Stop valves with bellows seal saddle. The bellows assembly as a seal is essentially “a kind of diaphragm valve”: a corrugated tube is used as a force transmission assembly, which is stretched and compressed. Naturally, the working medium is physically separated from the control rod - it is on one side of the tube, and the rod is on the other. Thus, bellows fittings are tight, but limited by the number of movements and, almost unrepairable. 

Valves with bellows seal provide COMPLETE TIGHTNESS connections and eliminates the need for maintenance during operation, but the bellows seal structurally more complex, more expensive and has a limited resource stem movements.