October 2016

Flange Gaskets: Full-Face or Ring

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.

Flange Basics

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.

  1. Pick two holes diametrically opposite. One we’ll call left hole, the other will be right hole.
  2. 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.
  3. 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.

Spiral Wound Gaskets Demystified

Spiral Wound Gaskets are what you reach for when a regular gasket won’t do the job. Regular flat gaskets seal parallel flanges with good surface finish. Over time though, those properties deteriorate and the joint becomes harder to seal. That’s when it’s time to go spiral-wound.

Spiral wound gaskets also work well in high-temperature and high pressure conditions and with corrosive media. That’s why they’re the gasket of choice in many petrochemical plants where their long life helps reduce downtime.

Edge Sealing

Most gaskets are cut from sheet material and that means they seal on their flat surfaces. When flanges are out of parallel the gap between them varies, forcing the use of thicker material. Where the gap is smaller the material must compress more. Pretty quickly the material reaches the limits of what it can do.

In contrast, the spiral wound gasket seals on its edges rather than a flat surface. To do this a long thin strip of gasket material is coiled up to create a ring. Placed between flanges, it’s the sides of the strip that make the seal.

Built-In Spring

To add elastic recovery, ensuring that a seal is maintained regardless of how the joint moves, the gasket material is interleaved with a metal strip. The combined materials are then formed into an approximate ‘V’ section during the coiling process. That metal ‘V’ acts like a spring, pushing outwards against the flanges.

Spiral Wound Gasket Material Options

The choice of gasket material is critical. Usually a spiral-wound gasket is made from PTFE or graphite, both of which come in various grades. In addition, the metal is also selected for the appropriate degree of springiness combined with corrosion resistance and durability.

Spiral-wound gaskets usually come with both an inner metal core and an outer metal ring. These are thinner than the coiled gasket material, so limiting how much the gasket can compress. The inner core also increases blowout resistance; one reason this design works well with high pressures.

For ease of identification, the outer edge of every spiral-wound gasket is color-coded. These codes follow an ASME standard, make it easy to identify and order the replacement part before the joint is taken apart. That helps minimize downtime!

Ask About Going Spiral Wound

The spiral-wound gasket was invented by Flexitallic back in 1912 but today there are a number of manufacturers. The gaskets follow ASME standards but are not all the same as each company has their own materials and designs.

If you have a challenging joint to seal, ask if a spiral-wound gasket would be a good option. If you’re replacing an existing spiral-wound gasket, check the color-code and order the new part before taking the joint apart.  As a spiral wound gasket supplier, Hennig Gasket stocks a large variety of spiral wound gaskets ready to ship.  Contact us today.