Gasket, Stud Bolt & Nut Buyer’s Guide: ASME B16.5, B16.20, A193, A194, EN 1092, EN 1515 Materials and Mill-Direct Sourcing

What the Three Components Actually Do in a Flanged Joint

A pressure-boundary flanged joint is a load-carrying, sealing, and disassemblable mechanical assembly. The pipe flanges are the structural members — usually two of them, weld-neck to the pipe, with a gasket face machined to a defined surface finish (RF, FF, RTJ, male-and-female, tongue-and-groove). The gasket is the sealing element, a ring of deformable material (graphite, PTFE, compressed fibre, spiral-wound metal-and-filler, kammprofile, or a solid metal ring) that fills the micro-gap between the two flange faces under bolt load. The stud bolt and nut assembly is the fastener that generates and maintains the clamping load on the gasket.

The relationship between those three components is governed by the ASME Pressure Vessel and Piping Code, by ASME B16.5 (Pipe Flanges and Flanged Fittings NPS ½ through 24), by ASME B16.47 (Large Diameter Steel Flanges NPS 26 through 60), and by ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly). The relevant bolting standard is ASME B16.5 Appendix E, supplemented by ASME B16.20 (Metallic Gaskets for Pipe Flanges) and B16.21 (Non-Metallic Flat Gaskets). The corresponding European body of work is EN 1092-1 (Steel Flanges), EN 1515-1/-2 (Bolting for Flanged Joints), and EN 1591-1 (Calculation of Bolted Flanged Joints). The Japanese counterpart is JIS B2220 (Steel Pipe Flanges) and the Russian system is GOST 33259 (Flanges for Valves, Fittings and Pipes).

The practical reality, however, is that the standard the flange is rated to, the standard the gasket is cut to, and the standard the bolt is rolled to are not always the same document. A pipe spools to ASME B16.5 Class 300 can be bolted to an EN 1092-1 PN 40 companion flange with a spiral-wound gasket cut to ASME B16.20 — provided the gasket OD, the bolt-circle, the number and size of bolt holes, and the gasket seating dimensions all line up. The procurement file that omits any one of those four dimensions is the procurement file that arrives on site with parts that do not fit.

Matching the Gasket to the Pressure Class, the Medium, and the Temperature

The first decision on the gasket line item is the type. Six families cover essentially every pressure-boundary service in hydrocarbons, chemicals, power, and marine.

Compressed Non-Asbestos Fibre and Graphite — The Class 150/300 Workhorse

For low-pressure hydrocarbon, water, steam, and general chemical service at temperatures up to about 450 °C, a compressed fibre sheet (CFS, non-asbestos aramid-with-NBR binder) or a flexible graphite sheet (with or without metal foil ｉｎｓｅｒｔ) is the default. The CFS sheet is rated to ASME B16.21, comes in thickness 1.5 mm and 3.0 mm, and is die-cut or hand-cut to the RF (raised face) or FF (flat face) dimension of the flange. Graphite sheet is preferred where the service is steam at 200 °C to 450 °C, or where the medium is mildly aggressive. The bolt load required to seat a 1.5 mm graphite sheet is roughly 25–35 MPa of gasket stress — well within the capacity of a stud-bolt assembly on a Class 150 or Class 300 flange. Both materials are inexpensive, the cutting tolerance is forgiving, and the procurement file is short.

PTFE and Modified PTFE — The Corrosion Specialist

For strong acid, strong alkali, or pharmaceutical service, a PTFE (virgin or filled with glass, carbon, or graphite) gasket to ASME B16.21 is the right answer. PTFE is chemically inert to almost everything except molten alkali metals and fluorine at high temperature. The downside is cold flow: under sustained bolt load, PTFE creeps and the bolt tension drops, which is why PTFE is rarely used above Class 300 and why retorquing is scheduled into the maintenance plan. Modified PTFE (e.g., filled with 25 % glass fibre) has roughly twice the creep resistance of virgin PTFE and is the preferred material for sulfuric, hydrochloric, and mixed-acid service. The procurement cost is roughly 5–10× CFS and the operating temperature is capped around 230 °C for modified PTFE, 260 °C for virgin.

