A pipe flanges is a disc-shaped connector welded, threaded or loose-fitted to the end of a pipe so the line can be joined to another pipe, a valve, a fitting or a pressure vessel, with a gasket and bolts providing the seal. Flanges are rarely chosen on shape alone — pressure class, media, temperature, corrosion exposure, the joining method and the inspection regime all matter. This guide walks through the decisions a procurement or engineering team faces when they sit down to specify flanges for a real project, drawing on the standards and product range that EZ Steel Industrial supplies to oil & gas, petrochemical, power and marine customers.

1. Start with the Flange Type, Not the Material

Many buyers look at material grade first and type second. In practice the type is decided by how the line is going to be installed, dismantled and inspected, and that decision drives the material you can realistically use.

• Weld Neck (WN): butt-welded to the pipe, stress transfers through the hub. The default choice for high pressure, high temperature and cyclic service.
• Slip-On (SO): slipped over the pipe and fillet-welded. Lower cost, easier alignment, limited to lower pressure classes.
• Blind (BL): solid disc used to close a pipe end, vessel nozzle or for hydrostatic testing.
• Socket Weld (SW): pipe sits in a socket, fillet weld at the hub. Used on small-bore, high-pressure lines where a smooth bore is wanted.
• Threaded (TH): tapered internal thread, no welding needed. Common on low-pressure utility lines and firewater.
• Lap Joint (LJ): backing flange rotates around a stub end. Convenient when bolt alignment is awkward or frequent dismantling is expected.

Once the type is fixed, the standard (ASME B16.5, ASME B16.47, EN 1092-1, GOST, JIS B2220) is usually selected by the project specification, not by the buyer. Different standards mean different pressure classes, different bolt patterns and different facing types, so mixing them on the same line should be avoided unless the engineer has approved it.

2. Pick the Material Group That Matches the Service

2.1 Steel flanges for general piping and high-pressure service

For the majority of oil & gas, refinery, power plant and structural piping, steel flanges are the workhorse. The three families below cover most line-pipe and process-pipe needs.

• Carbon steel (ASTM A105, A350 LF2): the default for ambient and moderate-temperature service. A105 covers non-corrosive fluids including hydrocarbons, steam and air up to about 425 °C; A350 LF2 is required when the project is low-temperature (down to −46 °C) such as LNG and winter pipelines.
• Stainless steel (ASTM A182 F304/F316, duplex F51/F55): chosen when corrosion resistance or cleanliness is the driver. 316/316L is the standard pick for chemical, food, pharmaceutical and offshore process systems; duplex and super-duplex grades are used for high-chloride seawater and aggressive sour service.
• Alloy steel (ASTM A182 F11/F22/F91): matched to the matching pipe grade (e.g. A335 P11/P22/P91) for high-temperature headers, superheater connections and refinery hot lines.

2.2 Copper-nickel flanges for seawater and marine service

When the fluid is seawater, brine or a marine cooling medium, carbon and even stainless steel can suffer pitting, crevice corrosion and galvanic attack. copper nickel flanges (90/10 Cu-Ni and 70/30 Cu-Ni) are specified because the alloy forms a stable protective film in seawater, resists biofouling, and tolerates the velocities typical in ship and platform cooling systems. 90/10 is the standard for shipboard piping, cooling water and firefighting mains; 70/30 is selected where the water is more aggressive (higher salinity, warmer temperatures, higher velocities).

3. Pressure Class, Size and Facing

With the type and material fixed, three numbers define the rest of the specification: the pressure class, the nominal size and the facing.

3.1 Pressure class

Under ASME B16.5 the classes run from 150 to 2500. In practice, Class 150 and Class 300 cover most utility, cooling water, firewater and low-pressure hydrocarbon lines. Class 600, 900 and 1500 are selected for high-pressure gas injection, wellhead manifolds and critical steam headers. Class 1500 and above usually justify a weld neck geometry and a full-quality NDT package.

3.2 Size and facing

ASME B16.5 covers NPS ½ through NPS 24; larger sizes move to ASME B16.47 Series A or Series B. Facing is normally raised face (RF) for general service, flat face (FF) when the mating flange is cast iron or a non-metallic-lined component, and ring-type joint (RTJ) for high-pressure, high-temperature or hydrocarbon service where a metal-to-metal seal is required. Tongue-and-groove and male/female facings are kept for special small-bore or self-aligning assemblies.

4. The Joint Is a System: Flange, Gasket, Stud Bolt

It is tempting to source the flange, the gasket and the stud bolt from three different suppliers to get the lowest price on each line. The leakage record in industry suggests otherwise: most field leaks start at a joint whose components were not selected as a system. A consistent gasket stud bolt nut package — correct hardness, correct bolt grade (typically B7/B7M for carbon steel, B8/B8M for stainless, B16 for high-temperature alloy), correct washer and nut — is what makes the pressure class and facing of the flange actually perform on site.

5. Matching Flange Type to Real Applications

The table below summarises how a procurement team typically maps flange choice to service conditions.

6. Why Procurement Teams Consolidate Flange Sourcing

On a typical project, the same line needs flanges in carbon steel, stainless, alloy and copper-nickel — and each one is paired with its own gaskets, stud bolts and nuts. Sourcing each line from a different supplier means four quality systems, four MTC formats, four shipping schedules and four non-conformance paths. Most EPC and plant-operating customers eventually consolidate to a single supplier that can deliver a matched bundle: flanges, gaskets, stud bolts and the matching pipe or fittings, all on one MTC and one inspection plan.

This is the model that EZ Steel Industrial has built since 1994. With eight product families under one quality system — carbon and alloy steel, stainless steel, copper & nickel alloy, heat efficiency tubes, pipe fittings, pipe flanges, gasket/stud bolt/nut, and industrial valves — a buyer can place one purchase order that covers the flange, the matching pipe, the bolts, the gaskets and even the valves at the line ends. MTCs, hydrotest certificates and PMI reports arrive in one package, which is what the project QA team actually needs to close out a line.