Gasket, Stud Bolt & Nut Selection: A Practical Guide to Flange Sealing Integrity, Standards, and Bundled Sourcing

Why Gasket and Bolting Decisions Are the Leak Line of a Project

A flanged joint only performs as well as its three coordinated components: the gasket, stud bolt & nut set, the flange facing finish, and the bolt-up procedure. In real projects, the leak path almost always traces back to one of three causes: a gasket type that was not matched to the service, a bolting material that lost strength at operating temperature, or a bolt-up sequence that loaded the gasket unevenly. None of these are exotic problems — they are procurement and workmanship problems, and they can be engineered out with the right specification.

The phrase "gasket, stud bolt and nut" looks like a single line item, but it is actually three engineering decisions stacked together. The gasket handles the seal. The stud bolt supplies the seating load. The nut (often with a washer) protects thread integrity and distributes load. If any one of the three is under-specified, the whole joint is at risk. That is why experienced buyers treat them as a system, not a commodity, and they source them together with the pipe flange and pipe supply from the same mill.

Step 1 — Identify the Flange Facing and Match It to a Gasket Family

Before selecting a gasket, confirm the flange facing type. ASME B16.5 / B16.47 covers the five most common facing geometries, and each one accepts a specific gasket family. Mismatching the two is the most common procurement error on legacy piping work.

Raised Face (RF) and Flat Face (FF)

RF is the default for ASME B16.5 Class 150 to Class 300 pipe flange products in carbon & carbon alloy steel. FF is typical for cast iron, fiberglass, and lined piping where the gasket must bear on the full face. RF and FF both accept soft cut gaskets (non-asbestos sheet, flexible graphite) for low-pressure service, spiral-wound gaskets for higher pressure, and ring-type joints only on specially grooved RF (FF is not rated for RTJ).

Ring-Type Joint (RTJ)

RTJ is a metal-to-metal seal where the gasket is a solid ring (oval or octagonal cross-section) pressed into machined grooves. RTJ is the standard for high-pressure service from Class 600 upward, and is mandatory for hydrocarbon and sour service on most operator specs. The ring itself is sized to the flange groove and the pressure class; do not mix R, RX, or BX style rings between flanges without a compatibility check.

Tongue & Groove (T&G) and Male & Female (M&F)

T&G and M&F facings are designed for self-aligning semi-metallic gaskets — typically spiral-wound with an inner ring (Type B), kammprofile, or corrugated metal. They are common in chemical, petrochemical, and high-temperature process piping where the gasket must be centered precisely and protected from blowout.

Step 2 — Map Gasket Type to Service Conditions

Once the facing is set, the next decision is the gasket material. There is no universal best gasket — only the right gasket for the combination of pressure, temperature, and media. The table below summarizes the most common industrial choices and the services they handle well.

A note on a material that is sometimes still seen on legacy RFQs: asbestos. It is banned under REACH in the EU, restricted in most operator specs, and effectively obsolete. Any modern gasket, stud bolt & nut package sourced from a mill-direct supplier should substitute compressed non-asbestos sheet, flexible graphite, or vermiculite for the legacy asbestos specification.

Step 3 — Spiral-Wound Gasket Selection (the Workhorse Choice)

For most refinery, petrochemical, and power projects from Class 150 to Class 600, the spiral-wound gasket is the right default. It is forgiving of flange surface finish, handles thermal cycling, and is available in five standard configurations.

Spiral-Wound Types (A / B / C / D / Special)

Type A (basic): winding metal and filler only, no rings. Used on T&G and M&F facings where the flange itself centers the gasket. Type B (with inner ring): adds a solid metal inner ring that prevents winding collapse and acts as a wind/splash baffle. Used on T&G and M&F; not used on RF/FF because the inner ring would sit inside the bore and obstruct flow. Type C (with outer ring): adds an outer guide ring that centers the gasket on the bolt circle and acts as a compression stop. Used on RF and FF facings. Type D (with inner and outer rings): combines both, for higher-pressure RF/FF service and for aggressive media. Special (ribbed): used on heat-exchanger pass partitions where the gasket must fit a narrow rib.

