When sealing raised or flat face flanges there are two choices of gasket shape: full-face gasket and ring-type. Each has advantages, so before ordering you should know which will suit your application best. You should also understand the measurements your gasket supplier needs before cutting material.
ASME standards describe several designs, but the most common are the raised face and the flat face. The difference between them is that the raised face flange has a raised region surrounding the pipe bore. The bolt holes are outside of this. The flat face flange has no such step.
The Ring-Type Gasket
This is positioned inside of the flange bolts and around the pipe bore. In a raised face design it sits on that surface. This design:
- Requires less material and less cutting.
- Can be installed without completely dissembling the joint, (making it a “drop in” gasket.)
- Is harder to clamp in position.
When specifying a ring type gasket only three measurements are needed: ID (which corresponds to the pipe bore,) OD (which is the same is the OD of the raised face,) and gasket thickness.
The Full-Face Gasket
Like the ring-type gasket, this seals on raised flange faces, but has an OD the same as the flange. That means it needs holes for the securing bolts to pass through, and these help locate it on the flange, making alignment easier. Extending out to the flange OD has the added benefit of filling the gap between bolting surfaces, which stops dirt getting in. However, the joint must be completely dissembled for installation.
Specifying a full-face gasket requires these measurements:
- ID (same as the pipe bore.)
- OD (same as the flange OD.)
- Bolt circle diameter (the diameter on which all the bolt hole centers are located.)
- Number of bolt holes (and spacing if they’re not be regular – which would be very unusual.)
- Gasket thickness.
Finding The Bolt Circle Diameter
While ID and OD can be measured with calipers or even a tape, this dimension is harder to determine.
- Pick two holes diametrically opposite. One we’ll call left hole, the other will be right hole.
- Measure from the outer edge of left hole, (the side nearest the flange OD) to the inner edge of right hole, (the side nearest the bore.) That dimension is the bolt circle diameter.
- As a check, measure a second pair of bolt holes and make sure the distance is the same. Remember the rule: outer edge to inner edge!
Full-face and ring gaskets will do an equally good job of sealing the joint. The difference really boils down to installation preferences and priorities.