A flange joint is the most over-specified and under-engineered connection in a piping system. On paper, it is just a ring, a sheet, and a few threaded rods. On site, it is the only thing standing between a refinery at full throughput and a maintenance shutdown. The flange body, the pipe flanges themselves, can be correctly machined and pressure-rated, yet the joint still fails — because the wrong gasket was used, the studs were a grade too soft, or the nuts were tightened in the wrong sequence.

After three decades of supplying pipes, fittings, flanges, and bolting to refineries, power plants, shipyards, and the world’s largest water-transfer projects — including China’s West-East Gas Pipeline and South-to-North Water Diversion — we have learned that a leak-tight joint is a system problem, not a parts problem. This guide walks through the four decisions that determine whether a bolted joint performs for 20 years or returns as a hot flange in month two: the gasket family, the stud bolt grade, the nut class, and the assembly procedure.

1. Start with the Service Envelope, Not the Catalog

Before selecting any sealing element, capture the operating envelope of the line. The five numbers below decide almost every downstream choice and should appear on every requisition:

• Design pressure and temperature — the maximum working pressure at the design temperature, including cyclic peaks and depressurization transients.
• Fluid — water, steam, hydrocarbons, sour service (H₂S / NACE MR0175), seawater, cryogenic LNG, oxygen, or abrasive slurry.
• Flange facing — Raised Face (RF), Flat Face (FF), Ring-Type Joint (RTJ), Tongue & Groove, or Male & Female. The facing dictates the gasket geometry before material is even discussed.
• Code and jurisdiction — ASME B16.5, ASME B16.47, EN 1092-1, GOST, JIS B2220, HG/T 20615, or project-specific specs.
• Inspection regime — hydrotest pressure, leak-tightness class (API 6FA, EN 13555), and any third-party witnessing (SGS, BV, TÜV).

Once these are fixed, the gasket style, the stud bolt grade, the nut type, and the torque values follow logically. Without them, even an experienced supplier is forced to quote the most common stock item, which is rarely the optimal one.

2. Choose the Gasket by Family, Then by Material

Gaskets are commonly grouped into three families based on construction and stress response. Picking the right family is more important than picking a brand.

2.1 Spiral Wound Gasket Details

A spiral wound gasket is a V-shaped or W-shaped metal strip wound alternately with a non-metallic filler (graphite, mica, PTFE, or ceramic). The result is a resilient, multi-ply seal that recovers under thermal cycling and tolerates differential flange relaxation. Common configurations include:

• Basic style — winding only; suitable for tongue-and-groove and male-female joints.
• With inner ring — protects the winding from media erosion and prevents over-compression; standard for RF facings on Class 600 and above.
• With outer ring — centres the gasket on the bolt circle and acts as a compression stop; common on flat-faced and heat exchanger applications.
• With both inner and outer rings — the most specified configuration for refinery and hydrocarbon service.

2.2 Ring-Joint Gasket Details

Ring-joint gaskets are solid metal rings (oval or octagonal cross-section) machined to fit the RTJ groove of the flange. They are selected for high pressure and high temperature service, particularly above Class 600 and on hydrocarbon, steam, and offshore piping. Materials are matched to the flange hardness to avoid groove damage: soft iron, low-carbon steel, 304, 316, Monel, and Inconel are common choices.

3. ｓｅｌｅｃｔ the Stud Bolt Grade and Nut Class

Stud bolts and nuts carry the load that keeps the gasket compressed. Undersize the grade and the joint relaxes; overspec and you waste money and risk galling on every disassembly. The two standards that govern almost every industrial application are ASTM A193 (for the stud) and ASTM A194 (for the nut).

3.1 Threading, Length, and Engagement

For ASME B16.5 flanges, stud bolt length is dictated by the flange thickness, the gasket thickness, the nut thickness, and a small allowance for washer and jam nut where used. The general rule of thumb is that the stud should protrude through the nut by at least one thread, ideally two. Under-length studs prevent proper nut engagement; over-length studs cause the nut to bottom out on the thread runout before the gasket is fully compressed.

