1. Why can't a double seal valve be used as a shut-off valve?
The advantage of the double seat valve spool is the force balance structure, which allows a large pressure difference, while its outstanding disadvantage is that the two sealing surfaces cannot be in good contact at the same time, resulting in large leakage. If it is artificially and forcibly used for cutting off occasions, the effect is obviously not good. Even if many improvements are made for it, it is not desirable.
2. Why is the stem of the straight stroke control valve thin?
It involves a simple mechanical principle: large sliding friction and small rolling friction. The valve stem of the straight-stroke valve moves up and down, and the packing is slightly compressed, it will wrap the valve stem tightly, resulting in a large return difference. For this reason, the valve stem is designed to be very small, and the packing is usually made of tetrafluoro packing with a small friction coefficient to reduce the return difference, but the problem is that the valve stem is thin, easy to bend, and the packing life is short. The solution to this problem is to use a travel valve stem, that is, an angular-stroke type regulating valve. Its stem is 2 to 3 times thicker than a straight-stroke valve stem, and a long-life graphite packing is used. The stem has good rigidity and packing. Long service life, small friction torque and small hysteresis.
3. Why is the cut-off pressure difference of the quarter-turn valve larger?
The cut-off pressure difference of the angular stroke valve is large, because the resultant force generated by the medium on the valve core or the valve plate has a very small torque on the rotating shaft, so it can withstand a large pressure difference.