Spiral-Wound Gaskets — The Class 300 to Class 900 Default

A spiral-wound gasket (SWG) is a pre-formed metal strip (usually 304, 316L, 321, or 316L Ti-stabilised stainless) wound in a spiral with a non-metallic filler (graphite, PTFE, or mica). The metal gives the mechanical strength and the spring-back; the filler gives the seating. The combination handles temperatures from cryogenic (-196 °C) up to about 870 °C (with mica filler and Inconel strip), pressures up to Class 900 (and beyond with a passivation ring), and cyclic thermal loads that a soft cut gasket cannot survive. The standard is ASME B16.20 for ASME-class flanges and EN 1514-2 for EN 1092-1 PN flanges. SWG is the workhorse of refinery and petrochemical flanged joints.

Ring-Joint (RTJ) — The Class 600 to Class 2500 Default

Above Class 600, the bolt load required to seat a soft or spiral-wound gasket becomes impractical. A ring-joint gasket (RTJ) — a solid metal ring, oval or octagonal in cross-section, machined to ASME B16.20 — is pressed into the ring groove machined into the flange face. The bolt load is high, but the gasket seating is metal-to-metal: 100 % of the seating stress goes into deforming the ring into the groove, and the seal survives at temperatures and pressures where any soft gasket would fail. Materials are soft iron (low-pressure), 304 SS, 316 SS, 321 SS, 347 SS, Monel 400, Inconel 600/625/825, and titanium. The trade-off is that the joint is non-self-centring and the groove is machined into the flange face, so RTJ flanges are not interchangeable with RF flanges. RTJ is the default at ASME B16.5 Class 600 and above, and at API 6A wellhead and Christmas tree equipment.

Kammprofile and Metal-Jacketed — The Specialty Service

A kammprofile gasket is a metal core (typically 304 SS or 316L) with concentric grooves on both faces, faced with a thin layer of graphite or PTFE. It combines the resilience of a soft gasket with the strength of a metal core and is used in heat-exchanger channel covers, vessel head joints, and valve bonnets. A metal-jacketed gasket is a soft filler (asbestos substitute, graphite, or ceramic) wrapped in a metal envelope (typically 304 SS or 316L); it is used in heat-exchanger header joints and in older refinery service where a non-asbestos substitute for the legacy asbestos-jacketed gasket is required. Both are specialty items supplied against an engineering drawing rather than a stock catalogue.

Stud Bolts and Nuts: Materials, Grades, and Temperature Limits

The stud bolt is the part of the assembly that actually generates the clamping force. The nut holds it. Both are made to a material standard that defines the chemistry, the heat treatment, the mechanical properties, and the upper temperature limit. The procurement file that does not name the material standard is a file that will be filled with the cheapest grade the mill has on the shelf, and that grade is rarely the right one for the service.

ASTM A193 — The Stud-Bolt Material Standard

ASME B16.5 Appendix E and ASME PCC-1 both reference ASTM A193 for stud-bolt material. The most frequently specified grades are:

• A193 B7 — quenched and tempered alloy steel, 4140/4142/4145 chemistry, minimum tensile 125 ksi (860 MPa) and yield 105 ksi (725 MPa). The default for ASME B16.5 Class 150 through Class 600 service at temperatures up to about 425 °C. Roughly 70 % of the stud-bolt orders placed in the world are B7.
• A193 B16 — chromium-molybdenum-vanadium alloy steel, similar to B7 but with a higher alloy content and a higher upper temperature limit. Specified for Class 600 and above steam service at temperatures between 425 °C and about 575 °C (boilers, superheaters, reheat lines).
• A193 B8 / B8M / B8T / B8C — austenitic stainless grades: B8 is type 304, B8M is type 316, B8T is type 321, B8C is type 347. The carbon-content Class (B8 Class 1, Class 1A, Class 1B, Class 1D, Class 2) determines whether the stud has been carbide-solution-treated for high-temperature service or carbide-solution-treated and strain-hardened for ambient service. Specified for stainless flange bolting, food/pharma, and cryogenic-to-ambient service.
• A193 B8M2 / B8M3 / B8T2 / B8C2 — the Class 2 versions of the austenitic grades, with higher tensile strength (125 ksi) achieved by strain-hardening rather than by changing the chemistry. Used where stainless bolting is required for corrosion reasons but B7-class strength is also required.
• A193 B5 — 5 % chromium alloy (e.g., 501), used in hydrogen service at moderate temperatures where B7 would be subject to hydrogen embrittlement. Specified for catalytic reformer and hydrodesulphurisation reactor bolting.
• A193 B6 — 12 % chromium alloy (e.g., 410), used in steam turbine bolting at temperatures up to about 525 °C.