Why Winding-Metal-to-Filler Ratio Matters

A common procurement pitfall is buying the cheapest spiral-wound that meets the dimensional standard. ASME B16.20 (Metallic Gaskets for Pipe Flanges — Ring-Joint, Spiral-Wound, and Jacketed) sets the geometry, but the recipe — the ratio of winding metal to filler — drives the seal. A "soft" gasket with too much filler compresses until the flange face contacts the outer guide ring, and the seal is then made by metal-to-metal contact at the guide ring, not by the sealing element itself. That is a much less reliable seal. The 2017 revision of B16.20 tightened the performance expectations for this reason. When sourcing, ask the supplier to confirm the winding-to-filler ratio and the seating stress (y) of the specific lot, not just the dimensional callout.

Filler & Strip Combinations

Graphite filler with 316L stainless winding is the default for hydrocarbon service up to about 550 °C. PTFE filler is used where media compatibility is the issue but temperature is moderate (below 260 °C). Vermiculite or mica fillers push the upper limit past 700 °C in oxidizing media. The outer guide ring is usually carbon steel (with anti-corrosion paint) for carbon-steel flanges, and 304 or 316 stainless for stainless flanges — this should match the flange material to avoid galvanic concerns in wet service.

Step 4 — Stud Bolt and Nut Selection by Service

The gasket is half the system. The stud bolt is the other half — it must generate and hold the seating load without yielding, relaxing, or failing at operating temperature. ASTM standards have split the bolting landscape into clear families.

Common Stud Bolt Materials (ASTM)

ASTM A193 Grade B7 is the workhorse stud bolt for carbon & carbon alloy steel flanges from -29 °C to +400 °C. It is a chromium-molybdenum steel, quenched and tempered, with a typical UTS around 860 MPa. ASTM A193 Grade B7M is the same chemistry but capped to a lower hardness for sour service (NACE MR0175). ASTM A193 Grade B8 / B8M / B8T / B8C / B8P are the austenitic stainless alternatives for stainless steel flanges. ASTM A320 Grade L7 / L7M is the low-temperature equivalent of B7 / B7M, mandatory for service below -29 °C (down to -101 °C for L7). ASTM A453 Grade 660 is the high-temperature bolt for service above 400 °C, used in power and steam headers.

Common Nut Materials (ASTM)

Nuts must always be softer than the bolt to take the thread wear during make-up and break-out. ASTM A194 Grade 2H is the standard nut for B7 studs, used in service -29 °C to +400 °C. ASTM A194 Grade 2HM is the NACE-compatible equivalent. ASTM A194 Grade 8 / 8M / 8T is the stainless nut family for B8 / B8M studs. ASTM A194 Grade 7 / 7M is the standard nut for L7 / L7M low-temperature studs. ASTM A194 Grade 8C / 8F / 9C are higher-alloy nuts for A453 Grade 660 studs at elevated temperature.

Sour Service and NACE Selection

For sour service (NACE MR0175 / ISO 15156), bolting must be sourced in the "M" condition (B7M, B8M, L7M, 2HM, 7M) to limit hardness and prevent sulfide stress cracking. Hardness is typically capped at 22 HRC for carbon / low-alloy bolting, 35 HRC for austenitic stainless. Any NACE project that ships B7 studs by mistake is a quality hold on site — confirm the callout is on every MTC and on the carton label.

Step 5 — Bolt-Up, Torque, and the Pass Sequence

Even a perfect gasket, stud bolt & nut set will leak if it is installed badly. Two rules of bolt-up matter most.

Target the Gasket Stress (y), Not Just the Torque

Torque values in a supplier table are starting points, not finish lines. The actual seating stress on the gasket must hit the published y-value (typically 68 MPa for graphite spiral-wound). For a Class 150 RF flange with a standard spiral-wound, this means a calculated torque in the 100–200 N·m range for M16 studs, scaling with bolt size. Hydrostatic testing after the first pass is the practical confirmation.

