1. The Standard Call-Out: ASME, EN, GOST, JIS or GB/T?

The first line on any flange spec sheet is the governing standard, and that single designation controls nearly every other dimension on the page. ASME B16.5 covers NPS ½ through NPS 24 in pressure classes 150 through 2500; ASME B16.47 takes over for NPS 26 and above as Series A (MSS SP-44) or Series B (API 605). EN 1092-1 organizes flanges by PN pressure rating and Type number (01 plate, 02 loose, 05 blind, 11 weld neck, 12 hubbed slip-on, 13 threaded, 33 weld ring neck). For cross-border EPC work you may also see GOST 33259, JIS B2220, or GB/T 17241 on the same requisition.

The standard drives the dimensional table, the bolt circle, the number and size of bolt holes, the facing finish, and the maximum allowable pressure-temperature rating. Mixing dimensions from one standard with a pressure class from another is one of the most common reasons a flange is rejected at the dock. When the spec sheet says "ASME B16.5 Class 150," every other number on the page must come from the ASME B16.5 Class 150 table. EZ Steel Industrial's documentation team flags any cross-standard mix in the mill's MTR (Mill Test Report) before the order is cut, so the inspector sees one consistent set of dimensions on the certificate and the flange itself.

2. Pressure Class vs. Service Pressure: Don't Read Them as the Same Number

A Class 150 flange is not designed for 150 psi. The pressure class is an identification number tied to a pressure-temperature table inside the standard. ASME B16.5 Class 150 carbon steel is rated at 285 psi at -29 °C up to 150 psi at 38 °C, with progressively lower ratings as the temperature climbs. Class 300, 600, 900, 1500, and 2500 follow the same pattern, and stainless steel ratings are different again. EN flanges use PN 6, 10, 16, 25, 40, 63, 100 and so on, where PN equals the maximum allowable working pressure in bar at room temperature.

The right way to read the spec sheet is to check the design pressure and design temperature of the line, then look up the required pressure class in the standard's rating table for the chosen material. A 6-bar steam line at 200 °C in carbon steel may need a PN 40 flange, not a PN 16. EZ Steel Industrial's technical team performs this check against the customer's piping class specification before recommending a flange type, so the steel flange on the quotation matches the service, not just the line on the datasheet.

3. Flange Type: Match the Geometry to the Maintenance Strategy

Weld neck, slip-on, socket weld, threaded, lap joint, and blind are not interchangeable. A weld neck flange has a long tapered hub that butts to the pipe and is full-penetration welded, transferring stress into the pipe and tolerating thermal cycling, pressure cycling, and high temperature. It is the default for refinery, power plant, and process piping above Class 600 or in any high-temperature or high-cycle service.

A slip-on flange slides over the pipe and is fillet welded at the hub and at the front face. It is easier to align, cheaper, and acceptable for low-pressure fire protection, plumbing, and HVAC, but it is not a substitute for a weld neck on a hot reheat line. Socket weld flanges are used on small-bore (typically NPS 2 and below) high-pressure piping where a smooth bore is needed. Threaded flanges suit low-pressure utility lines and are commonly used in galvanized carbon steel for building services. Lap joint flanges pair with a stub end and are rotated for bolt alignment on systems that need frequent dismantling, such as offshore modules. Blind flanges close pipe ends, valve bonnets, and pressure vessel nozzles and must be specified separately because their pressure-temperature rating is the same as the mating flanges in the joint.

The spec sheet should always state the type by name (Welding Neck, Slip-On, Socket Weld, Threaded, Lap Joint, Blind) and the short code (WNRF, SORF, SWRF, LJRF, BLRF). EZ Steel Industrial produces all six types in carbon, stainless, alloy, and copper-nickel material families, with EN 1092-1 Type numbers and ASME B16.5 short codes cross-referenced in the mill's RFQ response so the buyer's piping class spec and the supplier's offer speak the same language.

4. Material: The Four Families and Where Each One Belongs

Material selection drives cost, lead time, and corrosion allowance. The four families that cover the majority of industrial flange orders are carbon steel, low-temperature carbon steel, stainless steel, and copper-nickel. ASTM A105 is the workhorse carbon steel forging for -29 °C and above, while A350 LF2 Class 1 is required for low-temperature service down to -46 °C. Stainless grades span A182 F304/L, F316/L, F321, F347, and the duplex F51/F55/F60 for chloride-rich service. For marine and offshore cooling systems, copper nickel flanges in Cu-Ni 90/10 (UNS C70610/C70620) and 70/30 (C71500) resist seawater corrosion and biofouling.

