Valve Flange Gasket Selection: Spiral Wound, Ring Joint, and Sheet Gaskets

The gasket seals the pressure boundary at every flanged joint in a piping system, including the flange connections at each end of a valve and the body-bonnet joint in valves with bolted bonnets. Gasket selection determines whether the flange joint will maintain sealing integrity over the required service life under all operating conditions including temperature cycling, pressure fluctuation, vibration, and mechanical loads. Using the wrong gasket material, wrong gasket style, or wrong gasket dimensions leads to leakage at best and catastrophic joint failure at worst. Systematic gasket selection considers the process fluid chemical compatibility, operating temperature and pressure, flange facing type, and bolting load available to seat the gasket.

Spiral Wound Gaskets

Spiral wound gaskets are the most widely used gasket type for process piping flanges in Class 150 through 2500. They consist of a spiral winding of alternating metal strip (typically 316 stainless steel or other alloys) and filler material (typically flexible graphite, PTFE, or ceramic fiber) formed into a ring. Inner and outer guide rings of solid metal control the compression of the winding and prevent over-compression or blow-out. The flexible filler conforms to minor flange face imperfections to provide sealing, while the metal windings provide resilience and recovery under thermal and pressure cycling. The compressible filler material makes spiral wound gaskets excellent choices for services with temperature cycling where solid metal ring gaskets might relax and leak.

• Spiral wound with graphite filler: standard for general service to 600°C

• Spiral wound with PTFE filler: chemical service, corrosive fluids, food and pharmaceutical

• Ring type joint (RTJ): solid metallic ring for Class 600 and above, very high sealing stress

• Oval ring: older RTJ profile, replaced by octagonal ring in most modern applications

• Sheet gaskets (compressed fiber, PTFE): lower-pressure service, raised face flanges, class 150-300

Ring Type Joint (RTJ) Gaskets

Ring type joint gaskets are solid metallic rings machined to precise dimensions that seat in matching grooves machined in the flange faces. As the flanges are bolted together, the bolting load forces the ring into the grooves, plastically deforming the ring into the groove surfaces to create a metal-to-metal seal of very high contact stress. RTJ gaskets provide the highest achievable sealing performance and are specified for high-pressure (Class 600 and above) and high-temperature service where the resilience of spiral wound gaskets is unnecessary and the very high bolt loads available can seat the metal ring adequately. Oval and octagonal ring profiles are the two standard shapes; octagonal rings are preferred in new designs because they provide more even contact stress distribution. RTJ gaskets are single-use: the plastic deformation that creates the seal means the gasket cannot be reused after disassembly.

Gasket Installation and Torque Requirements

Correct gasket installation requires that the gasket be seated on a clean, undamaged flange face without radial scratches or tool marks in the sealing contact area. The gasket must be centered on the flange bore, and the correct bolt torque applied in a proper cross-pattern sequence to achieve uniform gasket seating stress. Under-torquing leaves the gasket insufficiently stressed to seal under operating pressure and thermal transients. Over-torquing can crush spiral wound gaskets past their optimal thickness, reducing resilience and potentially causing the filler to extrude. Gasket manufacturers provide bolt torque recommendations based on the gasket dimensions, materials, and required gasket seating stress, and these recommendations should be followed rather than applying arbitrary uniform torque values regardless of gasket type.