Start With the Joint, Not the Flange

A flange never works alone. It sits between the pipe, the gasket, the stud bolts and the mating equipment flange. Before choosing a flange face, confirm the pipe material, the design pressure and temperature, the medium, the bolt-loading capacity, and the inspection regime. EZ STEEL INDUSTRIAL, a steel and alloy pipe manufacturer in China operating since 1994, supplies the full pressure boundary as one integrated package — carbon steel pipe and stainless steel pipe matched to pipe fittings, gaskets and steel flanges with full MTC traceability.

Treating the joint as a single engineering unit is the most reliable way to avoid material mismatch, dimensional surprises and field rework.

Match Flange Type to the Job

Each flange geometry is designed for a specific duty. The right choice protects the joint, simplifies installation and keeps cost under control.

Weld Neck Flange (WN)

The bore of a weld neck flange matches the pipe bore, and the long tapered hub distributes mechanical stress into the pipe. Weld neck flanges are the default choice for high pressure, high temperature and cyclic service such as boiler headers, superheaters, hydrocracker reactors and offshore process piping.

Slip-On Flange (SO)

A slip-on flange slides over the pipe and is fillet welded from both sides. It is faster to align and cheaper to fabricate, and is widely used in low- to medium-pressure water, air and utility lines where the operating temperature is moderate.

Socket Weld Flange (SW)

Socket weld flanges accept the pipe into a recessed socket and are fillet welded at the hub. They are common in small-bore, high-pressure instrument and process lines, where a smooth bore and vibration resistance are required.

Threaded Flange

Threaded flanges are used where welding is not allowed on site, or where the line must be dismantled frequently. They are limited to lower pressure classes and to services where galling and thread corrosion are controlled.

Blind Flange

Blind flanges close pipe ends, valve outlets and pressure-vessel nozzles. They are also used for pressure testing and for future line extensions. Always match the blind flange class to the mating flange and confirm the facing finish.

Lap Joint Flange

Lap joint flanges pair with a stub end and rotate freely for easy bolt-hole alignment. They are typical in stainless or copper-nickel systems where the flange material can be lower-grade carbon steel and only the stub end contacts the corrosive medium.

For buyers who want a single supplier covering both standard and custom geometries, pipe flanges can be quoted by drawing with material, facing, drilling and coating specified together.

Pick the Right Material for the Service

Material selection should follow the medium and the environment, not the lowest unit price. The most common flange materials, and where they fit, are summarized below.

For seawater lines, copper nickel flanges are usually combined with Cu-Ni pipe and Cu-Ni stub ends so the entire wetted path shares the same corrosion mechanism. For high-temperature steam and refinery service, the flange and the connecting pipe should be ordered from the same heat number whenever possible to simplify welding procedure qualification.

Match Standards and Pressure Classes

Standards govern dimensions, tolerances, materials and pressure-temperature ratings. Mixing standards inside one joint is a common cause of bolt-pattern mismatch and gasket blow-out. The dominant systems are ASME B16.5 (NPS 1/2–24) and ASME B16.47 Series A and B for larger sizes, plus EN 1092-1, JIS B2220, GOST and BS 4504 for regional projects.

• Class 150: low-pressure water, air, general utility
• Class 300 / 600: process, refinery, mid-pressure steam
• Class 900 / 1500 / 2500: high-pressure feedwater, supercritical steam, critical hydrocarbon service

The same pressure class can carry a very different allowable stress in ASME versus EN, so always re-check the pressure-temperature rating chart against the design temperature before placing the order.

Don't Forget the Facing and the Gasket

A correct flange with the wrong facing will not seal. The four most common facing choices are:

• Raised Face (RF): default for most ASME Class 150–600 services with a soft or spiral-wound gasket
• Flat Face (FF): used with non-metallic gaskets and where cast iron or non-metallic flanges are joined
• Ring Type Joint (RTJ): metal ring gasket, used for Class 600 and above and for hydrocarbon service
• Tongue and Groove / Male and Female: specialized process lines requiring positive gasket location

The facing finish (stock finish, 125–250 AARH, 63 AARH for metal gaskets) must be specified in the RFQ together with the gasket material. EZ STEEL INDUSTRIAL also supplies matching gasket, stud bolt and nut sets so the joint arrives on site as a complete, traceable assembly.

Bundle the Pressure Boundary to Reduce Risk

On multi-line projects such as petrochemical plants, marine cooling systems or utility packages, sourcing pipe, fittings, flanges, gaskets and valves from one integrated supplier reduces documentation gaps and material-mismatch risk. EZ STEEL INDUSTRIAL supports a project-centric bundled supply covering pipe fittings such as butt-weld elbows, reducers and tees, steel flanges in carbon, stainless and copper-nickel grades, plus industrial valves for the same pressure classes.

A Practical Pre-Order Checklist

• Confirm design pressure, design temperature and the medium
• ｓｅｌｅｃｔ flange type (WN, SO, SW, threaded, blind, lap joint) for the duty
• ｓｅｌｅｃｔ material grade consistent with the connected pipe and fittings
• Specify standard (ASME B16.5 / B16.47, EN 1092-1, JIS B2220) and class
• Specify facing type, facing finish and gasket material
• Request MTC EN 10204 3.1 / 3.2, PMI report and NDT scope as required
• Confirm marking, coating, packaging and shipping documents before release