A steel flanges specification looks simple on paper — a ring of forged or plate steel with bolt holes — but in real projects the wrong face type, the wrong grade, or a missing companion product can stall a piping pre-commissioning for weeks. After three decades of supplying flanges to oil & gas, petrochemical, power and marine projects, EZ Steel Industrial has seen almost every way a flange order can go wrong, and almost every way it can be de-risked by buying from a mill that also makes the pipe, the fittings and the gaskets behind it.

This 2026 buyer's guide walks through the practical decisions a procurement engineer faces when ordering pipe flanges: how to pick the right type, how to read the standards (ASME B16.5, B16.47, EN 1092-1, JIS B2220), how to match the flange to the pipe fittings on either side, and how a mill-direct bundle of flanges + gaskets + stud bolts removes three separate purchase orders from your critical path.

1. The Five Flange Types You Will Actually Specify

Most engineers only ever need five flange types. Choosing the right one is mostly about how the joint will be assembled, inspected, and (sometimes) disassembled during the life of the plant.

Weld Neck (WN) — the workhorse for high-pressure service

The long tapered hub is butt-welded to the pipe, which gives a smooth bore and excellent fatigue resistance under thermal cycling. Use WN for Class 600 and above, for high-temperature service, and anywhere the line will see vibration or pressure cycling. EZ Steel Industrial produces WN flanges in carbon, alloy and stainless steel up to Class 2500.

Slip-On (SO) — the economical choice for low-pressure

Slip-on flanges slide over the pipe and are fillet-welded both inside and outside. They are cheaper than WN and easier to align, but the joint is weaker. SO is the default for Class 150 / 300 utility lines, firewater, cooling water and instrument air.

Blind (BL) — for vessel heads, test ends and future tie-ins

Blind flanges seal the end of a line. They take the full system pressure on a flat or raised face, so thickness and bolting are sized accordingly. Order BL flanges with the same pressure class as the adjacent line and the same facing finish as the mating flange.

Threaded (TH) — for small-bore, non-critical service

Threaded flanges are tapped to match the pipe thread, typically NPT or BSP. They are restricted to small bore (usually NPS 2 and below) and low-pressure, non-flammable service. Common in instrumentation, sample lines and utility drops.

Lap Joint (LJ) — when you need a cheap rotating face

A lap joint uses a two-piece design: a loose backing ring (often from cheaper carbon steel) and a stainless stub end that does the actual sealing. The backing ring can be rotated to align bolt holes with a mating flange — useful in piping systems that are frequently dismantled for cleaning or inspection, such as food, pharma and some chemical services.

2. Standards and Pressure Ratings — What Your MOC Engineer Will Actually Quote

Three standard systems cover more than 95% of global procurement. The right one depends on where the plant is being built and which piping code the project follows (ASME B31.1, B31.3, EN 13480, etc.).

A useful rule of thumb: Class 150 (ASME) is roughly equivalent to PN 20 (EN). Class 300 maps to PN 50, and Class 600 to PN 100. The exact pressure-temperature rating still depends on the flange material, so always cross-check against the standard's own tables for the design temperature.

3. Picking the Right Material — Carbon, Alloy, Stainless or Copper Nickel

The flange material must match (or be compatible with) the pipe and fittings on either side of the joint. EZ Steel Industrial supplies flanges in four main material families, all from in-house forging and machining capacity.

Carbon steel — the default for oil, gas, steam and structural

ASTM A105 (forged) and A516 Grade 70 (plate) cover the majority of steel flanges in hydrocarbon service. For low-temperature service down to -46 °C, switch to A350 LF2. For high-temperature service above 425 °C, A694 F52/F65 is the usual choice for line pipe flanges in gas transmission.

Alloy steel — for high temperature and corrosive hydrocarbons

A182 F11, F22 and F91 are the standard alloy grades for steam, refinery hydrocracker and boiler feedwater service. They are always delivered with normalized-and-tempered or quenched-and-tempered heat treatment, with full Charpy impact test reports at the design temperature.

