Appendix 2 Flange - Gasket

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Last updated: Aug 20, 2019 by Mike ClarkVersion comment
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Flange Neck Dimensions

Inside Diameter(B): For non-reverse integral flanges and reverse flanges, this is the ID of the flange host. For non-reverse loose type flanges and spherically dished covers, this is the bore of the flange.

Inside Diameter(B’): Only applicable for reverse flanges, this value represents the bore of the reverse flange.

Mating Flange I.D.: The ID of the mating flange that is exposed to pressure in blind flanges.

Head Factor C: This value is only applicable for blind flanges and it used to determine the required thickness for the blind flange. See UG-34 for more information.

Weld Efficiency: This value is only applicable for blind flanges and it used to determine the required thickness for the blind flange. See UG-34 for more information.

Lever arm (hR): The lever arm for the radial component of the membrane load of the spherical segment Hr. This field is only required for the design of a spherically dished cover for Figure 1-6(d). The magnitude of the moment arm is most easily determined by reviewing the mentioned figure; it is the axial distance from the centroid of the flange thickness to where the mid thickness of the dish intersects the flange. If the mid-thickness of the dish is closer to the gasket face of the flange (in the axial direction) than the centroid of the flange is to the gasket face, the sign of hR is positive; otherwise, it is negative. It may be more or less conservative to determine this value in the corroded condition.

Hub thickness (g0): The flange hub thickness at its thinnest point. This dimension will differ depending on the flange type and attachment detail (e.g., if the flange is a certain type, g0 may be the thickness of the host nozzle neck).

Weld leg size: The fillet weld between the back of the flange and the host. In several cases this value, or the sum of this value and the host thickness, act as the g1 value.

Flange face bevel size: This value is only available for lap joint flanges. It affects the calculation of the moment arms because the bevel size removes area of the flange that is acting on the lap.  Refer to ASME Section VIII, Division 1 - Appendix 2 Figures 2-4(1) and 2-4(2).  In the second figure this value is the dimension for the material removed on the flange face the contacts the back of the lap. 

Hub Thickness (g1): The flange hub thickness at its thickest point at the back of the flange. This dimension will differ depending on the flange type and attachment detail (e.g., if the flange is a certain type, g1 may be the sum of the thickness of the host nozzle neck and the fillet weld on the back of the flange).

Hub Length (h): The hub length is the distance from the back of the flange to the point where the hub is thinnest. Some of the flange configurations have requirements on this length.

Gasket and Facing Details

Material: A description of the gasket that will appear on the report but will not directly affect the results.

Type: A secondary description of the gasket. This will also appear on the report without directly affecting the results.

Facing Sketch: Review Table 2-5.2 from Appendix 2 for more information. This is used to determine the Effective Gasket Width, bo.

Seating Column: Review Table 2-5.2 from Appendix 2 for more information. This is used to determine the Effective Gasket Width, bo.

Factor m: Suggested values for the gasket factor may be obtained from Table 2-5.1 in Appendix 2. More accurate values should be available from the gasket manufacturer.

Seating Stress y: Suggested values for the gasket seating stress may be obtained from Table 2-5.1 in Appendix 2. More accurate values should be available from the gasket manufacturer.

O.D. contact face: The outside diameter of the gasket on the face of the flange. Depending on the "Facing Sketch," this value may be the actual outside diameter of the gasket or it may only reflect the diameter of actual contact with the flange (e.g., on a raised face flange where part of the gasket extends outside the raised face).

O.D. lap face: The outside diameter of the lap, not the flange, used with lap joint flanges.

Gasket width (N): The width of the gasket that is actually in contact with the flange. Depending on the "Facing Sketch," this value may be the actual width of the gasket or it may only reflect the width of actual contact with the flange (e.g., on a raised face flange where part of the gasket extends outside the raised face).

Check seating conditions for self-energizing gaskets: Select this box to run the gasket seating calculations for self energizing gaskets. Appendix 2 has a gasket seating bolt load W that can be significantly higher than the operating bolt load. This bolt load is determined as 0.5*(Am + Ab) where Ab is the actual total bolt cross-sectional area and Am is the higher of the area required for gasket seating or operating. W is used in the gasket seating check for determining stresses. Running the gasket seating case is optional because it may greatly increase the flange thickness due to the apparent excessive conservatism for gaskets of this type.