Gasket, Stud Bolt and Nut Joint Walkthrough: How to Specify the Pressure-Boundary Closure Around Pipe Flanges, Fittings and Valves

Why the Joint Closure Deserves Its Own Specification Document

Most piping specifications list the flange, the fitting and the valve, then write "bolting and gaskets per ASME B16.5 / B16.34" as a single line. In practice, the closure is the most failure-prone part of the pressure boundary. A spiral-wound gasket that was correct for 300# but landed on a 600# joint, a stud bolt that was specified as B7 but delivered as B16, a nut that was ordered in the wrong hardness class, or a washer placed where a self-centering ring should be — each of these turns a designed joint into a leak path.

EZ STEEL INDUSTRIAL has been supplying pipe flanges, pipe fittings, industrial valves and matching gasket, stud bolt and nut kits from a single mill in Changsha, China since 1994. The lessons below come from how closure components actually arrive on site, how they assemble on the flange face, and how they behave when the line is heated, cooled, and opened for the next inspection — not from a single datasheet line.

Map the Service Envelope Before You Pick a Gasket

The right gasket, stud bolt and nut combination is driven by four inputs that come from the line, not the catalog. Skipping any of them is the most common reason a closure fails its first hot cycle, its first hydrotest, or its first unplanned shutdown.

Fluid, Temperature and Pressure Class

A 150# class joint in ambient water service is sealed by a compressed non-asbestos sheet gasket with B7 stud bolts and 2H nuts — a routine closure. The same nominal size in a 600# hydrocarbon line at 450°C is sealed by a spiral-wound gasket with a graphite or PTFE filler, matched to a raised face (RF) or tongue-and-groove (T&G) flange, with B16 stud bolts and 2H or grade 660 nuts, and a controlled bolt-up sequence. A 900# or 1500# joint steps up to a ring-type joint (RTJ) gasket in soft iron or low-carbon steel, with the bolt cross-section sized to seat the ring without cracking it. In every case the closure is selected from the same data sheet as the pipe flanges themselves, not as a follow-up item.

Flange Face Finish and Gasket Style Compatibility

The flange face finish has to match the gasket. A stock finish of 125–250 microinch Ra (AARH) is fine for a soft non-asbestos or graphite sheet gasket. A spiral-wound gasket needs a serrated finish in the 125–500 microinch Ra range, because the windings need micro-channels for the filler and the metal-to-metal seat needs surface roughness. An RTJ gasket needs the precise groove dimensions and hardness called out in ASME B16.5, with the ring hardness deliberately below the flange groove hardness so the ring crushes, not the groove. A mismatch in any of these three is the most common cause of a leak that appears immediately on first pressurisation.

Thermal Cycling, Fire Case and Steam

A closure that runs at a steady 300°C is a different design problem from one that cycles between ambient and 500°C every shift. Spiral-wound gaskets with graphite filler carry the bulk of high-temperature cyclic service. Pure PTFE and filled PTFE are reserved for low-temperature, low-pressure, chemically aggressive service — typically below 200°C. A fire-case requirement, common in hydrocarbon and offshore service, pushes the closure toward a flexible graphite or mica-based spiral-wound design, with the stud bolt and nut material upgraded to maintain residual gasket stress at the elevated temperature. Steam service, including the saturated-steam envelopes in power and process plants, takes spiral-wound gaskets with graphite filler and B16 stud bolts in either B7 or B16 depending on the temperature ceiling.

Corrosion, Crevice and the Bolting Itself

Stud bolts and nuts in carbon and low-alloy steel (B7 / 2H) are the default for most refinery and process service. Chloride-bearing service, offshore exposure, and low-temperature acid condensate push the closure toward B8 / B8M stainless bolting, with B8M nuts and a controlled hardness range so the threads do not gall. Sour service (NACE MR0175) requires the closure to meet the same hardness ceiling as the pipe and the flange — usually HRC 22 maximum — and the certificate set has to cover the bolting, the nut, and the washer together. For very low-temperature cryogenic service (LNG, ethylene, LPG), austenitic stainless bolting (B8 Class 1 or Class 2) is used with PTFE- or flexible-graphite-filled spiral-wound gaskets rated for the cryogenic envelope.

The Gasket Families You Will Actually Specify

A small set of well-understood gasket families covers the vast majority of industrial piping. Knowing the difference between them is more useful than knowing every variant on the catalog page.

Compressed Non-Asbestos and Graphite Sheet

Compressed sheet gaskets — non-asbestos fibre, graphite, or graphite with metal ｉｎｓｅｒｔ — carry the bulk of low- and medium-pressure, low- to medium-temperature service. They sit on a raised face (RF) or flat face (FF) flange, are cut from a single sheet, and are easy to handle in the field. They are the right answer for water, steam, air, oils, and mild chemicals up to 150# or 300# class, and up to about 450°C for graphite. They are not the right answer for high pressure, for cyclic service, or for any joint where the flange face is serrated, where the bolt load is high, or where the fluid is hazardous.

