Gasket parameters – Part I – Gasket factors ”m” and ”y”
The tightness of the assembly is a very important issue for the integrity and safety of the installation, as well as for operating in prescribed parameters and obtaining desired efficiency. However, for quite some time now, the design of flange joints only takes into account the mechanical integrity and stability. With the increasing importance given to environmental protection and fugitive emissions, new norms and methods for design and calculation are also being developed.
There are 3 most commonly used calculation methods, each having different parameters – “m” and “y” for ASME sect. VIII div. 1 App 2, “Gb”,”a” and “Gs” for ASME BFJ and “Qsmin(L)”, “Qsmax”, “Qmin(L)”, “PQR” and “EG” for EN 1591-1+A1.
The most common method of calculation is ASME Boiler and Pressure Vessel Code, Section VIII, based on the factors ”m” and ”y”, mentioned for the first time in 1942.
- Factor ”m”– defined by ASME code as gasket factor or tightening factor. This is a multiplier applied to the value of the internal fluid pressure in order to obtain the necessary working gasket seating pressure. The maintenance factor ”m” is dimensionless. To be noted that the factor ”m” was established before tightness classes were developed.
- Factor ”y” – is the required load in order to close the gasket porosity and to ensure compliance with the flange surface. This is not the required assembly stress which will be defined by the new standards. The unit of measurement is psi. According to ASME code, ”y” is the pressure to seat the gasket in the flange and does not take account of the internal pressure which has to be contained in the service application. The ”y” factor is an empirical estimate only.
Factors ”m” and ”y” play an important role in design calculation. The experimental support for these factors in the norm ASTM F586, first introduced in 1979. This standard was withdrawn in 1998, following the need for a complete change in the theoretical system and the actions of the PVRC in this regard.
It is clearly stated in the ASME code that the gasket stress obtained usig the ”m” and ”y” factors are valid only for the mechanical calculation of the flanges and bolts and do not ensure sealing. Actually, there is no reference to a certain level of tightness in this calculation, as if the gasket had only two possible cases, to leak or not to leak.