Spiral Wound Gasket Inner Ring

Quick Answer

An inner ring is a solid metal ring fitted inside the bore of a spiral wound gasket (SWG). It prevents the windings from buckling inward under bolt load. ASME B16.20 requires inner rings for gaskets in Class 900 and above, and they are strongly recommended for vacuum service, high-temperature cycling, and corrosive or toxic media.

The inner ring serves two purposes: it acts as a compression stop on the bore side (preventing over-compression that would push windings into the flow path) and it protects the innermost sealing element from erosion by the process fluid. Without an inner ring, high bolt load or thermal cycling can cause winding inversion, leading to gasket fragments entering the piping system.

When an Inner Ring Is Required

ASME B16.20 sets specific rules for inner ring usage, but many engineering companies add inner rings in services where the standard does not mandate them.

Condition	Inner Ring Required?	Reason
ASME Class 900 and above	Yes (per ASME B16.20)	High bolt loads risk winding buckling
Vacuum service	Strongly recommended	Negative pressure can pull windings inward
Thermal cycling > 200 degC swing	Recommended	Repeated expansion/contraction loosens windings
Toxic or lethal fluid	Recommended	Any gasket fragment in the line is unacceptable
NPS 24 and above	Recommended	Large diameters increase buckling risk
Class 150 / 300 general service	Optional	Lower bolt loads reduce buckling risk
Sour service (NACE MR0175)	Recommended	Winding damage creates leak paths for H2S

Inner Ring Materials

The inner ring must resist corrosion from the process fluid. It is typically the same alloy as the winding strip or one grade higher.

Service Fluid	Winding Metal	Inner Ring Metal
Hydrocarbons, steam	SS 316	SS 316
NaOH, caustic	Monel 400	Monel 400
HCl, wet chlorine	Hastelloy C-276	Hastelloy C-276
High-temp H2 service	SS 321	SS 321 or SS 347

SWG Designation Codes

ASME B16.20 classifies spiral wound gaskets by their ring configuration:

Type	Outer Ring	Inner Ring	Typical Use
CG (or “with centering ring”)	Yes	No	Standard RF flange, Class 150-600
CGI (or “with centering and inner ring”)	Yes	Yes	Class 900+, vacuum, toxic service
No rings	No	No	Tongue-and-groove or male-female flanges

The CGI configuration is the most reliable option and the de facto standard for critical process piping in most EPC projects. The additional cost of the inner ring is negligible compared to the risk of a gasket failure in high-pressure or hazardous service.

Installation Notes

The inner ring must sit flush with or slightly below the flange face. Verify the bore clearance: the inner ring ID should be equal to or slightly larger than the pipe bore to avoid flow restriction. During bolt torquing, apply load in a star pattern to ensure even compression across the gasket. An improperly installed inner ring (e.g., cocked or protruding) defeats the purpose and may itself become a leak source.

Pro Tip

On purchase orders, specify “SWG with inner ring” explicitly, even for Class 150/300. Vendors may supply the cheaper CG style by default, which leaves the bore side unprotected.

Pair inner-ring SWGs with the correct stud bolt length and torque for the flange class. The outer ring thickness sets the compression stop on the bolt side; the inner ring does the same on the bore side. Both are needed for a reliable seal in demanding services.

Read the full guide to gasket selection