A pipe flange may look like a simple ring of steel, but in a real project it is one of the most consequential components in the entire piping system. A mis-specified flange can lead to gasket blow-out, bolt failure, leakage across welded joints, and costly on-site rework. A correctly specified flange, by contrast, helps a piping system reach its design life with no unplanned shutdowns.

After more than three decades of supplying flanges, pipes, fittings and valves to refineries, power stations, shipyards and water authorities, EZ Steel Industrial has learned that specifying a flange is less about memorizing standards and more about answering a small number of practical engineering questions. This guide walks through those questions in the order a procurement or engineering team would normally handle them.

1. Start with the Service: What Is Actually Inside the Pipe?

The first decision is not the flange type — it is the service. Service dictates temperature, pressure, corrosion profile and cleanliness requirements, and those four parameters drive almost every downstream choice.

• Hydrocarbon service (oil, gas, condensate): usually demands ASME B16.5 / B16.47 steel flanges in ASTM A105, A350 LF2 or A182 grades, with raised-face (RF) or RTJ sealing.
• Seawater and marine cooling: almost always pushes the specification toward copper nickel flanges (90/10 Cu-Ni or 70/30 Cu-Ni), which resist biofouling and chloride attack far better than carbon steel.
• Boiler feedwater and high-temperature steam: requires alloy flanges (F11, F22, F91) or stainless grades, with the matching tube specified as ASTM A335 or ASTM A213.
• Potable water transmission: is usually handled by AWWA C207 flanges, often with epoxy coating or cement lining for long service life.

2. Match the Flange to the Pipe Itself

A flange and its pipe are a system, not two independent items. Before locking the flange grade, the engineering team should confirm the pipe specification, because the flange material is normally selected to be equal to or higher than the pipe material in the corrosion allowance.

For projects that bundle pipe, fittings and flanges from a single source, this matching problem is handled at the quotation stage, which is one reason why EZ Steel Industrial organizes its production around project packages rather than individual SKUs.

3. Pick the Right Flange Type for the Job

Once the service and the matching pipe are fixed, the next step is the flange facing and geometry. The table below summarizes how the most common flange types are used in industrial practice.

4. Pressure Class, Facing and the Gasket Equation

Pressure class is the easiest number to specify but the easiest to oversimplify. A Class 150 flange is rated for roughly 150 psi at temperatures up to 300 °F in ASME B16.5, but the same flange drops in allowable pressure as service temperature climbs toward 800 °F. Always cross-check the pressure-temperature rating chart for the chosen material, not the headline class number.

The facing — flat face (FF), raised face (RF) or ring-type joint (RTJ) — must be selected together with the gasket. A common on-site failure is a Class 300 RF flange paired with a flat gasket, which never seals properly. For hydrocarbon and steam service, RF with a spiral-wound gasket is the workhorse combination. For seawater Cu-Ni systems, flat face with a nitrile or EPDM gasket is more typical, because RF surfaces can trap water and accelerate crevice corrosion.

5. Where Buyers Lose Money in Real Projects

Across hundreds of project orders, the same specification errors tend to appear again and again. The most common ones are worth flagging:

• Specifying a flange by class alone, without the pressure-temperature chart for the chosen material.
• Mixing imperial bolt-hole patterns with metric pipe, or vice versa, when shipping across regions.
• Choosing standard thickness for cyclic or vibration service, where the bolt load has to be recalculated for fatigue.
• Using carbon steel flanges in seawater service to save cost, then seeing severe pitting within 12–18 months.
• Buying flanges from a supplier that cannot also supply the matching pipe, fittings and gaskets, which leads to a multi-vendor coordination problem on site.

6. How EZ Steel Industrial Sources Flanges as Part of a Project Bundle

EZ Steel Industrial was founded in 1994 and currently operates with 500+ employees and an annual capacity above 480,000 metric tons. Rather than selling a single flange in isolation, the company typically delivers a matched package: pipe, fittings, flanges, gaskets and stud bolts, all released against the same heat numbers and mill test certificates.

This project-bundled approach is particularly relevant for buyers managing multi-discipline procurement. It removes the most common cause of traceable documentation gaps — when the flange MTC, the pipe MTC and the fitting MTC come from three different mills and three different inspection regimes. For the buyer, one set of documents against one project number is materially easier to manage than three.

7. Certifications Worth Asking For

For any flange going into a regulated plant, request the following documentation up front, before purchase order release. It is far more expensive to chase it after the material has shipped.

• Mill Test Certificate (MTC) to EN 10204 3.1, with full chemical and mechanical results.
• Dimensional report against the relevant ASME / EN / GOST / JIS standard.
• Non-destructive examination records (RT, UT, MT, PT) where the service demands them.
• Traceable heat number, marked on the flange body and on the certification.
• For Cu-Ni marine service: confirmation of the Cu:Ni ratio and any required EEMUA 144 / 234 documentation.

8. A Short Checklist Before You Release the PO

Before a flange purchase order is released, a practical specification should answer the following: service fluid, design pressure, design temperature, pipe material and schedule, applicable standard (ASME B16.5, B16.47, EN 1092-1, GOST, JIS, AWWA), flange type and facing, gasket and bolt material, and required documentation. Any item left as TBD is a future RFQ — and future RFQs almost always cost more than the original specification effort.