O-Ring Seals And Design Considerations

An O-ring is a tubular piece of material, either an elastomer or metal, with a cross-section in the shape of a circle that can be compressed to fit tightly into a circular grove and seal against the sides of the track. An O-ring, also known as a packing, is a flexible gasket in the shape of a loop — it is a elastomer with a round cross-section designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface. The term “O” in “O-ring” refers to the shape of the cross-section being round.

The O-ring may be used in static applications or in dynamic applications where there is relative motion between the parts and the O-ring. Dynamic examples include rotating pump shafts and hydraulic cylinder pistons. O-rings are one of the most common seals used in machine design because they are inexpensive, easy to make, reliable, and have simple mounting requirements.

Successful O-ring joint design requires a rigid mechanical mounting that applies a predictable deformation to the O-ring. This introduces a calculated mechanical stress at the O-ring contacting surfaces. As long as the pressure of the fluid being contained does not exceed the contact stress of the O-ring, leaking cannot occur. Fortunately, the pressure of the contained fluid transfers through the essentially incompressible O-ring material, and the contact stress rises with increasing pressure. For this reason, an O-ring can easily seal high pressure as long as it does not fail mechanically. The most common failure is extrusion through the mating parts.

The seal is designed to have a point contact between the O-ring and sealing faces. This allows a high local stress, able to contain high pressure, without exceeding the yield stress of the O-ring body. The flexible nature of O-ring materials accommodates imperfections in the mounting parts. But it is still important to maintain good surface finish of those mating parts, especially at low temperatures where the seal rubber reaches its glass transition temperature and becomes increasingly crystalline. Surface finish is also especially important in dynamic applications. A surface finish that is too rough will abrade the surface of the O-ring, and a surface that is too smooth will not allow the seal to be adequately lubricated by a fluid film.

O-rings are widely used in plumbing and other applications to seal and prevent leakage from pipes, joints, valves, tanks, and boilers. They are also used for sealing mechanical components where shafts pass through holes in plates or housings. They have tensile strength but little compressive strength relative to their size. They are inexpensive and can be used in almost any situation where a seal is required. They are available in various sizes, materials, cross-sections, and durometers (hardness).

They have many vital applications in petroleum refining and petrochemical processing. They are used as seals on pipelines and process equipment and to indicate the line location of equipment. They also provide isolation between hot process streams and cold fluids or between two parts of a system at different pressures.They withstand high pressure with minimal leakage but must be replaced when damaged or worn out.

They are also essential machinery to separate uranium isotopes for the nuclear power industry. The centrifuges used for enriching uranium use rotating machines made almost entirely from their materials. They are widely used as seals on pressure vessels and pipes in boilers, where they operate at temperatures up to many hundreds of degrees.

They have been used for decades, mainly with rotating machinery in oil and gas drilling. They are also found in machinery used in nuclear power generation, although some types of reactors now use non-rotating equipment. NDE has had an active O-ring and diffuser technology program, including the development of new materials and processes.

O-ring Seals

Prolonged exposure of the O-ring to high and low pressures causes the material to fracture or leak. The key to a good seal is the proper material selection and sealing techniques. There are three basic types of this seals: pressure, temperature, and vacuum seals.

When a material is heated, it expands and contracts. For O-rings, the ratio of its cross-sectional area to its length decreases with increasing temperature. Therefore, as the internal diameter increases and the outer diameter decreases with increasing temperature, the O-ring seals by friction and stretching will shrink at both ends.

Even slight temperature variations can cause O-rings to leak. The fluid being pumped through the system can be hot or cold (freshwater), and unless the system has been designed for this purpose, all of this temperature variation will cause wear on the O-ring sealing surfaces.

Temperature swings can also cause swelling and contraction of the O-ring. Seals manufactured from elastomers, such as EPDM, exhibit a degree of self-compression in warmer temperatures. The material will shrink and become more resilient as the temperature drops due to moisture content, elasticity, and molecular structure. The result is that the O-ring will tend to be tighter upon system shutdown and require pressurization of the system to release it.

Design considerations

The O-ring seal must withstand high pressure, but it is only as strong as its weakest point. The thickness of the material affects minimum design pressure, and this thickness must also be tested for compression strength. The geometric dimensions of the O-ring are directly related to the operating forces at which it will be used because, under these pressures, the material is under extreme stress. The O-ring dimensions are a function of the gap created between its surfaces in sealing engagement, which results from compression and extension and the thermal expansion characteristics of materials used in manufacturing

Spring Balance Valves: This valve is easy to work with, and most people have experience replacing O-ring with this type of valve. When you pull out the stem, then push down on the O-ring, it will seat itself. Remember not to over tighten the O-ring , as this will cause the O-ring to break prematurely.

Stem Balanced Valves: Stem balanced valves require more pressure, but they are less likely to break, and they can be used with higher-temperature O-ring. With all flat stem valves, you will have to push down on the O-ring while pulling out the stem to seat it. The proper application of pressure is critical in obtaining reliable results with all types of O-rings. The seal must not be over-compressed with excessive force, as this may lead to premature wear or leakage of fluid from contact with the surface of the O-ring.

If you have any questions, please always feel free to contact the experts at High Vac Depot. We are more than happy to answer any questions you have!