EZ Steel Industrial  |  Pipe Flanges Sealing & Bolting Buyer's Guide

Flange Gaskets, Stud Bolts and Nuts: A B2B Buyer's Guide to Sealing & Bolting Selection, Standards, and Mill-Direct Sourcing

A leaking flange joint rarely fails because someone picked the wrong pipe. It fails because someone picked the wrong gasket, the wrong stud bolt grade, or the wrong nut — then never verified the combination against a standard. This guide is written for procurement engineers, EPC sub-contractors, and plant maintenance leads who would rather specify the correct joint on paper than debug a leak at 3 a.m.

Why This Guide Matters Before You Send the Next RFQ

Walk through any oil refinery, LNG terminal, desalination plant, or ship engine room and you will find the same truth repeated in slightly different words: a flanged joint is only as reliable as the three components that hold it together — the flange faces, the gasket between them, and the gasket stud bolt nut set that clamps everything shut. Buy the flange and forget the seal, and the line leaks. Buy the seal and forget the bolt grade, and the seal crushes unevenly. Buy all three from three different low bidders, and the documentation does not match when the inspector arrives.

At ezindustrialtube.com, we see this pattern every week. A buyer will spec out a 24-inch Class 300 pipe flanges system to ASME B16.5, then ask for a "suitable gasket" and "B7 bolts" in the notes. The flange itself might be perfect. The joint will still leak, because "suitable" was never defined and the bolt length was never verified against the gasket thickness and flange recess.

This guide walks through how to specify a complete gaskets-and-bolting package the way a mill-direct manufacturer would: classify the service, choose the gasket type, match the bolting material, verify against the flange standard, and only then quote.

1. Classify the Joint First: Service, Fluid, Pressure, Temperature

Before anyone talks about R-type versus spiral wound, the joint needs a service classification. Five questions decide almost everything downstream:

• What fluid? Hydrocarbons, steam, seawater, ammonia, cryogenic LNG, sour service (H₂S), or potable water all push the choice in different directions. Sour service in NACE MR0175 environments rules out standard carbon steel bolting; seawater rules out standard graphite filler above 200 °C.
• What pressure class? Class 150 to Class 300 is a spiral-wound world with semi-metallic options. Class 600 and above, especially at smaller NPS, is increasingly an RTJ (ring-type joint) world.
• What temperature? Below −46 °C (cryogenic) you are usually looking at stainless or nickel bolting and a flexible graphite or PTFE gasket. Above 500 °C you are looking at metallic gaskets, often with a graphite or mica filler that survives the heat.
• Cycling or static? A joint that cycles from ambient to 400 °C every shift needs a gasket that recovers. A static joint only needs to seal once and hold.
• Fire-safe requirement? API 607 / ISO 10497 fire-safe tests exist for a reason. If the line is in a hydrocarbon fire zone, soft PTFE will not pass without a metal core.

Once those five answers are on the datasheet, the rest of the conversation is mechanical.

2. Match the Gasket Type to the Flange Face and Service

The four workhorse gasket families — compressed non-asbestos, spiral wound, ring-type joint, and jacketed — each map cleanly to a flange face and a service envelope. The mistake most buyers make is treating the gasket as a generic "soft" or "hard" choice instead of a family-by-family decision.

Compressed Non-Asbestos (CNA) & Flexible Graphite

CNA sheet gaskets with aramid fibre and NBR binder are the default for Class 150/300 RF (raised face) flanges in water, air, low-pressure steam, and mild chemicals. Flexible graphite (often with a stainless steel tang ｉｎｓｅｒｔ) extends the temperature range upward and is the standard upgrade when the buyer needs chemical compatibility that CNA cannot deliver. Both are cut to ASME B16.21 dimensions, which is why procurement typically calls them "B16.21 sheet gaskets" rather than by material.

Spiral Wound (SW)

The spiral wound gasket is the most-specified semi-metallic gasket in refinery and petrochemical piping. A metal strip (usually 316L stainless, Monel, or Inconel) is wound alternately with a non-metal filler (graphite, mica, PTFE) around a centering ring. The result seals Class 150 through Class 2500 raised-face and tongue-and-groove flanges, tolerates thermal cycling, and is the standard choice for ASME B16.5 flanges from NPS ½ through NPS 24. ASME B16.20 is the governing dimensional standard; the SW gasket sits in a male-female or tongue-and-groove recess when the user wants to be sure the gasket cannot blow out.

Ring-Type Joint (RTJ)

When the pressure class climbs past Class 600 or the service includes hydrocarbons at high temperature, the conversation turns to RTJ gaskets. The R, RX, and BX profiles are made to ASME B16.20 dimensions and seat in the machined groove of a ring-joint flange face. The seal is metal-to-metal, achieved by controlled compression. RTJ is the only choice for many offshore and wellhead applications, and it is the default when the buyer wants a joint that can be re-tightened under full pressure after a controlled blowdown.

Jacketed & Kammprofile

Jacketed gaskets wrap a non-metal filler inside a metal envelope; Kammprofile uses a serrated metal core with soft faces on both sides. Both fill the niche between CNA sheet and spiral wound — higher temperature than CNA, easier to handle than SW, and more compliant than RTJ. They are common in heat exchanger applications, where the gasket has to seat on a narrow face without overloading the bolts.

