Most flange leaks do not start with a bad flange. They start with three parts that were ordered as if they were three different projects: a valve rated to one pressure class, a pair of flanges rated to another, and a gasket-and-bolt set picked from a catalogue by bore size. They show up on site, they look like they belong together, and they pass the torque-up — until the line is heated, or the fluid is sour, or the first hydrotest pushes past the class limit that only one of the three parts was built for.

This article is built around the small set of decisions that have to line up on every joint: the pressure-temperature class stamped on the body, the facing machined on the flange, the gasket style sitting between them, and the stud grade and nut that pull the joint together. When those four are coordinated, the joint is repeatable. The aim is practical: a single set of choices you can apply to a new line spec, a P&ID, or a vendor drawing — and a way to spot the mismatch before the goods leave the mill.

Start with the Pressure Class on the Line

Everything else on the joint — the flange, the facing, the gasket, the stud grade — follows from one number: the pressure class of the line. The P&ID will express this as an ASME Class number (150, 300, 600, 900, 1500, 2500) or, on metric and EN projects, as a PN number (PN10, PN16, PN25, PN40, PN63, PN100). The valve body, the pipe flanges, and the gasket stud bolt nut set all have to be qualified to that same class for the design temperature of the line.

Two practical traps to avoid:

• Class vs. PN confusion. ASME Class 150 and PN20 are close but not interchangeable. Class 300 and PN50 also are not a 1:1 match. On a multi-region project, get the class system confirmed at the line-list stage — not at the receiving inspection.
• Class at design temperature, not at room temperature. ASME B16.34 and B16.5 pressure-temperature ratings fall as temperature rises. A Class 600 valve at 38 °C is not a Class 600 valve at 540 °C. Always qualify the rating at the maximum design temperature of the line.

The Flange Facing Decides the Gasket

The facing on the flange is what the gasket actually seals against. The four that cover almost every refinery, petrochemical, and process-piping joint are:

• Raised Face (RF). The default for ASME Class 150 and Class 300. The gasket sits on a 1.5–3 mm raised face. Concentric serration (stock finish) or spiral serration (phonograph finish) is specified at the quote stage — the two are not interchangeable.
• Flat Face (FF). Used on cast iron, glass-lined equipment, and low-pressure water service. Requires a full-face gasket that reaches the bolt circle — never a ring gasket on a flat face.
• Ring Type Joint (RTJ). The default from ASME Class 600 upwards and for high-temperature hydrocarbon service. The gasket is a metal ring (oval or octagonal cross-section) that sits in machined grooves. Pair only with RTJ flanges and RTJ valve ends.
• Tongue and Groove (T&G), Male and Female (M&F). Specified where a self-aligning, self-centring joint is needed — typically on heat-exchanger covers and special process equipment. Less common on pipeline work.

The single most common mismatch on a fresh project is an RF flange ordered with an RTJ gasket, or an RTJ flange ordered with a soft spiral-wound gasket. Neither is forgiving: the RF soft gasket will not seat in the RTJ groove, and the RTJ ring will not seal on the raised face. The rule is simple — the facing on the flange dictates the gasket style, full stop.

Matching Gasket to Service

Once the facing is set, the gasket material and style follow. The list below covers the four gasket families that show up on the bulk of industrial valves and process piping.

Compressed Non-Asbestos Sheet

Aramid fibre with a nitrile or graphite binder. Used for low-pressure water, air, and mild chemical service up to Class 300. Cheap, forgiving on RF facings, and tolerant of minor flange face damage. Not for hydrocarbon above 200 °C, not for steam above 250 °C, and not for any sour or oxygen service.

Spiral-Wound

A stainless steel strip wound with a non-metallic filler (graphite, PTFE, or mica). The workhorse of refinery and chemical service from Class 150 to Class 600. The filler sets the temperature and chemical limit: graphite for hydrocarbons and steam, PTFE for aggressive chemicals, mica for very high temperatures. Specify a centering ring and an inner ring on Class 300 and above to prevent turbulence erosion and over-compression.

Ring Joint (RTJ)

A solid metal ring — soft iron, low-carbon steel, 304, 316, or Monel — that bites into the flange groove under bolt load. Used from Class 600 up, and on any joint that has to be made and broken repeatedly. Brinell hardness of the ring is set about 30–40 BHN below the flange groove to ensure the ring — not the flange — is the consumable.

Kammprofile and Metal-Jacketed

A solid metal core with a soft facing — typically graphite — bonded to both sides. Used on heat-exchanger joints and in services where a spiral-wound would compress too much. Specified by the equipment manufacturer on the data sheet; do not substitute without their sign-off.

Stud Bolts and Nuts: The Forgotten Half of the Joint

The gasket does not seal itself. The stud-and-nut set pulls the flange faces together with enough residual load to keep the gasket seated through pressure cycles, temperature swings, and vibration. Get the stud grade wrong and the joint either leaks (under-tightened) or cracks the flange (over-tightened). Get the thread length wrong and the nut bottoms before the joint is made.