3.2 Coatings and Lubrication

Most ASTM A193 / A194 bolting is supplied black, galvanized, PTFE-coated, or zinc-flake coated (such as Geomet or Delta-Protekt). The coating choice depends on the environment: zinc-flake is now standard for offshore and subsea bolting, PTFE for chemical resistance, and black for general indoor service. Always ｄｅｃｌａｒｅ the desired coefficient of friction — typically K = 0.16 to 0.20 — so the supplier can match the lubricant or coating to the torque calculation.

4. Assembly Procedure: The Bolting Sequence Most Teams Get Wrong

A gasket can be correctly specified, a stud bolt correctly graded, and the joint still leaks — because the bolting sequence or torque was wrong. The reference document for this is ASME PCC-1, which is freely available and worth downloading for any EPC or maintenance team that assembles flange joints on a regular basis.

4.1 The Four-Pass Star Sequence

The widely accepted approach for raised-face flanges with a soft or semi-metallic gasket is a four-pass star sequence:

• Pass 1 — hand-snug all nuts, ensuring the gasket is centred and the flanges are parallel.
• Pass 2 — tighten each nut to approximately 30 % of the target torque, in a star pattern (opposite bolts first).
• Pass 3 — tighten to 60 % of the target torque, same star pattern.
• Pass 4 — final pass at 100 % of the target torque, again in a star pattern. Re-check all bolts in a fifth pass to confirm uniform compression.

4.2 Calibrated Torque or Direct Tension?

For general service, calibrated torque wrenches (mechanical, hydraulic, or battery) are acceptable when the bolt lubricant is known and consistent. For high-pressure, high-temperature, or critical-service joints, direct-tension indicators (such as load-indicating washers) or hydraulic bolt tensioners are preferred because they control the actual clamping force on the gasket rather than the torque on the nut. ASME PCC-1 covers both methods in detail.

4.3 Round-the-Loop Cross-Check

After the final pass, walk around the flange one more time. Any stud that feels loose, or any nut that turns further, indicates uneven gasket compression. Stop, release all bolts, and re-do the sequence from pass one. Do not continue tightening only the loose bolts — that bends the flange face.

5. Common Buyer Mistakes and How to Avoid Them

• Specifying the gasket only by facing. “RF, Class 300, 4-inch” tells the supplier nothing about the winding material, inner ring, or pressure class. Always include the full style code: spiral wound, 316L/graphite, with inner and outer ring, ASME B16.20.
• Mixing stud and nut grades. B7 with 2H is a matched pair. Mixing B7 with 8, or B8 with 2H, is a frequent shop error that fails in service.
• Ignoring the lubricant coefficient. The same torque value on a black bolt and a PTFE-coated bolt gives a different clamping force. ｄｅｃｌａｒｅ the coating or the K-factor in the requisition.
• Forgetting traceable certification. For pressure-containing parts in oil & gas, refinery, and nuclear service, EN 10204 3.1 / 3.2 certificates on every heat are non-negotiable. Many trading houses cannot supply this.
• Reusing gaskets. Once compressed, a soft or semi-metallic gasket should not be reused. Ring-joint gaskets can be re-used only if visually inspected, dimensionally checked, and re-lubricated to OEM guidance.
• Ordering from several suppliers. Stud, nut, and gasket sourced from three different vendors may have three different cert packages, three different finishes, and three different lead times. A single, mill-direct supplier removes all of this.

6. Why Source Sealing Packages From a Mill-Direct Manufacturer

There is a real difference between a mill-direct bolting package and one assembled from parts passed through several intermediaries. When you buy from a manufacturer with its own forging, machining, and inspection facilities, you control the heat number, the chemistry, the heat treatment, and the test records on every stud and every nut. When you buy through a trading house, you inherit the supplier they used that month.

EZ STEEL INDUSTRIAL has been a mill-direct producer since 1994, with 500+ technical staff, an annual capacity above 480,000 tons, and a fully accredited ISO 9001 laboratory. Our carbon steel pipe, stainless steel pipe, fittings, and flanges are produced under API, EN, and ASME certifications, and every gasket stud bolt nut set is delivered as a matched, traceable package — gaskets, stud bolts, and heavy hex nuts with the same heat number, the same MTR, and the same lead time.