2 Structural characteristics
2.1 The hemispherical body of the hemispherical double eccentric semi pneumatic ball valve adopts the double eccentric design, with reliable sealing effect, instant separation, small torque and long service life. It can be used in the harsh working conditions of conveying granular media. The structure principle is that the center line of the hemisphere is offset from the center line of the valve stem, and offset from the center line of the valve seat. When fully opened, the sphere is completely disconnected from the valve seat, and there is a certain gap. The radius of gyration is divided into long radius and short radius. The tangent line of the rotation track in the long radius will be formed with the sealing surface of the valve seat θ Angle, when the valve is opened and closed, the hemisphere has a gradual separation and gradual extrusion effect on the valve seat surface, thus reducing the mechanical wear and scratches between the valve seat and the hemisphere during opening and closing, and improving the service life.

Failure analysis of stainless steel pneumatic butterfly valve
The stainless steel pneumatic butterfly valve is structurally composed of a flexible bellows and a joint. The flexible bellows is externally wrapped with a stainless steel wire mesh sleeve for maintenance and pressure bearing. The joint is divided into a part connected with the bellows (internal connection part) and a part connected with the engineering pipeline (or body) (external connection part).
1. The bellows in the stainless steel pneumatic butterfly valve cracked and leaked due to the following reasons:
(1) Weld cracking The flexible bellows section in stainless steel pneumatic butterfly valve mostly adopts longitudinal seam welded pipe. The pipe strength is often insufficient due to the quality defect of longitudinal seam welding itself, leading to cracking and destruction of the weld. This destruction is a quality problem of the longitudinal weld of the bellows, so the quality control of the inch weld should be strengthened.
(2) The bellows pressure is not enough to burst because the pressure and temperature of the recommended medium are too high, and the bellows pressure is not enough to burst. This kind of damage is caused by the design and selection error, so the selection should be stopped strictly according to the hose design and selection specification, and the parameters of corrugated pipe and net sleeve should be affirmed.
(3) Corrosion, perforation and damage due to the large material of bellows
Most of them are austenitic stainless steels, which contain many non-metallic inclusions (such as silicate, oxide, TiN, etc.). When the pipeline is filled with corrosive media, corrosive penetration micropores will be formed at the edge of the weld fusion line or other thin spots. The corrugated pipe material is very thin, usually between 0.15 and 0.8. It is easy to form corrosion penetration on such a thin material, and this damage should be avoided. It is necessary to select qualified stainless steel strips (sensitization and other series experiments are qualified), and at the same time, it is necessary to avoid the destruction of bellows caused by external mechanical damage during consumption and transportation.
(4) Fatigue damage Under the dynamic working state, the stainless steel pneumatic butterfly valve is often damaged by fatigue cracks under the cyclic load, which is a normal failure. In order to extend the service life, it is necessary to ensure that the hose design and selection are correct, the device is accurate, and no additional stress such as twisting occurs to the hose.