In any piping system — whether it carries steam in a power plant, crude oil across a terminal, or seawater through a ship's engine room — pipe flanges are the joints that decide whether the line holds. A wrong flange choice does not fail on day one. It fails eighteen months later, on a welded seam you cannot easily reach. This guide walks you through the three decisions every buyer must get right: flange type, base material, and governing standard — and shows how a full-scope mill partner simplifies the entire order.

A pipe flange is a forged or plate ring welded, screwed, or lapped to the pipe end. Two flanges are pulled together with bolts and a gasket to form a sealed, demountable joint. That demountability is the whole point: it lets you isolate a valve, change a pump, or hydro-test a section without cutting the line.

Because the joint carries both mechanical load and internal pressure, every flange must be specified four ways: type (geometry), facing (RF, FF, RTJ), material grade, and pressure class. Skip any one of those and you end up with a fitting that looks right on paper but leaks in service. EZ Steel Industrial, founded in 1994 in Changsha, China, manufactures and stocks the full range so procurement teams can source steel flanges, gaskets, stud bolts, and matched pipe fittings from one supplier with a single mill test certificate chain.

Most published standards cover the same eight geometries. The differences look subtle on a drawing, but they change how the joint handles stress, temperature cycling, and disassembly.

2.1 Slip-On Plate Flange (SO)

A flat ring that slides over the pipe and is fillet welded on both faces. The cheapest flange to produce and the easiest to align in the field. Used for low-pressure water, fire protection, and utility lines — typically Class 150 and PN 10/16 service. EZ Steel supplies SO plate flanges to ASME B16.5, EN 1092-1 Type 01, BS4504, and DIN 2573.

2.2 Slip-On Hubbed Flange (SORF)

The same slip-on fit, but with a short hub that gives the bore extra reinforcement. SORF is the default choice for ASME Class 150/300 process piping — chemical, petrochemical, and HVAC. It is also the most common flange in the stainless steel pipe and carbon steel pipe inventories we keep for project call-offs.

2.3 Weld Neck Flange (WN)

Butt-welded to the pipe with a long tapered hub. The hub distributes mechanical stress into the pipe wall, which is why WN is mandatory for high temperature, high pressure, and cyclic service — think boiler feedwater, superheated steam, and hydrocracker reactor loops. Specify WNRF (raised face) for general service and WN RTJ (ring-type joint) for Class 600 and above.

2.4 Blind Flange

A solid disc used to close a pipe end, a valve end, or a vessel nozzle. Blinds are also used for pressure testing and for future tie-in points. They must be rated to the same pressure-temperature as the mating flanges, and the gasket facing (RF, FF, RTJ) must match exactly.

2.5 Lap Joint and Loose Flanges

The lap joint is actually two parts: a backing flange and a stub end welded to the pipe. The backing flange can rotate around the stub end, which makes bolt-hole alignment painless — useful in large-diameter carbon steel pipe and stainless steel pipe headers, and in systems that need frequent dismantling for inspection. EN 1092-1 Type 02 and Type 33, plus DIN 2642, cover this family.

2.6 Threaded and Socket Weld Flanges

Threaded flanges carry tapered NPT or BSPT threads and are used where welding is not practical — small-bore instrumentation lines, stainless steel pipe ｄｒｏｐ legs in hygienic systems, and repair situations. Socket weld flanges receive the pipe into a socket and are fillet welded; they are limited to small-bore high-pressure service where a smooth bore is required.

2.7 Backing Ring and RTJ Flanges

Backing rings (SANS 1123, BS T/D Table D/E) support flanged joints in waterworks and mining. RTJ flanges use a machined groove and a metal ring gasket for the highest pressure classes (ASME Class 600 and above, especially in oil and gas).

2.8 Custom and Galvanized Flanges

When the standard catalog does not fit — non-standard OD, special drilling pattern, or a project-specific coating — a custom flange is quoted by drawing. For outdoor or mildly corrosive service, hot-dip galvanizing to ASTM A153 is the most common finish; the coating specification (HDG, cold galvanizing, electro-galvanizing, or anti-rust oil) should always be written into the RFQ.

