1.   Why does the double seat control valve easily oscillate when working at a small opening?

For a single seated control valve , the valve stability is good when the medium is flow-opening; the stability of the valve is poor when the medium is flow-closed. The double seat valve has two spools, the lower spool is closed, and the upper spool is open. Thus, when working at a small opening, the flow-closed spool is liable to cause vibration of the valve. This is why a double seat valve cannot be used for small opening work.

2. 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-balanced structure, which allows a large differential pressure, and its  disadvantage is that the two sealing faces cannot be in good contact at the same time, resulting in a large leak. If it is artificially and forcibly used for cut off the application, it is obviously not effective, even if it has made many improvements (such as double-sealed sleeve valves), it is not advisable.

3. Why is the anti-blocking performance of the linear regulating valve unsatisfactory while that of the rotary valve good?

The straight-stroke valve spool is vertically throttled, and the medium flows horizontally in and out, and the flow passage in the valve chamber is inevitably turned and inverted, making the flow path of the valve quite complicated (the shape is inverted "S" type). Thus, there are many dead zones that provide space for the precipitation of the medium, which in the long run causes blockages. The direction of the rotary valve throttling is the horizontal direction. The medium flows horizontally and horizontally, which makes it easy to take away the dirty medium. At the same time, the flow path is simple, and the space for the medium to precipitate is also small, so the anti-blocking performance of the rotary valve is good.

4. Why is the valve stem of the linear control valve thinner?

It involves a simple mechanical principle: large sliding friction and low rolling friction. The valve stem of the linear control valve moves up and down, and the packing is slightly pressed tightly, which will tightly seal the valve stem and produce a large backlash. For this reason, the valve stem is designed to be very small, and the packing is also commonly used with a small friction coefficient of tetrafluoro packing to reduce the backlash, but the problem is that the valve stem is thin, the bend is easy, and the packing life is short. The best way to solve this problem is to use the travel valve stem, that is, the rotary type regulating valve. Its valve stem is 2 to 3 times thicker than the linear control valve stem, and the graphite packing with long life is selected. So the packing has a long service life, and the friction torque is small and the backlash is small.

5. Why is the cut-off pressure difference of the rotary type valve larger?

The cut-off pressure difference of the rotary type valve is large because the resultant force of the medium on the valve trim or the valve plate generates a very small torque to the rotating shaft, and therefore, it can withstand a large pressure difference.

6. Why does the demineralized water medium use a rubber-lined butterfly valve and a fluorine-lined diaphragm valve with a short service life?

Demineralized media contain low concentrations of acids or bases which are highly corrosive to rubber. The corrosion of rubber is characterized by expansion, aging, and low strength. The butterfly valve and diaphragm valve with rubber lining have poor effect, and the essence is that the rubber is not resistant to corrosion. The back-lined rubber diaphragm valve is improved to a fluorine-lined diaphragm valve with good corrosion resistance, but the diaphragm of the fluorine-lined diaphragm valve can be broken by being folded up and down, causing mechanical damage and shortening the life of the valve. The best way to use a special ball valve for water treatment, which can be used for 5 to 8 years.

7. Why should the shut-off valve be metal seated type as much as possible?

The shut-off valve requires that the leakage should be as low as possible. The leakage of the soft-sealed valve is the lowest, but it is not wear-resistant and has poor reliability. From the standard of small leakage and high reliability, the soft seal valve is not as good as the hard seal valve. Jinghua Flow now presents full-featured ultra-light regulating valve with seat coated by wear-resistant alloy protection, resulting in reliable sealing and low leakage rate.

8. Why does the cage guided sleeve type control valve can’t replace the single and double seat control valve?

The sleeve control valve that was introduced in the 1960s was widely used over the globe in the 1970s. The proportion of the sleeve control valve in the petrochemical plant introduced in the 1980s was large. At that time, many people believed that the sleeve control valve could replace the single and double seat control valves and became a second generation product. This is not the case today, single-seat control valves, double-seat control valves, and sleeve valves are equally used. This is because the sleeve control valve only improves the throttling form, stability and maintenance compared with the single seat valve, but its weight, anti-blocking and leakage indicators are equal to single and double seat control valves.

9. Why is selection more important than calculation?

Compared with the calculation, the model selection is much more important and more complicated. Because the calculation is just a simple formula calculation, it is not in itself the accuracy of the formula, but whether the given process parameters are accurate. There are many elements involved in the selection. Improper selection causes waste of manpower, material resources and financial resources, but also has unsatisfactory effect on use, which brings about some usage problems such as reliability, longevity, quality and so on.

10. Why are there more and more rack and pinion type pneumatic actuators used for control valves?

For pneumatic valves, the rack and pinion type pneumatic actuators can make full use of the air supply pressure, providing smaller actuator size, larger thrust, and more reliable O-ring than diaphragm type.

If you are interested in more information, freely contact us via sales@jhflow.com.