Gasket, Stud Bolt & Nut Buyer's Guide: ASME B16.20 / B16.5 Selection, Material Pairing, Bolt-Up Math and Mill-Direct Sourcing for B2B Procurement

Why a Single Mismatch Can Wreck a Flanged Joint

A flanged connection is a system, not a part list. The pipe flanges, the gasket stud bolt nut package, and the operating envelope (pressure, temperature, media, cycling) all have to agree. The fastest way to leak is to specify a spiral wound gasket (SWG) on a flat-face (FF) flange, or to pair a Class 600 gasket with a Class 300 flange — the geometry simply will not compress correctly. The second-fastest way to leak is to use B7 studs with B7M nuts when the hydrotest or sour-service clause calls for controlled hardness, or to torque through a graphite filler that has not been heat-treated to its service temperature.

This guide is built around how a mill-direct package from EZ STEEL INDUSTRIAL is actually specified: the gasket type, the stud bolt grade, the nut grade, the inner and outer ring materials, and the mill test documents you should expect in your inbox. EZ Steel has been making tube, pipe, fittings, flanges, and fastener packages since 1994 in Changsha, China, with a 480,000+ ton annual capacity and full API / EN / ASME certification, so the guidance here mirrors the same logic their export team uses to match the package to the project.

Step 1: Start with the Flange, Not the Gasket

Before you even open a gasket catalog, fix the flange standard, the pressure class, the facing type, and the bore. Everything downstream — gasket style, ring materials, stud length, nut grade — falls out of those four numbers.

1.1 Flange Standard and Pressure Class

For most B2B buyers shipping internationally, you are choosing between ASME B16.5 (NPS ½ to 24, Class 150 to 2500) and ASME B16.47 Series A or B (NPS 26 to 60, large-diameter). On the EN side, EN 1092-1 covers PN-rated flanges. The pressure class dictates the gasket thickness, the bolt count, the bolt size, and the allowable gasket stress.

1.2 Facing Type Drives Gasket Style

The facing type is the single biggest gasket-style decision. The general rule for buyers is shown in the table below, and the same rule applies whether you are sourcing steel flanges in carbon, stainless, or alloy.

If your flange is RTJ, the gasket must be RTJ. If your flange is RF, the gasket almost always needs an outer guide ring to keep the winding centered during installation. Trying to mix these is the single most common source of field leak calls.

Step 2: Pick the Gasket Type to Match the Service

Once the flange facing is fixed, the gasket material and construction are driven by three numbers: design temperature, design pressure, and the chemistry of the fluid. EZ Steel's gasket stud bolt nut package is typically built around three families.

2.1 Spiral Wound Gaskets (SWG) for the Bulk of Refinery and Petrochemical Work

A spiral wound gasket is a V-shaped metal strip (typically SS 304, SS 316L, or Monel 400) wound in alternating layers with a non-metallic filler (graphite, PTFE, or flexible ceramic). The combination gives you the high recovery and thermal cycling resistance of a soft gasket with the blowout resistance of a metal ring.

Standard ASME B16.20 SWG configurations buyers will see in EZ Steel's package:

• SWG with inner and outer ring (IR + OR): the most common style for ASME B16.5 RF flanges from Class 150 to Class 600. The outer ring centers the gasket and acts as a compression stop. The inner ring protects the winding from erosion and prevents turbulence at the bore.
• SWG with outer ring only (OR): used where the inner ring is not required, typically on higher-class RF flanges and on non-corroding service.
• SWG without rings: used in tongue-and-groove and male-and-female flanges, where the flange geometry itself provides the centering and compression limit.

2.2 Ring-Type Joint (RTJ) Gaskets for High-Pressure and Sour Service

For Class 600 and above, especially in oil & gas transmission, refinery hydrocrackers, and high-pressure steam, an RTJ gasket is the default. The ring is solid metal — typically soft iron, low-carbon steel, SS 304, SS 316, Monel 400, or Inconel — with an oval or octagonal cross-section that bites into the flange groove when torque is applied.

RTJ hardness must be at least 30 BHN less than the flange groove hardness. A 304 SS ring on a 304 SS flange will not seal: it will gall. The material must be matched to the metallurgy of the flange face, and the hardness must be lower. EZ Steel's package ships RTJ rings with full material certificates and hardness charts for NACE MR0175 sour service.

2.3 Non-Metallic and Flexible Graphite for Low-Pressure Utility Service

For water, air, low-pressure steam, and utility lines, compressed non-asbestos gaskets (aramid + NBR binder) and flexible graphite sheets are the cost-effective choice. They are also the right call on flat-face cast iron flanges where an SWG cannot be used.

