Engineers need critical design information when choosing a seal for a particular valve application. This includes:
- Operating temperature and pressure ranges
- What fluids or gases are being processed
- The environmental conditions
- Whether the seal is static or will be exposed to dynamic motion
- Expected life of the seal
In a recent presentation at the Valve Repair Council’s seminar, Nathan Sowder, business development manager for oil & gas and chemical processing at Parker’s O-Ring Division, explained the basics of elastomeric seal design. Starting out with an overview of the various polymers used in seals, he compared the pros and cons of various reinforcing fillers, plasticizers, process aids and cure systems. A key component of seal performance is the compound that it is made of.
Sowder pointed out that the best O-rings in the world won’t work if the gland they’re in isn’t designed correctly. About 25 to 50% of all leakage issues seen our applications engineers can be attributed to gland design, Sowder noted. “The remainder are caused by poor seal material selection, seal damage, and/or reaching the end of the seal’s service life.”
When designing a seal for an application, the goal is to eliminate tangential leaks, which are leaks around the seal, and permeation, which is leakage through the seal. The primary design considerations are:
- Material Selection – Includes temperature range, extrusion resistance, abrasion resistance, permeability, fluid compatibility and rapid gas decompression resistance.
- Squeeze – The amount of compression on the seal. Squeeze is expressed as a percentage of the free-state cross-sectional thickness: too little shortens seal life, too much overstresses the material. The optimum nominal design squeeze is typically around 20 to 25%.
- Gland fill – The amount of space filled by the seal in the gland. This is expressed as gland volume vs. seal volume, and it’s recommended there be a 75% nominal fill, and a 95% maximum gland fill. Have some vacant space allows for volume swell, thermal expansion and increasing width due to squeeze. Overfill can damage the seal by causing extrusion into the clearance gap.
- Stretch – The amount the seal is stretched in the installed state. The general rule is that it should be around 0 to 5%. Excessive stretch can overstress material and can thin the cross section and reduce the percentage of squeeze.
Static O-Ring Sealing
There are four configurations most commonly used for O-rings:
- Face Seal – Two flat plates, one containing a groove, which are bolted or clamped together. No stretch and zero clearance, with a 20 to 30% squeeze is recommended. The surface finish of sealing substrates is normally 32 micro-inches RMS for liquids, and 16 micro-inches RMS for gases.
- Radial Seal – Also referred to as a piston or rod seal. Recommend 0 to 5% stretch and a 15 to 30% squeeze, which can vary with the cross sectional thickness of the seal. The area of greatest concern, especially at higher pressures, is the clearance gap between the piston and bore. Too large a gap, or the use of the incorrect material can cause extrusion damage to th- seal. The surface finish is the same as that for the face seal.
- Dovetail/Half-Dovetail – Used primarily to retain the seal in the groove. This groove has undercuts that are created by pre-designed cutting tools but can be more difficult and expensive to machine.
- Crush Seal – Typically recommended as a 45-degree angle and can be utilized in threaded fittings. In most instances this is a seal configuration that results in significant permanent deformation of the O-ring which limits the ability to be removed and re-used multiple times.
Other considerations to be taken into account when designing seals are sharp corners, which create installation difficulties, pinching and cutting, the surface finish, tolerance stack-ups, eccentricity and side loading and pressure vs. clearance – extrusion and back-up rings.
O-rings can easily be damaged during installation. Primary causes can be from sharp corners, insufficient lead-in chamfer, no lubrication or an improperly sized O-ring. Damages can occur when the seal is being sheared, torn, nicked or cut in a variety of manners.
Figure 1 is a graphic from a Finite Element Analysis animation showing installation damage caused by an insufficient lead-in chamfer. With a sharp corner contacting the elastomeric O-ring it is evident that significant damage may be caused.
Solutions to damage occurring during installation include covering threads during installation, using lubrication when possible, having proper lead-in chamfers, smooth edges and corners and making sure the correct size seal is being utilized.
To ensure proper performance of seals in your valve applications, it is important to evaluate the material being used as well as the gland design where they are being used.