The advantage of the double seat valve spool is that the force balance structure allows a large pressure difference, and 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 to cut off the occasion, obviously the effect is not good, even if it has made many improvements (such as double sealed sleeve valve), it is also not desirable.
For a single core, when the medium is a flow-open type, the valve has good stability; when the medium is a flow-closed type, the stability of the valve is poor. The double seat valve has two valve cores, the lower valve core is closed, and the upper valve core is open. This way, when the valve is operated at a small opening, the flow-closed valve core can easily cause the vibration of the valve. This is the double seat valve. The reason why it cannot be used for small opening work.
Straight stroke valve spool is vertical throttling, and the medium is the horizontal flow in and out, the flow path in the valve cavity will inevitably turn and turn, so that the flow path of the valve becomes quite complex (shaped inverted S-shape). In this way, there are many dead zones, which provide space for the sedimentation of the media and, over time, cause blockages. The throttle valve throttle direction is the horizontal direction, the media flow into the horizontal outflow, easy to take away the dirty media, while the flow path is simple, the medium precipitation space is also very little, so the angle valve anti-blocking performance is good.
The cut-off pressure difference of the angular stroke valve is larger because the resultant force generated by the medium on the valve plug or the valve plate generates a very small torque on the rotating shaft, so that it can withstand a large pressure difference.
It involves a simple mechanical principle: large sliding friction and low rolling friction. The valve stem of the linear valve moves up and down, and the packing is pressed slightly. It will pack the valve stem tightly and generate a large return difference. For this reason, the valve stem is designed to be very small, and the filler is often a PTFE filler with a small friction coefficient in order to reduce the return difference, but the problem is that the valve stem is thin, it is easy to bend, and the filler life is short. To solve this problem, the best way is to use the trip valve stem, which is a quarter-rotor type control valve, its stem is 2 to 3 times thicker than the straight stroke stem, and the use of long-life graphite filler, stem rigidity Well, long filler life.
The cut-off valve requires a lower leakage, the better, and the leakage of the soft-seal valve is the lowest. The cut-off effect is certainly good, but it is not wear-resistant and has poor reliability. From the double standards of small leaks, sealing and reliability, soft seal cutting is not as good as hard seal cutting. Such as full-featured ultra-light control valve, sealed and piled with wear-resistant alloy protection, high reliability, leakage rate of 10 to 7, has been able to meet the requirements of the shut-off valve.