Step 3: Specify the Stud Bolt and Nut Correctly

A stud bolt is a fully threaded rod with a nut on each side. It is preferred over a hex bolt for flanged joints because it allows even clamping on both sides of the flange and tolerates small thickness variations between mating flanges. In the EZ Steel fastener package, stud bolts are typically supplied to ASTM A193 and the matching nut to ASTM A194. The grade pair is the single most important number on the callout.

3.1 The Three Grade Pairs You Will See 95% of the Time

The "M" suffix on B7M and 2HM means the hardness is controlled to a maximum of 235 HB. That is not optional for NACE sour service: higher hardness risks sulfide stress cracking. If your purchase order says B7 and your service is sour, the inspector will reject the package.

3.2 Stud Bolt Length and Thread Engagement

Stud bolt length is calculated from the flange thickness on each side, the gasket thickness, the washer thickness (if any), and the nut thickness. A practical formula for RF flanges with two heavy hex nuts is:

L = 2 × (flange thickness) + gasket thickness + 2 × (nut thickness) + washer allowance ± tolerance

EZ Steel's engineering team supplies a stud bolt length table per ASME B16.5 / B16.47 flange size and class on request, so the buyer does not have to recalculate for every RFQ. Thread engagement should be at least one diameter (1 × d) and ideally 1.5 × d into the nut for high-pressure cyclic service.

Step 4: Bolt-Up Math — Torque, Pattern, and Passes

Specifying the right stud bolt and gasket is only half the job. If the joint is not tightened correctly, it will leak regardless of grade. EZ Steel's package is typically delivered with a bolt-up procedure sheet tailored to the flange size and class. Three points are worth highlighting for the procurement team.

• Target gasket stress. SWG with graphite filler: typically 30–45 MPa seating stress. Compressed fiber: 20–30 MPa. RTJ: the seating stress is set by the geometry of the ring biting into the groove, not by torque alone.
• Four-pass cross pattern. Tighten in four passes at 30%, 60%, 80%, and 100% of target torque, in a star / cross pattern around the flange. This seats the gasket evenly and avoids the "smile" face on the flange.
• Final pass after thermal cycle. For hot service, a fifth pass at 100% torque after the first hot cycle (or after the hydrotest, depending on the project spec) is the industry standard. EZ Steel's bolt-up sheet includes this step explicitly.

Step 5: Mill Test Documents You Should Insist On

For any B2B export order, the gasket stud bolt nut package must travel with traceability. The minimum set of documents that EZ Steel ships with the standard package includes:

• Mill Test Certificate (MTC) to EN 10204 3.1 for the metal strip in SWG, the RTJ ring, the stud bolt, and the nut
• Hardness report for stud bolts (HB / HRC) and nuts, with values per ASTM A193 / A194
• Dimensional inspection report to ASME B16.20 (gasket) and ASME B18.2.1 / B18.2.2 (fasteners)
• Material certificate for the filler material (graphite density, ash content, chloride content) on request
• NACE MR0175 compliance certificate when the order is for sour service
• Hydrotest or leak-test report when the project specification requires it

Without these, the package is just metal. With them, the package is a documented joint that the end client can trace back to a specific heat of steel.

Step 6: Tie the Package to the Right Pipe and Fitting Family

In real project procurement, the gasket stud bolt nut package is almost never bought in isolation. It is bought alongside the pipe fittings, the matching pipe flanges, and the line pipe or tube. Consolidating the package with one mill-direct supplier has three practical benefits for B2B buyers:

• Single heat-number traceability. If the line pipe, flanges, fittings, and gasket rings all come from one mill system, the heat numbers line up across MTCs. The end client does not have to reconcile five different traceability chains.
• Single inspection point. One QA team, one release, one shipping document set, one customs entry. This compresses the schedule for EPC and refinery turnaround work.
• Single point of accountability. If there is a leak, the supplier owns the joint design across the whole system — gasket, stud, nut, flange, and fitting — not three or four companies pointing fingers at each other.

Common Sourcing Mistakes (and How to Avoid Them)

A few patterns show up repeatedly in failed or delayed RFQs. Flagging them up front saves a round of clarification emails.

• Mixed SWG and RTJ on the same line. Pick one facing per line unless the spec explicitly calls for a transition. Mixing them is fine in design but a headache in procurement.
• Specifying inner ring on tongue-and-groove. T&G geometry does not leave room for an inner ring. The callout should be "SWG without IR."
• Carbon steel nuts on stainless studs. Galvanic corrosion will lock the nut to the stud in service. Match the materials or use PTFE anti-seize.
• Quoting the gasket and the stud bolt in two separate purchase orders. You will pay two freight bills, receive on two dates, and lose consolidated traceability.

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