ASTM A194 — The Nut Material Standard

The nut is matched to the stud bolt by grade, hardness, and operating temperature. ASTM A194 defines the most common nut grades:

• A194 Grade 2H — heavy hex nut, quenched and tempered carbon steel, used with B7 and B16 stud bolts. The default across ASME B16.5 Class 150 through Class 600 service.
• A194 Grade 4 — quenched and tempered carbon-molybdenum, used with B7 and B16 at moderate temperatures.
• A194 Grade 7 — quenched and tempered alloy steel (similar to B7), used with B7 and B16 stud bolts in high-temperature service.
• A194 Grade 8 / 8M / 8T / 8C — austenitic stainless hex nuts (304, 316, 321, 347), matched to the corresponding A193 B8/B8M/B8T/B8C stud bolts.
• A194 Grade 8F — 304 stainless with added graphite-free lubrication, used where galling (thread seizure) is a risk.
• A194 Grade 3 / 6 / 9 — specialty grades for low-temperature, silicon bronze, and high-temperature service respectively.

Low-Temperature and Sour-Service Bolting

Below about -29 °C, ASME B31.3 requires impact-tested bolting. ASTM A320 L7 (quenched and tempered carbon steel, Charpy 18 J minimum at -46 °C) is the default for LNG, ethylene, and ammonia service. A320 L43 is the alloy equivalent. For NACE MR0175 / ISO 15156 sour H₂S service — refinery hydroprocessing, amine contactors, sour water strippers — the stud bolt must meet a maximum hardness of 22 HRC (237 HB). A193 B7M is the standard B7 variant for sour service; the "M" denotes the maximum-hardness restriction. The corresponding nut is A194 Grade 2HM or 7M. NACE compliance is rarely checked at goods-in inspection and is one of the most frequent audit findings in refinery QA.

Cross-Standard Bolting Equivalents

A purchase order written against a European or Asian standard is not a barrier to a mill-direct Chinese supply. The cross-walks are well-defined:

• A193 B7 ↔ EN 10269 42CrMo4 / 1.7225 / 1.7228 ↔ JIS G4105 SCM440 ↔ GOST 4543 40X / 30XMA
• A193 B16 ↔ EN 10269 40CrMoV4-6 / 1.7711 ↔ JIS G4110 SNB16 ↔ GOST 4543 30XMA / 25X1M1Φ
• A193 B8 (304) ↔ EN 10269 X5CrNi18-10 / 1.4301 ↔ JIS G4303 SUS304
• A193 B8M (316) ↔ EN 10269 X5CrNiMo17-12-2 / 1.4401 ↔ JIS G4303 SUS316
• A193 L7 ↔ EN 10269 42CrMo4 / 1.7225 (low-temp variant) ↔ JIS G4105 SCM440 (LT)
• A194 2H ↔ EN 10269 42CrMo4 ↔ JIS G4105 SCM440 (nut temper)

International Standards for Flanges, Gaskets, and Bolting

The flange rating, the gasket seating dimensions, the bolt-circle diameter, the number of bolt holes, and the stud-bolt length are not free-form numbers — they are defined by a pressure-class standard and a flange-face standard. Cross-border EPC projects routinely mix flanges from one standard with gaskets from another, and the procurement file is the only place where those mismatches are caught before they become site rework.

ASME B16.5 / B16.47 / B16.20 / B16.21

ASME B16.5 covers NPS ½ to NPS 24 flanges at Class 150, 300, 400, 600, 900, 1500, and 2500. ASME B16.47 covers NPS 26 to NPS 60 in Series A (ASME B16.47-2017) and Series B (MSS SP-44 legacy). Each pressure class has a defined pressure-temperature rating table (the "B16.5 Table 2-1.1" family) and a defined bolt circle, number of bolts, and bolt size. The flange face is RF (raised face), FF (flat face), RTJ (ring-type joint), or — for specialty service — male-and-female (M&F) and tongue-and-groove (T&G).

