A gasketed joint in a Hi-Hi construction -with the tube-side pressure being higher than the shell-side pressure – is not critical as the net pressure is from the tube-side. This pressure works with the bolts, not against them, since, being directly on the gasket, it is automatically distributed uniformly.
However, when the shell side pressure is in excess of the tube side pressure, this joint becomes highly critical, as the pressure works against the bolts. These bolts transmit their load to the gasket via the internal cylinder. The load on the gasket has to be uniform.
This condition imposes several challenges, such as:
1 To achieve the highest levels of leak-proofing, the use of a kamprofile gasket demands accuracy in the manufacture of the gasket and the gasket face, as also in uniform bolt-tightening procedures.
2 These gaskets must be especially designed for the specific joint that has to be leak proof as per EU standards.
To ensure this:
1 The hardness of the gasket face after stress-relieving and machining must be checked throughout the gasket face.
2 After fabrication, the gasket has to be heat-treated to achieve the hardness 10-15% lower than the gasket face it is tailored for.
3 Tightening procedures should be such, as to apply load which is uniformly distributed throughout the gasket. It must be borne in mind that it’s not the torque applied, but the axial load on the gasket at each bolt location which must be the same.
4 When equal torque is applied to all bolts, the axial load of the bolts on the gasket is far from uniform. Up to 70% of the applied torque may be consumed by the frictional losses. These frictional losses may not be the same for each and every bolt, hence the axial load on the gasket at each location may not be the same. Therefore, an extensive procedure must be adopted to ensure uniform distribution of load.
Therefore, an especially designed and manufactured gasket tailored to the shell and tube-sheet joint is the first and foremost requirement of this procedure.