3. Bolting Is Not "B7 by Default" — Match the Stud Bolt and Nut to the Service

Once the gasket family is set, the bolting has to be specified in the same conversation. A wrong bolt grade is a slow leak or, in sour service, a stress-corrosion failure. The standard material selection is:

• ASTM A193 B7 — chromium-molybdenum steel, the default stud bolt for ASME B16.5 Class 150 to Class 2500 flanges in hydrocarbon service up to about 450 °C. Pairs with ASTM A194 2H heavy hex nuts. This is the workhorse combination and what most buyers actually mean when they write "B7."
• ASTM A193 B8 / B8M / B8T — stainless steel classes (304, 316, 321) used in low-temperature, cryogenic, or corrosion-sensitive service. Pairs with A194 8 / 8M / 8T nuts. Mandatory for LNG and many chemical services.
• ASTM A193 B16 — chromium-molybdenum-vanadium steel for high-temperature bolting above 450 °C, common in steam and power plant piping.
• ASTM A320 L7 / L7M — low-temperature carbon and alloy bolting, mandatory for −46 °C and below, paired with A194 7 or 7M nuts.
• NACE MR0175 compliance — for sour service (H₂S-containing hydrocarbons), the bolt material must meet NACE hardness limits. B7M, B7M with supplementary testing, or L7M are typical.

The length of the stud bolt is calculated from the flange face geometry, the gasket thickness, the centering ring thickness (if any), and the nut height. ASME B16.5 stud bolt lengths are tabulated in standard flange datasheets, but the moment a non-standard gasket is inserted the length must be re-verified. A stud that bottoms out before the gasket reaches design compression is a field failure waiting for a thermal cycle to expose it.

4. Quick Reference: How the Families Map to Flange Standards

The table below summarises how the four main gasket families align with the most common flange standards. It is intentionally simplified; full selection is always confirmed against the line-specific P&ID and the project's piping class.

5. What a Mill-Direct Sealing & Bolting Package Should Look Like

A complete gaskets-and-bolting delivery from a mill-direct supplier is more than a list of part numbers. Five documents should arrive with every shipment, and the absence of any one of them is a sign that the supplier is trading, not manufacturing:

• Material test certificate (MTC / mill certificate) traceable to the heat number, showing chemical composition and mechanical properties for every stud bolt, every nut, and every metal component of the gasket.
• Dimensional inspection report confirming that the gaskets meet ASME B16.20 (or B16.21) tolerances, and that the stud bolt lengths match the flange datasheet.
• Hardness report for bolting — required for B7, B7M, B16, and any NACE MR0175 application.
• Marking and traceability on every component: heat number, material grade (B7, B8M, L7M, etc.), manufacturer mark, and standard reference. The colour codes on spiral wound gaskets must match the material.
• Packaging that preserves the joint — gaskets individually wrapped, stud bolts and nuts bagged by heat number, and any PTFE or graphite components protected from UV and mechanical damage.

The factories that actually make these components, rather than re-label them, can hand over all five documents in a single PDF the day the goods leave the warehouse. Trading houses and online resellers usually cannot.

6. A Pre-RFQ Checklist That Catches 80% of Mistakes

Before sending a gaskets-and-bolting RFQ, the following list will catch the vast majority of field problems:

• Confirm the flange standard: ASME B16.5, B16.47 Series A, B16.47 Series B, or API 6A.
• Confirm the flange face: RF, FF, RTJ, MF, TG, or specialty.
• Confirm the pressure class and NPS — then pick the gasket family from the table above.
• Confirm the fluid, temperature, and whether the service is sour (NACE) or fire-safe (API 607).
• Specify the stud bolt material and nut material as an explicit pair (for example, A193 B7 with A194 2H), and flag NACE if relevant.
• Verify the stud bolt length against the chosen gasket thickness, not against a generic flange datasheet.
• Request MTCs, hardness reports, and dimensional reports as a contractual deliverable, not as a "nice to have."
• Confirm packaging and labelling will survive containerised ocean freight without mixing heat numbers.

7. Where the Cost Actually Hides

Gaskets and stud bolts look like small-ticket items compared with the carbon steel pipe and stainless steel pipe runs they sit between, which is why they are often the last line of a procurement package and the first to be de-scoped. That is the wrong place to save.

A single unplanned shutdown on a 24-inch hydrocarbon line can cost more than the entire bolting package for a year. The genuine cost of gaskets and stud bolts is the cost of a mis-specified joint, the cost of the emergency callout, and the cost of the production loss — not the unit price on the PO. The right spec, written on the RFQ, is the cheapest part of the joint.

8. Bring Us Your Piping Class — We Will Quote the Whole Joint

If you are about to issue a flange, gasket, and bolting package and you want one manufacturer responsible for the whole joint, send us the piping class, the P&ID extract, or the flange datasheet. Our engineering team in Changsha will:

• Match the gasket family to the flange face and service.
• Specify the stud bolt and nut grade as a paired set, including NACE or low-temperature variants when required.
• Quote the flange, the gasket, the stud bolt, and the nut on one line item each, with MTCs across the package.
• Hold the order against your MTR / heat number register for the next project.