Choosing the Right Gasket
Correct gasket choice is an important step in the design of a process system as choosing the correct gasket from the beginning can result in significant savings due to longer gasket life, reduced gasket failure and lower leak rates.
Limitations in steam
Steam duty is perhaps the most common and one of the most arduous gasket applications. It is difficult in part because
1. Steam is a powerful hydrolyser capable of changing the nature of many polymers and fibres.
2. An increase in the pressure of saturated steam results in a corresponding increase in temperature. Gaskets under increased stress (ie higher internal pressure) in general require a reduced operation temperature.
3. Many materials harden in steam leading, in some cases, to embrittlement. This is especially applicable to asbestos free calendered materials.
When discussing temperature limits in steam only approximate guide-lines can be given due to considerations such as
Flange design (ie spigot and recess is far better than raised face)
Gasket thickness (thinner gaskets perform better)
Required service life
Degree of acceptable embrittlement
The use of jointing compounds in an assembly
Bearing these factors in mind we suggest that very careful consideration be given to the above parameters, especially where operation is near the borderline of the PT diagrams available.
Low Temperature Duty
The following conditions need to be met before determining suitability of materials at low temperatures.
The gasket is completely dry when installed
The flange is assembled at ambient temperature
The flange and bolt materials are suitable for low temperature use
A general rule is that gaskets cut from a sheet can be successfully used at smaller diameters, lower pressures and/or lower temperatures. As diameter, temperature or pressure increase the use of semi-metallic gaskets are more common. This is largely due to the effect of hydrostatic end thrust in reducing stress on the gasket and increasing the chance of a blow-out.
As a general rule, special care should be taken for gaskets above 600mm diameter, or operating above 200°C or 50 bar.