The spec sheet should state the material by ASTM/EN grade and, where relevant, by UNS number, the heat-treatment condition (normalized, quenched and tempered, solution annealed), and the supplementary requirements such as SSCC (Sulfide Stress Cracking) test per NACE MR0175 for sour service, or impact testing at a specified temperature. EZ Steel Industrial's MTR package records the heat number, the chemical analysis, the mechanical test results, the NDT, and the supplementary tests so the inspector can trace the forging back to the heat of steel.

5. Facing and Surface Finish: The Detail That Decides a Leak-Free Joint

The facing is the sealing surface that mates with the gasket. Raised Face (RF) is the most common, with a serrated spiral finish of 125 to 250 microinches Ra (AARH 125-250) for carbon steel and 125 to 200 microinches for stainless. Flat Face (FF) is used with non-metallic gaskets on cast iron or fiberglass flanges. Ring Type Joint (RTJ) cuts a precision groove for a metal ring gasket and is the standard for high-pressure Class 600 and above hydrocarbon service. Tongue and Groove (T&G) and Male and Female (M&F) facings suit specialized gaskets in chemical and steam service.

A spec sheet that only says "RF" is incomplete. The surface finish, the stock finish, and the spiral pattern should be explicit. A 125 RMS finish on a Class 150 RF will seat differently from a 250 RMS finish on a Class 300 RF, and the gasket manufacturer needs to match. EZ Steel Industrial's QA records note the facing type, the finish measurement, and the gasket compatibility class so the same line of information shows up on the flange face, the MTR, and the shipping documents.

6. Size, Bore, and Wall Thickness: Where the Inch vs. Millimeter Confusion Lives

Pipe flanges are specified by Nominal Pipe Size (NPS) in inches or by DN in millimeters. NPS 2 and DN 50 are not the same outer diameter, but they do represent the same nominal bore. The standard inside diameter of the flange bore typically matches the pipe inside diameter, with a small bore difference (B12.5, B16.5, etc.) to allow for welding. The spec sheet must show the NPS or DN, the bore schedule (standard or customer-specified), and the corresponding pipe wall so the welder can confirm fit-up.

A weld neck flange carries the same wall schedule as the pipe it is welded to. If the pipe is Schedule 80, the flange hub is machined to that wall. The spec sheet entry "Weld Neck, NPS 6, Class 300, Sch 40" is incomplete without confirming the pipe schedule. Cross-referencing the flange, the pipe, and the pipe fittings against one piping class spec eliminates most of the dimensional mismatches that surface during site erection.

7. Bolting, Gasket, and the Valve on the End: Treat the Joint as One System

A flange never works alone. The joint is the flange, the gasket, the stud bolts and nuts, and the mating flange on the other side. The spec sheet should reference the gasket style (spiral wound, ring joint, compressed non-asbestos, graphite, PTFE), the bolt material (A193 B7/B7M/B8/B8M for steel service, A320 L7/L43 for low temperature, A453 660 for high temperature), and the nut grade (A194 2H/2HM/8/8M/7/7M). Bolting that is one grade off can stretch during torque-up and leak on the first hydrostatic test.

Where the line ends at equipment or a valve, the mating flange belongs to the same joint. An industrial valve with a flanged body is supplied with its own set of integrally cast or forged flanges; these are not interchangeable with the line pipe flanges unless they share the same standard, class, facing, and material. EZ Steel Industrial bundles the flange with the matching gasket, stud bolt and nut set, and the line valve under one MTR trail so the inspector checks one document stack instead of five.

What a Mill-Direct Spec Sheet Looks Like

A mill-direct spec sheet on an EZ Steel Industrial quotation does more than echo the buyer's data sheet. It records the standard, the type code, the material grade, the heat number, the facing and finish, the bore schedule, the dimensional table, the bolting and gasket recommendation, the test and inspection plan, and the supplementary requirements on a single page. The same data appears on the MTR, on the heat number stencil on the flange hub, and on the shipping document, so the receiving inspector, the third-party inspector, and the document controller are looking at one consistent set of information.

For projects that source pipe flanges as part of a bundled piping package, the mill also issues a matching spec sheet for the pipe, the fittings, the gasket and stud bolt set, and the line valve, so the entire joint from one welded pipe end to the next can be reconstructed from a single document trail. That is the practical difference between buying a flange and buying a flange that has been documented for the inspector who will eventually sign it off.