Gasket, Stud Bolt & Nut Buyer's Guide: ASME B16.20 / B16.5 / B16.47 Selection, Bolt-Up Math, and Mill-Direct Sourcing from a Chinese Flange-Connection Hardware Supplier

1. Why the gasket, stud bolt, and nut are one BOM line, not three

A flanged joint is a system. The gasket , stud bolt & nut group lives together on the same stress loop: the bolt load has to seat the gasket to its target gasket stress, hold it through pressure and thermal cycles, and release cleanly at the next TA without seizing. If any one of the three is wrong, the joint drifts. Industry failure-data studies repeatedly show that more than 60% of ASME B16.5 flange leaks originate from gasket selection error or improper bolt-up — not from flange or pipe defects. That is why most refinery procurement specs now call out a single "bolting kit" line item rather than three separate part numbers.

For B2B buyers, the practical consequence is that sourcing the pipe fittings, the matching pipe flanges, and the gasket , stud bolt & nut kit from one mill-direct supplier eliminates the geometry mismatch, the MTC fragmentation, and the freight split that a multi-source BOM creates. EZ Steel Industrial ships all three categories out of one mill-direct program with EN 10204 3.1 mill test certificates bundled per joint.

2. ASME B16.20 gasket families — RTJ, spiral-wound, jacketed, and grooved metal

ASME B16.20-2017 is the controlling U.S. standard for metallic gaskets used on ASME B16.5 (NPS ½–24), ASME B16.47 Series A & B (NPS 26–60), and API 6A wellhead flanges. It defines four gasket families, each designed for a specific pressure/temperature window and flange face.

2.1 Ring-Joint (RTJ) gaskets — R, RX, BX

RTJ gaskets are solid metallic rings with an oval (R) or octagonal (R/oval) cross-section that sit in a machined groove on the flange face. They are self-energizing: as internal pressure rises, the ring is pushed harder against the groove walls. Common materials are soft iron (R-style for Class 150–300), 304/316 stainless (Class 600+), Monel 400, Inconel 600/625, and duplex 2205 for sour service. BX-style rings are a high-pressure API 6A geometry (10,000 psi and above). Maximum hardness is specified per material to avoid brinelling the flange groove.

2.2 Spiral-wound (SW) gaskets

A spiral-wound gasket is a V-shaped metal strip (304SS, 316SS, 321, Monel, Inconel) and a non-metallic filler (graphite, PTFE, mica, or non-asbestos fibre) wound in alternating layers under tension. The result is a gasket that can handle 800 °F+ and thermal cycling without losing stress. An inner ring (typically the same alloy as the winding) is mandatory on ASME B16.5 Class 600 and above, and on all ASME B16.47 flanges, to prevent inward buckling and to limit the filler exposure to the process stream. Outer guide rings are optional but common for centering.

2.3 Jacketed gaskets (JA)

A jacketed gasket has a soft filler (graphite or PTFE) enclosed inside a metal envelope (typically 304SS or 316SS). Jacketed gaskets are mostly used in heat-exchanger channel covers and low-pressure bonnets where a low-stress seal is required; their use on piping flanges has largely been replaced by SW. ASME B16.20 still publishes dimensional tables for ASME B16.5 and B16.47 Series A/B so that retrofit jobs can be quoted.

2.4 Grooved metal gaskets with covering layer (GM)

Introduced in ASME B16.20-2012, GM gaskets pair a serrated metal core (typically 304SS / 316SS) with a soft covering layer (graphite or PTFE). The serrations provide multiple sealing points; the soft layer provides conformability. GM gaskets are used in refinery and high-pressure petrochemical service where a robust reusable seal is needed and where a standard SW would be over-engineered for the cycle count.

3. Matching the gasket to the flange face (RF, FF, RTJ, tongue & groove)

The gasket you specify has to physically match the flange face finish. A mismatch — a soft SW gasket on an RTJ face, or a flexible graphite sheet on a flat face (FF) — will either crush the gasket beyond its working range or fail to seal. Below is the matrix that procurement should keep on hand when approving any pipe fittings or pipe flanges requisition that ships with bolting.

A common mis-spec in the field is shipping an SW gasket to a job that actually has an RTJ flange face, or vice-versa. The simplest safeguard is to require the gasket , stud bolt & nut supplier to match the gasket style code to the flange face type on the MTR. EZ Steel Industrial does this check as a standard pre-shipment QA gate.

4. Stud bolt material: B7, B7M, B8, B8M, B8M2, B8M3, B16

The stud bolt is the load-bearing element. The most common ASTM specifications are summarized below. The choice is driven by service temperature and whether the service is sour (NACE MR0175).

