A spiral wound gasket consists of spirally wound (usually V-section) metal strip with filler material and an outer metallic ring to hold the windings in place. Spiral wound gaskets are categorized as semi-metallic gaskets and their design is covered by ASME B16.20 - Metallic gaskets for pipe flanges. A spiral wound gasket is designed for one time use.
Retaining Ring or Centering Ring
The outer metallic ring of spiral wound gaskets is known as retaining ring or centering ring or outer guide ring. The centering ring just makes contact with the stud bolts and thus helps in aligning the gasket on the flange face. As per ASME B16.20 there is a clearance of 1/64" between the stud bolts and centering ring. If there is more gap between the stud bolts and centering ring, it is an indication that the correct size or rating of gasket is not being used on the flange connection.
The centering ring also acts as a compression limit stop to prevent over-compression of the gasket during the flange assembly as well as an anti-blowout device. As per ASME B16.20, the thickness of centering ring is required to be between 0.117 inches (2.97 mm) - 0.131 inches (3.33 mm). The outer guide ring is generally supplied as coated carbon steel but could be provided in other materials for corrosion protection against external environment. For operating temperatures below -45°C the centering ring may be supplied in stainless steel SS-304 material. It may be noted that the retaining ring does not come in direct contact with the contained fluid and hence need not be compatible with the process fluid.
Inner Metal Ring or Retaining Ring
As per ASME B16.20, inner metal rings are required to be specified for flexible graphite and other filler materials such as PTFE, particularly where experience has demonstrated an inward buckling of gaskets or lateral flow of gaskets towards the bore to be a problem. The inner ring acts as a compression limit stop and prevents the gasket from being overcompressed and inward buckling. Inner rings are required to be provided for the following flanges:
- 24 inch and larger in class 900 rating
- 12 inch and larger in class 1500 rating
- 4 inch and larger in class 2500 rating
The thickness of inner ring is same as outer ring. The material of the inner ring should normally match the material of the winding. Inner retaining ring increases the axial rigidity and resilience of the gasket. By filling the annular space between the gasket and flange, the inner ring reduces turbulence in the flange area and possibility of accumulation of solids or debris in the gap.
Additionally, inner rings are recommended for the following applications:
- Vacuum service
- High pressure or temperature service
- Aggressive service
- Flange finish smoother than 125 µ-inch
Filler Material
The commonly used filler materials and their temperature limits are provided below for guidance. The actual limits may vary with manufacturers.
| Material | Minimum °C | Maximum °C | Abbreviation |
|---|---|---|---|
| Ceramic | -212 | 800-1010 | CER |
| Flexible Graphite | -212 | 510-550 | FG |
| PTFE | -240 | 260 | PTFE |
| Mica Graphite | -212 | 288 | - |
Some process conditions may require that graphite based filler be coated by an appropriate corrosion inhibitor in order to prevent possible galvanic corrosion.
Gasket Thickness and Compression
The uncompressed thickness of spiral wound gasket as per ASME B16.20 is 4.45 mm and the thickness after compression is 3.3 mm ± 0.13 mm based on a uniform bolt stress of 30,000 psi. There are exceptions to this requirement for flange sizes 1/2", 3/4", 1 " in class 150, class 300 and class 600 rating. Other thicknesses available with manufacturers are tabulated below for reference. The actual compressed thickness is required to be verified with the manufacturer.
| Nominal Thickness "mm" | Compressed Thickness "mm" | Guide Ring Thickness "mm" |
|---|---|---|
| 2.5 | 1.9 - 2.1 | 1.9 - 2.1 |
| 3.2 | 2.3 - 2.5 | 2.0 - 2.2 |
| 4.5 | 3.2 - 3.4 | 3 - 3.3 |
| 6.4 | 4.6 - 4.8 | 4.6 - 4.8 |
| 7.2 | 5.0 - 5.4 | 5.0 - 5.4 |
Gasket Parameters for Bolting Calculations
The following spiral wound gasket parameters may be considered for flange bolting calculations as per ASME Code.
| Gasker Factor ( m value) | 3.0 |
| Minimum Design Seating Stress (Y value) | 10,000 psi |
| Maximum Seating Stress (Sg) | 30,000 psi |
The maximum gasket stress should not exceed the maximum seating stress.
Flange surface finish required for Spiral Wound Gaskets
The following surface finishes are considered most appropriate for spiral wound gaskets.
| Media (Process Fluid) | Flange Seating Surface Finish |
|---|---|
| General Use | 6.3 µm (250 µinch) |
| Hazardous Service | 3.2 µm (125 µinch) |
During flange assembly, as the bolts are tightened the flange faces compress the gasket. When internal system pressure acts on the inside of gasket, the windings are radially extended, which leads to the filler material being squeezed outward against the flange faces. Due to the chevron (V-shape) and the orientation of metal windings, as the internal pressure increases, a higher sealing force is generated against the flange faces as the v-shape tends to become straight.