How Gasket, Stud Bolt & Nut Combinations Seal Industrial Flanged Joints

A practical engineering guide to flanged-joint integrity in petrochemical, power and marine service

Most pipeline leaks do not start with the pipe itself. They start at the flange face, where three small components — the gasket, the stud bolt and the nut — decide whether the joint stays tight for thirty years or starts weeping the next quarter. This guide explains how those three components work together, how to match them to the right pipe flanges, and where EZ STEEL INDUSTRIAL's bundled supply can remove a great deal of the procurement risk.

1. The Flanged Joint Is a System, Not a Part

ASME B16.5 makes the point very clearly in clause 2.2: a flanged joint is composed of separate but interrelated components — the flanges, the gasket, and the bolting — assembled by a fourth influence, the assembler. Pressure-temperature ratings on the flange are only valid when all three components are chosen correctly and tightened in line with good practice. ｒｅｐｌａｃｅ the gasket with a softer style, or the stud bolt with a lower-grade material, and the published rating no longer applies.

This is why procurement teams that buy each item separately often end up with under-rated joints. The flange meets the code, the gasket meets the code, the bolt meets the code, and yet the assembly leaks. The fix is almost always to source the three components as a matched set from a single supplier who can vouch for the combination — which is exactly the working model behind our gasket stud bolt nut package.

2. Choosing the Right Gasket Style

Gasket selection is governed by the flange face, the pressure class, the temperature, and the medium. The same steel flanges can take very different gaskets depending on service:

Gasket Type	Typical Service	Pressure Class	Temperature Window
Spiral-wound with graphite / non-asbestos filler	Steam, hydrocarbons, refinery piping	ASME Class 150 – 600	-196 °C to ~550 °C
Ring-type joint (RTJ), oval or octagonal	High-pressure oil & gas, wellhead	Class 600 – 2500	Up to ~600 °C (ring material dependent)
Flexible graphite with metal ｉｎｓｅｒｔ	Hot oil, boiler manways	Class 150 – 300	Up to ~650 °C in steam
PTFE envelope, virgin or filled	Strong acids, chlorides, pharma	Class 150 – 300	-200 °C to ~260 °C
Compressed non-asbestos fibre	Water, low-pressure air, general utility	Class 150	Up to ~260 °C

A common mistake is to use a soft non-asbestos gasket on a Class 300 RF flange that actually sees Class 600 conditions because of water hammer. The seating stress is not enough, and the joint will creep. The right fix is a spiral-wound gasket matched to the flange rating — or, for the most demanding service, a ring-type joint with a metal ring hardened to the correct Brinell range.

3. Stud Bolt & Nut Grades That Actually Match the Flange

Bolting is the most commonly downgraded component in a flanged joint, because mild steel B7 stud bolts with grade 2H nuts look similar to a B16/B8 combination but behave very differently at temperature.

ASTM A193 B7 / A194 2H — the workhorse for ASME Class 150 to 600 service in carbon and low-alloy steel flanges, up to about 425 °C.
ASTM A193 B16 / A194 7 — required for high-temperature refinery and power service above 425 °C, where B7 would suffer from stress relaxation.
ASTM A193 B8 / B8M (Class 1 & 2) / A194 8 / 8M — austenitic stainless for stainless-steel flanges in chemical, food and cryogenic service, where galvanic compatibility with the stud bolt is important.
ASTM A320 L7 / L43 / A194 7 / 4 — low-temperature impact-tested bolting, mandatory for service below -29 °C per ASME B16.5 clause 5.3.

At EZ STEEL INDUSTRIAL, every stud bolt and nut we ship in a gasket stud bolt nut bundle is traceable by heat number, and the MTC clearly states the ASTM grade, the hardness range and the impact test result (where applicable). This removes a frequent cause of on-site rejection at hydrotest.

4. Tightening: The Step Most Buyers Forget

Even a perfectly specified joint will leak if it is tightened by feel. ASME PCC-1 describes controlled bolt tightening using one of four methods — the torque method, the torque-plus-nut-rotation method, the tension-control method, and the leak-free pre-tightening method. The choice depends on the gasket and the class:

A field-tested tightening sequence

Cross-tighten in at least four passes, each pass adding roughly 30% of the final target torque.
Achieve parallelism of the flanges — gap between flange faces must be uniform around the bolt circle before the final pass.
After 24 hours of thermal cycling, re-torque to compensate for gasket creep and bolt relaxation (especially with graphite-filled spiral-wound gaskets).

Buyers who treat this as a procedural formality lose far more flanges to leakage than buyers who invest in a torque wrench and a documented sequence. The bolt cost is roughly the same either way.

5. Special Case: Copper-Nickel Flanges in Seawater Service

Marine and offshore buyers are often surprised to learn that the bolting rule for copper-nickel pipe flanges is different from that of carbon steel. The reason is galvanic compatibility. A bare B7 stud bolt in a 90/10 or 70/30 Cu-Ni flange will pit quickly in seawater splash zones, and the joint can seize during the next overhaul.

Best practice is to specify a fully austenitic B8/B8M stud bolt with a PTFE or Xylan anti-seize coating, paired with a nitrile or EPDM-bonded gasket that is rated for marine cooling water. The fasteners then match the corrosion profile of the flange, and the joint can be broken out cleanly after ten years in service.

6. Procurement Checklist for a Leak-Free Joint

Before a purchase order is released, the following six items should be confirmed in writing — either on the datasheet or in the MTC:

Flange material, standard (ASME B16.5 / B16.47 / EN 1092-1), facing type (RF, FF, RTJ), and pressure class.
Gasket style, filler material, inner / outer ring material, and the target seating stress (m or y value per ASME).
Stud bolt ASTM grade, nut ASTM grade, thread, length, and any impact-test requirement.
Coating or anti-seize requirement (e.g. PTFE, Xylan, zinc-flake, hot-dip galvanizing).
Traceability: heat numbers on all three components, with the MTC and PMI report on file.
Packing: gaskets flat in wooden crates, studs and nuts bagged and labelled per flange size — nothing worse than mixing a 5/8" B7 with a 3/4" B7 on a congested offshore deck.

Source the joint, not just the parts

Since 1994, EZ STEEL INDUSTRIAL has supplied matched flange, gasket, stud bolt and nut packages to refineries, power plants, shipyards and pipeline contractors in more than 60 countries. Our pressure tubes, industrial valves, pipe fittings and flanges all come from audited mills with full ASTM, EN, ASME and ISO 9001 documentation. Send us your datasheet or P&ID and we will return a complete bill of materials — flanges, gaskets, stud bolts, nuts, and the matching gasket stud bolt nut set — packed, heat-numbered and ready to ship from a 480,000-ton annual capacity. Email export@ezsteelpipe.com or call +86 731 8870 6116 for a quotation.