M & Y Values in Gasket Design

18 Apr 2019 | Posted by Andrew Onions

We are often asked for the M & Y values for our materials by gasket cutters working with design engineers on new project applications or MRO personnel looking to improve or update the specification of the gasket materials in use.

In order for gaskets to work effectively a flange must be designed to create sufficient compressive load on the gasket to surface area to create an initial seal.  The gasket needs to confirm to the flange surface and must be compressed enough to fill any internal voids.

This load is typically expressed as stress in PSI and Is referenced as the ‘Y’ value.

Beyond the initial seal the gasket is required to maintain a seal when the vessel to which the flange is linked is pressurised. The flange requires sufficient design strength and bolting to hold the joint together against the internal force, and the gasket must be able to resist the additional stress being applied.

The ‘M’ value, referred to as the maintenance factor, is the compressive load required on the gasket to maintain the seal once pressurised.

The designer calculates the load required to seat the gasket, related to ‘Y’, and then a second calculation, using ‘M’ value and the design internal pressure.

The flanges are then built based on the larger of the two calculated values.     

The determination of these values is generally achieved through extensive testing of the material in accordance with the ASTM M & Y test method, where determination of the ‘Y’ value is achieved with an internal pressure of 2 psig and the ‘M’ value at 300 psig.

For the ‘Y’ value the compressive load is increased in steps until the compressive stress that achieves a maximum allowable leak rate for that gasket is achieved.

For the ‘M’ value the gasket is first compressed at a stress of 6770 psi and then pressurised to 300 psig. The stress is the lowered in steps until the gasket starts to leak at the maximum rate agreed.  The recorded ‘M' value is the stress value before the leakage exceeded the allowable rate.  

The cost of this testing can be prohibitive for low value materials such as commercial rubber and those used in low technology applications.  Consequently, MacLellan Rubber adopt the figure published in the existing standards for reinforced and unreinforced rubber.