Gasket standards: ASME B16.20 for metallic gaskets (spiral-wound, ring-joint, corrugated metal, kammprofile) and ASME B16.21 for non-metallic flat gaskets (compressed fibre, PTFE, graphite). The spiral-wound standard covers the OD, the ID, the winding strip width, the filler type, the centering ring (carbon steel or stainless, with or without passivation), and the bolt-hole numbers. The ring-joint standard covers the ring cross-section (oval R, octagonal R), the ring material, the hardness, and the dimensional fit in the groove.

EN 1092-1, EN 1514, EN 1515

EN 1092-1 is the European steel-flange standard. Pressure classes are PN 6, 10, 16, 25, 40, 63, 100, 160, 250, 320, 400. Face types are Type A (flat face), Type B (raised face), Type C (tongue), Type D (groove), Type E (spigot), Type F (recess), Type G (O-ring spigot), Type H (O-ring groove). EN 1514 is the European gasket standard (parts 1 to 8 cover non-metallic, spiral-wound, kammprofile, metal-jacketed, ring-joint, etc.). EN 1515-1 covers bolt selection and EN 1515-2 covers the classification of bolt materials for PN-designated steel flanges.

JIS B2220, GOST 33259

JIS B2220 is the Japanese standard for steel pipe flanges, with 5K, 10K, 16K, 20K, 30K, 40K, 63K pressure classes and FF / RF / TG / RJ face types. GOST 33259 is the Russian/CIS standard with PN designations similar to EN 1092-1, plus 0.1 / 0.25 / 0.6 / 1.0 / 1.6 / 2.5 / 4.0 / 6.3 / 10 / 16 / 20 MPa pressure classes. Flanges to JIS and GOST are not dimensionally interchangeable with ASME or EN flanges — the bolt circle, the bolt-hole diameter, and the flange OD all differ — and the procurement file that omits the standard prefix will be filled with the cheapest one that "looks right" at goods-in.

ASME PCC-1 and the Bolt-Up Procedure

ASME PCC-1 is the standard that governs how a flanged joint is actually pulled together on site. It defines four tightening methods:

• Manual (legacy) — the wrench and 1/3-rule method. Largely superseded but still used in non-critical service.
• Torque control — a calibrated torque wrench applies a target torque derived from the bolt size, the bolt material, and the desired preload. The most common method on small- and medium-bore flanges.
• Tension control — a hydraulic bolt-tensioner stretches the stud directly and the nut is run down while the stud is under load. Used on large-bore and high-pressure flanges where torque-control scatter is unacceptable.
• Stretch control — direct measurement of stud elongation using a micrometer or ultrasonic extensometer. The most accurate method and the only method that gives a verifiable preload, but it requires access to the stud and is rarely used in production service.

PCC-1 also specifies a multi-pass, star (four-corner) pattern sequence. The classic 4-pass sequence is 30 %, 60 %, 100 %, and 100 % (recheck) of the target torque, applied in a star pattern that moves the load around the flange gradually rather than locally. A single-pass tightening will leave the gasket unevenly seated and is the most common cause of small-bore flange leaks on commissioning day. The pattern is as important as the torque value — a 100 % torque applied in one pass will not give the same gasket seating as a 4-pass 30-60-100-100 sequence applied in the correct order.

Sizing the Bolt: Length, Thread, and the ASME B16.5 Calculation

The stud-bolt length is not the flange thickness plus the nut thickness plus a rule of thumb. It is defined by ASME B16.5 (and EN 1515-1) as the length that gives the correct stud thread engagement on the nut side, the correct gasket clearance on the flange side, and the correct preload at the design torque. The calculation is straightforward but unforgiving — an under-length stud will not engage enough thread on the nut and the joint will fail in service; an over-length stud will bottom out in the nut before the gasket is fully seated.

