The initial compression force applied to a joint must serve several purposes.
- It must be sufficient to initially seat the gasket and flow into the imperfections on the seating surfaces regardless of operating conditions.
- Initial compression force must be great enough to compensate for the total hydrostatic end force that would be present during operating conditions.
- It must be sufficient enough to maintain a residual load on the gasket/flange interface.
The m factor
What is m factor ?
From a practical standpoint, residual gasket load must be "K" times internal pressure if a tight joint is to be maintained. This unknown quantity "K" is known as "m" factor in ASME code and varies depending on the type of gasket that will be used.
The "m" value thus is the ratio of residual unit stess (bolt load minus hydrostatic end force) on the gasket to internal pressure of the system. The larger the number used for "m" the more conservative the flange design would be, and the more assurance there is of a leak tight joint.
The "m" factor also referred to as maintenance factor provides the additional preload needed in the flange fasteners to maintain the compressive load on a gasket after internal pressure is applied to a joint.
m = (W - A2P)/A1P
Where:
W = Total Fastener force (Ib. or N)
A2 = Inside area of gasket (in.2or mm2
P = Test pressure (psig or N/mm2)
A1 = Gasket area (in.2 or mm2)