Pipe Flange Buyer's Guide: ASME B16.5 / B16.47 / EN 1092-1 Types, Materials, Pressure-Temperature Ratings, and Mill-Direct Sourcing

What a Pipe Flange Actually Does — and Why the Joint Is the System

A pipe flange is the bolted, gasketed ring that joins two pipe ends, a pipe end to a fitting, or a pipe end to a valve or vessel nozzle. Mechanically, it transmits axial and bending loads between the connected items. Sealing-wise, it holds the line pressure in and the atmosphere out through a controlled compression of a soft or semi-metallic gasket. In almost every refinery, power plant, chemical complex, LNG terminal, ship, or water treatment facility you walk through, the flanged joint — not the pipe itself — is the most common site of either a routine maintenance event or an unplanned shutdown. Pick the wrong flange type, the wrong face, the wrong material, or the wrong bolt-up math, and the joint is a leak path from day one.

Because flanges are sold as part of a system, three sibling components travel with almost every flange order: the pipe fittings that transition size or direction upstream and downstream, the steel flanges or copper nickel flanges that match the service, and the gasket, stud bolt & nut set that actually makes the seal. The fastest way to eliminate joint-leak risk is to buy the whole stack from a single mill-direct supplier who can guarantee dimensional matching, MTC traceability and bolt-up math consistency across the four component families.

Flange Anatomy: Hub, Bore, Bolt Circle, Facing, and the Six Standard Body Types

Every pipe flange has the same five functional regions: the hub (the transition from the pipe or pipe-end to the flange ring), the bore (the through-hole for the fluid), the bolt circle (the pattern of bolt holes sized to clear the studs), the gasket face (the machined surface the gasket sits on), and the back face or backside (which may be smooth, serrated, or grooved for tongue-and-groove or RTJ). The way those regions are proportioned and welded to the pipe end defines which of the six standard body types the flange belongs to.

1. Welding Neck (WN) — for High-Pressure and High-Temperature Service

The welding neck flange has a long tapered hub that butt-welds to the pipe. The hub transition is the structural reason a WN is the right choice in Class 600 and above, and in any cyclic-temperature or fatigue-loaded service. The bore matches the pipe bore, so flow turbulence and crevice corrosion are minimized. WN flanges dominate the high-pressure side of refinery hydrocrackers, power-plant steam lines and any service where the joint sees bending or thermal-expansion stress. Standards: ASME B16.5 (NPS 1/2 to NPS 24), ASME B16.47 Series A and Series B (NPS 26 to NPS 60), EN 1092-1 Type 11.

2. Slip-On (SO) — for Low-Pressure, General-Service Piping

A slip-on flange slides over the pipe end and is fillet-welded both inside (socket) and outside. Because the weld is only fillet and the pipe does not continue through the bore, SO flanges are limited to Class 150 and Class 300 in most codes, and to non-toxic, lower-pressure service. The advantage is faster fit-up, lower installation skill requirement, and a lower unit cost. SO flanges are everywhere in utility water, plant air, firewater, and instrument-air lines. Standards: ASME B16.5 Type 1, EN 1092-1 Type 01 (Type A face) and Type 02 (Type B face).

3. Socket Weld (SW) — for Small-Bore, High-Pressure Stainless and Alloy

A socket weld flange has a recess the pipe end drops into before being fillet-welded. The design is essentially a forged boss with a socket. SW is the standard for NPS 1/2 to NPS 3 stainless and alloy systems at Class 600 and above, particularly in chemical and instrumentation lines where crevice corrosion at the pipe-to-flange interface must be minimized. The socket creates a small gap between pipe OD and socket ID; that gap vents, which prevents pressure-induced crevice cracking. Standards: ASME B16.5, MSS SP-79, EN 1092-1 Type 13.

4. Threaded (THD) — for Non-Weld, Disassembly-Friendly Service

Threaded flanges have an NPT or BSP thread that matches the pipe thread. They are used where welding is not permitted (e.g., near existing instrumentation, in explosion-risk areas with hot-work restrictions) or where frequent disassembly is required. THD flanges are limited to Class 150 and Class 300 in most codes and to lower temperatures; above ~ 260 °C the thread galling risk becomes unacceptable. They are not permitted in sour service (NACE MR0175) without a specific compatibility review.

