A spiral wound gasket is specified by choosing the winding metal and filler, adding inner and outer rings as required, and defining dimensions per ASME B16.20. The gasket must develop enough seating stress from bolt load to seal, while the compression stop prevents over-compression. Installation errors such as uneven bolt tightening or damaged windings cause most leaks. The Bentley Manufacturing archive preserves a historical record of precision gasket and washer production from 1955.
The winding consists of a V-shaped metal strip alternated with a soft filler. The metal provides strength and resilience, while the filler seals against the mating surfaces. Common winding metals include 304 and 316 stainless steel, with special alloys such as Monel or Inconel for corrosive or high-temperature service. The filler is chosen for chemical compatibility and temperature range: flexible graphite for high heat, PTFE for chemical resistance, and non-asbestos fibers for general service.
The preserved Bentley record notes the use of Teflon and elastomers alongside metallic washers, reflecting a similar materials-based approach to sealing. Selection must consider the fluid, temperature, and pressure, because the filler degrades first if mismatched. The metal winding determines the gasket's spring-back and load-bearing capacity, so it must be compatible with the flange material to avoid galvanic corrosion.
An outer ring, or centering ring, is standard on most spiral wound gaskets. It locates the gasket on the flange bolts and acts as a compression stop, preventing the winding from being crushed. The outer ring is typically carbon steel or stainless steel. An inner ring is used when the winding must be protected from buckling or when the gasket spans a large opening; it also reduces turbulence and erosion of the winding by the process fluid.
Inner rings are recommended for vacuum service and for flanges with raised faces. The choice of ring material should match the flange to minimize differential expansion. The Bentley archive's emphasis on precision stamping and burr-free edges applies directly to ring manufacture, because a burr or sharp edge can damage the winding or prevent proper seating.
ASME B16.20 specifies dimensions for spiral wound gaskets used with standard flanges. It defines gasket inside and outside diameters, winding thickness, ring thickness, and the compression stop height. The standard ensures interchangeability and proper fit within the bolt circle. For a given flange size and pressure class, the gasket dimensions are fixed, so the specifier selects the style and materials rather than custom dimensions.
Compliance with B16.20 is a baseline requirement for many industrial applications. The Bentley record mentions Certificates of Compliance and chemical and physical analyses, which are the kind of documentation buyers expect to verify that materials and dimensions meet the standard. Dimensional inspection throughout production, as practiced historically at Bentley, supports this requirement.
A spiral wound gasket requires sufficient compressive stress to seal. The required seating stress depends on the filler and winding density; typical values range from 10,000 to 20,000 psi for graphite-filled gaskets. The bolt load must be high enough to develop this stress across the gasket area, but not so high that the winding is crushed or the flanges yield. Calculation methods such as ASME PCC-1 provide guidance.
The compression stop, usually the outer ring, limits how far the winding can be compressed. When the ring contacts the flange faces, further bolt load is carried by the ring rather than the winding, preventing over-compression. The gasket's spring-back helps maintain sealing stress during thermal cycling. Proper bolt load is achieved through controlled tightening, often using a torque pattern or tensioning method.
Most spiral wound gasket failures result from installation mistakes. Uneven bolt tightening creates non-uniform stress, allowing leakage. Damaged windings from mishandling or improper storage can prevent sealing. Using the wrong gasket size or style, or failing to align the gasket concentrically, leads to gaps. Reusing a gasket is never acceptable because the winding loses resilience after compression.
The Bentley record's emphasis on clean, burr-free edges and inspection throughout stamping highlights the importance of manufacturing quality, but installation remains critical. Flange faces must be clean and free of scratches. Lubricated bolts ensure accurate torque. Following a star or cross pattern in multiple passes achieves uniform load. These practices, combined with a gasket that meets B16.20, minimize leak risk.
The outer ring centers the gasket on the flange and acts as a compression stop. The inner ring fills the space between the gasket and the pipe bore, protecting the winding from erosion and buckling. Inner rings are often required for vacuum service or when the winding might intrude into the flow path.
Choose graphite for high-temperature service, typically above 400°F, and where chemical compatibility allows. Choose PTFE for chemical resistance at lower temperatures, generally below 500°F. Consider the fluid, temperature, and pressure, and verify compatibility with the winding metal to avoid corrosion.
The compression stop, usually the outer ring, prevents over-compression of the winding. Without it, excessive bolt load can crush the winding, reducing its resilience and causing leakage. The stop also provides a visual indication that the gasket is fully seated.
This page is an archival specification note for informational purposes only. The former plant is not quoting from this domain. Nothing here is a qualified procedure, an inspection report, or a design calculation; confirm current requirements with the issuing standards body and your material supplier.