What types of gasket, stud bolt & nut are available for industrial piping?

1. Gaskets: three families, one job

A gasket is a mechanical sealing element placed between two flange faces. Because machined metal surfaces are never perfectly smooth, the gasket compresses under bolt load and fills the microscopic gaps, blocking the leak path. Gaskets are grouped into three families based on material and construction: non-metallic, semi-metallic, and metallic.

1.1 Non-metallic (soft) gaskets

Soft gaskets are cut from flat sheet material and are the workhorses of low-pressure utility and chemical service. They conform well to minor flange-face imperfections, are inexpensive, and are quick to ｒｅｐｌａｃｅ during maintenance. The common types are:

• Compressed non-asbestos fibre (CNAF) — aramid fibre with an NBR or similar binder. A general-purpose choice for water, air, steam, and mild chemicals, typically up to ASME Class 150 and around 200 °C.
• PTFE — outstanding chemical resistance, which makes it a favourite for aggressive acids and for food and pharmaceutical lines. It is limited in pressure and temperature and can cold-flow under load, so it is not used on high-pressure joints.
• Flexible graphite — the widest temperature range of the non-metallics, from cryogenic service up to roughly 650 °C in inert atmospheres. It has low mechanical strength, so it is often reinforced with a metal ｉｎｓｅｒｔ or eyelet on higher pressure classes.

1.2 Semi-metallic gaskets

Semi-metallic gaskets combine a metal component for strength with a soft filler for sealing. They cover a much wider pressure range than soft gaskets, from ASME Class 150 up to Class 2500, and are the standard choice for process and refinery piping.

• Spiral-wound gaskets (SWG) — the most widely used semi-metallic type. A V-shaped metal strip, usually 304 or 316L stainless, is wound together with a soft filler such as graphite or PTFE. An outer centring ring positions the gasket in the bolt circle, and an inner ring prevents the windings from buckling into the pipe. The construction absorbs temperature and pressure cycling well, which is why SWGs dominate refinery, petrochemical, and power plant service.
• Kammprofile (camprofile) gaskets — a solid metal core with concentric grooves machined on both faces, covered by a thin soft layer of graphite or PTFE. They offer a good balance of strength and conformability and are common on heat exchangers and vessel manways.
• Metal-jacketed gaskets — a soft filler enclosed in a thin metal jacket. They are easy to make in large diameters and are a cost-effective choice for heat exchanger shell flanges and channel covers, though they are less forgiving of uneven flange finishes.
• Metal-reinforced graphite gaskets — a rigid metal core, often 316L, sandwiched between flexible graphite layers. They are used in high-temperature, high-pressure, and corrosive services on flanges, valves, and pumps.

1.3 Metallic gaskets

Metallic gaskets are made from a single solid metal and are used where very high pressure or temperature rules out softer materials. The dominant type is the ring-joint gasket (RTJ), which seats into a machined groove on the flange face and forms a metal-to-metal, self-energised seal. RTJ gaskets are the standard for ASME Class 600 and above and for high-pressure oil and gas and steam service.

• Style R — oval or octagonal profile, the original ASME B16.20 and API 6A design, used up to Class 2500.
• Style RX — a modified R profile that becomes pressure-energised when the system is pressurised, improving the seal.
• Style BX — a pressure-energised profile for very high pressure wellhead service, where the gasket is fully confined between the flange faces.

Common RTJ materials include soft iron, low-carbon steel, 304/316 stainless, and nickel alloys such as Monel and Inconel, selected to match the flange material and the process media.

1.4 Gaskets by configuration

Beyond material, gaskets are also classified by how much of the flange they cover:

• Full-face gaskets — cover the entire flange face including the bolt holes, spreading the bolt load evenly. Best for flat-face flanges in low-pressure water and utility lines.
• Inner-bolt-circle (IBC) gaskets — sit inside the bolt circle and seal only the raised face. These are the standard choice for raised-face flanges in process piping.
• Ring-type-joint (RTJ) gaskets — precision metallic rings for grooved RTJ flanges, as described above.

2. Stud bolts: the threaded rod that clamps the joint

A stud bolt is a fully threaded rod, usually with no head, that passes through the bolt holes of two mating flanges and is tightened with a nut on each end. The two nuts pull the flanges together and compress the gasket, which is what actually creates the seal. Stud bolts are specified by material grade, thread form, diameter, and length, and the grade is chosen according to temperature, pressure, and the corrosiveness of the media.

