08

2022

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08

Valve 16 asks, which one is difficult for you?

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As one of the indispensable equipment in chemical and other industrial production, valves may encounter many problems from selection to use to maintenance. We have sorted out some common problem types and hope to help everyone.

As one of the indispensable equipment in chemical and other industrial production, valves may encounter many problems from selection to use to maintenance. We have sorted out some common problem types and hope to help everyone.

 


1. What are the three main factors that should be considered when selecting an executing 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, 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 valve core must be calculated.   3) Consider whether the response speed of the actuator meets the process operation requirements, especially the electric actuator.

2. What are the characteristics of electric actuators compared with pneumatic actuators, and what are the output forms?  The electric driving source is electricity, which is simple and convenient, with large thrust, torque and high stiffness. However, the structure is complex and the reliability is poor. They are more expensive than pneumatic ones in small and medium sizes. It is often used in situations where there is no air source or strict explosion-proof and flame-proof requirements are not required.   

Electric actuators have three output forms: angular stroke, linear stroke, and multi-turn.



3. Why does the cut-off pressure difference of the quarter-turn valve be larger? The cut-off pressure difference of the quarter-turn valve is large because the resultant force generated by the medium on the valve core or valve plate exerts a very small torque on the rotating shaft. Therefore, it can withstand a large pressure difference.        Butterfly valves and ball valves are the most common quarter-turn valves.

4. Which valves require flow direction selection? how to choose?  

Single-seal control valves such as single-seat valves, high-pressure valves, and single-seal sleeve valves without balancing holes require flow direction selection.   There are advantages and disadvantages to both open and closed flows. The flow-open type valve works relatively stably, but has poor self-cleaning performance and sealing performance, and has a short life. The flow-closing type valve has a long life, good self-cleaning performance and sealing performance, but has poor stability when the valve stem diameter is smaller than the valve core diameter. .   Single-seat valves, small flow valves, and single-seal sleeve valves usually choose flow-opening, and flow-closing is selected when there is severe flushing or self-cleaning requirements. The two-position quick-opening characteristic regulating valve selects the flow-closed type.

5. In addition to single and double seat valves and sleeve valves , what other valves have regulating functions?      

Diaphragm valves, butterfly valves, O-type ball valves (mainly for cutting off), V-type ball valves (large adjustment ratio, with shearing effect), and eccentric rotary valves are all valves with regulating functions.

6. Why is selection more important than calculation?

Comparing calculation with 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.         Selection involves a lot of content, and if you are not careful, it will lead to improper selection, which will not only cause a waste of manpower, material resources, and financial resources, but also the use effect is not ideal, causing several use problems, such as reliability, lifespan, and 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 core is that it has a force-balanced structure that allows a large pressure difference. However, its outstanding disadvantage is that the two sealing surfaces cannot make good contact at the same time, causing large leakage.         If it is used artificially and compulsorily in cutting-off situations, the effect will obviously not be good. Even if many improvements are made for it (such as double sealing sleeve valve), it is not advisable.

8. Why is the double-seat valve easy to oscillate when working with a small opening?  

For a single core, when the medium is flow-open, the valve has good stability; when the medium is flow-closed, the valve has poor stability. The double-seat valve has two valve cores, the lower valve core is in flow-closed state, and the upper valve core is in flow-open state.         In this way, when working with a small opening, the closed-flow valve core will easily cause valve vibration, which is why the double-seat valve cannot be used for small opening work.

9. What are the characteristics of the straight-through single-seat control valve? Where to apply?  

1) The leakage flow is small, because there is only one valve core and it is easy to ensure sealing. The standard leakage flow is 0.01%KV, and further design can be used as a shut-off valve.   2) The allowable pressure difference is small, and the thrust force is large due to the unbalanced force. The △P of the DN100 valve is only 120KPa.   3) Small circulation capacity. The KV of DN100 is only 120.   It is often used in situations 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 to apply?  

