than flange connection under harsh conditions. However, it is difficult to disassemble and reinstall the welded valve, so its use is limited to the occasions where it can normally operate reliably for a long time, or where the service conditions are harsh and the temperature is high.
4. Selection of valve material
Select the correct materials for the shell, internals and sealing surface of the valve. In addition to considering the physical properties (temperature, pressure) and chemical properties (corrosivity) of the working medium, we should also master the cleanliness of the medium (whether there are solid particles). In addition, we should also refer to the relevant regulations of the state and the user department.
The most economical service life and best service performance of the valve can be obtained by selecting the correct material of the valve correctly and reasonably. The material selection sequence of valve body is: cast iron – carbon steel – stainless steel, and the material selection sequence of sealing ring is: Rubber – Copper – alloy steel – F4.
In addition, the flow and pressure grade of fluid flowing through the valve shall also be determined, and the appropriate valve shall be selected by using the existing data (such as valve product catalog, valve product sample, etc.).
Selection description of common valves
Type selection description of gate valve
In general, gate valves shall be preferred. In addition to steam, oil and other media, gate valves are also suitable for media containing granular solids and high viscosity, as well as valves in vent and low vacuum systems. For the medium with solid particles, the valve body of the gate valve shall be provided with one or two purge holes. For low-temperature medium, low-temperature special gate valve shall be selected.
Type selection description of globe valve
- The stop valve is applicable to the pipeline with lax requirements for fluid resistance, that is, the pipeline or device with little consideration for pressure loss and high-temperature and high-pressure media, and it is applicable to the steam and other media pipeline with DN < 200mm;
- Stop valves can be selected for small valves, such as needle valve, instrument valve, sampling valve, pressure gauge valve, etc;
- The stop valve has flow regulation or pressure regulation, but the requirements for regulation accuracy are not high, and the pipeline diameter is relatively small, it is advisable to select stop valve or throttle valve;
- For highly toxic media, bellows sealed stop valve should be selected; However, the stop valve should not be used for medium with high viscosity and medium containing particles that are easy to precipitate, nor for vent valve and valve of low vacuum system.
Type selection description of ball valve
- The ball valve is suitable for medium with low temperature, high pressure and high viscosity. Most ball valves can be used in media with suspended solid particles, and can also be used in powdery and granular media according to the sealing material requirements;
- The full channel ball valve is not suitable for flow regulation, but it is suitable for occasions requiring rapid opening and closing, so as to realize emergency cut-off in case of accident; Generally, ball valves are recommended for pipelines with strict sealing performance, wear, necking channel, rapid opening and closing action, high-pressure cut-off (large differential pressure), low noise, gasification, small operating torque and small fluid resistance.
Ball valve is suitable for light structure, low pressure stop valve and corrosive medium; The ball valve is also the most ideal valve for low-temperature and cryogenic media. The low-temperature ball valve with valve cover should be selected for the pipeline system and device of low-temperature media;
When the floating ball valve is selected, the valve seat material shall bear the load of the ball and working medium, and the large-diameter ball valve needs large force during operation;
The 200mm ball valve shall be worm gear driven; Fixed ball valve is suitable for occasions with large diameter and high pressure; In addition, the ball valve used for the pipeline of highly toxic process materials and combustible media shall have fire-proof and anti-static structure.
Model selection description of throttle valve
The throttle valve is applicable to the occasions with low medium temperature and high pressure, and is applicable to the parts where the flow and pressure need to be adjusted. It is not applicable to the medium with high viscosity and containing solid particles. It should not be used as an isolation valve.
Type selection description of plug valve
Plug valves are suitable for occasions requiring rapid opening and closing. Generally, they are not suitable for steam and medium with high temperature, medium with low temperature and high viscosity, and medium with suspended particles.
Type selection description of butterfly valve
Butterfly valve is suitable for occasions with large diameter (such as DN ＞ 600mm), short structural length requirements, and rapid flow regulation and opening and closing requirements. It is generally used for water, oil, compressed air and other media with temperature of 80 ℃ and pressure of 1.0MPa; As the pressure loss of butterfly valve is relatively large compared with gate valve and ball valve, butterfly valve is suitable for pipeline system with lax pressure loss requirements.
Type selection description of check valve
- Check valves are generally suitable for clean media, not for media containing solid particles and high viscosity. When DN ≤ 40mm, lift check valve should be used (only allowed to be installed on horizontal pipeline); When DN = 50 ~ 400mm, swing lift check valve should be used (it can be installed on both horizontal and vertical pipelines. If installed on vertical pipelines, the medium flow direction 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 selected; The swing check valve can be made into a very high working pressure, and the PN can reach 42MPa. It can be applicable to any working medium and any working temperature range according to the material of shell and seal.
