PISTON VALVE
	The Sealing system of Piston Valve is astonishingly simple and extraordinarily effective: Always leak-tight across the port and to atmosphere.
	The shut-off assembly comprises a stainless, steel piston, two resilient valve rings and a ferrous-metal lantern bush. The sealing surface consists of the cylindrical surface of the piston and the corresponding inner surface area of the sealing rings. This provides a very large sealing surface compared with globe valves with conventional design - one of the reasons for the piston valve's excellent seal.
The upper valve ring provides a reliable seal to atmosphere. The middle layer of the ring is of PTFE and is fully contained between Kor-P laminates, the valve body and the piston, in both the open and shut positions. The pressure on the valve ring is provided by the nuts securing the bonnet to the valve body. Belleville washers are fitted under the bonnet nuts to ensure that the pressure of the bonnet on the valve ring is kept constant. This maintains uniform contact of the PTFE with the piston and the body : permeability to gases is reduced to a minimum and a optimum seal to atmosphere is guaranteed. The lower valve ring provides the seal across the ports. The lantern bush serves as a distance piece between the two rings.
The thrust produced by tightening the bonnet nuts acts axially from above, this thrust is transmitted from the upper valve ring via the lantern bush, to the lower valve ring, which is contained in the body. The resilient sealing rings are thereby compressed and transform this thrust into a radically acting pressure. Being supported against the wall of the valve body, the laminates of the valve rings are compressed around the piston and thereby produce an outstanding seal. The piston valves stay tight-without maintenance.
The valve rings are the heart of a piston valve. They are made from a material specially developed and exclusively used for the manufacture of valve ring which consists essentially of asbestos, a high quality elastomer and graphite.
Valve rings are not merely surface-graphited; the graphite content extends throughout the ring. For this reason its outstanding self lubricating properties are retained throughout the ring's entire service life.
Valve rings are resistant to water, steam, oils, solvents and a wide variety of other media.