It involves a simple mechanical principle: sliding friction is large, and rolling friction is small. The straight stroke valve stem moves up and down, if the packing is compressed a little tighter, it will wrap the valve stem very tightly, producing a large return difference. For this reason, the valve stem design is very small, and PTFE packing is commonly used with a small friction coefficient in order to reduce the return difference, but it resulting in the thin valve stem is easy to bend, so the packing life is also short. To solve this problem, the best way is to use rotary valve stem, that is, the Angle stroke type of the control valve, its valve stem is 2~3 times thicker than the straight stroke valve stem, choosing long life of graphite packing. The valve stem stiffness is good and the packing life is long.

The shut-off pressure differential of the angular stroke valve is large, because of the resultant of force generated by the medium on the valve plug or the valve disc produces a very small torque on the rotating shaft, so it can bear a large pressure differential.

For single plug valve, the valve stability is good when the medium is flow open, and poor when the medium is flow closed. Double seat valve has two plugs: the lower plug in flow closed, the upper valve plug in flow open, and when working in small travel, the plug in flow closed is easy to cause valve vibration, this is the reason why double seat valve can not be used for small travel work.

The advantage of the double-seat valve plug is the force balance structure, which allows for a large pressure differential, and its prominent disadvantage is that two sealing surfaces can not contact well at the same time, resulting in large leakage. If it is used artificially and forcibly to on-off occasions, it is obviously not good, even if it makes many improvements (such as double seal sleeve valve), it is not applicable.

Straight stroke valve plug is vertical throttle, and the medium flows it into and out horizontally, so flow path in the valve chamber must turn backwards which makes flow path becomes quite complex (shape like inverted S type). In this way, there are many dead areas, which provide room for the precipitation of the medium, and in the long run, blockage will be produced. The horizontal direction of the Angle stroke valve throttle means the medium horizontally flow in, out, easy to take away the unclean medium. At the same time, the flow path is simple, so the medium precipitation room is very little, so the Angle stroke valve anti-plugging performance is good.

Shut-off valve requires the lower leakage, the better will be. The soft seal valve leakage rate is the lowest and the shut-off result is good, but resistance and reliability is poor. From the double standard of small leakage and reliable seal, soft seal shutting is not as good as hard seal. For example, the full-function ultra-light control valve, sealed and stacked with wear-resistant alloy protection, high reliability, the leakage rate of 10~7, has been able to meet the requirements of the shut` -off valve.

The pneumatic ball valve pipeline is mainly used to shut off quickly, distribute and change the direction of the medium. The pneumatic ball valve is a new valve. So what's its advantages and disadvantages? 

1. Fluid resistance is small, and its resistance coefficient is equal to the pipe segment of the same length. 
2. Simple structure, small size, light weight.
3. Tight and reliable.   Plastic is widely used in the ball valve sealing surface material for good sealing, which also used in the vacuum system.
4. Easy to operate. Quick opening and closing, from full open to closed by rotating 90° only, easy to control remotely .
5. easy to maintain. The pneumatic ball valve structure is simple and the sealing ring is not fixed which make it convenient to disassemble and replace.
6. When in opening or closure state, the sealing surface of the ball and the valve seat is isolated from the medium. When the medium passes through, it will not cause the erosion of the valve sealing surface.
7. A wide range of applications: diameter from a few millimeters to a few meters, high vacuum and high pressure can be used.
8. because the pneumatic ball valve power source is gas, generally 0.2-0.8MPa, relatively safe. If the pneumatic ball valve leaks air, comparing to the liquid and electric valve, the gas can be directly discharged, without pollution to the environment, and has a high safety at the same time. 
9. Compared to manual and turbine rotating ball valves, pneumatic ball valves can be configured with large dimension (manual and turbine rotating ball valves are generally below DN300,
pneumatic ball valves can reach DN1200.）

The shortcomings are as follows:
1. Its adjustment performance is worse than that of the shut-off valve, especially the pneumatic valve;
2. Because the main seat sealing ring material of the pneumatic ball valve is PTeflon which is negative to almost chemicals. But the physical properties of PVC, including a higher expansion coefficient, sensitivity to cold flow and poor thermal conductivity require the seat seal to be designed according to these characteristics. When the sealing material becomes hard, the reliability of the seal is damaged. Moreover, PTFE’s temperature resistance grade is low and can only be used at less than 180℃. Above this temperature, the sealing material will ages. It is generally recommended to use it below 120℃.