Gaskets: Spiral Wound, Ring Joint and Sheet — Which Goes Where

Spiral wound gaskets

A spiral wound gasket is made of a V-shaped metal strip wound with a soft filler such as flexible graphite or PTFE. It has:

• an outer ring (centring ring) that locates the gasket and limits compression,
• an inner ring that protects the windings from the flow and prevents buckling into the bore.

Inner rings are required for some fillers (such as PTFE) and some size and class combinations by ASME B16.20, and many projects require them in all sizes. Typical winding material is 316L stainless steel, with other alloys for corrosive services.

Ring type joint (RTJ) gaskets

An RTJ gasket is a solid metal ring that sits in machined grooves in the flange faces. Types include:

• R type (oval or octagonal), for ASME B16.5 RTJ flanges,
• RX type, pressure-energised, for API 6A equipment,
• BX type, pressure-energised, for high-pressure API 6A flanges.

The ring must be softer than the flange groove so it deforms rather than damaging the groove. RTJ gaskets can’t be reused.

Sheet gaskets

Flat gaskets cut from sheet material such as compressed fibre, PTFE or flexible graphite. They are cheap and suitable for low-pressure, low-temperature services such as water and air, typically in Class 150. They are also used on flat-face flanges, where a full-face gasket is needed.

Insulating gasket kits

On pipelines with cathodic protection, an insulating gasket kit electrically isolates one side of a flanged joint: an insulating gasket plus sleeves and washers for the bolts. Insulating joints (monolithic) are an alternative that avoids the flange.

Gasket	Typical flange	Typical services
Spiral wound with inner and outer rings	Raised face, Class 150 to 2500	Hydrocarbons, steam, most process
RTJ (R / RX / BX)	Ring joint grooves	High pressure, wellhead, some offshore
Sheet (fibre, PTFE, graphite)	Raised or flat face, low class	Water, air, utilities
Insulating kit	Raised face	Cathodic protection boundaries

Bolting and assembly

A gasket only works if it is compressed evenly to the right stress. Use the right bolt material and lubricant, tighten in a cross pattern in stages, and use controlled torque or bolt tensioning for critical joints. ASME PCC-1 gives guidelines for pressure boundary bolted flange joint assembly.

What to take back to your desk

Check that the gasket type in the piping class matches the flange facing on every connected piece of equipment. A raised-face pipe flange bolted to an RTJ valve is a classic site surprise.