Valve Gasket Selection and Flange Sealing Systems

Gaskets are the pressure boundary element between valve flanges and connected piping, and their selection is as important to plant integrity as the valve body material itself. Gasket failures—blowouts, leaks, and chemical attack—cause unplanned shutdowns, safety incidents, and environmental releases. Correct gasket selection considers flange facing type, bolt load capability, operating temperature and pressure, fluid compatibility, and applicable sealing standards.

Flange Facing Types and Gasket Compatibility

The flange facing type determines which gasket design is compatible. Raised face (RF) flanges, the most common industrial standard, use full-face or spiral wound gaskets that are compressed by bolt load onto the raised seating surface. Flat face (FF) flanges, used with cast iron or non-metallic valve bodies, require full-face gaskets that extend to the bolt holes to distribute bolt load and prevent flange distortion. Ring type joint (RTJ) flanges use metal ring gaskets (oval or octagonal cross-section) that seat in machined grooves in the flange face, providing very high pressure sealing capability for Class 900 and above. Male-female (M-F) and tongue-and-groove (T-G) facing types confine the gasket laterally to prevent blowout and are used in high-pressure heat exchanger and specialty valve applications.

• Raised face (RF): most common—use spiral wound, kammprofile, or ring gasket

• Full face (FF): cast iron and plastic flanges—full-face gasket to prevent flange cracking

• Ring type joint (RTJ): Class 900+—metal oval or octagonal ring, excellent high-pressure seal

• Male-female (M-F): confined gasket—high sealing reliability, requires matched flange pairs

• Tongue and groove (T-G): similar to M-F—used in critical high-pressure services

Spiral Wound Gasket Construction and Grades

Spiral wound gaskets are the most widely used gasket type in process plant piping and valve connections. They consist of a v-shaped metal winding strip (usually 316 SS) alternating with a filler material (PTFE, graphite, ceramic, or mica), wound in a spiral with inner and outer centering rings. The outer ring positions the gasket and limits compression; the inner ring prevents inward collapse under high bolt load. Graphite-filled spiral wound gaskets are standard for steam and hot hydrocarbon service; PTFE-filled gaskets are used in corrosive chemical service; mica-filled gaskets serve high-temperature (above 500°C) applications. The ASME B16.20 standard governs spiral wound gasket dimensions and is referenced by ASME B16.5 for flange connections.

Bolt Load Requirements and Gasket Seating

Adequate bolt load is essential for gasket seating and maintaining a leak-free flange joint under operating conditions. ASME PCC-1 (guidelines for pressure boundary bolted flange joint assembly) and the ASME Boiler and Pressure Vessel Code Appendix 2 provide calculation methods for required bolt loads based on gasket seating stress (minimum load to seat the gasket) and operating load (minimum load to maintain seal under internal pressure). Insufficient bolt load is a leading cause of flange leakage; over-tightening can crush soft gaskets or yield bolts. Torque wrenches, hydraulic bolt tensioners, or ultrasonic bolt elongation measurement ensure correct and uniform bolt load application. Lubricating bolt threads and faces with the specified lubricant (typically molybdenum disulfide or PTFE-based) is essential for achieving the target bolt stress from the specified torque.