Selecting a pipe flange is rarely a one-line decision. The wrong facing type, material grade, or pressure class will show up within months as leakage, fatigue cracking, or unplanned shutdowns — and the cost of a re-do almost always exceeds the savings of buying cheaper. This guide walks through the four questions every specifier should answer before placing an order: what fluid, what pressure and temperature, what standard, and what inspection regime.

1. Match the Flange Material to the Service Fluid

Material choice drives both safety and total cost. The chart below is a starting point, not a substitute for a corrosion review on your specific chemistry.

For marine and offshore systems, the choice is almost always copper nickel alloy flanges: 90/10 for ship cooling loops and 70/30 for hotter or more aggressive brine service. The natural biofilm that forms on Cu-Ni surfaces also reduces macrofouling, which means lower maintenance on long sea voyages.

2. Pick the Right Flange Type for the Pressure and Temperature

Weld Neck (WN)

The long tapered hub reduces stress concentration at the pipe-to-flange transition, making WN the default for high-pressure and high-temperature service. Always specify WN for ASME Class 600 and above.

Slip-On (SO) and Socket Weld (SW)

SO flanges are easier to align in the field and about 30% cheaper to manufacture, but they only offer about two-thirds the fatigue life of a WN. SW is preferred on small-bore (NPS 2 and below) high-pressure systems where a fillet weld inside the socket is acceptable.

Blind, Threaded, Lap Joint, and Orifice

Blind flanges close off a line for testing or future tie-ins. Threaded flanges suit low-pressure, non-critical utility lines. Lap joint flanges pair with a stub end, which is convenient for systems that need frequent disassembly. Orifice flanges carry the taps and pressure-rings required for flow measurement.

3. Choose the Correct Facing and Pressure Class

The facing determines how the flange seals. Pair the facing with the right gasket and you avoid the most common cause of in-service leakage — gasket crushing or extrusion.

• Flat Face (FF) — low pressure, soft gaskets, usually cast iron flanges.
• Raised Face (RF) — the most common; pairs with spiral-wound or compressed non-asbestos gaskets.
• Ring-Type Joint (RTJ) — metal-to-metal seal for high-pressure hydrocarbon service (Class 600+).
• Tongue & Groove / Male & Female (T&G, M&F) — self-aligning; used in heat exchangers and small-bore chemical service.

Pressure class is governed by the applicable standard. ASME B16.5 covers NPS ½ through 24 up to Class 2500; ASME B16.47 Series A & B cover NPS 26 through 60. For wellhead and Christmas-tree equipment, API 6A is mandatory. European projects typically call EN 1092-1, with PN ratings that map roughly to the ASME Class system.

4. Don't Skip the Mandatory Inspection Regime

A flange that passes dimensional inspection can still fail in service if internal defects are present. The following tests are non-negotiable for pressure-containing flanges:

• Positive Material Identification (PMI) via OES or XRF to confirm alloy content.
• Tensile, hardness, and Charpy impact testing per ASTM A370 / ISO 6892.
• Dye penetrant (PT) and magnetic particle (MT) for surface and near-surface defects on carbon and alloy steel.
• Ultrasonic (UT) or radiographic (RT) for thick flanges, duplex stainless, and high-class ratings.
• Hydrostatic test at 1.5× design pressure for forged flanges in critical service.

Marking and traceability close the loop: each flange should be permanently stamped with manufacturer, material grade, size, pressure class, and heat number so a mill test certificate can be traced back to the original heat.

5. Why Specifiers Choose EZ STEEL INDUSTRIAL

Founded in 1994 in Changsha, China, EZ STEEL INDUSTRIAL is a full-cycle pipe products manufacturer with more than 500 specialists and annual capacity above 480,000 metric tons. Every step — from raw material inspection to final marking — happens under one roof, with an ISO 9001-certified lab and approvals to API, EN, and ASME specifications.

Our flanges are integrated with the rest of our pipework portfolio: carbon steel pipe, stainless steel pipe, and copper nickel alloy tubes, plus pipe fittings and heat efficiency tubes — so you can source a full piping package from one supplier rather than coordinating three. Our piping has been specified into landmark projects including the South-to-North Water Diversion, the West-East Gas Pipeline, petrochemical refineries, and modern marine fleets.