1: Why Can’t double Seat Valve be used as Shut-Off valve

  The advantage of the two-seat valve spool 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 artificial, mandatory for cutting occasions, obviously the effect is not good, even if it has made many improvements( such as double sealing sleeve valve), is not desirable.

2: Why is it easy to oscillate when the two-seat valve works at 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 valve cores, the lower valve core is in flow-closed, the upper valve is in flow-closed type. The spool is in the flow-open state, so that, the flow-closed valve spool will easily cause vibration of the valve when working with small opening, which is why the double-seat valve can not be used for small opening.

3: Anti-blocking performance of the straight-stroke regulating valve is poor, and the anti-blocking performance of the angular stroke valve is good ?
　Straight stroke valve spore is vertical throttling and the medium is horizontal flow into and out of the valve chamber flow channel must turn to turn, so that the valve flow path has become quite complex ( S type). There are many dead zones,providing space fore the medium to settle, which in the long run causes clogging. Angle stroke valve throttling direction is the horizontal direction, horizontal flow into the medium, horizontal outflow easy to take away the dirty medium. At the same time the flow patch is simple, medium precipitation space is very little, so the Angle stroke valve blocking performance is good.

4: Why does the Angle stroke valve cut off, the pressure difference is large　
 Because the resultant force of the medium on the spool or valve plate produces a very small torque on the rotating shaft, it can withstand a large differential pressure.

5: Why straight stroke regulating valve stem is thinner
　It involves a simple mechanical principle: Sliding friction is high, rolling friction is low. Straight stroke valve stem up and down movement, packing a little big tight, it will put the stem package is very tight, resulting in a large return difference. Therefore, the valve stem is designed to be very small and the packing is usually made of PTFE packing with a small friction coefficient to reduce the return difference, but the resulting problem is that, the valve stem is thin, easy to bend, and the packing life is short. To solve this problem, it is best solution is to use the stem of the travel valve that is the angular-stroke type regulating valve, its stem 2-3 times thicker than the straight-stroke valve stem, and the graphite packing with long life is sued, the valve stem has good rigidity, and the packing life is long. The friction torque is small and the hysteresis is small.
6: Why should the shut-off valve choose a hard seal as much as possible ?
 The cut-off valve requires the lower leakage the better. The leakage of the soft-seal valve is the lowest. Of course, the cut-off effect is good, but it is not-wear-resistant and has poor reliability. From the double standard of small leakage and reliable sealing, soft-seal cut-off is not as hard as possible. The seal is cut off well. For example, the full-function ultra-light control valve is sealed and piled with wear-resistant alloy protection, with high reliability and a leakage rate of 10-7, which can already meet the requirements of the cut- off valve.