In simple terms, a flange must be designed to create a sufficient compressive load on the gasket contact area to create an initial seal. The gasket must conform to the flange surface and must be compressed enough to seal off any internal voids or spaces.
The minimum stress to seal in installation is referred to as the "y" value. The "m" value allows the flange designer to determine the compressive load on the gasket required to maintain a seal when the vessel is pressurized. The flange must have sufficient strength and bolting to hold the joint together against the hydro-static end force and to apply the necessary additional stress on the gasket.
The "m" value is used as a multiplier or maintenance factor. The design will be such that the flange and bolting will hold the flanges together under pressure (the hydro-static end force) and exert additional stress on the gasket of "m" multiplied by the internal pressure. The designer calculates the load required to seat the gasket (related to "y") and performs a second calculation using the "m" value and the design internal pressure. The flanges are then designed based on the larger of the two calculated values.
It is important to note that there have been no established rules to decide what values of “m” & “y” to publish. A manufacturer can publish numbers that are “found by experience to be workable” and there are no clear rules to say what leakage rate the published “m” & “y” factors will lead to. Typically, the "y" factor is based on the minimum stress necessary to provide a seal at an internal pressure of 2 psig (0.14 bar). At Pipeotech however, we are working to eliminate fugitive emissions in all applications. Therefore, our “m” & “y” values are based on tests performed with helium at 40 bar.
Even though the DeltaV-Seal stands out and have values based off an internal pressure of 40 bar, it can relate to a "grooved metal gasket" as found in ASME B16.20 when it comes to "m" & "y". When calculating flange stresses, both the "m" & "y" factors of grooved metal gaskets and the given dimensions of grooved metal gaskets in the code (table GM-2.1-1) can be used for the DeltaV-Seal. This leads to:
The “m” & “y” factors provided here, and the dimensions from ASME B16.20, are for flange stress design only and are not to be used as gasket seating stress values in actual service. Use Pipeotech’s recommended installation values and you will create a lasting "gas-tight" bolted joint.