Gasket Function and Basics
A flange gasket works by compressing under bolt preload to fill irregularities in the flange facing surfaces. The compressive stress must be high enough to create a seal but low enough to avoid damaging the gasket or the flange. The gasket must also accommodate relative movement between the flanges caused by thermal expansion, pressure-induced forces, and external piping loads. Selecting the wrong gasket type or material leads to leakage, fugitive emissions, and potential system failure.
Non-Metallic Gaskets
Non-metallic gaskets are made from compressible materials and are typically used in low-pressure and low-temperature applications. Rubber (elastomer) gaskets serve low-temperature, low-pressure service up to 400°F. PTFE gaskets offer excellent chemical resistance across a wide pH range up to 500°F. Flexible graphite gaskets handle high temperatures up to 850°F with good chemical resistance. Compressed non-asbestos fiber (CNAF) gaskets are a common general-purpose option for moderate service conditions. The main limitations of non-metallic gaskets are lower pressure capacity and potential for blowout under high pressure.
| Gasket Type | Max Temperature | Max Class | Chemical Resistance | Relative Cost |
|---|---|---|---|---|
| Elastomer (NBR, EPDM) | 400°F | 150 | Good (media-specific) | Low |
| PTFE | 500°F | 300 | Excellent (all chemicals) | Medium |
| Flexible Graphite | 850°F | 600 | Good (avoid strong oxidizers) | Medium |
| CNAF | 700°F | 300 | Good (general service) | Low |
Semi-Metallic Gaskets
Semi-metallic gaskets combine the mechanical strength of metal with the sealing properties of a soft filler material. Spiral wound gaskets are the most common semi-metallic type, consisting of alternating layers of metal strip and filler material wound into a spiral shape. They typically include inner and outer rings for centering and reinforcement. Metal jacketed gaskets have a soft filler core inside a metal jacket. Camprofile (Kammprofile) gaskets feature a grooved metal core with a graphite or PTFE covering. Semi-metallic gaskets are suitable for medium to high-pressure and temperature applications up to Class 1500.
Metallic Gaskets
Metallic gaskets provide the highest sealing integrity for critical service. Ring joint gaskets are solid metal rings designed for RTJ flange facings. They deform into the groove under bolt load to create a metal-to-metal seal. Oval and octagonal cross-sections are available for different groove designs. Solid metal flat gaskets are used for extreme temperature or pressure applications. The gasket material must be softer than the flange material to prevent flange face damage, and metal selection must consider galvanic corrosion compatibility with the flange material.
Material Selection Guidelines
Temperature range is often the primary factor: elastomers to 400°F, PTFE to 500°F, graphite to 900°F, and metal to 1800°F. Chemical compatibility is critical: PTFE for aggressive chemicals, graphite for alkalis, and stainless steel for general corrosive service. Pressure capability follows a hierarchy: non-metallic gaskets up to Class 300, semi-metallic gaskets up to Class 1500, and metallic gaskets for Class 2500. Cost follows the same pattern: non-metallic is most economical, metallic is most expensive.
Dimensions and Standards
ASME B16.20 covers metallic and spiral wound gasket dimensions, while ASME B16.21 covers non-metallic flat gaskets. Gasket inner and outer diameters must match the flange facing dimensions. For spiral wound gaskets, the inner ring provides centering and prevents inward collapse, while the outer ring serves as a compression stop. Proper gasket sizing ensures that the gasket seats correctly within the bolt circle and that the sealing stress is uniformly distributed.
Installation Best Practices
Clean both flange faces thoroughly and inspect for damage before installing a new gasket. Apply proper bolt torque in a cross-bolting sequence to ensure uniform gasket compression. Never exceed the gasket manufacturer's recommended compression limits, as over-compression can damage the gasket and cause leakage. Always use new gaskets for every joint reassembly, and ensure the gasket is properly centered within the bolt circle before tightening. Following these practices will maximize joint integrity and service life.
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