Gaskets and Stud Bolts

Gasket Selection by Flange Face

The gasket must match the flange face. A raised face flange, which is the most common, uses a spiral wound gasket with a centering ring, a sheet gasket, or a jacketed gasket. A flat face flange, typically used where the flange is cast iron or where the joint is against a valve or vessel with a flat face, uses a full face gasket so that the bolts pass through the gasket. A ring joint flange has a grooved face and uses a solid metal ring joint gasket, which seals by the wedging action of the ring into the groove and is the choice for high pressure and high temperature service above class 900 and for critical gas tight applications. The wrong gasket type for the face will not seal regardless of bolt torque, so confirm the face type before ordering.

Bolting Grades and B7 versus B7M

The most common flange bolting for high temperature and high pressure service is ASTM A193 grade B7 studs with ASTM A194 grade 2H nuts. B7 is a chromium-molybdenum alloy steel, quenched and tempered, with a minimum tensile strength of 860 MPa at sizes up to 2-1/2 inch. For sour service, B7M is used instead. B7M has the same chemistry and strength as B7 but a lower maximum hardness limit, 235 HB or 22 HRC, and is supplied with 2HM nuts, so that the bolting satisfies NACE MR0175. For low temperature and cryogenic service, ASTM A320 grades L7 and L7M are used. Stainless bolting, B8 and B8M, is used for corrosion resistance or where non-magnetic properties are needed, but note that stainless bolting has lower tensile strength and a much higher risk of galling during tightening than alloy steel bolting.

Gaskets and bolting: the joint is one specification, not two

A flanged joint is made of three parts that have to be specified together: the flange facing, the gasket and the bolting. Ordering them separately is where most joint problems begin, because the gasket has to match the facing and the bolting has to be rated for the flange class and the temperature.

Ordering the joint as one package

Gaskets and bolting are bought separately and fail together. Four points are worth checking before the flange package is released.

• The gasket follows the facing, not the size. A raised face, a flat face and a ring joint take different gaskets at the same size and class, and the gasket type also depends on the class. Mixing a ring joint gasket into a raised face joint is not a field fix.
• The bolting follows the gasket. Ring joint joints are bolted harder than sheet gasket joints for the same class, so the stud grade and the tightening method belong with the gasket selection rather than with the flange order alone.
• Stud length follows the joint, not the flange. Where a blind, a spectacle blind or a spacer is in the joint, the stack is thicker and the studs are longer. Ordering studs from the flange list alone leaves the joint short.
• Material and fluid compatibility. The winding and filler of a spiral wound gasket, and the lubricant on the studs, have to suit the fluid and the temperature. Confirm the filler against the line specification rather than against the flange size.

Source: gasket dimensions from ASME B16.20, facing and pressure class from ASME B16.5, stud and nut grades from ASTM A193/A194. The range supplied and the combinations we hold are confirmed at quotation.

Grades supplied

The grade fixes the minimum strength and the chemistry. Select a grade for its full composition and mechanical tables.

• A193 B7
• A193 B7M
• A193 B8
• A193 B8M
• A194 2H
• A194 2HM
• A194 8
• A194 8M
• 304 graphite
• 316 PTFE

Download Gaskets and Stud Bolts data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.