1. Why Bolted Joints Leak More Often Than the Flanges Themselves

A flanged joint is a system, not a part. The flange ring, the gasket, the studs, the nuts, the facing finish, the bolt torque and the service fluid all have to be in agreement, or the joint will find a leak path. In practice, the flange and the line pipe are usually specified correctly because they appear on the isometric drawing. The gasket, stud and nut are often ordered as a "consumable" line item by the procurement team, which is why they are the most common source of unplanned shutdowns.

A useful mental model is to think of the gasket as the active sealing element, the studs and nuts as the clamping mechanism, and the flange facing as the sealing surface. Get any one of the three wrong, and the other two cannot compensate. The rest of this guide walks through how to specify each one for a refinery, petrochemical, power or marine piping system.

2. Picking the Right Gasket for the Service

Gasket selection starts with three numbers: design temperature, design pressure, and the chemical nature of the fluid. From there, the choice usually narrows to one of three families.

Ring-type joint (RTJ) gaskets are solid metal rings (oval or octagonal cross-section) that sit in machined groove on the flange face. They are the standard for ASME B16.5 Class 600 and above, and for most high-pressure wellhead and Christmas-tree assemblies. Once compressed, they can only be re-used after a full re-machining pass on both the ring and the groove.

Non-metallic gaskets — compressed fiber, PTFE, flexible graphite — still have a real place in low-pressure water, air, HVAC and chemical service. They are cheap, conform to imperfect faces, and do not need controlled torque. They do not belong on a steam line above 200 °C, in a hydrocarbon flare line, or anywhere the project calls for a fire-safe design.

A practical shortcut: if the line is on a process flow diagram, not a utility diagram, start with a spiral-wound gasket. The incremental cost over a compressed fiber sheet is small, and the gain in resilience and traceability is large. Where the line touches seawater — cooling loops, ballast, firefighting mains — confirm the gasket material against the same corrosion data used to pick the copper nickel alloy pipe and the matching copper nickel flanges.

3. Stud Bolt and Nut Grades That Match the Pressure Class

The stud bolt and the nut are a matched pair. The grade marking on the stud head has to agree with the grade marking on the nut, and both have to be appropriate for the design temperature. Mixing grades is one of the most common causes of brittle fracture in cold service and of creep rupture in high-temperature service.

The thread standard has to be agreed in writing. Most international projects mix UNC/UNF (ASME B1.1) and metric ISO threads on the same site, and a stud that is "almost the right size" will not pass a torque test. State the thread standard, the length (measured from end to end, not from under-the-head) and the projection of the stud beyond the nut face on every RFQ.

4. Facing, Finish and Flange Preparation

A spiral-wound gasket cannot seal on a flange face that has a 3.2 mm Ra finish; an RTJ gasket cannot seal in a groove that has been touched with a wire brush. The flange facing is part of the seal.

• Raised face (RF) with serrated concentric finish (typically 125–250 µin Ra, 3.2–6.3 µm Ra) — the most common facing for spiral-wound gaskets in Class 150 to Class 600. The serration helps the gasket bite and resist blow-out.
• Flat face (FF) — used with cast iron, FRP and other non-metallic flanges where a full-face gasket is needed to spread the load over the entire face.
• Ring-type joint (RTJ) — groove must be machined to ASME B16.5 dimensions with the correct profile number (R, RX, BX). Any corrosion, galling or out-of-roundness above the standard tolerance is a re-machine.
• tongue-and-groove (T&G) and male-and-female (M&F) — niche facings, often for heat exchanger covers and low-pressure chemical service.

Before assembly, the flange faces should be clean, dry and free of radial scratches, weld spatter and old gasket residue. A plastic scraper — never a metal one — is the right tool to remove stuck material. Any deviation above the surface roughness spec should be re-faced on-site or sent back to the machine shop. The mating flanges also need to be parallel within the limit set by the gasket manufacturer; misalignment concentrates the bolt load on one side of the joint and is a leading cause of leakage on first pressurization.

5. Torquing, Cross-Bolting and Re-Torque Schedules

A gasket seals at a target gasket stress, not at a target torque. The torque value is only a way to reach that stress, and it depends on the stud grade, the stud diameter, the nut factor and the lubrication state of the threads. The bolt-up sequence matters as much as the torque value.