5. Lap Joint (LJ) — for Stainless and Alloy Pipe with a Backing Ring

A lap joint flange is a two-piece assembly: a loose backing ring (usually carbon steel) and a stub end (the matching alloy or stainless). The stub end butt-welds to the pipe; the backing ring slips over the stub and rotates freely. The advantage is that the wetted side is the high-cost alloy, while the heavily-bolted, structurally-loaded ring is the cheap carbon steel. LJ flanges are the standard for stainless and nickel-alloy systems in low-pressure service where frequent bolt rotation or field alignment is needed. Standards: ASME B16.5, EN 1092-1 Type 02 / Type 04.

6. Blind (BL) — for Termination, Inspection Access, and Future Tie-In

A blind flange has no bore; it is a solid disk that bolts onto a flanged pipe end to terminate the line. BL flanges are the standard way to dead-end a line for hydrotest, for future extension, or for inspection access. They carry the full pressure load on the bolts, so the bolt pattern and the gasket seating must be designed to ASME B16.5 Class for the full design pressure. Standards: ASME B16.5, EN 1092-1 Type 05.

The Six Standard Facing Types and Where Each One Belongs

The flange facing is the machined surface the gasket sits on. The wrong face for the service is one of the most common procurement errors — and the most expensive, because the mistake is not visible until the joint is under pressure.

Three rules of thumb from real tenders. First, RF is the default for 80 % of process service; do not specify FF unless the mating flange is cast iron or the system is low-pressure water. Second, RTJ is mandatory for Class 600 hydrocarbon and hydrogen service above ~ 540 °C, and for any ASME B16.5 Class 900+ flange. Third, the surface finish of the face is part of the spec, not an option; a 250 µin finish on an RTJ groove will leak.

The Standards Map: ASME B16.5, B16.47, EN 1092-1, GOST, and GB/T 17241

A pipe flange is not interchangeable across all standards — the bolt circle, the facing diameter, the raised-face height and the pressure-temperature envelope are not the same. Below is the working map that shows which standard applies in which size and class range, and which customer-side equivalents line up across the four major systems.

On a real EPC tender the B16.5 dimension and the EN 1092-1 dimension rarely line up at the same nominal size. A DN 100 / NPS 4 flange has a different bolt circle, a different PCD, and a different raised-face diameter in each system. The fix is to specify the standard by name, not by class, and to require the mill to provide a dimensioned drawing before the order is released. A flange that is "Class 150 equivalent" with no further standard reference is almost always a problem waiting to happen.

Pressure-Temperature Ratings: How to Read the ASME B16.5 Table

The pressure-temperature rating is the single most important flange property to get right, and the one most often misquoted. The ASME B16.5 §6.3.2 table maps the maximum allowable non-shock pressure (in PSIG or bar) for each combination of Class (150, 300, 600, 900, 1500, 2500) and material group at a given service temperature. The same Class 600 flange made of ASTM A105 carbon steel has a different allowable pressure at 400 °C than the same Class 600 flange made of ASTM A182 F316L stainless.

The table tells the procurement story. At room temperature a Class 150 A105 flange is rated to 285 PSIG, but at 540 °C the same flange is rated to only 110 PSIG — the rating drops 61 % over 500 °C of temperature rise because of creep-allowance derating. A F316L Class 150 flange, by contrast, drops only 38 % over the same range, which is why stainless is the default for hot hydrocarbon and steam service above 425 °C. Three common mistakes: (1) using the 38 °C rating as the design pressure without derating for temperature; (2) selecting a flange class by guessing the pressure rather than reading the table at the actual operating temperature; (3) specifying a higher Class than needed because of the 38 °C headroom, which inflates cost and bolt-load demand without adding safety margin.

Materials: From Carbon Steel to Duplex, and the Cu-Ni Marine Set

A flange is forged, plate-rolled, or cast from a material that has to match the pipe material, the service fluid, the design temperature, and the corrosion allowance. The four families below cover about 95 % of industrial orders.

Carbon and Carbon-Alloy Steel (ASTM A105, A350 LF2, A694)

A105 is the workhorse carbon-steel forging for Class 150 to Class 1500 service from -29 °C up to ~ 425 °C. A350 LF2 is the low-temperature variant with Charpy impact tested at -46 °C, mandatory for low-temperature carbon-steel piping per ASME B31.3. A694 covers higher-strength carbon and alloy steel flanges for high-pressure transmission service, typically matched to API 5L X60 to X80 linepipe. All three are in the carbon & carbon alloy steel family of the mill-direct catalog and form the basis of most refinery, power-plant and pipeline orders.