For an ASME B16.5 RF Class 150 NPS 6 flange (DN 150) with a 1.5 mm graphite sheet, the stud-bolt length is approximately 4 inches (100 mm) with a ¾"-10 UNC thread (M20 in EN 1515-1). The corresponding Class 600 NPS 6 stud is roughly 5 inches (125 mm) with a 1"-8 UNC thread. The thread engagement on the nut side must be at least equal to the bolt diameter (1×d for steel-on-steel) and ideally 1.5×d for cyclic-loaded service. B16.5 Appendix E gives the canonical stud-bolt length and stud-bolt quantity for every NPS × Class combination, and an RFQ that does not cite the standard will produce a stud-bolt order that the mill is free to interpret.

Thread is UNC (Unified National Coarse) per ASME B1.1 for imperial studs and metric coarse (ISO 261) per EN 13 for EN studs. UNC-2A / UNC-2B fit is the default for ASME B16.5 service. The nut is heavy hex per ASME B18.2.2 or hex per ASME B18.2.4, and the washer (if used) is flat per ASME B18.21.1. The stud thread is rolled, not cut — rolled threads have higher fatigue strength and the procurement file should call this out.

Mill Test Certificates and the EN 10204 3.1 vs 3.2 Question

A stud-bolt order without an MTC is a stud-bolt order of unknown origin. The international standard for the MTC is EN 10204. Type 3.1 is a mill-issued certificate validated by the mill's quality department, independent of production. Type 3.2 is a 3.1 certificate additionally witnessed by a third-party inspector — usually a classification society (LR, DNV, BV, RINA, ABS, CCS), a notified body, or the buyer's inspector. Type 2.1 is a statement of compliance with no test data and is not acceptable for pressure-boundary service; Type 2.2 is a test report with no independent validation. Anything weaker than 3.1 should not pass a refinery or petrochemical QA gate.

The 3.1 MTC for a stud bolt should include: heat number, material standard (e.g., "A193 B7"), heat-treatment condition (e.g., "quenched and tempered at 595 °C minimum"), chemical composition (C, Mn, P, S, Si, Cr, Mo, V), mechanical properties (tensile strength, yield strength, elongation, reduction of area, hardness), impact test results (Charpy V-notch at the specified temperature for low-temp grades), and NDT results (magnetic-particle per ASTM A388 or ultrasonic per ASTM A388 for critical-service studs). The MTC for a gasket should include the material standard, the lot number, the batch dimensions, the test results for compressibility and recovery (per ASTM F36, F38, F146, F127), and the NDT results (visual + dimensional + leak per ASTM F112 for spiral-wound).

Sour-service NACE MR0175 / ISO 15156 compliance is documented on the MTC by hardness (≤ 22 HRC for most carbon-steel bolting grades in sour service) and by a statement of compliance with the standard. Third-party witness (Type 3.2) is mandatory for sour-service H₂S bolting on most refinery NDE regimes and is a routine requirement for offshore and subsea bolting.

Service-Specific Material Selection: What Actually Goes Wrong in Service

The most expensive flange-joint failures in service are not exotic. They are the predictable consequence of a material or standard that was acceptable on paper and not acceptable in the chemistry of the medium, the temperature of the service, or the load cycle of the process.

Steam Service Above 425 °C

A193 B7 stud bolts are rated to 425 °C. Above that, B7 starts to lose yield strength and the joint creeps. A193 B16 (Cr-Mo-V) is required for the 425 °C to 575 °C range, and A453 Grade 660 (austenitic, age-hardened) for 575 °C to about 700 °C. A refinery 100-day Turnaround will typically include retorquing of all Class 600 steam joints on the main steam line; a new-build that does not specify B16 or 660 for those joints will fail at the first extended run. The same applies to the gasket — graphite sheet does not survive above 450 °C, and the steam service requires a spiral-wound with 316L strip and graphite filler, or a corrugated metal gasket with graphite facing.

Sour H₂S Service (NACE MR0175 / ISO 15156)

In refinery hydroprocessing, amine contactors, and sour water strippers, the H₂S partial pressure is above the NACE threshold (0.0003 MPa partial pressure in aqueous phase). The stud-bolt material must be NACE-compliant: A193 B7M with a maximum hardness of 22 HRC, A193 B8M Class 2, or L7M. The nut is A194 2HM. A standard B7 stud with a hardness of 28-32 HRC will suffer sulphide stress cracking (SSC) or hydrogen-induced cracking (HIC) within weeks. The failure mode is a clean brittle fracture at the first engaged thread, with no plastic deformation. The cost of prevention — adding the "M" suffix and the NACE test report — is roughly 5-10 % of the stud-bolt price.