Hydraulic stud tensioners are worth specifying on large-bore Class 600 and above flanges, where manual torque wrenches cannot guarantee uniform bolt load around the circumference. They also remove the operator-dependence from the procedure, which matters on a multi-national project where crews of different nationalities will be working on the same line. Always record the torque values, the sequence, the time and the operator on the joint completion sheet — that document is the one the inspector will ask for at the hydrotest.

6. Standards, MTCs and Documentation That Survive Audit

The stud, the nut and the gasket each carry a separate standard, and the project specification will name which edition applies. The most common combinations are listed below.

• ASME B16.5 / B16.47 — flange dimensions and pressure-temperature ratings, including the bolt circle and the number of bolt holes. The stud length is usually derived from the flange thickness and the nut height given in the same standard.
• ASME B16.20 — spiral-wound and RTJ gaskets used with ASME B16.5 flanges. Covers style, materials, dimensions and markings.
• ASME B16.21 — non-metallic flat gaskets for ASME B16.5 flanges.
• ASME B18.2.1 / B18.2.2 — heavy hex bolts, studs and heavy hex nuts. Defines the dimensional envelope for the most common stud bolt designs.
• ASME PCC-1 — the assembly procedure reference for pressure-boundary bolted flange joints. Often called out on the ITP even when the flange is not ASME-stamped.
• EN 1514 / EN 12560 — European gaskets for pipe flanges. Pairs with EN 1092-1 flanges and PN pressure classes.

Material test certificates should be ordered to EN 10204 3.1 for the studs and nuts, and to 3.2 where the service is sour (NACE MR0175), nuclear, or otherwise high-consequence. A stud bolt that ships without a traceable heat number is the wrong stud bolt — even if it is the right size and grade. Keep the MTCs, the dimensional reports and the assembly records together; the inspector will ask for them as a single package at the final walk-down.

7. Common Field Failures and How to Prevent Them

Across refinery, power and shipbuilding sites, the same handful of mistakes show up on every reliability review. Most of them are specification and assembly issues, not material defects.

• Wrong gasket for the facing. A spiral-wound gasket is installed on a flat face, or an RTJ on a raised face. The joint cannot develop the seal stress it was designed for, and it leaks on first pressurization.
• Mixed stud and nut grades. A B7 stud with a grade 2H nut is fine; a B7 stud with a B8M nut is not. The lower-strength partner yields first, the joint relaxes, and the leak follows.
• Over-torqued studs. Yielding the stud on assembly feels like a tight joint, but the stud has lost its elastic reserve and will not follow the gasket through thermal expansion.
• Re-used RTJ gaskets. The ring is work-hardened from the first compression. A second compression does not reach the original seal stress, and the joint will not pass the next leak test.
• No re-torque on hot service. The first thermal cycle relaxes the gasket by 10–20%. A scheduled re-torque at ambient, before the line is handed over, is the cheapest insurance on the project.
• No torque record. If the joint leaks after hand-over, the first thing the inspector wants is the bolt-up sheet. A missing sheet is treated as a missing procedure.

8. Sourcing the Joint as a Single Package

A bolted joint is not the sum of three line items. It is a coordinated package of flange, gasket, studs, nuts, facing finish, assembly procedure and documentation. The cleanest way to keep it that way on a real project is to source all of it from one manufacturer on one PO, against one consolidated MTC set.

That is the structure the EZ Steel Industrial catalogue is built around. The site organizes the work the same way a piping engineer or procurement officer already thinks: pipe flanges in carbon, stainless and copper-nickel grades, the matching pipe fittings, the gasket stud bolt nut set, and the line pipe — carbon steel pipe and stainless steel pipe — that ties the system together. Specifying the whole joint against one supplier removes the most common hand-off errors: a stud length that is right for one flange and wrong for the next, a gasket style that drifts between packages, a documentation set that does not agree across line items.

9. Closing Notes and Next Steps

A flanged joint that stays tight for the life of the plant is not the result of an exotic material or a clever trick. It is the result of a small number of correct decisions made at the right time: the right gasket for the service, the right stud and nut grade for the temperature, the right facing for the gasket, and a documented assembly procedure that the crew can actually follow. None of those decisions is technically difficult. The hard part is keeping them in agreement across the whole package, from the line pipe to the industrial valves at the end of the run.

If you are at the RFQ stage on an active project, the fastest way to keep those decisions in agreement is to send the flange list, the bolt-up schedule and the service conditions to a supplier that can quote the whole package on one MTC set. That is the surest way to keep the joint sealed at hydrotest, sealed at start-up, and still sealed at the next turnaround.