Spiral-Wound Gaskets

A spiral-wound gasket is a metal wind strip and a non-metallic filler strip wound together on a mandrel, with an inner ring (compression stop) and an outer guide ring. It carries the bulk of refinery, petrochemical, power, and high-temperature process service from 300# to 600# and beyond, and from cryogenic temperatures up to about 1000°C depending on the filler. Graphite filler is the modern default. PTFE is reserved for aggressive chemicals. Mica and ceramic fillers handle the high-temperature envelopes where graphite would oxidise. The spiral-wound is the right answer wherever the closure has to recover from thermal cycling, where a hot blowout would be unacceptable, and where the flange face is serrated to grip the windings.

Ring-Type Joint (RTJ) Gaskets

RTJ gaskets are solid metal rings — oval or octagonal — that seat in a precision groove on the flange face. The ring is always softer than the groove, so the ring crushes on first bolt-up and the joint becomes metal-to-metal. RTJ is the standard closure for 900# and above, for high-pressure hydrocarbon and steam headers, and for any joint where a soft gasket would be displaced by the bolt load. The ring material is selected for the fluid and the temperature: soft iron for water and low-temperature hydrocarbon, low-carbon steel for general refinery, 304/316 stainless for corrosive service, Monel and Inconel for the most demanding chemical and high-temperature envelopes.

Kammprofile, Grooved and Jacketed Gaskets

Kammprofile gaskets combine a soft facing (graphite or PTFE) with a grooved metal core to give a high-stress, low-leakage closure on heat exchanger and vessel flanges. Grooved gaskets are used in tongue-and-groove and male-female flange pairs. Jacketed gaskets — solid metal jackets filled with soft material — are reserved for the few services where a single-material closure cannot meet the corrosion and temperature requirement at the same time. All three are specialty items, ordered when the catalog spiral-wound or RTJ is the wrong answer for the joint.

Match the Bolting to the Gasket and the Service

A gasket is only as good as the bolt that loads it. The stud bolt grade, the nut grade, the washer type, and the bolt-up sequence have to be designed as a single system. ASME PCC-1 is the standard reference for the assembly procedure; ASME B16.5 and B16.34 cover the dimensional envelope; ASTM A193 and A194 cover the bolting and nut material.

Washer, Self-Centering Ring and Lubricant

A plain washer under the nut distributes the load and protects the flange face from the rotating nut. A self-centering washer (or a beveled washer on RTJ bolting) keeps the stud bolt square in the tapped hole during assembly. A controlled thread lubricant — typically an anti-seize compound with a known coefficient of friction — is required to keep the bolt-up torque consistent from one joint to the next. ASME PCC-1 calls out the friction assumption; the lubricant and the bolting have to be qualified together, not separately. Skipping the washer or the lubricant is the most common reason two identical joints tighten to two different gasket stresses.

Bundle the Closure With the Rest of the Pressure Boundary

A standalone closure order is the most expensive way to buy a closure. The same items, ordered inside a project bundle that includes the pipe flanges, pipe fittings and industrial valves they sit between, lands on site with one heat number set, one mill test certificate pack, one delivery window, and one responsible supplier.

What to Put in a Single Closure Bundle

• All pipe flanges of the same pressure class, facing, and material, with full MTC 3.1 documentation
• Matching pipe fittings (BW, SW, threaded) of the same standard, in matched quantities per spool, with the same heat number family
• Industrial valves specified with the same body, trim and pressure class as the line, with the same MTC pack
• Gaskets, stud bolts, nuts, washers and self-centering rings packaged per joint, with the right hardness grade for the design temperature
• Controlled thread lubricant, included in the bundle, with documented coefficient of friction
• For hazardous or high-pressure service, traceability covering the bolting, the nut, the washer, and the gasket as a single closure kit

How Bundled Sourcing Cuts Real Cost on a Closure Order

Three savings show up on every bundled closure order. First, freight: one container or break-bulk slot instead of three, with no airfreight top-up for the missing 2% of small parts. Second, document control: one MTC pack per spool, not per part, which removes hours of indexing work at the receiving desk and shortens the QA review before bolt-up. Third, schedule: a single delivery window lines up with the spool assembly crew, instead of flanges arriving on day one and stud bolts arriving on day twenty. On a typical 50-tonne piping bundle the bundled approach removes two weeks of float and one round of expedited freight.

A Field Checklist Before You Send the Closure RFQ

Use the list below as a final filter on every closure order. If any of these answers is missing, the closure is not yet specified.

• Fluid, design pressure, design temperature, and corrosion envelope confirmed in writing
• Pressure class (150# / 300# / 600# / 900# / 1500# / 2500#) and flange face (RF / FF / RTJ / T&G) chosen for the service
• Gasket style, filler, ring material and hardness selected for the same envelope as the flange
• Stud bolt material (B7 / B7M / B8 / B8M / B16) and nut grade (2H / 2HM / 8 / 8M / 660) specified for the design temperature
• Washer type, self-centering ring, and thread lubricant specified as part of the bolt-up kit
• Bolt-up sequence, target gasket stress, and torque (or load) values written into the assembly procedure, per ASME PCC-1
• Traceability and MTC level (3.1 / 3.2) written into the purchase order, not left to the supplier
• End protection, sea-worthy packaging, and overseas transport preservation specified on the order