2.1 Common stud bolt material grades

Most stud bolts for industrial piping follow the ASTM A193 specification for high-temperature service, with a few other families for special conditions:

• A193 B7 — chromium-molybdenum alloy steel, quenched and tempered. The default grade for high-temperature hydrocarbon service, rated from about -20 °C up to 538 °C, and always paired with an A194 2H nut.
• A193 B7M — the same chemistry as B7 but with restricted hardness for sour (H&sub2;S) service, per NACE MR0175, paired with an A194 2HM nut.
• A193 B16 — chromium-molybdenum-vanadium, with higher creep resistance than B7, rated up to about 595 °C for steam and furnace service, paired with an A194 7 nut.
• A193 B8 and B8M — 304 and 316 stainless respectively, for corrosive and chloride-bearing environments such as marine and offshore service, paired with A194 8 and 8M nuts.
• A320 L7 and L7M — low-temperature grades with Charpy impact testing down to -46 °C, used for LNG, ethylene, and ammonia service, paired with A194 4 or 7 nuts.
• A182 duplex and super-duplex — for highly corrosive and high-strength applications where standard stainless grades are not enough.

2.2 Thread forms and lengths

Stud bolts are threaded to standard series such as UNC (unified coarse), UNF (unified fine), or metric. For ASTM A193 and A320 grades one inch and above, the 8-thread series (8UN) is standard. The required stud length depends on the flange thickness, the gasket thickness, washer thickness if used, and the minimum thread engagement into the nut. Getting the length right matters as much as the grade: too short a stud cannot develop the preload needed to seat the gasket, and too long a stud interferes with neighbouring pipework.

3. Nuts: the other half of the bolted joint

Nuts are threaded onto the ends of the stud bolt to clamp the flanges together. For flanged joints the standard is the heavy-hex nut, which is thicker and wider than a standard hex nut to spread the load over a larger contact area. Other types you may meet include lock nuts for high-vibration service and jam nuts used to lock a position.

Nut material is governed by ASTM A194, and the nut grade must be matched to the stud grade. The nut is normally made slightly harder than the stud so the threads do not gall and the load is distributed evenly. Common pairings are A194 2H with B7 studs, 2HM with B7M, 7 with B16, and 8/8M with B8/B8M. Nuts should also be made of a compatible material to avoid galvanic corrosion between the stud and the nut.

4. How to choose the right combination

The gasket, stud bolt, and nut are selected as a system, not as three independent items. A practical selection path looks like this:

1. Start with the flange. The flange standard, size, pressure class, and face type (flat, raised, or ring-joint) determine which gasket family and configuration you can use.
2. Define the service. Temperature, pressure, and the process media decide the gasket material and the stud grade. Steam and high-temperature service push you toward graphite-filled spiral-wound gaskets and B7 or B16 studs; chloride-bearing seawater points to stainless B8M studs and PTFE or graphite fillers.
3. Match the nut to the stud. Always pair the stud grade with its corresponding A194 nut grade so the joint behaves as designed.
4. Check the standards. Gasket dimensions follow ASME B16.20 (spiral-wound and ring-joint) and B16.21 (non-metallic), while flange and bolting data come from ASME B16.5 and B16.47. API 6A covers wellhead service.

5. Sourcing gasket, stud bolt & nut from a manufacturer

Because the three components must work together, buying them from a single manufacturer with matched sets and traceable documentation saves a great deal of trouble on site. A manufacturer that produces its own gasket, stud bolt & nut packages can supply all three from one source, pre-matched by grade, with mill test certificates and positive material identification on request. This matters most on regulated projects where every fastener must be traceable to its heat number.

EZ Steel Industrial has manufactured industrial piping components since 1994 and supplies gasket, stud bolt, and nut packages alongside its pipe flanges, pipe fittings, valves, and tubing across carbon steel, stainless steel, and copper-nickel materials. Its three manufacturing bases in Cangzhou, Yangzhou, and Lishui support everything from single flanged joints to complete project packages, with quality control that includes hydrostatic testing, ultrasonic inspection, and positive material identification.

6. Frequently asked questions

What is the most common gasket for refinery piping? The spiral-wound gasket, usually with a graphite filler and stainless windings, is the most widely specified type for raised-face flanges in refinery and petrochemical service.

When should I use a ring-joint gasket instead of a spiral-wound gasket? Use an RTJ gasket when the flange is machined with an RTJ groove, typically at Class 600 and above, or where the service demands a metal-to-metal seal with the highest resistance to blowout.

Can I mix studs and nuts from different suppliers? It is not recommended. The stud and nut grades are designed as a matched pair, and mixing sources makes it harder to verify material traceability and correct pairing.

What documentation should come with gasket, stud bolt & nut? For regulated service, ask for mill test certificates (EN 10204 3.1 or higher) that list the heat number, chemical composition, and mechanical properties, plus positive material identification on delivery.

Choosing the right gasket, stud bolt, and nut is not complicated once you understand the families and grades available. Match the trio to your flange, your service conditions, and the relevant standards, and buy them as a matched, traceable package from a manufacturer that can back them up with documentation.