PISTON RING AND SEAL RING DESIGN CONSIDERATIONS

The terms piston ring and seal ring are used somewhat interchangeably and refer to rings that have a gap or split and are used as a seal to prevent fluid passage. The gap or split improves sealing efficiency by providing better conformability to the sealed surface and thus less leakage and also facilitates installation. Design of the ring must take into account pressure differential, operating temperature, viscosity of sealed fluids, co-efficient of friction, material compatibility, corrosion, linear or rotational movement, and relative speed of movement.

Rings can be designed to seal on the OD of the ring (expanding) or the ID of the ring (contracting or rod ring). Since most piston and seal ring leakage occurs at the gap, many different gap designs have been developed. Gap designs such as step seal (gap ends overlap with the overlap surface at an angle to the ring OD) and step joint (gap ends overlap with the overlap surface parallel to the side of the ring) provide substantially better sealing over a standard butt joint or straight cut gap. Some gap designs such as hook joint are used to facilitate installation where access is limited.

Piston and Seal Rings utilize the fluid on the high pressure side plus ring tension to seal against the moving surface and the surface of ring groove. Ring tension is primarily a function of the wall dimension, the ring's free or relaxed gap and the modulus of elasticity of the material. Tension can be enhanced, if needed, by use of expanders or contractors that cause the ring to exert additional pressure against the mating surfaces.

Material selection, of course, is of great importance. Considerations include conformability to surface being sealed, operating temperature, relative thermal expansion, compatibility with mating parts, corrosiveness of fluids being sealed, coefficient of friction of the moving surfaces, and assembly stresses. Surface finish and dimensional accuracy obviously play a very important role in the sealing efficiency of rings.

At United Ring & Seal, inc. we understand these factors and apply them with prudence and cost effectiveness in our design recommendations.