Why can’t metal gaskets be used with Type A flanges?
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In heat exchangers, why can’t metal gaskets be used with Type A flanges? Asbestos rubber gaskets were chosen for these flanges, but leakage occurred – is it a problem with the gaskets? Is there water on the tube side and steam at 120°C on the shell side?
The selection specifications for Type A flanges are given in Standard P8, Table 2; there are no metal gaskets.
A flange leak is not necessarily due to a problem with the gasket; it could also be caused by deformation of the flange itself or an issue with the tightening sequence. Type A flanges are suitable for low working pressures; the bolts and gaskets used with them must have a strength level that matches that of the flange itself. Flanges designed for low stresses are not appropriate for use with metal linings or wound gaskets.
If you choose according to flange standards, you must adhere to the principle that flanges, studs, nuts, and gaskets must be compatible with each other. If you are designing custom flanges, it is fine to use metal gaskets or wound gaskets. However, when these gaskets are used in conditions with low pressure and temperature, the pre-tightening condition plays a dominant role; as a result, the area of the studs required, as well as the thickness of the flange, becomes quite large, which is clearly unreasonable.
Because type A flanges are equivalent to plate flanges, they are prone to deformation and have low stress levels.