Gaskets, Seals, Packings
Preventing leaks and contaminant ingression keeps a system operating as intended.
Seal selection is often an imprecise and time-consuming process, involving numerous compromises. Some qualities a seal must have are obvious such as containing the fluids for which it is designed. Also, the seal must be compatible with the fluids it contacts to maintain its physical integrity. Dynamic seals must have good wear resistance to ensure long life.
Other, not so obvious factors include having sufficient strength to resist extrusion under maximum temperature and pressure. Stability is required to resist twisting and deformation in the seal cavity. Finally, overall economics must be considered.
Unfortunately, some compromise is almost always necessary because the desired features conflict with one another. Loading of a dynamic seal is a good example. High loading between seal and moving surface results in good sealability but also produces high friction and wear. Reducing this load increases seal life but permits more fluid to escape at low pressure.
Seal selection is by no means an exact science. In any application, the designer must decide which factor has precedence. The seal often is so critical to the system that it should be considered early in the design. In struggling with these design factors, many OEM designers use the seal manufacturers' expertise.
Compression seals come in a variety of shapes and are characterized by the high unit load exhibited between seal and walls. These are generally the seals of choice in static sealing applications, which are presumed to be totally leak free. This is especially important at low pressures, when the seal alone and not system pressure must supply the sealing force. They can also be used in light-duty dynamic applications because of their adequate performance at low cost.
Probably the most widely used compression seal is the simple O-ring. An O-ring can be considered an incompressible viscous fluid with very high surface tension. This "fluid" is forced by mechanical or hydraulic pressure to flow into the sealing cavity, blocking the flow of the less-viscous fluid being sealed. Properly installed, the O-ring is squeezed about 10 to 15% of its original cross-sectional diameter. The compression absorbs the tolerance stack up between mating surfaces (or between shaft and gland in dynamic applications), and forces the elastomer into microscopic surface grooves on mating parts.
Under moderate pressure, the O-ring flows up to, but not into, the clearance gap between components. As pressure rises, both sealing force and contact area increase. At its pressure limit, which depends on seal dimensions and hardness, part of the O-ring starts to extrude into the clearance gap. At this point, the seal can shear, leading to failure.
Backup rings protect O-rings and other squeeze packings from extrusion by high pressures. They are generally used at pressures exceeding 1,500 psi and can be used at lower pressures when diametral clearances are large.
Like backup rings, caps or slippers prevent extrusion at high pressure. However, caps also protect against friction in reciprocating applications. They are made of PTFE and come in a variety of shapes.
Compression set of the elastomer is of prime importance when choosing a static seal. Excessive compression set found in lower grade rubber reduces the effective sealing force, resulting in leakage. High temperatures accelerate this condition.
To counteract compression set, a controlled amount of volume swell of the rubber, caused by absorption of hydraulic fluid, is preferred. In most applications, 5 to 15% swell is recommended for a static seal. In light-duty dynamic applications, volume swell can soften the elastomer and increase friction, leading to heat buildup. Thus, swell in dynamic applications should be limited to 5 to 6%.
O-rings are also suitable for light-duty rotary applications. Generally, they provide satisfactory life if running speeds are limited to 750 fpm, and sealed pressures to 200 psi. However, some seal manufacturers permit O-ring use at speeds to 1,500 fpm and pressures to 800 psi.
The key to using O-rings in rotary applications is avoiding the Gow-Joule effect -- the tendency of elastomers under tension to shrink when heated. This sets up a destructive cycle in which friction and heat give rise to even more friction and heat until the seal fails. This is avoided by using the O-ring in compression, rather than tension.
Compression seals are available in a wide variety of shapes. Rectangular rings, quad rings, and H-rings are examples of seals that do much the same job as O-rings, but fill the gland better and have a more stable geometry.
Teflon cap seals consist of a compression seal with a Teflon cap. They are primarily used in applications where high velocity may cause frictional heat buildup. PTFE is used because of a low coefficient of friction and excellent wear characteristics. But PTFE does not seal as well as rubber and is susceptible to damage from solid contaminants.
O-ring loaded lip seals are a good choice when wear resistance and low-pressure sealing are necessary. Urethane, for example, exhibits excellent abrasion and tear resistance, but has poor compression set characteristics. Adding an O-ring to a urethane lip seal provides the compressive sealing force needed at low pressure.