Why Steel Flanges Still Anchor Industrial Piping

Even with the rise of welded and threaded systems, pipe flanges remain the standard way to join, isolate and inspect industrial pipework. They allow valves to be removed, gaskets to be replaced and sections to be tested — all without cutting the line open. In refineries, power plants, desalination trains and offshore platforms, the bolted flanged joint is the joint you trust when downtime costs thousands of dollars an hour.

The catch is that a flange is not a generic commodity. Type, material, facing and bolting all have to be specified together, and every choice interacts with the others. EZ Steel Industrial, operating out of Changsha, China since 1994, supplies more than 480,000 tonnes a year of pipes, fittings, flanges, valves and bolting under API, EN, ASME and ISO standards. Three decades of mill work mean that the questions below are exactly the questions a good supplier should be helping you answer.

Step 1: Choose the Flange Type for the Service Duty

Flange type is dictated first by pressure, temperature and whether the joint will be taken apart often. Most steel flanges fall into a handful of well-defined structural families.

Slip-On (SO) Plate and Hubbed Flanges

A flat or hubbed ring that slides over the pipe and is fillet-welded on the outside and inside. SO plate flanges are economical for low-pressure water, air and fire-fighting lines, while SO hubbed flanges add extra bore support for mildly elevated service. EZ Steel stocks both EN 1092-1 Type 01/12 plate and hubbed slip-on flanges in carbon, stainless and alloy steel.

Weld Neck (WN) Flanges

The long tapered hub is butt-welded to the pipe, so stress flows smoothly from flange to pipe. Weld neck flanges are the default for high-pressure, high-temperature and cyclic service — boiler feedwater, superheater outlets, hydrocracker reactors and most ASME B16.5 Class 600 and above. They are also the only type allowed in many nuclear and offshore codes.

Blind Flanges

Solid discs used to close a pipe end, vessel nozzle or valve body. They double as test blanks during hydrostatic and pneumatic pressure testing. In a refinery, blind flanges are the most heavily inspected item on a piping isometric.

Lap Joint and Threaded Flanges

Lap joint flanges mate with a stub end, so the flange can rotate for bolt alignment — useful in systems that are frequently dismantled, such as non-corrosive chemical lines. Threaded flanges carry tapered NPT or BSPT threads and are reserved for low-pressure, non-flammable service where welding is undesirable, such as utility water and instrument air.

Step 2: Match Material to the Fluid and the Environment

The flange material is the joint’s first line of defence. EZ Steel groups its steel flange portfolio to match the parent piping material, so a single mill can supply both the pipe and the matching pipe flanges on the same heat.

For marine and ship-building service, copper-nickel is often the most economical option because it eliminates additional cathodic protection on seawater lines. EZ Steel’s 90/10 and 70/30 Cu-Ni flange range shares the same metallurgical history as its Cu-Ni pipe, eliminating galvanic mismatch.

Step 3: Lock Down the Facing and the Bolted Joint

A flange is only as good as the gasket that seals it. Once the type and material are fixed, the buyer must specify the facing finish and match it to the correct gasket and stud bolt assembly.

• Raised Face (RF): the default for ASME B16.5 and most refinery service. A 1.6 mm (Ra 125–250 µin) spiral-serated finish with a soft or semi-metallic gasket.
• Flat Face (FF): used with cast iron or non-metallic flanges and full-face gaskets; common in water and HVAC.
• Ring Type Joint (RTJ): machined metal groove for ring gaskets; mandatory for high-pressure Class 600 and above, and standard in oil & gas block valves.
• Tongue and Groove / Male & Female: self-aligning facings for heat exchangers and process vessels where gasket blow-out is a concern.

The stud bolt and nut kit is not a consumable — it is part of the pressure boundary. Gasket stud bolt nut assemblies should be ordered to ASTM A193 B7/B7M, B16, B8 or A320 L7 depending on the service temperature, and matched to the gasket hardness. EZ Steel’s gasket line includes spiral-wound, ring-joint, compressed non-asbestos and graphite, allowing the mill to ship a flange, a gasket and the correct bolting as one coordinated package.

Step 4: Bundle Flanges With the Rest of the Joint

The most common cause of a flange joint failing in service is not the flange itself — it is mismatched components. A weld neck flange on an A106 pipe needs a matching A105 or A234 WPB pipe fittings, an ASME B16.5 stud bolt of the right length, and a gasket rated for the design pressure and temperature. Buying each of these from a different source multiplies paperwork, lead time and risk.

A bundled procurement approach — pipe, fittings, flanges, gaskets and bolting from a single mill under one MTR set — is the cleanest way to control quality on refinery, desalination and power projects. EZ Steel Industrial has supplied bundled packages to projects including China’s West-East Gas Pipeline, the South-to-North Water Diversion scheme, and shipbuilding yards in South Korea and Singapore, with full ISO 9001 mill certification and third-party inspection on request.

A Buyer’s Checklist Before Sending the RFQ

• Standard and pressure class: ASME B16.5 / B16.47, EN 1092-1, GOST, JIS B2220, BS 4504.
• Type: SO plate, SO hubbed, WN, blind, lap joint, threaded, RTJ.
• Material grade and heat treatment condition.
• Size, bore (standard or pipe schedule), facing finish.
• Compatible gasket type and matched stud bolt/nut specification.
• Coating or finish: black, hot-dip galvanised, anti-rust oil, paint system.
• Documentation: MTR, hydrostatic test, NDT, third-party inspection certificate.

Send this list with your RFQ and the right steel flanges — matched to the right pipe, fittings and bolting — will arrive on the same vessel.