Common problems of valve selection III

Valve as one of the indispensable equipment in industrial production, from selection to use to maintenance may encounter many problems, here Xiaobian collected some common problems, see if it can help you?

1、 What are the three main factors that should be considered in selecting the actuator?

(1) The output of the actuator should be greater than the load of the valve, and it should be matched reasonably.

(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 valve core should be calculated.

(3) It is necessary to consider whether the response speed of the actuator meets the requirements of process operation, especially the electric actuator.

2、 Compared with pneumatic actuator, what are the characteristics and output forms of electric actuator? 

      The electric drive source is electric power, simple and convenient, with large thrust, torque and stiffness. But the structure is complex and the reliability is poor. It is more expensive than pneumatic in small and medium size. It is often used in the occasions without gas source or strict explosion-proof and flame-proof.

      The electric actuator has three output forms: angular stroke, straight stroke and multi rotation.

3、 Why is the cut-off pressure difference of angle stroke valve larger?

     The larger cut-off pressure difference of angle stroke valve is because the resultant force generated by medium on the valve core or valve plate has very small torque on the rotating shaft, so it can bear larger pressure difference. Butterfly valve and ball valve are the most common angle stroke valves.

4、 Which valves need flow direction selection? How to choose?

      For single seal control valves, such as single seat valve, high pressure valve and single seal sleeve valve without balance hole, the flow direction should be selected.

      Flow opening and flow closing have their own advantages and disadvantages. The flow open valve works stably, but its self-cleaning performance and sealing performance are poor, and its service life is short; The flow closed valve has long service life, good self-cleaning performance and sealing performance, but the stability is poor when the stem diameter is less than the spool diameter.

      Single seat valve, small flow valve and single seal sleeve valve are usually flow on, and flow off can be selected when the scouring is severe or there is self-cleaning requirement. The two position quick opening regulating valve is of the flow selective closed type.

5、 In addition to single seat valve, double seat valve and sleeve valve, what other valves have regulating function?

Diaphragm valve, butterfly valve, O-type ball valve (mainly cut-off), V-type ball valve (large regulation ratio and shear function) and eccentric rotary valve are all valves with regulation function.

6、 Why is selection more important than calculation

      Compared with calculation and selection, selection is much more important and complicated. Because the calculation is only a simple formula calculation, it does not depend on the accuracy of the formula itself, but on the accuracy of the given process parameters.               

The type selection involves a lot of contents. A little carelessness will lead to improper type selection, which not only leads to the waste of manpower, material resources and financial resources, but also leads to some use problems, such as reliability, service life and operation quality.

7、 Why can’t double seal valve be used as shut-off valve?

      The advantage of double seat valve core is the force balance structure, allowing large pressure difference, and its outstanding disadvantage is that the two sealing surfaces can not be in good contact at the same time, resulting in large leakage.              

If it is used artificially and compulsorily in the cutting off situation, the effect is obviously not good. Even if many improvements have been made to it (such as double seal sleeve valve), it is not desirable.

8、 Why is the double seat valve easy to oscillate when it works with small opening?           

      For single core, when the medium is flow open, the valve has good stability; When the medium is flow closed, the stability of the valve is poor. The double seat valve has two valve cores, the lower valve core is in the flow closed position, and the upper valve core is in the flow open position.               

      In this way, when working in small opening, the flow closed spool is easy to cause the vibration of the valve, which is the reason why the double seat valve can not be used in small opening.

9、 What are the characteristics of straight through single seat control valve? Where is it applied?        

(1) Small discharge, because there is only one spool, easy to ensure sealing. The standard discharge is 0.01% kV, which can be used as a shut-off valve in further design.

 (2) The allowable pressure difference is small and the thrust is large due to unbalanced force. The valve △ P of DN 100 is only 120 kPa.

 (3) The circulation capacity is small. The kV of DN 100 is only 120. It is often used in the situation of small leakage and small pressure difference.

10、 What are the characteristics of straight through two seat control valve? Where is it applied?  

( 1) The allowable pressure difference is large, because it can counteract many unbalanced forces. The valve △ P of DN 100 is 280 kPa.

  (2) Large circulation capacity. The kV of DN 100 is 160.

  (3) The leakage is large because two valve cores cannot be sealed at the same time. The standard discharge is 0.1% kV, which is 10 times of single seat valve. Straight through double seat control valve is mainly used in high pressure difference, leakage requirements are not strict occasions.

11、 Why is the anti blocking performance of straight stroke control valve poor and that of angle stroke control valve good?

      The valve core of straight stroke valve is vertical throttling, while the medium flows in and out horizontally. The flow channel in the valve cavity must turn upside down, which makes the flow path of the valve quite complicated (such as inverted “s” shape). In this way, there are many dead zones, which provide space for the medium precipitation. In the long run, it will cause blockage.               

     The throttling direction of the angle stroke valve is the horizontal direction. The medium flows in and out horizontally, which is easy to take away the unclean medium. At the same time, the flow path is simple, and the space for medium precipitation is also small, so the angle stroke valve has good anti blocking performance.

12、 Under what circumstances do valve positioners need to be used?     

( 1) In the case of high friction and precise positioning. For example, high temperature and low temperature control valve or control valve with flexible graphite packing;

   (2) Slow process needs to improve the response speed of the control valve. For example, temperature, liquid level, analysis and other parameters of the regulation system.

  (3) It is necessary to improve the output force and cutting force of the actuator. For example, single seat valve with DN ≥ 25 and 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) In the operation of split range control system and control valve, it is sometimes necessary to change the form of air opening and air closing( 5) It is necessary to change the flow characteristics of the control valve.


Post time: Jun-04-2021

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