Sixteen Common Questions in Selection of Valves (Part One)

A valve is one of the indispensable devices in industrial production, and you may encounter many questions from selection, use to the maintenance of valves. Some common questions are listed in this article.
 
1. Which main factors should be considered when we choose an actuator?　　
(1) The output of the actuator should be greater than the load of the valve and should be reasonably matched.　　
(2) When the standard combination is checked, it is necessary to consider whether the allowable pressure difference specified by the valve meets the process requirements. When the pressure difference is great, the unbalanced force on the valve spool must be calculated.　　
(3) It is necessary to consider whether the response speed of the actuator meets the requirements of the operation, especially the electric actuator.
 
2. Compared with pneumatic actuators, what are the characteristics of electric actuators and how many kinds of outputs are there?
The electric drive source is electric power, which is simple, convenient, and has great thrust, torque and rigidity. However, the structure is complex and the reliability is poor. It is more expensive than the pneumatic one of small and medium sizes. It is often used in places where there is no gas source or that don't have strict requirements of explosion-proof and flame-proof. The electric actuator has three output forms: angular travel, linear travel, and multi-turn travel.
 
3. Why is the cut-off pressure difference of the quarter-turn valve large?　　
The cut-off pressure difference of the angular travel valve is relatively big because the resultant force generated by the medium on the valve spool or valve plate produces a very small torque on the rotating shaft. Therefore, it can withstand a big pressure difference. Butterfly valves and ball valves are the most common valves with angular travel.
 
4. Which valves need to be selected for flow direction? How to choose?
Flow directions need to be considered for regulating valves with single sealing such as single seated valves, high-pressure valves, and single-seal sleeve valves without balance holes.
 
There are pros and cons to media flowing toward the opening or closing direction of valves. The valve with media flowing toward the opening direction of valves works relatively stable, but its self-cleaning and sealing performance are poor, and its service life is short. The valve with media flowing toward the closing direction of valves has a long service life, good self-cleaning and sealing performance. However, when the diameter of the stem is smaller than that of the valve spool, the stability is poor. 　
 
Media flowing toward the opening direction of valves is usually selected for single seated valves, valves with small flows and single-seal sleeve valves, but media flowing toward the closing direction of valves can be chosen when there is a requirement for severe scouring or self-cleaning. Media flowing toward the closing direction of valves can be chosen for the two-position type quick opening control valve.
 
5. In addition to single seated, double seated valves and sleeve valves, which other valves have regulating functions?　
Diaphragm valves, butterfly valves, O shaped ball valves with mainly cutting off functions, V shaped ball valves with great regulating ratios and shearing effects, and eccentric rotary valves are all valves with regulating functions.
 
6. Why is selection more important than the calculation?　　
Model selection is much more important and more complicated compared with calculation because the calculation is just a simple formula calculation. It itself does not lie in the accuracy of the formula, but the accuracy of the given process parameters. The selection involves a lot of matters. A little carelessness will lead to improper selection, which not only causes waste of manpower, materials and financial resources, but also brings unsatisfactory use effects, posing several use problems, such as reliability, service lives, and operating quality.
 
7. Why can't the double seated valve be used as a shut-off valve?
The advantage of the double seated valve spool is the force balance structure, which allows a big pressure difference. Its outstanding disadvantage is that the two sealing surfaces cannot be in good contact at the same time, resulting in a lot of leakages.
 
If it is used for cutting off conditions, it is not effective. Even if many improvements are made, such as double seated sleeve valves, it is not advisable.
 
8. Why is the double seated valve easy to shake when working at a small opening?　
For a single valve spool, when the medium is flowing toward the opening direction of the valve, the valve has good stability. When the medium is flowing toward the closing direction of the valve, the stability of the valve is poor. The double seated valve has two valve spools; the lower valve spool is at the medium flowing toward the closing direction, and the upper spool is at the medium flowing toward the opening direction.
 
In this way, the valve spool with medium flowing toward the closing direction is likely to cause the vibration of valves when the opening angle is small. This is the reason why the double seated valve cannot be used for a small opening angle.
 
9. What are the characteristics of the straight-through single seated control valve? Where is it used?
The straight-through single seated control valve has the following features:
(1) The leakage is small because there is only one valve spool, which is easy to ensure the sealing. The standard amount of leakage is 0.01%KV, and further design can be used as a shut-off valve.　　
(2) The allowable pressure difference is small. Because the thrust of the unbalanced force is great,  the △P of a valve DN100 is only 120KPa.　　
(3) The flow capacity is poor. The KV of DN100 is only 120. It is often used in places where the leakage amount is small and the pressure difference is not big.

10. What are the characteristics of the straight-through double seated control valve? Where is it used?　

The straight-through double seated control valve has the following features:
(1) The allowable pressure difference is big because it can offset many unbalanced forces. The pressure change range of the valve of DN100 is 280KPa.　　
(2) The flow capacity is good. The KV of DN100 is 160.　　
(3) The leakage is great because the two valve spools cannot be sealed at the same time. The standard leakage rate is 0.1%KV, which is 10 times higher than that of a single seated valve. The straight-through double seated control valve is mainly used in places with requirements of high-pressure difference and a small amount of leakage.