Page 164 - Machinery Component Maintenance

Process Muchinet-\ Piping 147

Dirty or Damaged Flange Faces. These are obvious causes for leakage
since damage or dirt (including scale) can create a leakage path along the
flange face. Damage includes scratches, protrusions (e.g., weld spatter)
and distortion (warpage) of the flange.

Excessive Forces in the Piping System at Flange Locations. This can occur
because of improper piping flexibility design, or by excessive application
of force to attain flange alignment. Improper location of temporary or
permanent restraints or supports will also cause high flange bending mo-
ments and forces.

The Importance of Proper Gasket Selection

The following discussion covers some of the more important factors
relating to gasket size and type. Flanges are designed to accommodate
specific sizes and types of gaskets. When the gasket does not meet the
requirements necessary to ensure good seating, or is crushed by the bolt
load, leakage will result. Heat exchanger girth flanges are more closely
tailored to one specific gasket than are piping flanges per ANSI B16.5.
Therefore, somewhat greater latitude is possible with the latter.

Gasket Width

The width of a gasket is considered in the design of a flange. For a
given bolt load, a narrow gasket will experience a greater unit load than a
wide gasket. It is, therefore, important to determine that the proper width
gasket has been used.

For asbestos piping gaskets consult Group 1. Appendix E of ANSI
B16.5.
For double-jacketed, corrugated asbestos gaskets consult API 60 I.
For spiral-wound gaskets consult API 601.
For heat exchanger girth flanges, consult the exchanger drawings.

A common reason for gasket leakage is the use of gaskets which are too
wide because of the erroneous impression that the full flange face must
be covered. This is not true. The above standards should always be fol-
lowed.

Gasket Thickness
Gasket thickness determines its compressibility and the load required
to seat it. The thicker the gasket, the lower the load necessary for seating.