A flange is rarely the most expensive line item on an industrial bill of materials, but a wrong specification can stop a refinery, a desalination train, or a chemical plant in its tracks. After three decades of supplying pipe flanges to projects across petrochemical, marine, power, and pipeline works, the EZ Steel Industrial engineering team has seen the same handful of mistakes repeat themselves: the carbon steel weld neck on a sour service line, the flat-face gasket on a Class 600 steam joint, the 90/10 Cu-Ni flange ordered to the wrong EEMUA clause.

This guide walks through how a procurement engineer should specify a flange based on the service environment first, the material second, and the paperwork—mill test, marking, NDT—last. It draws on the same logic we apply when our customers send us a datasheet for review.

1. Start with the Service Environment, Not the Schedule

Before anyone quotes a flange, four questions have to be answered on the datasheet. If any of them is missing, treat the enquiry as incomplete and ask the project engineer to fill it in. The cost of getting it wrong is always higher than the cost of one extra email.

• What is the fluid? Hydrocarbons, seawater, steam, acids, alkalis, cryogenic LNG, or a mixed-phase slurry. Each fluid has a different corrosion and erosion mechanism.
• What is the design temperature and pressure? These set the pressure class (150# up to 2500# for ASME B16.5, or PN 6 to PN 100 for EN 1092-1) and the material grade.
• Is the service sour (NACE)? Anything above the NACE MR0175 / ISO 15156 limits for H₂S partial pressure requires restricted-grade materials and NACE-certified steel flanges.
• How often will the joint be broken? Frequent maintenance favors lap joint with a stub end; a permanent high-pressure line favors weld neck.

2. Match the Flange Type to the Job

The seven common flange types in ASME B16.5 are not interchangeable. Each was developed for a specific stress, alignment, and maintenance profile. When in doubt, use weld neck for the process line, slip-on for utility and low-pressure water, and blind for the end-of-line.

Weld Neck (WN) – the default for process piping

The tapered hub absorbs butt-weld stresses and the bore matches the pipe ID, which eliminates turbulence. Use WN for Class 300 and above, for thermal cycling service, and for any line scheduled for in-service weld repair. In a refinery hydrocracker or a high-pressure steam line, weld neck is non-negotiable.

Slip-On (SO) – economical for low pressure

Slip-on flanges slide over the pipe and are fillet-welded both inside and outside. They are about 30% cheaper than weld neck and easier to align, but the joint is weaker. Reserve SO for Class 150 firewater, cooling water, and compressed air. Do not use SO on cyclic temperature service.

Blind, Lap Joint, Threaded, Socket Weld, Orifice

Blind flanges terminate a line for hydrotest or future tie-in. Lap joint flanges pair with a stub end and are used where the system is regularly dismantled—common in chemical plants with lined pipe. Threaded flanges belong on low-pressure utility lines only. Socket weld flanges are limited to small-bore (≤ NPS 2) high-pressure instrumentation. Orifice flanges carry the pressure taps for flow measurement.

3. Material Selection: Carbon, Stainless, Alloy, or Copper-Nickel

Once the service envelope is fixed, the material table usually picks itself. The four families below cover more than 95% of industrial flanges.

Material Family	Typical Grades	Best Fit For
Carbon Steel	ASTM A105, A350 LF2, A516	General hydrocarbon service, -29°C to 425°C, moderate pressure. The default for pipeline and refinery headers.
Low-Temp Carbon Steel	A350 LF2, LF3	Refrigeration, LNG vapor lines, ammonia plants with impact-tested sub-zero service.
Stainless Steel	A182 F304/F304L, F316/F316L, F321, F347	Chemical, food, pharma, and any chloride-bearing environment below 400°C.
Alloy Steel	A182 F11, F22, F91	High-temperature steam, syngas, and hydrocracker loops up to 650°C.
Duplex / Super Duplex	A182 F51, F53, F55	Offshore topside, seawater with high chlorides, sour service. Excellent pitting resistance.
Copper-Nickel	C70600 (90/10), C71500 (70/30)	Seawater cooling lines, shipbuilding, desalination intake and outfall. Specified per EEMUA 234 and ASME B16.24.
Nickel & High-Nickel Alloys	Monel 400 (N04400), Inconel 625 (N06625), Hastelloy C276	Sour service, hot acid, and aerospace or nuclear safety-class lines.