Hydrogen Service at High Pressure

In hydrocracker and hydrotreater service, hydrogen at 10-20 MPa partial pressure will diffuse into the stud bolt and cause hydrogen embrittlement, internal cracking, and delayed brittle fracture. A193 B5 (5 % Cr) or B6 (12 % Cr) is specified for hydrocracker stud bolts. A193 B7 is acceptable only for moderate-hydrogen service with documented operating-temperature control. The gasket is a 316L-graphite spiral-wound with an Inconel 825 or Monel 400 inner ring; the inner ring prevents the graphite from extruding into the bore under the high differential pressure.

Marine and Seawater Service

Seawater ballast, firewater, and cooling-water flanges are typically Class 150 or Class 300 RF with a 316L-graphite spiral-wound gasket. The stud bolt is B8M (316) Class 1 or 2. The nut is 8M. For shipboard service, classification society certification (LR, DNV, BV, RINA, ABS, CCS) is mandatory. For offshore service, NACE MR0175 may also apply if the service is sour.

Cryogenic Service Below -29 °C

LNG, ethylene, and ammonia storage and processing require impact-tested bolting. A320 L7 at -46 °C and L43 (alloy) at -101 °C are the standard grades. The gasket is a spiral-wound with PTFE filler (down to about -196 °C) or a flexible graphite with a 316L strip (down to about -40 °C). LNG-class vessels often specify LNG-specific spiral-wound gaskets with a centering ring sized for the cryogenic flange-face finish.

Procurement Workflow: From the Flange BOM to the Crate on the Dock

A clean gasket, stud bolt and nut order runs through a well-defined sequence. Sloppy execution at any step costs weeks and frequently turns into a Turnaround-day re-order.

1. BOM freeze by flange rating and service. Every line item is a (NPS, Class, service, gasket type, stud-bolt material, nut material, MTC type) tuple. Anything left undefined becomes a clarification round that delays the order by one to two weeks.
2. Material split by service severity. Critical-service stud bolts (B16, B7M, L7, 660) come from a mill with documented heat-treatment records and a Type 3.1 MTC minimum. Non-critical stud bolts (B7, B8) can come from a mill-direct inventory with a Type 3.1 MTC.
3. RFQ and quotation. A serious mill responds within 3-5 working days with a per-piece price for stud bolts (priced by diameter and length, not by weight), a per-piece price for gaskets, a per-piece price for nuts, the lead time, and a draft MTC layout. A response that arrives without an MTC layout is incomplete.
4. Mill audit (for new suppliers). For first-time orders above roughly 1,000 stud-bolt pieces, a mill audit covering material traceability, the heat-treatment furnace records, the threading equipment, and the calibration log is well worth the week it takes. The audit catches the kind of issue no MTC will reveal, such as re-labelled heat numbers or a tempering furnace that has not been calibrated in 18 months.
5. PO release with retention. Standard practice is 10 % retention against the MTC pack, the dimensional report, and any third-party inspection report. Without retention, the mill has no incentive to deliver a complete document pack.
6. Pre-shipment inspection. A 100 % visual and dimensional check on the stud-bolt thread, plus a sample-based hardness test (10 pieces per lot) and a sample-based MTC verification, is standard for shipments above 5,000 stud-bolt pieces. For shipments below that, an ISO 2859-1 Level II sampling plan is sufficient.

Mill-Direct vs Stockist: Why the 15-30 % Margin Exists

A gasket stud bolt nut order sourced mill-direct is materially different from the same order sourced through a stockist. The stockist does no rolling, no heat treatment, no threading, no spiral winding — it adds a margin on top of the mill's price and forwards the mill's MTC. The mill-direct path is one fewer set of hands on the part, one fewer margin layer, and one fewer opportunity for the documentation to be altered en route.