Multi-Pass Torque-Up (the "Star" Sequence)

Never torque a flange to final value in one pass. Use a four-pass sequence at 30%, 60%, 80%, and 100% of target torque, in a star or 4-bolt-cross pattern that always skips the next bolt. For a 16-bolt flange, the star sequence is 1-9-5-13, then 3-11-7-15, then 2-10-6-14, then 4-12-8-16. This evens the load around the gasket and prevents the half-side loading that causes weeping. A second full re-torque pass at hot operating temperature (hot torque) is required for steam and high-temperature hydrocarbon service.

Step 6 — Standards You Should Be Calling Out on the RFQ

Procurement specifications should reference the standard that the supplier will be audited against. The list below covers the most common international references and the products they govern.

Step 7 — MTC and Documentation: the 12-Point Checklist

A well-written PO for gasket, stud bolt & nut supply should require the supplier to deliver the following documentation. Anything missing is a flag to hold the shipment at receiving.

• Mill Test Certificate (MTC) per EN 10204 3.1 for each heat / lot, with chemical analysis and mechanical results
• Type 3.2 MTC (third-party witnessed) when the project spec calls for it — typical for sour service and nuclear
• Hardness test report for bolting (Brinell or Rockwell per ASTM A193 / A194 / A320 / A453)
• Sour-service compliance statement (NACE MR0175) when applicable, with hardness ≤ 22 HRC for carbon / low-alloy bolting
• SpiraI-wound gasket test report: winding-to-filler ratio, density, compressibility, recovery per ASTM F36 / F112 / F127 / F152
• RTJ ring hardness verification (must be ≤ flange groove hardness by 30–40 HB)
• PMI (positive material identification) report on stainless items
• Visual and dimensional inspection report (size, thickness, ID, OD, cross-section, centering)
• Hydrostatic or pneumatic test report when the project spec requires it (rare for gaskets, more common for assembled joints)
• Lot traceability labels on every carton and on every individual bolt / ring / gasket (where supplied individually)
• Country of origin and HS code on the packing list
• Storage and handling instructions (gaskets should be stored dry, flat, and free of mechanical damage)

Common Procurement Mistakes and How to Avoid Them

Across hundreds of joint failure investigations, the same seven mistakes show up again and again. Each one is preventable at the specification stage.

• Ordering a basic-type (A) spiral-wound for an RF flange — the gasket will extrude. Specify C or D for RF/FF, B for T&G / M&F.
• Using a non-asbestos sheet gasket above Class 150 steam — the gasket will blow. Switch to spiral-wound with graphite filler at Class 150 hot steam or higher.
• Quoting A193 B7 studs in sour service — it is a quality hold. Specify B7M (or L7M for low temperature) plus NACE statement on the MTC.
• Mixing B7 studs with stainless nuts — galling on make-up. Match the family: 2H nuts for B7, 8 / 8M nuts for B8 / B8M, 7 / 7M nuts for L7 / L7M.
• Substituting 304 stainless for 316L in marine or chloride service — pitting corrosion follows. Match the stud / nut to the flange metallurgy.
• Single-pass torque-up to final value — uneven gasket seating and a guaranteed leak. Use a four-pass star sequence at 30 / 60 / 80 / 100% torque.
• Buying spiral-wound on dimensional spec only — too-soft gaskets fail in service. Confirm the winding-to-filler ratio and seating stress y from the supplier.

Where the Gasket, Stud Bolt & Nut Package Fits in the Wider Project

The gasket, stud bolt & nut package is rarely a standalone order. On a real EPC project it ships with the pipe flange supply, the pipe itself (in carbon & carbon alloy steel, stainless steel, or copper-nickel for marine), the pipe fittings (BW / SW / threaded), and the industrial valves. Sourcing these together from one mill cuts three real costs: mismatched MTCs that fail receiving inspection, late deliveries that hold up flange make-up, and customs paperwork that has to be re-cut for each separate vendor.

A mill-direct supplier that owns the flange machining, the bolt threading, the spiral-wound winding line, and the RTJ machining under one roof can deliver a complete joint package with one set of MTCs, one shipping schedule, and one point of contact for non-conformance. For a 5,000-joint project, that is the difference between a clean punch-list closeout and a two-month delay chasing missing documentation.