For three decades EZ Steel Industrial has supplied flanges to power plants, petrochemical complexes, marine yards, and water projects across more than 60 countries. In that time we have learned one thing: most flange problems on site trace back to a misstep in the specification stage, not in the workshop. This guide walks through how we help buyers and engineers get the specification right the first time, drawing on the same selection logic our technical team uses every day.

1. Start With the Service Conditions, Not the Catalog

Before you look at flange types, write down four numbers: design pressure, design temperature, the medium being carried, and the pipe size. These four inputs will narrow your options far more than any standard or material brochure. A 150# flange that works perfectly on a cooling water line will fail in a high-pressure steam header; a stainless weld neck that lasts decades in chemical service will be wasted money on a low-pressure air line.

Once those numbers are fixed, you are really choosing between two decisions: which flange facing type to use, and which material grade matches the medium and temperature. The flange body style falls out of those two answers almost every time.

2. The Five Flange Types You Will See Most Often

Whether you are looking at ASME B16.5, EN 1092-1, GOST 33259, or JIS B2220, the same five structural types dominate global piping. Each is engineered for a specific duty, and using the wrong one is the most common cause of in-service leakage.

Weld Neck (WN) — the high-pressure workhorse

A weld neck flange has a long tapered hub that is butt-welded to the pipe. The taper distributes mechanical stress and avoids the high stress concentrations that a sharp transition would create. This makes WN the default choice for service above Class 600, for high-temperature headers, for cyclic loading, and for any line that will see frequent inspection. In a steam power plant or ethylene cracker you will find WN flanges on almost every critical joint.

Slip-On (SO) — economical for low-pressure duty

The pipe slides into a slip-on flange and is welded both inside and outside. SO flanges are easier to align, cheaper, and faster to install, which is why they are the most common flange in low-pressure firewater, HVAC, and utility lines. The trade-off is strength: SO flanges are roughly two-thirds as strong as a comparable weld neck, so they should not be used where pressure, temperature, or fatigue is high.

Socket Weld (SW) — for small-bore, high-pressure

A socket weld flange has a recess that the pipe is inserted into before being fillet-welded. It is the standard small-bore connection (typically NPS 1/2 to NPS 2) in chemical, instrumentation, and small-bore high-pressure systems. The fillet weld is easy to make, the joint is compact, and the design gives smooth flow characteristics. Care must be taken to avoid crevice corrosion between pipe and socket in aggressive media, so the gap is usually vented by a small bore drill at the bottom.

Threaded (TH) — where welding is impractical

Threaded flanges have machine threads matching the pipe thread. They are used in low-pressure, non-hazardous services where welding is not practical — for example, utility lines, compressed air, or galvanised steel pipework. They must not be used in high-temperature or high-cyclic services because thread galling and crevice corrosion will shorten the service life.

Blind (BL) — sealing a system

Blind flanges have no bore. They are used to close the end of a pipe, isolate equipment for maintenance, or pressure-test a section of piping. In a refinery or offshore platform, blinds are one of the most safety-critical items because they hold full system pressure on a flat plate. They are almost always supplied as weld neck or forged designs so the joint can be torqued without distortion.

At a glance: which flange fits which job

3. Material Selection: Carbon, Stainless, or Alloy?

The flange material has to match the pipe material, the medium, and the temperature. Get this wrong and you either waste money on corrosion resistance you do not need, or you watch the joint rust away in months. Three families cover the vast majority of industrial flanges.

Carbon and carbon-molybdenum steel

ASTM A105, A350 LF2, and A694 are the workhorses for oil, gas, water, and steam service. They are strong, weldable, machinable, and economical. For a project carrying water, oil, or air at moderate temperatures, an A105 weld neck paired with an A106 or A53 carbon steel pipe is usually the correct economic choice. In low-temperature service (down to -46°C), A350 LF2 is the standard alternative.

Stainless steel

304/304L and 316/316L stainless flanges are the default for food, beverage, pharmaceutical, chemical, and offshore duties. The molybdenum content of 316 gives it a clear edge in chloride-rich environments such as seawater cooling and coastal chemical plants. Where post-weld corrosion in the heat-affected zone is a concern, the L-grade (low carbon) variants should be specified. For higher temperatures or aggressive chemicals, duplex (F51/F55) and super-austenitic grades (AL-6XN, 254 SMO) extend the envelope considerably. Our full stainless steel pipe programme covers the same material families so the flange, the pipe, and the fittings can be supplied as a matched package.

Copper-nickel and nickel alloys

Where the medium is seawater, brackish water, or a corrosive chemical stream, 90/10 and 70/30 copper nickel alloy flanges are the established choice. They resist biofouling, tolerate high velocities, and have a long track record in shipbuilding, desalination, and offshore platforms. Our copper nickel flanges are produced to EEMUA 234, ASME B16.5, and GB/T standards for exactly these duties. For highly aggressive or high-temperature chemical service, Monel, Inconel, and other nickel alloys are available on request.

4. Standards Matter More Than Most Buyers Realise

A flange that satisfies one standard does not automatically satisfy another. The bolt circle, the facing height, the pressure-temperature rating — all of these shift between ASME B16.5, EN 1092-1, GOST 33259, and JIS B2220. Mixing standards on the same joint is one of the most common reasons for leaks after a retrofit or an international procurement.

For projects in the oil, gas, and petrochemical sectors, ASME B16.5 is the global benchmark and pairs naturally with ASME B16.47 for large diameters. In Europe, EN 1092-1 is mandatory for pressure-rated lines under the Pressure Equipment Directive. Russian and CIS projects still call for GOST 33259. Japanese and Korean shipyards, as well as most Taiwanese EPC contractors, specify JIS B2220. Confirming the governing standard at the procurement stage — not at site — saves a great deal of cost and delay.

5. Bolting, Gaskets, and the Forgotten Half of the Joint

A flange is only as good as the bolts and the gasket. ASTM A193 B7 studs with A194 2H nuts are the standard bolting for carbon and alloy steel flanges in non-corrosive service. For stainless flanges, A193 B8 class 1 or B8M bolts are used to avoid galling. For low-temperature duty, A320 L7/L43 studs are mandatory. Each of these bolting sets has its own temperature limit — using standard B7 studs above 400°C, for example, will cause premature creep failure.

On the gasket side, the most common choices are spiral-wound graphite (for ASME Class 150 to 600, high temperature), ring-type joint (RTJ) for Class 600 and above, compressed non-asbestos sheet for low-pressure water, and PTFE envelopes for chemical service. Matching the gasket style to the facing (RF, FF, RTJ, tongue-and-groove) is non-negotiable. Our gasket stud bolt nut kits are factory-matched to the flange so bolting, gasket, and flange arrive as one engineered package.

6. Why Buy Flanges as Part of a Bundled System

Procurement teams that buy pipe, flanges, fittings, and valves from a single source routinely report two gains: faster delivery on site, and a sharp ｄｒｏｐ in interface problems. When a pipe fittings and its companion flange are machined to the same tolerance and the same standard, the field welder spends his time welding instead of shimming. When a line includes industrial valves, flanges, and matching gaskets, the line is pressure-tested as a system rather than as loose parts.

At EZ Steel Industrial we stock flanges in carbon, stainless, alloy, and copper-nickel grades, with full traceability under API, EN, and ASME specifications. Our ISO 9001 laboratory issues mill test certificates for every batch, and our bundle model lets buyers pull a complete steel flanges package from a single purchase order. From heat-exchanger bundles in heat efficiency tubes to marine cooling in copper-nickel, we have built our reputation on the principle that a flange should be the easiest part of the project, not the most painful one.