Two standards cover almost every stud-bolt spec on a process joint:

• ASTM A193 B7 / A194 2H. The default for ASME Class 150 to Class 600 hydrocarbon service up to about 425 °C. Chromium-molybdenum alloy steel, quenched and tempered. The nut is the matching heavy-hex 2H.
• ASTM A193 B16 / A194 7. Specified for high-temperature service above 425 °C — the chromium-molybdenum-vanadium grade that keeps its strength at the temperatures B7 starts to soften at.
• ASTM A320 L7 / A194 4. Required for low-temperature carbon and alloy steel service below -29 °C. Impact-tested to 18 J at -101 °C. The default for LNG, LPG, and light-hydrocarbon service.
• Stainless B8 / B8M (Class 1 or 2). Specified for stainless pipe flanges and stainless lines where galvanic corrosion between a carbon steel stud and a stainless flange would be a concern.

A practical habit: every stud-and-nut set on a project should arrive with a mill test certificate, a hardness report, and (for low-temperature or sour service) an impact-test report. Keep them in the same traceability file as the flange and the valve body. When the inspector asks for the heat number, the answer is one click away, not a phone call to the warehouse.

Putting It Together: A Reference Table

The table below is the cheat-sheet version of the rules above. It is meant to be applied at the line-class stage, then cross-checked against the project piping specification before the PO is cut. The "Class" column refers to ASME B16.5 / B16.34 classes; the equivalents on a metric project are listed alongside.

A Procurement Workflow That Catches the Mismatch Early

Most flange-and-valve mismatches are visible on the line-list long before the parts leave the mill. The five-step workflow below is the one we run on every coordinated bundle.

1. Lock the Line Class

For every line on the isometric, capture the line number, the service, the design pressure, the design temperature, the fluid, and the ASME Class (or PN). The line-class sheet is the single source of truth — every valve, every flange, every stud set is qualified against it.

2. Pull the Mating Parts from One Source

A valve, two flanges, a gasket, a stud-and-nut set — that is one joint. Order it as one joint. The supplier that ships all five on one packing list, with one heat-number family and one test plan, is the supplier you want. It is the simplest way to avoid the situation where the valve arrives from one mill, the flanges from another, and the studs from a third — all stamped to the right class, none of them tested together.

3. Match the Body Material to the Pipe

A carbon steel pipe line should not end in a stainless steel valve body unless there is a written reason. The WCB / CF8M / LCC choice on the valve has to be the same family as the pressure tubes on either side. Mixed-material joints work, but only when the galvanic, thermal-expansion, and NACE implications have been written down — not when they are a surprise at the torque-up.

4. Confirm the Test Plan in Writing

Hydrotest, pneumatic test, NDT, PMI, hardness survey — these are the items the inspector will check at the receiving bay. Get the test plan, the acceptance criteria, and the certificate format in writing at the quote stage, not at the dock. A coordinated bundle ships with one consolidated MTR and one test report covering the valve, the flanges, and the stud set.

5. Run a Receiving Inspection That Mirrors the Test Plan

Visual, dimensional, material verification (PMI on stainless and alloy), hardness check on studs, and a check of the class stamp and facing against the line-class sheet. Five minutes at the receiving bay saves five weeks of rework on site.

What a Coordinated Bundle Looks Like in Practice

A refinery shutdown, a gas-compressor station, a seawater cooling train — these are the projects where the cost of a mismatch is measured in lost production rather than rework hours. The bundle that arrives on site for these projects typically looks like this:

• A trunnion-mounted ball or API 600 gate valve, body in WCB or LCC, trim in 13Cr or SS316, flanged RTJ ends, marked per ASME B16.34.
• A pair of steel flanges in the same material family and the same heat number, faced and serrated to the project piping-class spec.
• A spiral-wound or RTJ gasket, batch-marked and traceable to the same job number.
• A stud-and-nut set in the correct grade (B7, B16, or L7), with nuts, washers, and a lubrication note from the supplier.
• A single packing list, a single MTR file, and a single point of contact for any post-shipment question.

On the back of that bundle, the right connection is already specified for the pipeline works on either side of it. The line-class sheet is satisfied, the inspector is satisfied, and the joint is ready to make up. That is the whole point of coordinating the procurement — moving the engineering decision from the torque-up back onto the line-list, where it belongs.

Frequently Asked Questions

Can I mate an RF valve to RTJ flanges with a spiral-wound gasket?

Not safely. The RTJ groove is machined into the flange face; a spiral-wound will not seat in it. Either order the valve with an RF end (and use an RF gasket) or ｒｅｐｌａｃｅ the flanges with RTJ flanges matched to the valve.

Do I need inner rings on every spiral-wound gasket?

Yes on Class 300 and above for hydrocarbon service. The inner ring prevents turbulence erosion of the winding, centres the gasket, and acts as a thermal block during make-up. It is a small cost; skip it and the gasket will fail early.

Can I use B7 studs on stainless flanges?

You can, but you should not, especially in wet or coastal service. The galvanic couple between a carbon steel stud and a stainless flange is a known corrosion cell. Specify B8 / B8M (Class 2) for stainless lines; the cost difference is small compared to a flange change-out ten years later.

What is the practical difference between an oval-section and an octagonal-section RTJ ring?

The octagonal section gives a slightly higher blowout resistance and is the modern default. The oval section is interchangeable in modern grooves but is being phased out. New specification should call for octagonal, type R, in the material matched to the service.