For the matching flanges and bolting, the EZ Steel steel flanges program covers ASME B16.5 / B16.47 forged flanges in A105, A182 F11/F12/F22, A182 F304/F304L/F316/F316L, and A350 LF2 low-temperature grades. The gasket , stud bolt & nut kit is then matched to the flange material on the same MTC family.

5. Heavy-hex nut grade: 2H, 2HM, 8, 8M, 8M2, 8M3

The nut has to be the same alloy family as the stud and rated for the same service. The most common ASTM A194 nut grades used with ASTM A193 stud bolts are listed below. A nut that is rated lower than the stud will yield before the stud reaches design load, and the joint will not seal.

6. Stud-bolt length and thread engagement math

For ASME B16.5 flanges, the stud-bolt length is calculated from:

L = 2 × (Flange thickness RF) + 2 × (Washer thickness) + (Gasket thickness + allowance) + (Nut thickness) + (Thread allowance, ¼ in / 6 mm)

Thread engagement is the controlling check: the nut must engage the stud by at least the diameter of the stud (1D engagement) for carbon steel and at least 1.5D for austenitic stainless, to avoid thread stripping at design load. ASME B16.5 stud-bolt length tables are published in the standard; for non-standard combinations (e.g., Class 1500 tongue-and-groove), the engineering team has to compute the length per joint. A common rule of thumb for a 1-inch B7 stud on a 4-inch Class 150 RF flange is approximately 5½ inches effective length; on a Class 600 RF flange of the same NPS, the length grows to roughly 7 inches. The exact length depends on flange thickness and gasket seating height, so always cite the actual flange callout in the gasket , stud bolt & nut RFQ.

7. Controlled bolt-up per ASME PCC-1 (gasket-stress target)

ASME PCC-1-2013 is the controlling U.S. guideline for the assembly of pressure-boundary bolted flange joints. It replaced the older "torque-only" method with a controlled-bolt-up procedure based on the gasket's target seating stress. For a graphite-filled spiral-wound gasket, the target gasket stress is typically 30 MPa (4,400 psi); for a flexible graphite sheet, 15–25 MPa; for a compressed non-asbestos, 25–35 MPa; for an RTJ ring, the seating is by plastic deformation, so the bolt load is the API 6A / ASME B16.5 prescribed value rather than a free stress target.

The PCC-1 controlled-assembly sequence has four steps:

1. 1Verify gasket, stud, nut, and washer part numbers against the joint sheet.
2. 2Apply the first pass at 30% of target bolt load using a star / four-bolt pattern.
3. 3Apply the second pass at 60% of target load using the same star pattern.
4. 4Apply the final pass at 100% of target load using a full star pattern, then a final circumferential pass to equalize.

Hydraulic torque wrenches calibrated within the last 12 months are the standard tool. Pneumatic impact wrenches are only acceptable for the first pass on non-critical service. For sour service or hydrogen service, the same sequence is applied but with the bolts and nuts pre-tested for hardness per A193 B7M / A194 2HM. The MTC for every gasket , stud bolt & nut shipment should include the recorded assembly torque value at 100% load, and the bolt-up record should be filed in the joint QA package.

8. Five procurement pitfalls (and how to avoid them)

8.1 RTJ gasket on a raised-face flange

A solid metal RTJ ring cannot seal on a smooth RF face — there is no groove for the ring to deform into. The joint will appear to be tight at hydrotest, then leak the first time the line is in service. Fix: pull the MTR for the flange face and match the gasket style to the face. EZ Steel includes a face-style cross-check on every gasket , stud bolt & nut quote.

8.2 Spiral-wound without inner ring on Class 600+

An SW gasket without an inner ring on an ASME B16.5 Class 600 (or higher) flange is a known failure mode — the SW body buckles inward under bolt load, the filler extrudes, and the line leaks at the bore. ASME B16.20 makes the inner ring mandatory at Class 600 and above.

8.3 Undersized stud-bolt length

A stud bolt that does not protrude beyond the nut face by at least one thread plus the thread runout cannot be properly torqued. A common misspec is to copy the bolt length from a Class 150 flange table and apply it to a Class 300 or Class 600 joint of the same NPS. The flange gets thicker as the class goes up, so the bolt length has to grow accordingly. Always re-derive bolt length when the class changes.

8.4 A194 8 nut on A193 B7 stud (or vice versa)

Pairing an austenitic SS nut with a Cr-Mo stud (or vice versa) creates a galvanic and hardness mismatch. The softer component yields, the joint loses preload, and the gasket relaxes. Always match stud and nut alloy family on the MTC.

8.5 No MTC for individual lots

A gasket , stud bolt & nut kit shipped as a generic "carbon steel stud bolt + graphite gasket" without an EN 10204 3.1 MTC is untraceable. If a leak occurs, the root-cause analysis cannot move forward. Require 3.1 MTC per heat number for studs, per lot for gaskets, and per heat number for nuts.

