Gaskets, Bolts and Flange Joint Integrity: Getting the Bolted Joint Right

A flanged joint has four parts that must work together: two flanges, a gasket and a set of bolts, assembled by a person following a procedure. Leaks are almost never caused by the flange. They come from the wrong gasket, the wrong or under-loaded bolts, or assembly that did not seat the gasket evenly. This article walks a buyer through the standards that govern each part, the choices that matter for typical process and building services duties in the Gulf, and the assembly practice that turns correct materials into a tight joint.

The standards that define the joint

ASME B16.5 covers flanges and flanged fittings from NPS 1/2 to NPS 24 in classes 150, 300, 400, 600, 900 and 1500, with class 2500 up to NPS 12. Besides dimensions and pressure-temperature ratings it includes requirements and recommendations for flange bolting, gaskets and flange joints, so it is the first reference for what goes between and through the flanges (ASME B16.5). Gasket dimensions come from two companion standards: ASME B16.20, 2023 edition, for metallic gaskets, which covers ring-joint, spiral-wound and metal-jacketed types dimensionally suitable for B16.5, B16.47 and API 6A flanges; and ASME B16.21, 2021 edition, for nonmetallic flat gaskets (ASME B16.20; ASME B16.21).

Assembly is covered by ASME PCC-1, Pressure Boundary Bolted Flange Joint Assembly. Its scope is pressure-boundary flanged joints with ring-type gaskets that sit entirely within the circle enclosed by the bolt holes, and it includes guidance on training and qualifying assemblers and on troubleshooting joints that do not seal (NORMSERVIS; ASME). Project specifications normally call up a specific edition, so confirm which one applies before writing the site procedure.

Choosing the gasket

Gasket choice is driven by temperature, pressure, the medium and the flange facing. Soft sheet gaskets seal with little bolt load and are forgiving of flange condition, but have the lowest temperature and pressure limits. Spiral wound gaskets combine a metal winding with a soft filler and cover most process duties. Ring-type joints are solid metal and need grooved flanges.

The temperature figures for rubber, PTFE and spiral wound PTFE are maximums published in a general engineering reference (Engineering ToolBox) and manufacturers publish their own limits; use the datasheet of the product offered. On spiral wound gaskets, the outer ring centres the gasket on the flange, adds radial strength against blow-out and acts as a compression stop, while the inner ring, present on the style usually designated CGI, is specified for high pressure or temperature and for corrosive or toxic media because it supports the winding and prevents inward buckling (Seal and Design). Fillers are commonly flexible graphite or PTFE, with high-temperature mica-based fillers available for hot gas duties.

Two rules save most gasket problems. Match the gasket to the facing: flat-face cast iron flanges need full-face soft gaskets, raised-face steel flanges take ring gaskets inside the bolt circle, and RTJ grooves take only the matching ring. And never reuse a gasket; PCC-1 recommends against it (ASME).

Choosing bolts and nuts

Bolting for flanged joints in pressure service is specified to ASTM A193 for studs and bolts and ASTM A194 for nuts, and the two must be paired correctly.

Mechanical properties are from a published summary of the A193 tables and fall with increasing diameter, for example B7 drops to 115 / 95 ksi between 2-5/8 and 4 inches (Baco Enterprises). Service temperature ranges and nut pairings are from a materials reference (Projectmaterials). The practical consequence is that stainless Class 1 bolting has roughly a quarter of the yield strength of B7, so a joint designed for B7 cannot simply be swapped to B8 for corrosion resistance without checking the available bolt load against the gasket's seating stress.

Grade 2H nuts are quenched and tempered carbon steel with a hardness range of 248 to 352 HB and are the standard partner for B7; grade 8 and 8M nuts are 304 and 316 stainless for B8 and B8M. Every nut must be marked with the manufacturer's symbol, the grade and heat or lot traceability (Projectmaterials). On receipt, check the head and nut markings against the certificate, because unmarked or mis-marked bolting is the easiest substitution to miss on site.