1) The allowable pressure difference is large because it can offset many unbalanced forces. The △P of the DN100 valve 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 leakage flow 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 situations where high pressure difference and leakage requirements are not strict.

11. Why do straight-stroke control valves have poor anti-blocking performance, while angle-stroke valves have good anti-blocking performance?

The valve core of the straight stroke valve is a vertical throttling, while the medium flows in and out horizontally. The flow path in the valve cavity must turn and reverse, making 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, which in the long run will cause blockage.         The throttling direction of the quarter-turn valve is the horizontal direction. The medium flows in horizontally and flows out horizontally, which can easily take away the dirty medium. At the same time, the flow path is simple and there is little space for the medium to settle, so the quarter-turn valve has good anti-blocking performance.

12. Under what circumstances is it necessary to use a valve positioner?    

1) Occasions where the friction is large and accurate positioning is required. For example, high temperature and low temperature control valves or control valves using flexible graphite packing;   2) Slow processes where the response speed of the control valve needs to be improved. For example, regulating systems for parameters such as temperature, liquid level, and analysis.   3) When it is necessary to increase the output force and cutting force of the actuator. For example, single-seat valves with DN≥25 and double-seat valves with DN>100. The pressure drop across the valve △P>1MPa or the inlet pressure P1>10MPa.   4) Sometimes it is necessary to change the air opening and air closing modes during the operation of the split-range regulating system and regulating valve.   5) Occasions where the flow characteristics of the regulating valve need to be changed.

13. What are the seven steps to determine the caliber of a regulating valve?  

1) Determine the calculated flow rate - Qmax, Qmin   2) Determine the calculated pressure difference - select the resistance ratio S value according to the system characteristics, and then determine the calculated pressure difference (when the valve is fully open);   3) Calculate the flow coefficient - choose the appropriate Use calculation formulas, charts or software to find the max and min of KV;   4) KV value selection - based on the max value of KV that is closest to the KV of the first grade in the selected product series, get the primary caliber;   5) Opening degree calculation - requirements Qmax ≯ 90% valve opening; Qmin ≮ 10% valve opening;   6) Actual adjustable ratio check - general requirements should be ≮10; R actual > R requirements   7) Caliber determination - re-select if unqualified KV value, verify again.

14. Why did sleeve valves fail to replace single and double seat valves?

Sleeve valves, which came out in the 1960s, were widely used at home and abroad in the 1970s. Sleeve valves accounted for a large proportion of petrochemical plants introduced in the 1980s. At that time, many people believed that sleeve valves could replace single and double valves. Seat valve becomes 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 has improved throttling form, stability and maintenance than single-seat valves, but its weight, anti-blocking and leakage indicators are consistent with single- and double-seat valves. How can it replace single- and double-seat valves? Woolen cloth? Therefore, they can only be used together. Intelligent electric sleeve regulating valve

15. Why should the cut-off valve use a hard seal as much as possible?

The leakage of the shut-off valve is required to be as low as possible. The leakage of the soft seal valve is as low as possible. The shut-off effect is certainly good, but it is not wear-resistant and has poor reliability. From the dual 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 control valve is sealed and protected by wear-resistant alloy. It has high reliability and a leakage rate of 10 to 7, which can already meet the requirements of a shut-off valve.

16. Why is the stem of a straight stroke regulating valve thinner?

It involves a simple mechanical principle: large sliding friction and small rolling friction. The valve stem of a straight stroke valve moves up and down. If the packing is pressed a little tighter, it will wrap the valve stem very tightly, resulting in a large hysteresis. To this end, the valve stem is designed to be very small, and the packing often uses PTFE packing with a small friction coefficient in order to reduce the hysteresis. However, the problem derived from this is that a thin valve stem is easy to bend, and the packing life is also short.         To solve this problem, a better way is to use a rotary valve stem, that is, a quarter-stroke regulating valve. Its valve stem is 2 to 3 times thicker than a straight-stroke valve stem, and it uses graphite filler with long life and high valve stem stiffness. Good, the filler has a long life, its friction torque is small, and the hysteresis is small.