4. Copper-Nickel Flanges: Where the Specification Gets Subtle

Marine engineers often order copper nickel flanges by simply writing "Cu-Ni 90/10, ASME B16.5, Class 150" and assume any supplier can deliver. In practice, three clauses decide whether the flange will survive five years or fifteen.

Pick 90/10 or 70/30 deliberately

C70600 (90/10) is the marine workhorse—good corrosion resistance in seawater, easier to weld, lower cost. C71500 (70/30) carries more nickel and is reserved for higher-velocity seawater (above 3.5 m/s), polluted harbor water, or hotter service. If the datasheet does not specify velocity and temperature, ask before quoting.

Lock down the standard

ASME B16.24 covers cast copper-alloy flanges. EEMUA 234 covers seamless Cu-Ni pipe and fittings but is widely accepted for flanges in offshore projects. MIL-F-17191 is required for some naval applications. A mismatch between flange standard and pipe standard is one of the top causes of dimensional rejection at site.

Watch the facing

Copper-nickel is soft. Raised face (RF) finish on Cu-Ni must be carefully machined to avoid galling, and spiral-wound gaskets with graphite or PTFE filler are preferred over compressed non-asbestos. Ring-type joint (RTJ) is generally avoided on Cu-Ni flanges because the hardness mismatch with carbon steel RTJ rings accelerates groove deformation.

5. Bolted Joint Assembly: Where Flanges Actually Fail

More flange leaks start at the bolt-up than at the flange itself. A few field rules that we share with every customer receiving a delivery of gasket stud bolt nut assemblies from our mill.

• Match the bolt material to the flange. A193 B7 studs with A194 2H nuts is the carbon steel default. For stainless flanges above 200°C, A193 B8 (304) or B8M (316) is appropriate, but never mix galvanically.
• Use the correct gasket for the service. Spiral-wound for Class 300 and above, flexible graphite for steam, PTFE for chemical, RTJ for high-pressure hydrocarbons. Never re-use a spiral-wound gasket.
• Torque in a star pattern, in at least three passes. First pass to 30% of target, second to 60%, final to 100%, then a fourth circular check 24 hours after hot commissioning.
• Re-torque after the first thermal cycle. Differential thermal expansion between the flange and the bolts will relax the joint on the first heat-up. A re-torque pass at operating temperature is a standard practice in power plants.

6. Documentation that Survives an Audit

For an EPC or refinery project, the flange arrives with paperwork that often matters as much as the flange itself. A complete mill package for a Class 600 sour-service weld neck should include:

• Mill Test Certificate (MTC) per EN 10204 3.1, with full chemistry, tensile, hardness, and impact data.
• Positive Material Identification (PMI) report, 100% on stainless and high-nickel alloys.
• NDT reports: UT or RT for thick flanges, MT or PT for surface defects, Charpy impact at the specified test temperature.
• Hydrostatic test certificate if the flange is to be used in forged high-pressure service.
• Heat treatment records, including soak time and temperature.
• Traceable marking on every flange: manufacturer, material, size, class, heat number, and standard.

Skipping any of these on a small-bore utility line may be forgiven. Skipping them on a Class 600 sour-service flange will stop the shipment at the warehouse.

7. Bundling Flanges with the Rest of the Piping Package

A flange does not arrive on site alone. It is bolted to a butt weld fittings elbow, welded to a piece of pipe, and connected to a valve. The most expensive line-item mismatch on a project is not the flange, but the dimensional mismatch between flange, fitting, pipe, and valve. When you source from a single integrated mill that produces pipe, fittings, flanges, and gaskets under one quality plan, you remove the dimensional and documentation risk that comes with juggling four different suppliers in three different time zones.

EZ Steel Industrial has been producing pipe, fittings, flanges, valves, and gaskets in Changsha, China, since 1994, with an annual capacity above 480,000 metric tons. Every flange we ship is matched to a corresponding pipe, fitting, or valve on the same MTC line, with the same heat number traceability, and tested in our ISO 9001-accredited lab. That is how an EPC contractor avoids the late-night "why doesn't the bolt circle line up" phone call.