How to select industrial valve?
Most of the medium in the chemical production plant has the characteristics of large toxicity, inflammable, explosive and corrosive, the working conditions are complex and harsh, operating temperature and pressure is high, once the valve failure, the light will lead to the leakage of the medium, the heavy will lead to the shutdown of the device, and even cause a malignant accident. Therefore, scientific and reasonable choice of 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 and operating control mode.
2. Select the correct type of valve
The correct choice of valve type is based on the designer's full grasp of the whole production process, operating conditions as the prerequisite, in the selection of valve type, the designer should first master the structural characteristics and performance of each valve.
3. Determine end connection of valve
In threaded connection, flange connection, welding end connection, the first two are most commonly used. Threaded valves are mainly nominal sizes under 50mm, if the size is too large, connection installation and sealing is very difficult.
Flange connected valves, its installation and disassembly is more convenient, but compared to the screw connected valves bulky, higher price, so it is suitable for all sizes and pressures of the pipeline connection.
Welded connections are more reliable than flange connections under more demanding conditions. But soldered valves are difficult to disassemble and reinstall, so their use is limited to situations where they are usually reliable for long periods of time, or where conditions are harsh and temperatures are high.
4. Selection of valve material
Choose the shell, inner parts and sealing surface material of the valve, in addition to considering the physical properties of the working medium (temperature, pressure) and chemical properties (corrosion), but also to master the cleanliness of the medium (there is no solid particles), in addition, but also to refer to the relevant provisions of the state and use departments.
The most economical service life and the best service performance can be obtained by choosing valve material correctly and reasonably. Body material selection order is: cast iron - carbon steel - stainless steel, sealing ring material selection order is: rubber - copper - alloy steel -F4.
5. Other
In addition, also should determine the flow rate through the valve fluid and pressure levels, using the available information (such as valve product catalog, valve product samples, etc.) to choose the appropriate valve.
Description of common valve selection
1. Gate valve selection instructions
Generally, gate valves should be preferred. Gate valves are not only suitable for steam, oil and other media, but also for medium containing granular solids and high viscosity, and suitable for vent and low vacuum system valves. For medium with solid particles, the gate valve body shall have one or two purge holes. For low temperature medium, low temperature special gate valve should be selected.
2. Globe valve selection instructions
Globe valve is applicable to the fluid resistance requirements are not strict on the pipeline, that is, the pressure loss is not considered, and high temperature, high pressure medium pipeline or device, suitable for DN < 200mm steam and other medium pipeline;
Small valve can choose globe valve, such as needle valve, instrument valve, sampling valve, pressure gauge valve;
Globe valve has flow regulation or pressure regulation, but the accuracy of the regulation requirements are not high, and pipe diameter is relatively small, it is appropriate to choose the globe valve or throttle valve;
For the highly toxic medium, appropriate to choose the bellows seal globe valve; But the globe valve should not be used for viscosity of the medium and contain particles easy to precipitate medium, also should not be used as vent valve and low vacuum system of the valve.
3. Ball valve selection instructions
Ball valve is suitable for low temperature, high pressure, high viscosity medium. Most ball valves can be used with suspended solids in the medium, according to seal material requirements can also be used in powder and granular medium;
Full channel ball valve is not suitable for flow regulation, but suitable for quick opening and closing occasions, convenient to achieve emergency cut off; Usually in the sealing performance is strict, wear, shrink mouth channel, quick opening and closing action, high pressure cut-off (pressure difference is large), low noise, gasification phenomenon, operating torque is small, fluid resistance is small in the pipeline, recommended to use the ball valve.
Ball valve is suitable for light structure, low pressure cut-off, corrosive medium; Ball valve or low temperature, cryogenic medium of the most ideal valve, low temperature medium of the pipeline system and equipment, it is appropriate to choose the low temperature ball valve with the valve cover;
When choosing the floating ball valve, the valve seat material should accept the load of the ball and the working medium.
The ball valve of 200mm should choose the worm gear transmission form; Fixed ball valve is suitable for larger diameter and higher pressure occasions; In addition, for the process of highly toxic materials, combustible medium pipeline ball valve, should have fire, anti - static structure.
4. Throttle valve selection instructions
The throttle valve is suitable for the medium temperature is lower, the pressure is higher occasions, suitable for the need to adjust the flow and pressure of the part, is not suitable for the viscosity of large medium containing solid particles, not for the blocking valve.
5. Cock valve selection instructions
Cock valve is suitable for quick opening and closing, generally not suitable for steam and higher temperature medium, for lower temperature, high viscosity medium, also suitable for medium with suspended particles.
