Cast steel ball valve has compact structure, reliable sealing, simple structure, easy maintenance, sealing surface and the ball is often closed state, not easily eroded by the media, easy to operate and repair. It is suitable for general working medium such as water, solvent, acid and natural gas. It is mainly used for cutting or turning on the medium in the pipeline, and can also be used for fluid regulation and control. After using for a long time, cast steel ball valve will appear poor fluid phenomenon, what is the reason for this?