Austenitic Stainless Steel (ASTM A182 F304, F304L, F316, F316L, F321, F347)

Austenitic stainless flanges are the default for chemical, food, pharmaceutical, and any service where corrosion resistance to chlorides, acids or organic media is required. F316L is the workhorse for welded systems because the low-carbon (≤ 0.03 % C) chemistry avoids sensitization in the heat-affected zone. F304 / F304L is the lower-cost option for less-corrosive service. F321 (Ti-stabilized) and F347 (Nb-stabilized) are specified for high-temperature service above 540 °C where sensitization is a concern. All are in the mill's stainless steel range.

Duplex and Super-Duplex (ASTM A182 F51, F53, F55)

Duplex stainless flanges (22 % Cr, 5 % Ni, 3 % Mo, with N) have roughly twice the yield strength of 316L and superior chloride stress-corrosion-cracking resistance. They are the standard for offshore, seawater, and high-chloride process service. F51 is the standard 2205 duplex, F53 is 2507 super-duplex, F55 is the 6 % Mo super-austenitic. The trade-off is weldability: duplex flanges need controlled heat input and a nitrogen-bearing shielding gas to retain the austenite-ferrite balance.

Copper-Nickel (Cu-Ni 90/10 and 70/30) for Marine and Seawater

Cu-Ni 90/10 (UNS C70600) and 70/30 (UNS C71500) are the standard materials for shipboard piping, seawater cooling, firemain, and bilge systems. They have intrinsic resistance to biofouling and chloride corrosion that stainless and carbon steel cannot match in continuous seawater immersion. Standards: EEMUA 144, EEMUA 234, MIL-T-16420K, DIN 86019, GB/T 26291. Cu-Ni flanges are typically delivered with the matching copper nickel flanges in the same lot, with EEMUA 234 traceability documentation per order.

Nickel Alloys (Inconel 600, 625, Monel 400, Hastelloy C276)

For aggressive chemicals, high-temperature petrochemical, or nuclear service, nickel-alloy flanges are specified. Inconel 600 (N06600) for hot caustic and chlorine, Monel 400 (N04400) for hydrofluoric acid and seawater, Hastelloy C276 (N10276) for the most corrosive acid-chloride mixtures. These are typically forged to ASTM B564, machined to ASME B16.5 dimensions, and supplied with full MTC including heat-number traceability. They are usually sourced with the matching pipe fittings in the same heat to avoid mixed-material service failures.

Dimensional and NDT Inspection: What a Mill-Direct Test Certificate Has to Show

A pipe flange is a forged or plate-rolled component that has been machined to ASME B16.5 or EN 1092-1 dimensions, then tested to a standard that is as much about traceability as it is about mechanical properties. The minimum test package for a B2B shipment should include the items below.

• Material Test Certificate (MTC / Mill Test Report) per EN 10204 3.1 or 3.2. The 3.1 certificate is mill-issued and accepted for most process service. The 3.2 is third-party witnessed and required for nuclear, offshore, or pressure-bearing equipment under the Pressure Equipment Directive (PED 2014/68/EU) Category III and IV.
• Chemical composition. Heat-by-heat C, Mn, Si, S, P, Cr, Ni, Mo, Cu, V, Nb, Ti as applicable. For stainless and duplex, the PRE (pitting resistance equivalent) number should be reported.
• Mechanical properties. Tensile strength, yield strength (0.2 % offset for austenitic), elongation, reduction of area, and — for low-temperature service — Charpy V-notch impact at the design minimum temperature per ASTM A370.
• Hardness. Brinell or Rockwell per the material spec. For sour service (NACE MR0175), hardness must be ≤ 22 HRC for carbon and low-alloy steels, ≤ 35 HRC for some austenitic grades.
• Dimensional inspection. OD, ID, raised-face diameter, raised-face height (RF = 1.6 mm for Class 150/300, 6.4 mm for Class 600+), bolt-circle diameter, number and size of bolt holes, flange thickness, and hub length. The drawing is normally signed by QC and traceable to the MTC.
• Non-destructive testing. 100 % ultrasonic testing (UT) per ASME B16.5 §8 for Class 600 and above, and for all Classes in sour service. Surface MT or PT on machined faces is common for austenitic stainless and nickel alloy. RTJ grooves are dimensionally inspected with a profilometer and visually at 10x.
• Pressure test (when required). Hydrostatic test of the flange blank at 1.5 × the Class rating pressure is occasionally specified for critical-service orders. It is not a routine requirement for standard ASME B16.5 deliveries.