6. Butterfly valve selection instructions
Butterfly valve is suitable for large diameter (such as DN > 600mm) and short structure length requirements, as well as the need for flow regulation and quick opening and closing requirements, generally used for temperature 80℃, pressure 1.0MPa of water, oil and compressed air and other media; Due to the butterfly valve relative to the gate valve, ball valve pressure loss is relatively large, so the butterfly valve is suitable for the pressure loss requirements are not strict in the pipeline system.
7. Check valve selection instructions
Check valves are generally suitable for clean medium, not for medium containing solid particles and viscosity. When DN≤40mm, it is advisable to use the lift check valve (only allowed to install on the horizontal pipe); When DN = 50 ~ 400mm, it is appropriate to use swing lift check valve (can be installed in both horizontal and vertical pipelines, if installed in the vertical pipeline, the flow of media from the bottom up);
When DN≥450mm, buffer type check valve should be used; When DN = 100 ~ 400mm, the pair check valve can also be used. Swing check valves can be made to work at very high pressure, PN can reach 42MPa, depending on the shell and seal material can be applied to any working medium and any operating temperature range.
Medium for water, steam, gas, corrosive media, oil, drugs and so on. The operating temperature range of the medium is -196 ~ 800℃.
8. Diaphragm valve selection instructions
Diaphragm valve is suitable for operating temperature less than 200℃, pressure less than 1.0MPa oil, water, acid medium and containing suspended medium, not for organic solvents and strong oxidizer medium;
Weir diaphragm valve should be selected for abrasive granular medium, and the flow characteristics of weir diaphragm valve should be referred to. Viscous fluid, cement slurry and precipitating medium should choose straight diaphragm valve; Diaphragm valves should not be used for vacuum piping and vacuum equipment except for specific requirements.
Questions on valve selection
1. Which three main factors should be considered in selecting the actuator?
The actuator output should be greater than the valve load and reasonably matched.
When checking the standard combination, consider whether the allowable differential pressure specified by the valve meets the process requirements. Large differential pressure to calculate the valve core by the unbalanced force.
It is necessary to consider whether the response speed of actuators can meet the requirements of process operation, especially electric actuators.
2. What are the characteristics of the electric actuator compared with the pneumatic actuator? What are the output forms?
The electric driving source is simple and convenient for electric power, large thrust, torque and stiffness. But the structure is complex and the reliability is poor. It is more expensive than pneumatic on medium and small specifications. It is often used in occasions where there is no air source or explosion proof or fire prevention is not strictly required. The electric actuator has three output forms: angular stroke, straight stroke, and multi-turn.
3. Why is the cutting pressure difference of Angle stroke valve larger?
The cutting pressure difference of Angle stroke type valve is large, because the resultant force of the medium on the spool or valve plate produces very small torque on the rotating shaft, therefore, it can bear a large pressure difference. Butterfly valve, ball valve is the most common type of Angle stroke valve.
4. Which valves should be selected for flow direction? How to choose?
Single seal type of regulating valve such as single seat valve, high pressure valve, no balance hole single seal sleeve valve to choose the flow direction. Flow open, flow close have advantages and disadvantages. The valve with open flow type works stably, but its self-cleaning performance and sealing performance are poor, and its service life is short. Flow closed type valves have long service life, good self-cleaning performance and sealing, but poor stability when the diameter of the stem is less than the diameter of the valve core.
Single seat valve, small flow valve, single seal sleeve valve usually optional flow open, when the scour is severe or there is a self-cleaning requirements when the optional flow closed. Two - position fast - opening characteristic control valve for closed - flow type.
5, in addition to single, double seat valve and sleeve valve, which have regulating function of the valve?
Diaphragm valve, butterfly valve, o-type ball valve (mainly to cut off), v-type ball valve (large adjustment ratio, with shear action), eccentric rotary valve is a regulating function of the valve.
6. Why is selection more important than calculation?
Calculation and selection, selection is much more important, much more complex. Because the calculation is only a simple formula calculation, it does not depend on the accuracy of the formula itself, but depends on the accuracy of the given process parameters.
The selection involves more content, a little careless, will lead to improper selection, not only cause the waste of manpower, material resources, financial resources, and the use of the effect is not ideal, bring a number of use problems, such as reliability, life, quality of operation.
7. Why can't double-seal valves be used as cut-off valves?
The advantage of the two-seat valve spool is the force balance structure, allowing a large pressure difference, while its outstanding disadvantage is that the two sealing surface can not be good contact at the same time, resulting in large leakage.
If it is used artificially and compulsively to cut off, the effect is obviously not good, even if many improvements have been made to it (such as double seal sleeve valves), it is not desirable.
8. Why is it easy to oscillate when the two-seat valve works with small opening?
For a single core, when the medium is open flow, the stability of the valve is good; When the medium is flow closed, the stability of the valve is poor. Two-seat valve has two spool, the lower spool in the flow closed, the upper spool in the flow open.
