Gaskets are sealing elements installed between mating flange faces. The compressive force applied when flanges are mated (due to bolt torqueing) keeps the gasket in position between the two flanges. Flange gaskets are split into three main categories.
- Non-metallic / Soft – Compressed non-asbestos fibre (CNAF), PTFE, rubber, ceramic fibre.
- Metallic / Hard – oval ring, octagonal ring.
- Semi-metallic / Composite – spiral wound, metal jacketed, camprofile.
Soft gaskets (non-metallic gaskets) have a large contact sealing area and are suitable for low pressure applications only. The usage of soft gaskets is mostly restricted to flat face flanges, although it is possible to use them with raised face flanges also. Irrespective of the type of flange used, a soft gasket should be installed on a serrated flange surface to avoid poor sealing.
Compressed Non-Asbestos Fibre Gasket (soft gasket)
The general appearance of most soft gaskets is that they are thin and malleable (can be bent by hand); they are often delivered in rolls and have to be unwound before use. Typical construction materials for soft gaskets are:
- Elastomers (natural and synthetic rubber), compressed non-asbestos fibre (CNAF), PTFE, rubber, ceramic fibre, flexible graphite.
Soft gaskets are ill suited for medium to high pressure applications and are rarely used for system pressures exceeding 20 bar (290 psi). As this type of gasket is mostly used in conjunction with flat face flanges, its pressure class usage is limited to a maximum of 250.
Soft gaskets are cut to include or exclude the bolt holes. If the bolt holes are excluded, the gasket is termed a ‘flat ring’ gasket and should be used with a raised face flange. If the bolt holes are included, the gasket is termed a ‘full face’ gasket and should be used with a flat face flange.
Tip – its possible to use a soft flat ring gasket with a full-face flange, but this should be avoided. The contact sealing face of a full-face flange is designed to extend from the interior diameter of the flange, to its exterior diameter. If a flat ring flange is used, the actual sealing area is far less than in the original design, thus the possibility for leakage increases considerably.
Hard gaskets (metallic gaskets) have a small contact sealing area and are used for medium to high pressure applications. Because metallic gaskets are ‘hard’, they must have a small sealing face in order to achieve the pressure required to deform the gasket and obtain a reliable seal. The most common type of metallic gasket is the ring-type joint, although others exist e.g. lens ring.
Typical construction materials for hard gaskets are:
- Various grades of steel (using chromium and/or molybdenum as alloy material), copper, soft iron.
Ring-Type Joint (RTJ) Gasket
Composite gaskets consist of two or more materials. The soft material is placed between the sealing faces of a flange to create the required seal. The hard material gives the gasket greater rigidity and sturdiness, which helps the softer material resist deformation effects such as squeezing, buckling, and blow-out. Composite gaskets are used across a wide range of pressures and temperatures and are thus available in all pressure classes.
Spiral Wound Composite Gasket
Composite gaskets can be split into three main categories, these are the spiral wound, metal jacketed and camprofile. Of these three, the spiral wound type is by far the most common. A composite gasket may be used with a raised face flange, tongue-and-groove flange, or a male-and-female flange.
Spiral Wound Composite Gasket Labels