For a typical 5,000-piece stud-bolt order, the mill-direct vs. stockist price gap lands in the 15-30 % range on a delivered basis, with the larger savings on bigger lots. The savings are not free: the mill-direct buyer is the importer of record, the buyer of the MTC, and the buyer of the freight booking. For procurement teams that have already built that capability — most large OEMs, EPCs, and refinery Turnaround contractors have — the mill-direct path is unambiguously better. For first-time buyers, a hybrid path (mill-direct with the stockist retained for freight and customs clearance) often gives the best of both worlds.

A mill-direct catalog worth a look is the EZ Steel Industrial line, which covers steel flanges to ASME B16.5, B16.47, EN 1092-1, JIS B2220, and GOST 33259, spiral-wound and ring-joint gaskets to ASME B16.20 and EN 1514, A193 B7/B16/B8/B8M/L7 stud bolts, and A194 2H/7/8/8M nuts, all with EN 10204 3.1 MTCs and the option of 3.2 third-party witness. The same mill also supplies the companion pipe fittings and industrial valves lines, so a bundled order for flanges, gaskets, bolting, fittings, and valves can move on a single MTC discipline and a single freight booking — a real saving on EPC and Turnaround orders where the bolt-and-gasket portion is rarely a line item on its own.

Five Common Procurement Pitfalls and How to Avoid Them

The most expensive mistakes on a flanged-joint bill of materials are not exotic. They are routine, predictable, and almost all of them trace back to a procurement decision that can be made correctly the first time.

• B7 stud bolt specified above 425 °C. A193 B7 starts to lose yield at 425 °C. Steam at 480 °C will creep the joint within 6-12 months. Specify A193 B16 for 425-575 °C, A453 660 for 575-700 °C. The price premium is 20-40 % and the avoided shutdown is many multiples of that.
• NACE MR0175 omitted on a sour-service order. Standard B7 (28-32 HRC) will fail by SSC in weeks in refinery H₂S service. Specify B7M, B8M, or L7M. The audit finding is the most common in refinery QA, and the failure mode is a clean brittle fracture at the first thread.
• Spacer ring omitted on a ring-joint flange. A ring-joint flange requires a spacer ring to keep the gasket centred during transport and installation. Without the spacer ring, the RTJ ring can be displaced and the joint will not seat. The cost of the spacer ring is trivial; the cost of a misaligned RTJ is a Turnaround-day re-order.
• Gasket standard mis-specified. A spiral-wound cut to ASME B16.20 is not interchangeable with one cut to EN 1514-2 even if the OD and ID happen to match. The standard-prefix must be on the PO and on the MTC. Mixing standards produces leaks that are impossible to diagnose from the outside.
• Plain (non-galvanised, non-passivated) carbon-steel centering ring on a stainless spiral-wound. The centering ring is the visible part of the gasket assembly and the most common site of rust and thread seizure. A 304 or 316L centering ring with a passivation coating is the default. Specifying "carbon steel" because the price is lower is a common short-term saving that becomes a long-term maintenance problem.

Quick-Reference Sourcing Checklist

A short list to print and keep next to the flanged-joint BOM:

• Flange standard and pressure class written out in full (e.g., "ASME B16.5 Class 150 RF, NPS 6, A105 forged").
• Gasket standard and type written out in full (e.g., "ASME B16.20 spiral-wound, 316L strip, graphite filler, 304 centering ring with passivation, NPS 6 Class 150").
• Stud-bolt material standard, length, thread, and quantity (e.g., "A193 B7, ¾"-10 UNC-2A × 4 in length, 8 pieces per joint").
• Nut material standard (e.g., "A194 2H heavy hex, ¾"-10 UNC-2B").
• MTC type — EN 10204 3.1 minimum, 3.2 for nuclear, offshore, and most sour-service and hydrogen-service joints.
• Service-specific compliance (NACE MR0175, ISO 15156, PED 2014/68/EU, classification society) on the PO and on the MTC.
• Packaging: stud bolts waxed or oiled, threaded ends protected, gaskets flat-packed with separator sheets, all in a seaworthy crate or carton.
• Lead time, in calendar weeks from PO acceptance, not from PO issuance.
• Retention clause: 10 % against complete MTC and dimensional document pack.
• Pre-shipment inspection right defined and dated in the PO.