The medium is water, steam, gas, corrosive medium, oil, medicine, etc. The working temperature range of the medium is – 196 ~ 800 ℃.
Type selection description of diaphragm valve
- The diaphragm valve is applicable to oil products, water, acidic media and media containing suspended solids with working temperature less than 200 ℃ and pressure less than 1.0MPa, and is not applicable to organic solvents and strong oxidant media;
- For grinding granular medium, weir diaphragm valve should be selected, and its flow characteristic table should be referred to when selecting weir diaphragm valve; Straight through diaphragm valve shall be selected for viscous fluid, cement slurry and sedimentary medium; In addition to specific requirements, diaphragm valves should not be used on vacuum pipelines and vacuum equipment.
Q & A on valve selection
Question 1. Which three main factors should be considered in selecting an actuator?
The output of the actuator shall be greater than the load of the valve and shall be reasonably matched.
When checking the standard combination, consider whether the allowable differential pressure specified by the valve meets the process requirements. When the pressure difference is large, the unbalanced force on the valve core shall be calculated.
Consider whether the response speed of the actuator meets the process operation requirements, especially the electric actuator.
Question 2. Compared with pneumatic actuator, what are the characteristics and output forms of electric actuator?
The electric drive source is electric, simple and convenient, with large thrust, torque and stiffness. But the structure is complex and the reliability is poor. It is more expensive than pneumatic in small and medium specifications. It is often used in occasions without air source or strict explosion-proof and flame-proof. The electric actuator has three output forms: angular stroke, straight stroke and multi rotation.
Question 3. Why is the cut-off pressure difference of angle stroke valves large?
The cut-off pressure difference of angular stroke valve is large because the resultant force generated by the medium on the valve core or valve plate has a very small torque on the rotating shaft. Therefore, it can bear a large pressure difference. Butterfly valve and ball valve are the most common angle stroke valves.
Question 4. Which valves need flow direction selection? How to choose?
Flow direction selection is required for single seal control valves, such as single seat valve, high-pressure valve and single seal sleeve valve without balance hole. Flow opening and flow closing have their own advantages and disadvantages. The flow opening valve works stably, but its self-cleaning performance and sealing performance are poor and its service life is short; The flow closed valve has long service life, good self-cleaning performance and sealing performance, but its stability is poor when the valve stem diameter is less than the valve core diameter.
Single seat valve, small flow valve and single seal sleeve valve are usually opened by flow, and closed by flow when there is severe scouring or self-cleaning requirements. The two position quick opening characteristic regulating valve is flow selective and closed.
Question 5. In addition to single seat valve, double seat valve and sleeve valve, what other valves with regulating function?
Diaphragm valve, butterfly valve, O-type ball valve (mainly cut-off), V-type ball valve (with large regulation ratio and shear effect) and eccentric rotary valve are all valves with regulation function.
Question 6. Why is selection more important than calculation?
Compared with calculation and type selection, type selection is much more important and complex. Because the calculation is only a simple formula calculation, it does not lie in the accuracy of the formula itself, but in whether the given process parameters are accurate.
There are many contents involved in model selection. A little carelessness will lead to improper model selection, which will not only cause a waste of human, material and financial resources, but also the use effect is not ideal, resulting in several use problems, such as reliability, service life, operation quality, etc.
Question 7. Why can’t the double seal 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 differential pressure. Its prominent disadvantage is that the two sealing surfaces can not be in good contact at the same time, resulting in large leakage.
If it is artificially and forcibly used in cutting off occasions, the effect is obviously not good. Even if many improvements have been made for it (such as double seal sleeve valve), it is not desirable.
Question 8. Why is the two seat valve easy to oscillate when working with a small opening?
For single core, when the medium is flow open, the valve has good stability; When the medium is flow closed, the stability of the valve is poor. The double seat valve has two valve cores, the lower valve core is closed and the upper valve core is open.
In this way, when working with a small opening, the flow closed valve core is easy to cause the vibration of the valve, which is the reason why the two seat valve can not be used for working with a small opening.
Question 9. What are the characteristics of straight through single seat regulating valve? Where is it used?
The discharge is small, because only one valve core is easy to ensure sealing, the allowable differential pressure is small, and the thrust is large due to unbalanced force.