In this way, in the small opening of the work, the flow of the closed valve core is easy to cause the valve vibration, this is the two-seat valve can not be used for the small opening of the work reason.
9. What are the characteristics of straight through single-seat regulator? Where is it applied?
Discharge is small, because there is only one valve core easy to ensure sealing, the allowable pressure difference is small, because of the imbalance force is large.
Small circulation capacity. KV for DN100 is only 120. It is often used in situations with small leakage and pressure difference.
10. What are the characteristics of straight through two-seat regulator? Where is it applied?
The allowable pressure difference is large because many unbalanced forces can be offset.
Large circulation capacity. KV to DN100 is 160.
Large leakage, the reason for the two spool can not seal at the same time. The standard discharge is 0.1%KV, 10 times that of a single-seat valve. Straight - through two - seat valve is mainly used in high - pressure differential, leakage requirements are not strict occasions.
11, why the straight stroke control valve poor anti-blocking performance, Angle stroke valve good anti-blocking performance?
Straight stroke valve spool is vertical throttling, and the medium is horizontal flow in and out, the valve chamber of the channel is bound to turn back, so that the valve flow path becomes quite complex (shape such as inverted "S"). In this way, there are many dead zones, providing space for the precipitation of the medium, in the long run, causing blockage.
Angle stroke valve throttle direction is the horizontal direction, the medium horizontal flow in, horizontal outflow, easy to take away the unclean medium, at the same time the flow path is simple, the space of medium precipitation is also very few, so the Angle stroke valve anti-blocking performance is good.
12. When should valve positioner be used?
Occasions where high friction force requires accurate positioning. Such as high or low temperature control valves or valves with flexible graphite packing;
Slow process to improve the response time of the regulator. For example, temperature, liquid level, analysis and other parameters of the regulation system.
Need to improve the actuator output force and cutting force of the occasion. For example, single seat DN≥25, double seat DN > 100. Pressure drop at both ends of the valve △P > 1MPa or inlet pressure P1 > 10MPa.
Sometimes it is necessary to change the air opening and air closing forms in the operation of the range regulating system and regulating valve.
Where the flow characteristics of the regulator need to be changed.
13. What are the seven steps to determine the diameter of the control valve?
Determine the calculated flow -- Qmax and 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 open);
Calculate the flow coefficient -- select the appropriate formula to calculate the Max and min of KV;
Selection of KV value -- according to the Max value of KV, the KV that is closest to one gear in the selected product series is selected to obtain the primary caliber;
When opening calculation, which requires Qmax ≯ 90% valve opening; Qmin ≮ 10% when the valve opening;
Actual adjustable ratio calculation - general requirements shall be ≮ 10; R is actually >, R is required
Calibre determined -- if not qualified, re-select the value of KV, and then verify.
14, why the sleeve valve instead of a single, two-seat valve did not get what you want?
The sleeve valve came out in the 1960s, in the 1970s at home and abroad a large number of use, the petrochemical equipment introduced in the 1980s accounted for a large proportion of the sleeve valve, at that time, many people think that the sleeve valve can replace the single, double seat valve, become the second generation of products.
Today, this is not the case, single-seat valve, double-seat valve, sleeve valve are equally used. This is because the sleeve valve only improved the throttle form, stability and maintenance better than the single seat valve, but its weight, plug and leakage indicators are consistent with the single, double seat valve, how can it replace the single, double seat valve? So, they can only be used together.
15, why cut off valve should try to choose hard seal?
Cut off the valve requirements of the lower the better, the leakage of the soft seal valve is the lowest, cut off the effect of course good, but not wear-resisting, poor reliability. From the double standard of small leakage and reliable seal, soft seal cut off is better than hard seal cut off.
Such as full - function ultra - light valve, sealed and stacked with wear - resistant alloy protection, high reliability, leakage rate up to 10 ~ 7, has been able to meet the requirements of the cut-off valve.
16. Why is the straight stroke valve stem thinner?
It involves a simple mechanical principle: large sliding friction, small rolling friction. Straight stroke valve stem movement up and down, a little bit of packing pressure, it will pack the stem very tight, resulting in a large return error.
For this purpose, the stem is designed to be very small, packing and often small friction coefficient of tetrafluoride packing, in order to reduce the return error, but the problem sent is the stem is small, easy to bend, packing life is short.
To solve this problem, the best way is to use the travel valve stem, that is, the Angle stroke type of regulating valve, its stem than straight stroke stem thick 2 ~ 3 times, and choose long life graphite packing, stem stiffness is good, packing life is long, its friction torque is small, small return difference.
