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What should I do about flange face damage: accept, repair or replace?
Soft Gaskets Versus Hard Gaskets
The ability of a gasket to seal a damaged area is similar to how a gasket seals against the surface finish of the flange. It depends on how conformable the gasket is to the particular area of the flange face surface. Soft gaskets are more capable of filling (and therefore sealing) a defective area than hard gaskets. For this reason, the appendix divides its guidance into two categories of gaskets: soft gaskets and hard gaskets. Each type has a different limit.
For example, if a single gouge has a radial length of less than one-fourth of the gasket width, the permissible depth of the gouge is 0.050 inch for a soft gasket. The identical gouge on a hard gasket has a depth limit of only 0.030 inch. See Table 1 for additional information. For any damage type, end users must correctly categorize the type of gasket used and match it to the respective table provided in the standard and noted in this article.
Flange Flatness Limits
The flatness of the flange also affects a gasket’s ability to seal. A lack of flange flatness can also be considered damage. This can occur because of excessive loads on the flange or as the result of errant machining of the flange surface. Soft gaskets will also tend to conform to and seal areas of poor flatness better than a hard gasket. Again, the tolerance of a lack of flatness depends on whether the gasket is soft or hard.
For example, a hard gasket can seal a lack of flatness in the radial direction that is less than 0.006 inch. The limit for a soft gasket is 0.01 inch. Guidelines are also included that set limits on a lack of flatness in the circumferential direction. See Table 2 for additional information.
Table 2. Flange seating face flatness tolerances. Reprinted from ASME PCC-1-2010, by permission of ASME. All rights reserved.
Repair, Replace or Accept?
The best policy is always to protect the flange faces from any form of damage. Plants should also establish a quality assurance program that verifies the flatness of new flanges and provides training for maintenance personnel on the acceptable limits as provided in the guidance of ASME PCC-1-2010, Appendix D. This guidance provides the best available information for field personnel to determine whether to accept flange face damage, repair it or replace it.
Author’s Note: ASME reviews and rewrites its standards every three years. PCC-1-2010 has recently undergone its review and rewrite and is currently out for ballot. Once approved via the ballot process, PCC-1-2013 is expected to be published. The new document will offer further clarification regarding the details in this article. However, minor revisions to the information offered in this “Sealing Sense” are expected. P&S