Frequently encountered valve knowledge sixteen questions, which one is difficult for you?


As one of the indispensable equipment in chemical and other industrial production, valves may encounter many problems from selection to use to maintenance. The editor has sorted out some common problem types, hoping to help everyone.
1. What are the three main factors that should be considered when selecting an implementing agency?
1) The output of the actuator should be greater than the load of the valve, and should be reasonably matched.
2) 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 should be calculated.
3) It is necessary to consider whether the response speed of the actuator meets the process operation requirements, especially the electric actuator.
Second, compared with pneumatic actuators, what are the characteristics of electric actuators, and what are the output forms?
The electric drive source is simple and convenient, with large thrust, torque and rigidity. But the structure is complex and the reliability is poor. It is more expensive than pneumatic ones in small and medium size. It is often used in occasions without air source or without strict explosion-proof and flame-proof.
Electric actuators have three output forms: angular stroke, linear stroke, and multi-turn.
3. Why is the cut-off pressure difference of the angular stroke valve larger?
The cut-off pressure difference of the angular stroke valve is large because the resultant force generated by the medium on the valve core or the valve plate has a very small torque on the rotating shaft, so it can withstand a large pressure difference.
Butterfly valve and ball valve are the most common angular stroke valves.
Fourth, which valves need flow direction selection? how to choose?
Flow direction selection is required for single-sealed control valves such as single-seat valves, high-pressure valves, and single-sealed sleeve valves without balance holes.
There are pros and cons to opening and closing. The flow-open type valve is relatively stable, but has poor self-cleaning performance and sealing performance, and has a short service life; the flow-closed type valve has a long service life, good self-cleaning performance and sealing performance, but when the diameter of the valve stem is smaller than the diameter of the valve core, the stability is poor. .
Single-seat valve, small flow valve, and single-sealing sleeve valve are usually selected to flow open, and can be selected to flow closed when flushing is severe or self-cleaning is required. The two-position quick-opening characteristic regulating valve selects the flow-closing type.
Five, in addition to the single, double seat valve and sleeve valve, what other valves have regulating functions?
Diaphragm valve, butterfly valve, O-type ball valve (mainly cut off), V-type ball valve (large adjustment ratio, shearing effect), eccentric rotary valve are all valves with adjustment function.
6. Why is the selection more important than the calculation?
Compared with calculation and selection, selection is much more important and complicated. Because the calculation is just a simple formula calculation, it does not depend on the accuracy of the formula, but on whether the given process parameters are accurate.
The selection involves a lot of content. A little carelessness will lead to improper selection, which not only causes waste of manpower, material resources and financial resources, but also the use effect is not ideal, resulting in several use problems, such as reliability, life, operation quality etc.
7. Why can’t the double seal valve be used as a shut-off valve?
The advantage of the double seat valve spool 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 forcibly used for cutting off occasions, the effect is obviously not good. Even if many improvements are made for it (such as double-sealed sleeve valve), it is not desirable.
8. Why is the double seat valve easy to oscillate when 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 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 valve core of the flow-closed type is easy to cause the vibration of the valve, 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 applied?
1) The allowable pressure difference is small, and the thrust is large due to the unbalanced force. The valve △P of DN100 is only 120kPa.
2) The circulation capacity is small. The Kv of DN100 is only 120.
It is often used in occasions with small leakage and small pressure difference.
10. What are the characteristics of the straight-through double-seat control valve? Where is it applied?
1) The allowable pressure difference is large, because it can offset many unbalanced forces. The valve ΔP of DN100 is 280KPa.
2) Large circulation capacity. The KV of DN100 is 160.
3) The leakage is large, because the two valve cores cannot be sealed at the same time. The standard discharge flow is 0.1%KV, which is 10 times that of the single-seat valve.
The straight-through double-seat control valve is mainly used in the occasions where the high pressure difference and the leakage requirements are not strict.
11. Why the anti-blocking performance of the straight-stroke regulating valve is poor, and the anti-blocking performance of the angular stroke valve is good?
The spool of the straight stroke valve is a vertical throttling, and the medium flows in and out horizontally. In this way, there are many dead zones, which provide space for the sedimentation of the medium, which will cause blockage in the long run.
The throttling direction of the angle stroke valve is the horizontal direction. The medium flows in horizontally and flows 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 the medium to settle, so the anti-blocking performance of the angle stroke valve is good.
12. Under what circumstances should a valve positioner be used?
1) When the friction force is large and precise positioning is required. Such as high temperature, low temperature control valve or control valve with flexible graphite packing;
2) The occasions where the slow process needs to improve the response speed of the control valve. For example, adjustment systems for parameters such as temperature, level, analysis, etc.
3) When 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 across the valve ΔP>1MPa or the inlet pressure P1>10MPa.
4) Sometimes it is necessary to change the form of air-on and air-off during the operation of the split-range regulating system and regulating valve.
5) When it is necessary to change the flow characteristics of the regulating valve.
13. What are the seven steps to determine the diameter of the regulating valve?
1) Determine the calculation flow – Qmax, Qmin
2) Determine the calculated pressure difference – select the resistance ratio S value according to the characteristics of the system, and then determine and calculate the pressure difference (when the valve is fully opened);
3) Calculate the flow coefficient – select the appropriate calculation formula chart or software to find the max and min of KV;
4) KV value selection – according to the KV max value, the KV of the selected product series is closer to the first grade, and the primary selection caliber is obtained;
5) Check calculation of opening degree – ≯90% valve opening degree when Qmax is required; ≮10% valve opening degree when Qmin is required;
6) Actual adjustable ratio check calculation – general requirements should be ≮10; R actual > R requirements
7) Determination of caliber – if unqualified, re-select KV value and verify again.
14. Why did the sleeve valve replace the single and double seat valve but not get what you wanted?
The sleeve valve, which came out in the 1960s, was widely used at home and abroad in the 1970s. In the petrochemical plants introduced in the 1980s, the sleeve valve accounted for a large proportion. At that time, many people believed that the sleeve valve could replace the single and double valves. The seat valve has become the second generation product.
Up to now, this is not the case, single seat valve, double seat valve, sleeve valve 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, it can only be used together.
15. Why should the shut-off valve use hard seals as much as possible?
The lower the leakage of the shut-off valve, the better. The leakage of the soft-sealed valve is low. Of course, the shut-off effect is good, but it is not wear-resistant and has poor reliability. From the double standard of small leakage and reliable sealing, soft sealing is not as good as hard sealing. For example, the full-function ultra-light control valve is sealed and piled with wear-resistant alloy protection, with high reliability and a leakage rate of 10 to 7, which can already meet the requirements of shut-off valves.
16. Why is the stem of the straight-stroke regulating 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 wrap the valve stem tightly, resulting in a large return difference. For this reason, the valve stem is designed to be very small, and the packing is usually made of tetrafluoro packing with a small friction coefficient to reduce the return difference, but the problem is that the valve stem is thin, easy to bend, and the packing life is short.
To solve this problem, the solution is to use a travel valve stem, that is, an angular stroke type regulating valve. Its valve stem is 2 to 3 times thicker than the straight stroke valve stem, and the graphite packing with long life is used, and the valve stem has good rigidity and packing. Long service life, small friction torque and small hysteresis.

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