Frequently questions arise regarding the proper selection, use, storage and handling of gaskets. Unfortunately these questions usually emerge after a gasket has failed in service. It must be remembered that a seal is much more than a spacer which can take all types of abuse and still function as intended. Generally, a gasket is placed between two surfaces in order to prevent leakage. Even though that is a static piece of material and is generally not subjected to constant mechanical action or chemical exposure, it must, nonetheless, meet several criteria to be able to perform its function.
A good gasket must have the following properties
It must deform (compress) with the application of the force required to seal it. Otherwise, a leak-proof seal will not be achieved.
It must be malleable enough to fill any irregularities in flange face surfaces.
It must be able to withstand the force used to seal the flange faces without tearing, shredding, splitting, etc.
- Rapid recovery
It must rebound to an appreciable amount of its original gauge when the pressure or load is removed. This is directly related to Compression Set.
- Minimal relaxation
This is also 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.
- Heat resistance
Since at least part of the gasket will be exposed to existing service temperatures, it must be resistant to those temperatures.
- 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. The acceptable gasket must cover at least outer 75% of the flange face.
- 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, could be formed small cracks and is very likely premature failure to occur.
Pay Attention When Handling a Gasket
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.
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 gaskets. This is caused by over-tightening and/or under-curing of the gaskets among other reasons. If this should be a potential concern there may be placed a spacer 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 a type of the gasket or its material from which it may be cut. Тhe gaskets must not be excessively compressed. Тhe durometer and state of cure of the gaskets must be verified as well as the accuracy of the torque wrench.
А gasket is not a simple piece of material which acts only as a spacer. The proper selection, installation and use of a gasket can be as complex as that of the lining material itself.