1. Why is the two-seat valve prone to oscillation when working under small opening conditions?
For single-core valves, when the medium is flow-open type, they’re stable. When the medium is flow-closed type, they’re unstable. There’re two valve cores in the two-seat valve. The lower core is in flow-closed condition while the upper core is in flow-open condition. In this case, under small opening conditions, the flow-closed type of valve core will easily cause oscillation of the valve. This is why two-seat valves can be used for small opening conditions.
2. Why can’t double-sealing valves be used as sanitary shut-off valves?
Two-seat valve core has the advantage of balanced force double-sealing valve structure, allowing a large pressure difference. And its shortcoming is that the two sealing surfaces cannot be in good contact at the same time, causing a large leakage. If it’s purposefully and mandatorily used in shut-off occasions, the effect won’t be good, even if a lot of improvements have been made on it. (such as double-sealing sleeve valve)
3. Why does the straight stroke sanitary regulating valve have a poor anti-blocking performance and rotary stroke regulating valve have a good anti-blocking performance?
The core of straight stroke valve is vertical throttle, but the flow direction of the medium is horizontal. So there must be lots of twists and turns in valve cavity flow channel, making the flow paths of the valve very complex. Therefore, there’re a lot of blind zones that provide space for deposition of the medium, which in the long run causes clogging. The throttle direction of rotary stroke valve is horizontal, the medium horizontally flows in and out, which can easily take away unclean medium. Meanwhile, the flow path is simple, and there’s little room for medium deposition, so rotary stroke valve has a good anti-blocking performance.
4. Why is the stem of straight stroke regulating valve thin?
It involves a simple mechanical principle: the bigger the sliding friction, the smaller the rolling friction. The stem of straight stroke valve moves up and down. If the stuffing slightly pressed a little tighter, it would wrap up the stem tightly, resulting in a large hysteresis. To this end, the valve stem is designed to be thin and small and the stuffing use PTFE that has a small friction coefficient in order to reduce the hysteresis. But the problem that derives from this is that a thin valve stem is easy to bend and the stuffing has a short service life. To solve the problem, the best way is to use rotary valve stem, a regulating valve similar to the rotary stroke ones. Its valve stem is 2 to 3 times thicker than that of the straight stroke valve. And it uses graphite stuffing that has a long service life. The valve stem stiffness is good, the service life of stuffing is long, the friction torque and hysteresis are instead small.
5. Why does the rotary stroke valve have a relatively large shut-off pressure difference?
It’s because the torque on the rotating shaft produced by the force of medium on valve core or valve plate is very small.
6. Why is the service life of desalination water medium short when using rubber lined butterfly valve and fluorine lined diaphragm valve?
The medium of desalination water contains low concentrations of acid or alkali, which are pretty corrosive to rubber. The corrosion of rubber is exemplified by expansion, aging and low strength. The poor effect of using rubber lined butterfly valve and diaphragm valve is actually caused by the intolerance of corrosion of rubber. Rear rubber-lined diaphragm valve is improved to be the fluorine lined diaphragm valve that has a good tolerance of corrosion. But the lining of diaphragm of fluorine lined diaphragm valve can’t withstand the up-and-down folding and is broken, resulting in mechanical damage and shorter service life of the valve. Now the best way is to use water treatment special ball valves, which can be used for 5 to 8 years.
7. Why should shut-off valves try to use hard sealing?
Sanitary shut-off valves require the leakage to be as low as possible. The leakage of soft sealing valve is the lowest, so the shut-off effect is certainly good. But it’s not wear-resistant, not reliable. Judging from the double standard of small leakage and reliable sealing, soft sealing shut-off is not as good as hard sealing shut-off. Take full-featured ultra-light regulating valve for example, it’s sealed and piled with wear-resistant alloy for protection, highly reliable, has a leakage rate of 10 to 7, all of which already have met the requirements of shut-off valves.
8. Why can’t sleeve valves replace single-seat and double-seat valves?
Sleeve valves, which first appeared in the ’60s, were widely used in the ’70s worldwide. Sleeve valves account for a large percentage in petrochemical equipment imported during the ’80s. At that time, many people thought sleeve valves could replace single-seat and double-seat valves to become the second-generation products. Until now, that is not the case. Single-seat valves, double-seat valves and sleeve valves are equally used. That’s because sleeve valves only improve the way of throttle, have better stability and maintenance than single-seat valves. But its weight, anti-blocking and leakage indicator are the same as those of single-seat and double-seat valves.
9. Why is selection more important than computation?
Compare computation with selection, selection is more important and more complicated. Computation is only a simple formula. It’s not the formula itself, but rather the accuracy of given parameters that matters. Selection involves a lot of things, being a bit careless will lead to improper selection, which not only results in the waste of manpower, material, and money, but the effects of use are not good, causing a number of problems in use, such as reliability, service life and quality, etc.
10. Why is piston actuator increasingly used in pneumatic valves?
For pneumatic valves, the piston actuator can make full use of air pressure to make the size of the actuator smaller than that of the film type. The thrust is bigger. The O-ring in the piston is also more reliable than that of the film. All in all, it’s increasingly used.