1. Field of the Invention
The present invention relates to the field of valves and the like, and more particularly but not by way of limitation, to an improved seal ring assembly for operating pressure retention on pistons and the like.
2. Discussion
There are many applications in which valve operation is assisted, or primarily achieved, by use of controlled operating pressure. That is, many valves are actuated by controlled routing of fluid pressure against movable surfaces, such as spool pistons, to effect interaction with valve components to selectively open and close fluid flow ports.
Such fluid operated valves can be found in virtually every industrial area, including aircraft installations using air streams often high in potential contaminants. For example, in high temperature gas turbine pneumatic valving applications, airborne contaminants can combine with particles from the compressor section (e.g. nickel and cobalt alloys) to form compounds that, if leaked past a seal, such as a piston ring seal, can collect in places which can impede the effectiveness of the seal.
Three-piece split ring seal assemblies are commonly used to seal valve pistons and the like, and in high temperature applications the use of graphite rings has found wide acceptance. Such three-piece ring seals comprise a seal ring backed by an inner ring, both of which are in radial juxtapositional contact with a larger backup ring equal in thickness to the combined thicknesses of the seal ring and the inner ring against which the backup ring is disposed. Since it is generally easier to produce accurate sealing surfaces either as flats that can be lapped or as cylindrical surfaces that can be cylindrically ground and lapped, the three-ring seal is a good selection as all the mating surfaces are of this type.
While three-piece ring seals offer advantages, and in many cases present the only positive seal for applications requiring rings less than one inch in diameter, these ring seals have limitations. Airborne contaminants in high temperature applications, such as that mentioned above, can form compounds that collect in between the piston and the ring inside diameter, as well as in the ring gaps. This trapped contamination tends to jam the piston ring, and as a consequence thereof, the flexibility of the ring seal is reduced considerably, if not altogether. This loss of flexibility can result in sticking pistons, or it can increase seal leakage due to the seal's inability to conform to the confining cylinder wall.
Attempts to solve these problems have included the uses of step seal rings; two-piece rings (straight cut inner and step seal outer); and single piece rings with no break or ring gap in conjunction with a multiple piece piston to support the unitary ring. Some designs include loose fitting pistons and seals, and have high pilot port flow to provide forces sufficient for actuation. These valves typically require higher pressures to operate.
Some designs use no seal rings and rely on precision ground, hardened pistons and cylinders to achieve very close fits for leakage control.
None of these prior art seals provides reliable low pressure sealing in a high temperature environment valve application subject to airborne contaminants where low friction, unidirectional sealing is required.