Flange material is selected to match the pipe material, the service fluid, and the operating temperature. The chart below maps the most common choices to real industrial conditions.

EZ Steel Industrial holds all of these families in its Changsha mill. Steel flanges in carbon, stainless, and alloy grades are produced to ASME B16.5, B16.47, EN 1092-1, GOST, JIS, and BS standards, while copper nickel flanges for marine service are stocked in 90/10 and 70/30 compositions per EEMUA 234 and ASME SB151.

A flange's dimensions, pressure-temperature rating, and material traceability are all set by the standard on its nameplate. Mixing standards on the same joint is the single most common cause of field rejection.

• ASME B16.5 — the dominant standard for NPS ½ through 24, Classes 150 to 2500, used in North American and most international oil and gas projects.
• ASME B16.47 — large-diameter flanges (NPS 26 through 60), Series A (MSS SP-44) and Series B (API 605).
• EN 1092-1 — the European standard covering PN 2.5 to PN 400, with Types 01 to 13 mapping to the geometries above.
• GOST 33259 — Russian and CIS standard, used in oil and gas projects in Russia, Kazakhstan, and the Middle East.
• JIS B2220 — Japanese standard, common in shipbuilding and LNG terminals across Asia.
• BS 4504 and BS 10 — legacy UK standards, still seen in water, fire protection, and older process plants.

A full-service mill should be able to cross-reference all of these. At EZ Steel, every flange shipment is released with a mill test certificate traceable to the heat number, the standard, and the project specification.

A flanged joint is only as strong as its weakest component. If the flange is ASME B16.5 Class 300 and the gasket is a soft compressed sheet, the joint will blow at the first thermal cycle. The standard practice is to specify the entire joint package together:

• Gasket — spiral-wound for Class 150 to Class 600, ring-type joint (R, RX, BX) for higher classes, non-asbestos sheet for low-pressure water. EZ Steel's gasket stud bolt nut range covers spiral-wound, RTJ, and soft cut gaskets matched to the same standards as the flanges.
• Stud bolts — B7 / B7M for carbon steel service, B8 / B8M for stainless, B16 for high-temperature alloy, with 2H heavy hex nuts.
• Washer and lubrication — required for proper torque transfer and to prevent galling on stainless bolting.

These are the issues our QA team sees most often on incoming RFQs:

• Specifying the standard but not the type (e.g., "ASME B16.5 flange" with no SORF / WNRF / Blind call-out).
• Mixing standards on the same joint — a Class 150 ASME flange mated to a PN 16 EN flange because both are 2-inch and look the same.
• Choosing carbon steel for a chloride or sour service without specifying a corrosion allowance or a clad alternative.
• Forgetting the facing finish — a stock RF (125–250 Ra) will not seal against a soft gasket the way an FF face does.
• Buying loose flanges without the matching stub ends or backing rings — and discovering the mismatch at the bolt-up stage.

Before a flange leaves the mill, it should carry:

• Heat number traceability linked to the MTC (EN 10204 3.1 or 3.2 as required).
• Dimensional inspection report covering OD, bore, thickness, facing, and drilling (PCD and number of holes).
• Material certificate confirming grade, chemical composition, and mechanical properties (yield, tensile, impact if specified).
• NDT records where required — ultrasonic, magnetic particle, dye penetrant for higher classes and low-temperature service.
• Hydrostatic or pressure test certificate for blind flanges used in test programs.

EZ Steel's ISO 9001-certified lab and API / EN / ASME production licenses mean every flange can be released to the documentation level your project requires — from standard 3.1 mill certificates to full third-party witnessed testing by SGS, TÜV, or Bureau Veritas.

• Standard and type (e.g., ASME B16.5 WNRF Class 300, or EN 1092-1 Type 11 PN 40).
• Size, NPS or DN, and pressure class.
• Material grade and any cladding or coating.
• Facing finish (RF, FF, RTJ) and surface roughness.
• Bore schedule (standard bore or pipe schedule to match).
• Quantity, delivery window, and packaging requirement.
• Documentation level: MTC 3.1, 3.2, PMI, NDT, third-party witness.
• Matched gaskets and stud bolts, or supply them separately.