Frequently questions arise regarding the proper selection, use and handling of gasketing materials. Unfortunately these questions tend to emerge after a gasket has failed in service. It must be remembered that a gasket is much more than a spacer which can take all types of abuse and still function as intended. Generally, a gasket is to be placed between two surfaces in order to prevent leakage. Even though a gasket is a static piece of material and is generally not subjected to constant mechanical action or chemical exposure, it must, nonetheless, meet several criteria before it can be expected to perform as expected. The following discussion will be limited to rubber gaskets but it must be realized that metallic, semi-metallic and other non-metallic gaskets also are available.
A good gasket must have the following properties –
The gasket must deform/compress with the application of the force required to seal it. Otherwise, a leak-proof seal will not be achieved.
The gasket must be malleable enough to fill any irregularities in flange face surfaces.
3) Rapid recovery
The gasket must rebound to an appreciable amount of its original gauge when pressure is removed. This is directly related to compression set.
4) Inherent strength
The gasket must be able to withstand the force used to seal the flange face without tearing, shredding, splitting, etc.
5) Minimal relaxation
This is closely related to compression set. The gasket must maintain its original stress-strain properties while under a sealing force. Rubbers with poor compression set resistance and thermoplastics will be seen to bulge out from the flange face.
6) Chemical resistance
If the gasket covers the flange face and extends into the vessel or pipe, it must be resistant to the chemicals within the pipe. IF an otherwise acceptable gasket does not have good chemical resistance, covering the outer 75% of the flange face with the gasket and lining up to it with the prescribed rubber lining is recommended.
7) Heat resistance
Since at least part of the gasket will be exposed to existing service temperatures, it must be resistant to those temperatures.
8) Weather resistance
The outer edge of the gasket will be exposed to external ambient conditions and therefore must be resistant to air, ozone, sunlight, etc. If the gasket is not weather resistant, small cracks may form and premature failure will most likely occur.
Since these properties depend on the gasket having an optimum state of cure, care must be taken to ensure that is does. This is fairly easily accomplished with autoclave and exhaust steam cured rubber lined vessels. Internal pressure curing does not yield optimally cured rubber on the flange faces since they have to be capped to accomplish the desired internal pressure. In these instances, a pre-cured gasket works best.
Blair has several pre-cured linings in various gauges suitable for gasketing. If a specialty gasket such as one with fiber reinforcement is required, you may need to contact a company whose sole business is gasket manufacturing.
Occasionally gaskets will block after being in service for extended periods of time. This makes separation of the flange services extremely difficult and usually results in destruction of the existing gasket. This is caused by over-tightening and/or under-curing of the gasket material among other reasons. If this should be a potential concern a spacer may be placed between the gaskets. This may be a simple polyethylene or fluorinated polymeric product which will not stick to the rubber gaskets.
As a final statement it must be said that tightening of bolts on flange faces is nearly as critical as the selection of gasket material. There is a Blair note on this called – RUBBER LINED PIPE FLANGE BOLT TORQUING PROCEDURE. It is imperative to follow this procedure. After following this procedure, the gaskets must be compressed 25-30 %( not to exceed 33%). If this is not the case, the durometer and state of cure of the gasket must be verified as well as the accuracy of the torque wrench.
This article is intended to be a reminder that a gasket is not a simple piece of material which acts only as a spacer but that proper selection, installation and use of a gasket can be as complex as that of the lining material itself.