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Frequently Asked Questions about Valve Selection

  • Author:cowinns
  • Source:cowinns
  • Release on:2021-03-11
Frequently Asked Questions about Valve Selection

Valves, as one of the indispensable equipment in industrial production, may encounter many textual problems from selection to use to maintenance. Here I have collected some common problems.

1. Which three main factors should be considered when choosing 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 must be calculated.  

(3) 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 which 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 complex 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 relatively large, 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 large pressure difference.

     Triple offset butterfly valve 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, which 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.

Sixth, why it is said that selection is 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, causing 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?

(1) 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.  

(2) The allowable pressure difference is small, and the thrust is large due to unbalanced force. The △P of DN100 valve is only 120KPa.  

(3) 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? 

(1) The allowable pressure difference is large, because it can offset many unbalanced forces. The valve △P 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 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 control valve poor, and the anti-blocking performance of the quarter-turn valve is good?  

      The straight-stroke valve spool 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.

fisher control valve.jpg

12. Under what circumstances need to use valve positioner? 

(1) 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;   

(2) The slow process needs to increase the response speed of the control valve. For example, the adjustment system of temperature, liquid level, analysis and other parameters.  

(3) Where 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.  

(4) When the split-range regulating system and regulating valve are in operation, it is sometimes necessary to change the air-opening and air-closing modes.   (5) Where 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?

(1) Determine the calculated 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 the calculation (valve fully open (3) pressure difference;   

(4) Calculate the flow coefficient-select the appropriate calculation formula chart or software to find the max and min of KV;   

(5) KV value selection-according to the KV max value in the selected product series closest to the KV of the first gear, the primary selection caliber is obtained;   

(6) Opening degree checking calculation-when Qmax is required ≯90% valve opening; when Qmin ≮10% valve opening;   

(7) Checking calculation of actual adjustable ratio—general requirement should be ≮10; R actual>R requirement  

(8) Determination of the caliber-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 What? 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. The shut-off effect is of course 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. However, 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 life of the packing is long, but the friction torque is small and the backlash is small.