9. Mill-direct vs. stockist: when consolidated sourcing pays off

For one-off emergency orders on standard Class 150 RF joints, a domestic stockist is often faster because the part is on the shelf. For any project with more than ~50 flanged joints, a mill-direct gasket , stud bolt & nut supplier consolidates the BOM and the QA package. The break-even is usually between 50 and 200 joints, depending on class and material mix. Below is a quick comparison.

For projects in the carbon-steel / stainless-steel piping family, EZ Steel also ships the pipe fittings (BW / SW / threaded) and pipe flanges on the same mill program. The QA manager gets one package of 3.1 MTCs per heat number, the freight forwarder handles one consolidated shipment, and the site stores one BOM line for the entire flanged joint.

10. Seven-item RFQ checklist + 5 FAQ

10.1 7-item RFQ checklist

1. Flange standard (ASME B16.5 / B16.47 / API 6A), pressure class, NPS, face type (RF / FF / RTJ / T&G / M&F).
2. Process fluid, operating temperature, operating pressure, cyclic vs. steady service.
3. Material family: carbon steel, low-temp, stainless, alloy, or sour (NACE MR0175).
4. Gasket type: RTJ (R/RX/BX), spiral-wound (with/without inner ring), jacketed, GM, or compressed non-asbestos.
5. Stud bolt: ASTM grade (B7, B7M, B8, B8M, B8M2, B8M3, B16), diameter, length, threads per inch (UNC / UN8 / metric).
6. Nut: ASTM A194 grade (2H / 2HM / 8 / 8M / 8M2 / 8M3), heavy hex, washer needed (yes/no).
7. Documentation: EN 10204 3.1 MTC per heat, lot traceability, PMI report, source country, packaging per joint kit.

10.2 5 frequently asked questions

Q1. Can I use a spiral-wound gasket on a Class 150 RF flange?
Yes. SW gaskets are acceptable from Class 150 up to Class 2500. On Class 150, an inner ring is optional; on Class 600 and above, an inner ring is mandatory. On Class 150 RF flanges, a flexible graphite sheet or compressed non-asbestos gasket is also common, depending on the application.

Q2. Are stud bolts and nuts reusable after a TA?
Sometimes. ASME PCC-1 allows reuse of studs and nuts if the threads are clean, the nuts spin freely by hand, and the stud is within 1–2% of its original length (no measurable necking). In sour service (NACE MR0175), reuse is generally not allowed. EZ Steel tags every gasket , stud bolt & nut shipment with a lot number and recommends that buyers track reuse count per stud.

Q3. Do I need a different gasket for hydrogen service?
Yes. Hydrogen service typically uses a spiral-wound gasket with a 304SS or 316SS inner ring and a flexible graphite filler, with a controlled gasket stress in the 30–40 MPa range. Pure graphite without metal reinforcement is not recommended for hydrogen because of permeation. EZ Steel's petrochemical facilities program includes hydrogen-service SW gaskets and corresponding A193 B7 / B16 stud bolts.

Q4. Is 2H nut always required with B7 stud?
For general refinery / petrochemical service, yes — 2H nut is the standard pairing. For sour service (NACE MR0175), the nut is upgraded to 2HM with the same hardness cap as the B7M stud. For high-temperature steam above 427 °C, B16 studs pair with grade 4 nuts. For low-temperature service below -29 °C, A320 L7 studs pair with A194 grade 4 or L7M nuts.

Q5. How is the gasket stress verified at site?
By controlled bolt-up. A torque wrench is calibrated to the bolt load required to reach the target gasket stress. The sequence is the four-pass star pattern from PCC-1-2013. The bolt-up record is filed in the joint QA package along with the MTC. The gasket , stud bolt & nut shipment from EZ Steel includes a recommended torque table based on the stud diameter, stud grade, and target gasket stress.

11. EZ Steel Industrial supply scope

EZ Steel Industrial supplies the full pressure-boundary hardware stack from one mill-direct program, including carbon & carbon alloy steel and stainless-steel pipe and tube, pipe fittings (BW, SW, threaded), pipe flanges (carbon, stainless, alloy, copper-nickel), the gasket , stud bolt & nut kit (RTJ, spiral-wound, jacketed, grooved metal, A193 B7/B7M/B8/B8M/B16 studs, A194 2H/2HM/8/8M nuts), and industrial valves. Heat-exchanger and boiler-side hardware — including u bend tube and finned tube from the heat efficiency tubes family — is also available on the same mill program, which means a single QA package covers the entire flanged joint plus the connected heat-transfer surface.