Every leakage incident on a process line traces back to one of three causes: wrong material, wrong pressure class, or wrong facing. If you are about to place a procurement order for a new plant, an expansion, or a maintenance turnaround, getting pipe flanges right the first time will save you weeks of field rework and thousands in lost production.

This guide walks through the same checklist our engineers at EZ STEEL INDUSTRIAL use when a client sends us a process data sheet. It is built around the most common industrial services — oil and gas, petrochemical, power generation, marine, and seawater cooling — and pulls in the standards we see most often in cross-border procurement packages.

Start With the Service, Not the Catalog

Buyers often begin by scrolling through a flange catalog and trying to match a drawing. That approach almost guarantees a mismatch. Start with three service-level questions instead:

• What fluid is inside the pipe, and at what temperature and pressure?
• Is the line indoors, outdoors, buried, or submerged?
• Is the service corrosive, erosive, or subject to thermal cycling?

Once you have those answers, you can map them to a material family and a pressure class. The same logic applies whether you are ordering pipe flanges alone, or as part of a bundled package that includes gasket stud bolt nut sets and matching butt weld fittings.

Match Material to the Fluid and the Environment

Material selection is the single most expensive decision in a flange order, because a wrong choice shows up as either accelerated corrosion or unnecessary over-spec cost. The four material families used most often in industrial projects are:

1. Carbon Steel — The Workhorse

ASTM A105 forged carbon steel is the default for non-corrosive services such as steam, air, oil, and glycol. It handles temperatures from -29 °C up to 425 °C and is available in pressure classes from 150# to 2500#. If your line is carrying hydrocarbons at moderate temperatures and you are watching budget, carbon steel is almost always the right answer.

2. Low-Temperature Carbon Steel

For services below -29 °C — LNG, refrigerated gas, cold climates — specify ASTM A350 LF2. The nickel content and impact-tested metallurgy prevent brittle fracture at low temperature. Skipping this step is one of the most common audit findings we see.

3. Stainless Steel — For Corrosion and Cleanliness

ASTM A182 F304 and F316 are the standard picks. F316L with low carbon is preferred for welded assemblies to avoid intergranular corrosion. Stainless flanges are typically paired with stainless steel pipe in pharmaceutical, food-grade, and chemical processing lines.

4. Copper-Nickel Alloy — For Seawater

When the service is seawater, firewater, or marine cooling, 90/10 (C70600) and 70/30 (C71500) copper-nickel flanges outperform stainless steel on resistance to chloride pitting and biofouling. They are a common item in shipbuilding, offshore platforms, and coastal power plants.

Choose the Right Pressure Class

Pressure class is not a guess. It is calculated from the design pressure, design temperature, and the allowable stress of the material at that temperature. The two main classification systems you will see in international procurement are:

A frequent error is using a 150# flange on a line that sees 300# cold working pressure. The flange will hold for a while, but the bolts will stretch, the gasket will creep, and you will see a leak within the first year. Always build in a margin of at least 1.5× between the design pressure and the flange rating, or higher if the line is subject to water hammer or pressure surges.

Pick the Flange Type That Fits the Joint

Material and pressure class decide what flange you can use. The joint design decides which one you should use. The four most common types in industrial procurement are:

Weld Neck (WN) — The First Choice for Process Lines

The long tapered hub distributes mechanical stress across the pipe, which makes WN flanges ideal for high-pressure, high-temperature, and cyclic services. They are butt-welded to the pipe and are the most fatigue-resistant option.

Slip-On (SO) — For Low-Pressure and Utility Lines

Easier to align and less expensive than WN, slip-on flanges are common in firewater, cooling water, and utility air. They are not recommended above 300# class or for high-cycle temperature service.

Socket Weld (SW) — For Small-Bore High-Pressure

Used on small-bore lines (NPS 2 and below) where a socket weld gives a strong, leak-tight joint. Often paired with socket weld fittings in steam and chemical systems.

Threaded (TH) — For Non-Welded Maintenance

Threaded flanges are used where welding is not allowed, such as in explosive atmospheres, on lined pipe, or in maintenance-friendly assemblies. They are typically limited to 300# class and lower.

Blind (BL) — For Vessel and End-of-Line Closures

Blind flanges seal off the end of a line, a vessel nozzle, or a header. They are sized to the same pressure class as the mating flange and are tested at 1.5× the design pressure during hydrotest.

ｓｅｌｅｃｔ the Facing and the Gasket Together

Flange facing and gasket selection cannot be separated. The most common facings are:

• RF (Raised Face) — Default for ASME B16.5 classes 150# to 600#.
• FF (Flat Face) — Used with cast iron and non-metallic flanges where the gasket sits flat.
• RTJ (Ring Type Joint) — For high-pressure classes 900# and above; metal ring gasket sits in machined grooves.
• Tongue and Groove / Male and Female — For self-aligning, high-integrity joints in chemical and petrochemical service.

Match the gasket to the service: spiral wound for hydrocarbon and steam, compressed non-asbestos for water, ring joint for high-pressure hydrocarbons, and PTFE envelope for chemical compatibility. Order the gaskets, stud bolts, and nuts together with the flanges so the metallurgy, hardness, and trace marks all line up on the receiving inspection.

Build the Procurement Package

A complete flange order should include the following in one document, not scattered across emails:

• Item number, service description, and line number from the P&ID.
• Size (NPS), pressure class, facing type, and material grade.
• Quantity per item, with spares and weldolet blanks listed separately.
• Required standards (ASME B16.5, EN 1092-1, GOST, JIS) and third-party inspection scope (EN 10204 3.1 / 3.2).
• Surface finish, traceability, and marking requirements.

This is also the moment to align your flange order with the rest of the piping package. Cross-border projects often bundle flanges together with the line pipe, the fittings, and the bolting so they ship together and arrive in matching heat numbers. The fewer separate shipments you manage, the lower the risk of a missing gasket or wrong-class stud bolt on site.

Common Pitfalls We See Every Month

After 30 years of cross-border flange supply, these are the issues that show up most often during receiving inspection or during commissioning:

• Mixing pressure classes on the same line because the P&ID was revised late in the project.
• Carbon steel flanges on stainless lines, or vice versa, after a sub-supplier swap.
• Missing traceability — no heat number, no MTC, no third-party certificate when the spec required 3.2.
• Bolting mismatch — B7 studs with B8 nuts, or studs that are too short for the gasket thickness.
• Wrong facing — RF on a system that should have been RTJ, or FF on a high-pressure line.

Each of these is cheap to fix on paper and expensive to fix on site. A 15-minute cross-check before the purchase order is released will save the project several days of field rework.

When You Need a Bundled Solution

If your project involves more than just flanges — line pipe, fittings, gaskets, stud bolts, and valves all on the same purchase order — it is worth asking your supplier to deliver the full package as a single shipment with unified documentation. At EZ STEEL INDUSTRIAL we routinely supply pipe flanges, carbon steel pipe, stainless pipe, butt weld fittings, and gaskets under one mill order, with one MTC bundle and one inspection plan.

Send us your data sheet, your piping class, or even just a list of line numbers, and our engineering team will return a material and class recommendation, a price quotation, and a delivery plan — usually within two business days.