A clean test package is the single biggest signal that you are buying from a mill rather than a stockist. Trading companies typically cannot produce a 3.2 certificate, do not own the forging record, and re-pipe the MTC from a different heat when asked. Mill-direct suppliers issue the 3.1 or 3.2 from their own QA department with a traceable heat number, and the certificate header matches the actual forging.

Bolt-Up and Gasket Math: How to Make the Joint Actually Seal

A correctly specified flange with the correct gasket and studs still leaks if the bolt-up is wrong. The basic rule is from ASME PCC-1 "Guidelines for Pressure Boundary Bolted Flange Joint Assembly": tighten the studs in a calibrated, multi-pass star pattern so the gasket stress is uniform around the face. The target gasket stress comes from the gasket manufacturer — typically 30 MPa to 50 MPa for compressed non-asbestos, 60 MPa to 100 MPa for graphite, 70 MPa to 200 MPa for spiral-wound, and 150 MPa to 250 MPa for RTJ metal rings.

Three practical points. First, lubricate the studs and nuts with the bolt manufacturer-recommended anti-seize; an unlubricated stud can lose 30 % to 50 % of its torque to thread friction, and the resulting gasket stress is below design. Second, retorque after the first thermal cycle — most soft and semi-metallic gaskets relax 5 % to 15 % on the first heat-up, and a second pass is the cheapest insurance against a hot-flange leak. Third, do not mix studs and nuts from different material specs on the same joint. A193 B7 stud with A194 2H nut is the standard carbon-steel bolting; mixing in a B8 stainless stud on the same flange creates a galvanic cell at the first moisture event.

Sourcing from a Mill-Direct Manufacturer: A 7-Point Pre-Order Checklist

The fastest way to lose margin and time on a flange order is to source from a stockist when a mill-direct order would have been cheaper, faster, and traceable. The seven checks below turn a stockist into a mill in the procurement records — and surface the cases where a stockist is genuinely the right choice (small, mixed-class spares orders, for example).

1. Forging or plate-rolling capability. Ask if the supplier owns open-die or closed-die forging hammers, or plate-rolling and welding equipment. If they do not, they are a fabricator, not a mill.
2. Heat-treatment furnace. Normalizing, quenching, tempering, solution annealing. The answer to "can you normalize A105 to 30 HRC max" tells you whether the mill is real.
3. EN 10204 3.1 / 3.2 issuing authority. The MTC must come from the mill's QA department, signed by a mill-employed certified inspector. Stockists re-issue MTCs from a different heat.
4. In-house NDT. UT, MT, PT, and hardness testing equipment. ASME B16.5 §8 mandates 100 % UT for Class 600+ — a mill that outsources UT is a fabricator.
5. Machining capability. CNC lathe or vertical turning lathe for the facing and bolt-circle. Manual machining is acceptable for Class 150 spares but not for Class 600+ production.
6. Standard coverage. ASME B16.5, B16.47, EN 1092-1, GOST 33259, GB/T 17241, JIS B 2220. A mill with only one standard on the price list is a regional stockist.
7. Joint delivery. Can the supplier ship the flanges, the matching pipe fittings, the pipe flanges or copper nickel flanges in special alloys, and the gasket, stud bolt & nut set in the same shipment, on the same MTC, with the same heat number? If yes, you have a mill.

Where EZ Steel Industrial Fits in the Pipe Flange Supply Chain

EZ Steel Industrial is a Chinese mill-direct manufacturer of industrial steel pipe, tube, fitting, flange and valve products, organized in eight catalog families. Within the flange family, the mill supplies steel flanges in carbon, alloy, stainless, duplex and nickel-alloy grades to ASME B16.5, ASME B16.47 Series A and B, EN 1092-1, GOST 33259 and GB/T 17241, with face options RF, FF, RTJ, T&G, and M&F from Class 150 up to Class 2500. The marine and shipbuilding line covers copper nickel flanges in 90/10 and 70/30 to EEMUA 144 / 234, MIL-T-16420K and DIN 86019. Each order ships with the matching pipe fittings, the gasket, stud bolt & nut set, and — for power, refinery and chemical EPC — the upstream and downstream industrial valves as a single traceable lot.

For procurement teams used to issuing three or four separate RFQs and reconciling the MTCs at goods-in, the mill-direct joint-delivery model removes the most common source of joint-leak risk in field service: components from different lots, with different hardness, different bolt-up math, and different traceable histories. The flange is the joint, and the joint is the system. Buying the whole stack from one mill, on one MTC, in one heat, is the simplest and most cost-effective improvement a B2B procurement team can make.