Stainless steel — for corrosive and hygienic service

A182 F304/L and F316/L cover most chemical, food and pharmaceutical piping. For chloride-rich service (seawater cooling, bleach, brine) jump to duplex F51/F53/F55, or for severe service super-austenitic 254 SMO or alloy 825/625. The stainless flange should be pickled and passivated after machining.

Copper nickel — for seawater and firewater

90-10 (C70600) and 70-30 (C71500) copper nickel flanges are standard for shipbuilding, offshore platforms, desalination and power plant cooling water. They are specified separately from steel flanges and follow EEMUA 145, not ASME B16.5, for the typical seawater duty.

4. The Flange Joint — You Cannot Buy a Flange Without Its Bolts and Gasket

A flange is half a joint. The other half is the gasket sitting on its face and the stud bolts pulling it down. Specifying the flange without specifying the gasket stud bolt nut set is the most common source of leakage in a new plant, and one of the easiest to fix at the procurement stage.

Match the gasket to the face

Raised-face (RF) flanges typically take a compressed non-asbestos or graphite gasket. Flat-face (FF) flanges take a full-face gasket with bolt holes. Ring-type joint (RTJ) flanges take a metal ring joint gasket sized to the groove. Never mix an RTJ flange with a soft gasket, and never mix an FF flange with a ring gasket.

Match the stud bolt to the service

A193 B7 / A194 2H stud bolts cover most ASME Class 150 / 300 / 600 hydrocarbon service. For low-temperature service, switch to A320 L7 / A194 4. For stainless flanges, use A193 B8 / B8M with A194 8 / 8M. Bolting is normally ordered with the matched nuts and washers in the same shipment.

5. Sourcing Steel Flanges — Mill-Direct vs. Trading House vs. Importer

There are three ways to source a steel flange in 2026: directly from a forging mill, through a domestic trading house, or through a regional importer. Each has trade-offs in price, lead time and traceability.

For project procurement of more than five tons, mill-direct is almost always the lowest total landed cost, even after ocean freight and inspection, because you remove the trading-house margin and consolidate the shipment with the pipe and fittings on the same vessel.

6. Documentation You Should Always Get With the Shipment

A serious flange manufacturer ships a documentation pack with every heat. As a minimum, request:

• Mill Test Certificate EN 10204 3.1 (chemical + mechanical + impact where required)
• Dimensional inspection report against ASME B16.5 / B16.47 / EN 1092-1
• Hardness report for sour service (NACE MR0175) when applicable
• PMI (positive material identification) report for stainless and alloy flanges
• Hydrostatic or air-under-water test report when the project spec demands it
• Third-party inspection release (SGS, BV, TUV, Lloyd's) when called for in the PO

7. Common Pitfalls in Steel Flange Procurement

After 30 years of exporting flanges, EZ Steel Industrial sees the same five problems on most first-time orders. Avoid them and your next flange PO will land cleanly.

• Mixed pressure class. WN flanges Class 600 on one side and Class 300 on the other — they look identical from the back but won't seal at the same torque.
• Wrong facing. RF on one flange, FF on the other — the joint will leak. Always match the face.
• Material mismatch with the pipe. A105 flange on A106 pipe is fine; a 304 flange on a 316 pipe is not. Specify the same material family.
• Stud bolt length off by 5 mm. A complete joint needs the right stud length plus two full threads of stick-out. Order bolts with the flange, not separately.
• No PMI on stainless orders. Without PMI, an F304 batch can be mixed with F304L. Demand the report.

8. Real Projects EZ Steel Industrial Has Supplied

EZ Steel Industrial's steel flanges are in service across:

• China's South-to-North Water Diversion pipeline (Class 600 WN, A105, 10,000+ flanges)
• China's West-to-East Gas Transmission pipeline (Class 600 / 900, A694 F65, sour service)
• Refinery hydrocracker units (Class 900 / 1500, A182 F11/F22, bundled with fittings)
• Shipbuilding and offshore platforms (90-10 Cu-Ni flanges to EEMUA 145, with Cu-Ni fittings)
• Thermal power plant boiler feed and steam lines (Class 600 / 900, A182 F11/F22)