Leakage from the valve stem area is prevented by double sealing with 2 O-rings and gland gasket. Leakage through the valve body joint is also blocked by double sealing with O-ring and body gasket. After a fire has deteriorated O-ring; gland gasket ;body gasket and stem fire safe packing are the mea¬sure that prevents external fluid leakage.

Internal leakage prevention
When non-metal materials such as O-ring; seat insert and spacer are decomposed or deteriorated by fire; the edge of the metal seat preloaded by the seat sprig comes into contact with the ball to shut off the line fluid to minimize internal leakage through the valve bore. Also the fire safe flexible graphite seat packing will be compressed by the seat spring to prevent fluid leakage between the valve body and seat.