The function of gaskets is to interpose a semi-plastic material between the flange facings, the material which, through deformation under load seals the minute surface irregularities to prevent leakage of the fluid. Gaskets can be generally divided into three main categories as follows:
1. Non-metallic Gaskets:
Usually composite sheet materials are used with flat face flanges and low pressure class applications. Non-metallic gaskets are manufactured non-asbestos material or Compressed Asbestos Fibre (CAF). Non-asbestos types include arimid fibre, glass fibre,
elastomer, Teflon (PTFE) and flexible graphite. PTFE or RPTFE are used in high corrosive applications.
Full face gasket types are suitable for use with flat-face (FF) flanges and flat-ring gasket types are suitable for use with raised face (RF) flanges.
2. Semi-metallic Gaskets:
Semi-metallic gaskets are composites of metal and non-metallic materials. The metal is intended to offer strength and resiliency while the non-metallic portion of a gasket provides conformability and sealability Commonly used semi-metallic are spiral wound, metal jacketed, Cam profile and a variety of metal-reinforced graphite gaskets.
Semi metallic gaskets are designed for the widest range of operating conditions of temperature and pressure. Semi-metallic are used on raised face, male-andfemale and tongue-and –groove flanges.
Spiral wound gaskets are used for high temperature and high-pressure applications. The main parts of spiral wound gasket are metallic windings and filler material. The criteria for material selection of windings are: corrosive nature, fluid concentration, operating temperature and material cost. Commonly used material for winding are Stainless steel 304, 316 and 321.Alternate winding materials also can be used depending upon the services. General filler material used are Asbestos and graphite. Graphite filler are recommended for very high temperature application.
For proper positioning, spiral wound gaskets are provided with outer centering ring made up of Carbon Steel. In Vacuum service application they are provided with inner ring. The material of the ring should be compatible with the process fluid.
Spiral wound gasket is recommended with flange face finish of 63-250 AARH.
3. Metallic Gaskets:
Metallic gasket is fabricated from one or a combination of metal to the desired shape and size. Common metallic gasket is ring-joint gasket and lens rings.
It is suitable for high-pressure and temperature applications and require high bolt load to seal.
API 601– Metallic Gasket for Refinery Piping
BS 3381– Spiral Wound Gaskets to suit BS 1560 Flanges
ANSI B 16.20– Metallic Gasket for Steel Pipe Flanges, Ring Joint, Spiral Wound and Jacketed
ANSI B 16.21– Non-Metallic flat Gasket for Pipe Flanges