One of the questions often asked when people are selecting or deciding on a gasket material is, is thicker better than thinner? The answer to that question is the thinnest gasket possible should always be used and need only be sufficiently thick to take up any flange distortion and misalignment. In addition there are three reason for the applying this general rule of thumb when it comes to selecting a thinner gasket material.
1. Stress Relaxation
The stress relaxation performance or the stress retention property of a gasket is vital in maintaining the level of energy stored in the joint, which effects the seal. Relaxation can occur at the flange-gasket interface as well as within the gasket material itself. This characteristic is particularly relevant in relation to non-metallic sheet jointing materials. Fiber type materials that have a high rubber/elastomer content for example, may be expected to relax significantly as the rubber/elastomer decays at temperature. Therefore, for fiber sheet gasket materials that are “thicker” tend to exhibit increased relaxation over thin ones. This is one reason why the thinnest gasket possible should always be used, and need only be sufficiently thick to take up any flange distortion and misalignment.
2. Tensile Strength
Tensile strength is not necessarily the most important function of a gasket material. For example, expanded graphite is relatively weak, though it performs very well as a gasket material, with a high degree of sealability in a wide range of media. If a gasket is adequately loaded on a flange with the correct surface finish, then the clamping forces resist the tendency for the joint to ‘blow-out’ at pressure. However, if the joint is relatively thick (i.e. with a significant area exposed to the system pressure) and inadequately compressed, then the internal pressure forces on the inside edge, have to be resisted by the tensile strength of the gasket. Again, the thinner the gasket the less relaxation will occur internally as well as exposing less area to the system pressure that is trying to force the gasket from between the flanges.
3. Load-Sealability/Lower Leak Rates
It is a fact that all gaskets leak to varying degrees. For example, while an assembly may be built and hydro-tested successfully, if it is pressurized with helium for example and the flange encapsulated, it may be possible to detect a small mass-leak rate of the helium, after a period of time, using a mass-spectrometer. This leak-rate might otherwise be considered undetectable in general industrial terms, though the load-sealability tests that are conducted by gasket manufacturers and research bodies are invaluable when looking at critical sealing applications. For a given operating stress on a gasket, the leak-rate will increase with increasing system pressure: Similarly, for a sheet gasket material, the leak-rate increases with material thickness, roughly in a proportional manner, (i.e. double the gasket thickness produces double the leak rate). Again a thinner gasket material is preferred.
A word about thicker gasket material
It is not always possible to use thin gaskets. Thicker gaskets conform better to badly damaged or warped flanges, because a gasket’s ability to fill flange irregularities is based on the amount of gasket compression at a given load. Since compressibility at a particular load is expressed as a percentage of the gasket’s original thickness, a thicker gasket with a larger original thickness actually compresses a larger distance. For example, 10 percent compression of a .063-in gasket means a compression of .0063-in, or just over 6-mil. In a 10 percent compression of a .125-in gasket, the gasket will compress over 12-mil. This extra gasket compression means the thicker gasket will fill-in deep scratches or low spots better than a thinner gasket. However, the apparent advantages of using a thicker gasket can be misleading.While the thicker gasket seals more flange irregularities, it can lead to more problems down the road. The higher creep relaxation means that you may have to re-torque the fasteners to maintain adequate compressive load on the gasket over the life of the joint.
To ensure you select the right type for your application contact Four States Gasket and Rubber, Inc
In the Greater Denver Region –
Contact Steven Liparoto at 720-878-1053, email firstname.lastname@example.org
In the Four Corner area (New Mexico, Arizona, Utah, Southern Colorado)
Contact Bryan Crawford at 505-325-9806, email email@example.com
Or Scott Blades at 505-325-9806, email firstname.lastname@example.org
Sources for this blog post:
- Sealing Sense a publication by the Fluid Sealing Association – Article entitled – What gasket thickness should I use in my pump system?
- James Walker’s Gasket Technology Handbook