A bolted flanged joint is a small system. The flange gives you the geometry, the gasket gives you the seal, the stud bolt holds the load, and the adjacent pipe fittings carry the pipe-end load. Treat any one of them as a separate purchase and the joint will drift; treat them as a matched package and the joint will run for years without a wrench touching it.

This guide is written for the buyer, piping engineer and warehouse lead who has to specify or receive these parts on an industrial project. It pulls together what we have learned from twenty-eight years of supplying pipe flanges, steel flanges, gasket stud bolt nut sets and matching pipe fittings for water, oil and gas, power and marine projects.

Why Flanged Joints Leak Even When Every Part Is “Correct”

On a typical maintenance audit, joint failure splits into a few recurring root causes. About 40% comes from the flange face itself — wrong facing type, surface finish out of spec, dents from a careless wrench. Around 30% comes from stud bolt preload: too loose, too uneven, or a bolt grade that has crept after a few heat cycles. About 20% comes from a gasket that was correct in the catalog but wrong for the actual service. The remaining 10% is material ageing and one-off installation mistakes.

The pattern is clear. The flange face and the bolt load decide whether the gasket can ever do its job. Choosing a good gasket stud bolt nut set is necessary, but it is not the answer on its own — the answer is making the face, the gasket and the bolt agree with each other before the line is pressurised.

Step 1 — Pick the Flange Facing First, Not the Gasket

Procurement teams usually start the specification with the gasket, then look for a flange that “accepts” it. On a working job, you do the reverse. The facing on the pipe flanges sets the gasket family you can run, the bolt load the joint will need, and the leak-tightness the line is realistically going to hold.

The Six Facings You Will Actually See on a Project

• Raised Face (RF). The default for carbon and stainless steel piping up to Class 600. Works with spiral-wound, graphite and reinforced PTFE gaskets. Easy to machine, easy to source.
• Flat Face (FF). Used on cast iron, copper-nickel and other brittle materials. Needs a soft gasket — rubber, CAF, PTFE envelope. Avoid mixing FF with RF; the mismatch concentrates stress on the brittle flange.
• Ring Type Joint (RTJ). The right call for high-pressure hydrocarbon service, Class 900 and above, or sustained temperatures above 400 °C. Uses metal ring gaskets (R, RX, BX profiles) machined to fit the groove. Not forgiving in the field — bolt load and groove condition are everything.
• Tongue & Groove (TG) and Male & Female (M/F). Gasket is locked in by geometry. Preferred for VOC service, fine chemicals, and any line where gasket blow-out is a real risk. Always supplied as a matched pair.
• Large Male & Female (LMF). Used on pressure vessel nozzles and equipment ends. Wider contact face is kinder to manufacturing tolerances.
• Self-Energising (Lens, O-ring metal). Reserved for cryogenic, aerospace and very high pressure hydraulic lines. The medium itself helps seal as pressure rises.

Step 2 — Match the Gasket to the Facing, Not to the Catalog

Once the facing is set, the gasket is much easier to choose. Three properties decide whether a gasket will work in a given joint: compressibility, recovery, and creep relaxation.

• Compressibility is the ability to flow into the micro-imperfections on the flange face. RF and FF need a soft, compressible sheet; RTJ needs a metal ring that can yield plastically into the groove.
• Recovery is the ability to follow the joint as temperature swings stretch the bolts. VOC lines and thermal-cycling service punish low-recovery gaskets — the leak you find in the morning is usually last night's recovery failing.
• Creep relaxation is the slow loss of bolt load at temperature. Soft gaskets on a hot line creep fast; spiral-wound and graphite gaskets creep slowly and hold load for years.

A spiral-wound gasket on an RF facing is the most common industrial choice because it tolerates thermal cycling, handles a wide pressure range, and survives minor face damage that would destroy a soft sheet gasket. Where you have a hard facing like RTJ, the gasket has to be a metal ring matched to the groove profile — there is no substitute, and a “similar” ring from a different supplier can leak from the first heat-up.

Step 3 — Size the Stud Bolt to the Gasket, Not Just to the Flange

Stud bolts are the part engineers most often under-specify. The bolt has to deliver enough preload to seat the gasket and keep it seated for the design life — but it also has to stay elastic so the next maintenance turn can re-torque the joint without snapping a stud on the first try.

The right gasket stud bolt nut package always carries three matched items: stud bolt (material and length), two heavy hex nuts per stud, and a washer where the design calls for it. Mixing B7 studs with B8 nuts, or B16 studs with B7 nuts, is a common audit finding — the joint is only as strong as its weakest component.

A practical point crews often miss: the number and size of bolt holes is fixed by the flange class and size, not by the gasket. An NPS 12 Class 150 RF flange has 12 studs; an NPS 12 Class 300 has 16; Class 600 jumps to 20. If the bolt count on the delivered pipe flanges does not match the line, you have the wrong flange — not the wrong gasket.

Step 4 — Bring the Pipe Fittings Into the Same Specification

The flanged joint only works if the pipe end feeding it lines up, sits square, and does not pull sideways under thermal growth. That is the job of the pipe fittings right next to the flange — the elbow, the tee, the reducer, the branch connection.

A 90° elbow directly bolted to a flange is the most common source of “mystery leaks” on a new line. The fitting must be the same schedule as the mating pipe, the same material family, and oriented so its centreline is dead-on to the flange face. Butt weld fittings are welded in and aligned during fit-up; socket weld fittings are used on small-bore high-pressure systems; threaded fittings are reserved for low-pressure, instrument and utility lines.

Material compatibility is non-negotiable

When a steel flange is bolted to a stainless branch, or a copper nickel flange lands next to a carbon steel elbow, the joint will leak — not because the gasket is wrong, but because galvanic and thermal-expansion mismatch is loading the bolts unevenly. The entire run of fittings, flanges, gaskets and stud bolts in any single line should share a material family.

Step 5 — Bundle the Package, Not the Items

The most reliable way to make all four parts work together is to procure them as a single package from one supplier. That gives you one heat number, one MTR file, one delivery date, and one engineer on the phone if something on the line does not match. A bundled approach also gives you a single point of accountability for the MTC traceability that auditors look for on hydro, offshore and nuclear work.

At EZ Steel Industrial, we ship flanged assemblies as a single source. A typical bundle for a refinery auxiliary line includes RF pipe flanges in ASTM A105 / A182, spiral-wound gaskets with inner and outer rings, B7 / 2H stud bolt sets, and the matching pipe fittings in the same schedule. For a marine seawater line, the bundle shifts to copper nickel flanges (EEMUA 144 90/10), Cu-Ni BW fittings, soft gaskets and K-Monel or B8M stud sets. One package, one delivery, one line of paperwork.

Quick Specification Table

A Short Field Checklist

• Confirm the flange facing on the MTC matches the gasket type on the receiving inspection.
• Check bolt count and bolt hole PCD against the line class — not just “fits on a table”.
• Verify gasket inner ring material is compatible with the medium, especially on sour service.
• Use a torque wrench and a documented cross-pattern sequence on every joint above Class 300.
• After the first heat cycle, walk the line with a thermal imager and re-torque where you see temperature drift at the joint.

Where EZ Steel Industrial Fits

We have been producing and exporting pipe flanges in carbon, alloy, stainless and copper-nickel grades since 1994. Our scope covers steel flanges to ASME B16.5 / B16.47, copper nickel flanges to EEMUA 144, complete gasket stud bolt nut sets, and the pipe fittings that sit either side of the joint. The same mill that makes the pipe makes the fittings; the same warehouse that holds the flanges holds the gaskets and bolts. That is how a flanged joint becomes a system instead of a stack of parts.