For hot-dip galvanised or PTFE-coated studs, remember that the coating changes the friction and therefore the torque needed for a given bolt load, and that oversize tapping of the nut is required for galvanised threads.

Assembly: load, pattern and lubrication

The purpose of assembly is to deliver a uniform bolt load that compresses the gasket to its seating stress and keeps it there in service. PCC-1 describes how to get there and its 2010 revision added lessons from leaks that could have been avoided by small procedural changes: check that nuts run freely on the stud during lubrication, because small thread imperfections significantly affect bolt load; remove thick paint between nut and flange, because it degrades and releases load; and treat temporary test gaskets with caution (ASME).

The familiar star or cross pattern is called the legacy pattern in PCC-1. In the sequence described by a bolting specialist, the first pass tightens each bolt in star order to 20 to 30 percent of target torque, the second to 50 to 70 percent and the third to 100 percent, followed by circular passes at final torque around the flange until no nut moves, which usually takes about two passes for spiral wound and kammprofile gaskets and more for RTJ. The legacy pattern is valid for B16.5 and B16.47 flanges and heat exchanger flanges with any gasket type. PCC-1 Appendix F adds faster alternative patterns; the circular-only pattern is restricted to hard gaskets such as kammprofile and must not be used on spiral wound, RTJ or double-jacketed gaskets (Hex Technology; ASME).

Torque values depend on stud grade and diameter, lubricant friction, gasket type and the target bolt stress, so refer to the standard and the gasket manufacturer rather than a generic table. Use a calibrated torque wrench or hydraulic tensioning on larger joints, and record the tool setting and the lubricant used.

Inspection and documentation

Before assembly, inspect flange faces for radial scores across the seating surface, check flange alignment and gap, and confirm the gasket type, size and class against the drawing. During assembly, record the pattern, passes and final torque or tension. After pressure testing, a hot retorque may be specified for soft gaskets that relax, within the limits the gasket maker allows.

For the procurement file, keep the gasket certificate showing material and the governing standard, the bolting certificate showing grade, heat number and mechanical test results, and the flange certificate. PCC-1 also recognises that the lack of standard qualifications for assemblers is a leading cause of leaks from poor practice, which is why its Appendix A sets out training and qualification; on critical service ask for qualified assemblers (ASME).

Key takeaways

• Flanges follow ASME B16.5; gaskets follow B16.20 (metallic) and B16.21 (nonmetallic); assembly follows ASME PCC-1.
• Choose the gasket for temperature, pressure, medium and facing, and use inner rings on spiral wound gaskets for high pressure and vacuum.
• Pair A193 B7 with A194 2H, B8 with 8 and B8M with 8M; stainless Class 1 studs have far lower strength than B7.
• Check bolt and nut markings against the certificates on receipt.
• Tighten in the legacy star pattern in staged passes and finish with circular passes until no nut moves; use the stated lubricant and a calibrated tool.
• Do not reuse gaskets; clean faces, free-running nuts and no paint under the nut are the cheapest leak prevention available.

Frequently asked questions

Can I use stainless steel bolts on carbon steel flanges to avoid rust?

Only after checking the bolt load. B8 and B8M Class 1 studs have a much lower yield strength than B7, so the gasket may not reach its seating stress at the allowable bolt stress. Strain-hardened Class 2 studs or coated B7 studs are usually the better answer.

Do I need an inner ring on every spiral wound gasket?

Not on low pressure utility duties, but it is the safer default. The inner ring prevents the winding buckling inward under pressure differential and under PTFE filler, and the cost difference is small compared with a leak during commissioning.

What torque should I apply?

Torque depends on stud grade, diameter, lubricant, gasket and target bolt stress, and the published tables differ with those assumptions. Take the value from the project procedure or the gasket manufacturer for the actual joint, and record the lubricant used because it changes the result.

How Al Safsaaf can help

Al Safsaaf Trading & Contracting Company supplies gaskets, bolts and nuts alongside flanges from its base in Riyadh, so the three parts of the joint arrive matched and certified together. Send us the flange class, facing, medium and temperature and we will put together a joint package from our fasteners and gaskets range, or contact us for a specific service.