Small circulation capacity. The KV of DN100 is only 120. It is often used in occasions with small leakage and small pressure difference.
Question 10. What are the characteristics of straight through two seat regulating valve? Where is it used?
The allowable differential pressure is large, because it can offset many unbalanced forces.
Large circulation capacity. The KV of DN100 is 160.
The leakage is large because two valve cores cannot be sealed at the same time. The standard discharge is 0.1% KV, which is 10 times that of a single seat valve. Straight through two seat regulating valve is mainly used in occasions with high pressure difference and lax leakage requirements.
Question 11. Why is the anti blocking performance of the straight stroke regulating valve poor and the anti blocking performance of the angular stroke valve good?
The valve core of the straight stroke valve is vertical throttling, and the medium flows in and out horizontally. The flow channel in the valve cavity must turn backward, which makes the flow path of the valve quite complex (such as inverted “s” shape). In this way, there are many dead zones, which provide space for medium precipitation. In the long run, it will cause blockage.
The throttling direction of the angle travel valve is the horizontal direction. The medium flows in and out horizontally, which is easy to take away the unclean medium. At the same time, the flow path is simple, and there is little space for medium precipitation, so the angle travel valve has good anti blocking performance.
Question 12. When is a valve positioner required?
Occasions with high friction and accurate positioning. For example, high-temperature and low-temperature regulating valves or regulating valves with flexible graphite packing;
Where the response speed of the regulating valve needs to be improved during slow process. For example, the regulation system of temperature, liquid level, analysis and other parameters.
Occasions where the output force and cutting force of the actuator need to be improved. For example, single seat valve with DN ≥ 25 and double seat valve with DN > 100. When the pressure drop △ P ＞ 1MPa at both ends of the valve or the inlet pressure P1 ＞ 10MPa.
In the operation of split range control system and control valve, it is sometimes necessary to change the form of gas on and gas off.
Occasions where the flow characteristics of the regulating valve need to be changed.
Question 13. What are the seven steps to determine the diameter of the regulating valve?
- Determine the calculated flow: Qmax, qmin;
- Determine the calculated differential pressure: select the resistance ratio s value according to the system characteristics, and then determine the calculated differential pressure (when the valve is fully open);
- Coefficient of flow calculation: select appropriate calculation formula, chart or software to calculate the max and min of KV;
- Selection of KV value: according to the max value of KV, the KV closest to the first gear in the selected product series is obtained;
- Checking calculation of opening: when Qmax is required, ≯ 90% valve opening; ≮ 10% valve opening at qmin;
- Checking calculation of actual adjustable ratio: General requirements shall be ≮ 10; R actual ＞ R requirement;
- Determination of caliber: if it is unqualified, reselect the KV value and verify it again.
Question 14. Why did sleeve valve replace single and double seat valve?
The sleeve valve, which came out in the 1960s, was widely used at home and abroad in the 1970s. The sleeve valve accounted for a large proportion in the petrochemical units introduced in the 1980s. At that time, many people believed that the sleeve valve could replace the single and double seat valve and become the second generation product.
Up to now, this is not the case. Single seat valve, double seat valve and sleeve valve are 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? Therefore, it can only be used together.
Question 15. Why should the shut-off valve be hard sealed as far as possible?
The lower the leakage of the shut-off valve, the better. The leakage of the soft seal valve is the lowest. Of course, the cutting effect is good, but it is not wear-resistant and has poor reliability. From the double standards of small leakage and reliable sealing, soft seal cutting is not as good as hard seal cutting.
For example, the full-function ultra light regulating valve is sealed and stacked with wear-resistant alloy protection, with high reliability and leakage rate of 10 – 7, which can meet the requirements of shut-off valve.
Question 16. Why is the stem of the straight stroke regulating valve thinner?
It involves a simple mechanical principle: large sliding friction and small rolling friction. The stem of the straight stroke valve moves up and down, and if the packing is slightly compressed, it will wrap the stem very tightly, resulting in large return difference.
Therefore, the valve stem is designed to be very small, and the PTFE packing with small friction coefficient is often used to reduce the return difference, but the problem is that the valve stem is thin, it is easy to bend, and the service life of the packing is also short.
The best way to solve this problem is to use the travel valve stem, that is, the angle stroke regulating valve. Its stem is 2 – 3 times thicker than the straight stroke stem, and the graphite packing with long service life is selected. The stem stiffness is good, the packing life is long, the friction torque is small and the return difference is small.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Stainless Steel Valves, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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