Start With Service Conditions, Not With the Drawing

Every project RFQ we receive lists a flange type, a standard and a size. That is the easy part. The information that actually decides whether a flange survives its first year of service sits further up the page: the fluid, the temperature, the pressure cycle, and whether the line will be opened for inspection. We ask for those four data points before we suggest a type or a grade, because the same nominal size can be supplied as a flat plate slip-on for utility water at ambient temperature, or as a forged weld-neck with a corrosion allowance for sour hydrocarbon service.

For a typical industrial pipeline, three numbers drive most of the design choice:

• Design pressure and temperature, including any upset or blowdown case
• Fluid — water, steam, hydrocarbons, seawater, chemicals — and its corrosivity
• Mechanical load — vibration, thermal cycling, external forces from connected equipment

Once those are clear, the standard (ASME B16.5, ASME B16.47, EN 1092-1, JIS B2220, GOST, etc.), the pressure class, the material grade and the facing finish are largely fixed. The flange type then becomes a choice driven by access, welding procedure and how often the joint will be broken.

The Five Steel Flange Types We Supply Most Often

Weld Neck (WN) — for high pressure and critical service

A weld neck flange has a long tapered hub that is butt-welded to the pipe. Stress transfers smoothly from the flange to the pipe wall, which is why it is the default for high-pressure steam, hydrocracker feed lines and boiler headers. We supply WN flanges in ASTM A105, A350 LF2, A182 F11/F22/F91, A182 F304/F316 and duplex grades, machined to ASME B16.5 from 1/2" to 24" and to ASME B16.47 Series A & B for larger sizes.

Slip-On (SO) — for low-pressure, cost-sensitive lines

Slip-on flanges slide over the pipe and are fillet-welded both inside and outside. They are easier to align on site and cheaper to produce, which is why they dominate in fire water, cooling water, compressed air and low-pressure process piping. We typically recommend them only up to Class 300 and for non-cyclic service, because the fillet weld is the weak link in a fatigue environment.

Blind (BL) — for termination and testing

Blind flanges close pipe ends, vessel nozzles and valve bodies. They are also used to isolate sections for hydrotest. We supply RF and RTJ blind flanges in matching pressure classes, and we keep the bolt-hole pattern aligned with the adjacent line flanges so the test head drops in without re-drilling.

Lap Joint (LJ) and Threaded (TH) — for systems that need to be opened

Lap joint flanges pair with a stub end and can rotate around the pipe, which makes bolt alignment much easier on large-diameter stainless lines. Threaded flanges are used where welding is undesirable — for example, in stainless lines that must not be heat-tinted, or in field repair work on painted carbon-steel systems. Both are typically limited to Class 300 and below.

Socket Weld (SW) — for small-bore, high-pressure instrumentation

Socket weld flanges are common in 1/2" to 2" instrument and utility lines where a smooth bore and a strong joint are needed. They are not recommended for corrosive service, because crevice corrosion can build up in the socket.

Material Is the Decision Above the Type

Buyers often fix the type and standard first, then ask for material recommendations. We work the other way around. For each inquiry, the material grade is set by corrosion allowance, temperature and any code requirement (ASME B31.1, B31.3, B31.4, etc.) before we look at type. The table below summarizes how we usually match service to grade when the standard is ASME B16.5.

For seawater service in particular, we steer customers to copper nickel alloy flanges and pipe instead of stainless, because 90/10 and 70/30 Cu-Ni tolerate biofouling and low-velocity flow far better than 316L. We cover this in detail in our copper-nickel seawater service guide.

Pressure Class, Facing and the Joint That Actually Seals

Pressure class (Class 150, 300, 600, 900, 1500, 2500 in ASME; PN 6, 10, 16, 25, 40, 63, 100 in EN 1092-1) sets the flange thickness, the bolt circle, the number of bolts and the facing. It must match the connected pipe and the design pressure of the line — never mix a Class 150 flange with a Class 300 valve on the same joint, because the bolt pattern and the gasket seating will not line up.

Facing is where most field leaks begin:

• Raised Face (RF) is the default for most ASME Class 150/300 hydrocarbon and water lines, paired with a spiral-wound or compressed-fiber gasket.
• Flat Face (FF) is used for cast iron or glass-lined equipment, where an RF could crack the flange.
• Ring Type Joint (RTJ) is required for high-pressure Class 600 and above, and for hydrocarbon service above 600 # where fire-safe sealing is critical.
• Tongue & Groove and Male & Female are niche but still used in heat exchanger and small-bore chemical service.

Finishes matter as well. A stock 125–250 µin Ra (3.2–6.3 µm) RF finish is fine for spiral-wound gaskets. For soft gaskets on flat faces, we recommend 63–125 µin Ra (1.6–3.2 µm) to avoid leakage. We can also supply machined and serrated finishes to the customer's gasket supplier's recommendation.

Why We Bundle Flanges With Fittings and Bolting

It is rare to see a real project RFQ that asks for flanges alone. A pipeline is a system, and the flange, the pipe fittings (elbows, tees, reducers) and the joint assembly (gasket, stud bolt, nut) all have to come from compatible materials, with matching traceability and shipping windows. We have built our business around that reality.

How an EZ STEEL Inquiry Is Usually Resolved

A typical RFQ for steel flanges comes to our export desk with a list of types, sizes, classes and a material callout. We respond within one working day with the following package:

• Confirmed dimensional compliance to the standard (ASME B16.5, EN 1092-1, JIS B2220, GOST 12820/12821)
• Material certificate plan — EN 10204 3.1 for general service, 3.2 for critical service, with TPI optional
• Compatible gaskets, stud bolts and nuts quoted together, with the same lead time
• Mill origin (Changsha, Hunan) and full traceability per heat number, available on request

We hold inventory of common pressure classes (Class 150, 300, 600) in A105, A350 LF2, F304 and F316 so that repeat orders can ship in 2–3 weeks. For non-standard or large-diameter orders, our forging capacity (annual output above 480,000 tons across pipes, fittings, flanges and valves) supports a typical 6–8 week lead time from PO to FOB.

What We Have Learned After 30 Years of Flange Supply

The cheapest flange in a quotation is rarely the cheapest flange in service. The number of leak points in a refinery or a power plant is, in practice, the number of bolted joints. A well-chosen flange grade, a clean facing, a matched stud bolt and a gasket that is qualified for the actual fluid will outlast the line around it. We have seen WN flanges in service for 25 years and we have seen 18-month-old slip-on flanges crack at the fillet weld. The difference was almost always in the upstream choices — material, class, facing and bolting.

If you are starting a new project, or auditing an existing piping package, our engineering team can review your flange schedule against the operating data and recommend a tighter specification. We are not a catalog reseller — we manufacture the pipe, the fittings, the flanges and the valves, and we can adjust grade, schedule and bolt specification to match what the line actually needs.