Most flange RFQs start with a 2D drawing, then drift into material questions, then facing questions, then a "by the way, can it be RTJ?" conversation that pushes the quotation two days past the bid date. The drawing is not the problem — the missing decision sequence is. For a 30-year manufacturer like EZ Steel Industrial, the fastest quotations always come from buyers who lock the structural type first, the material grade second, and the pressure-temperature envelope third.

This guide rebuilds that decision sequence for steel flanges used in process piping, marine cooling, and high-pressure power service — with practical tables and mill-floor recommendations drawn from real RFQs we have processed for buyers in 40+ countries.

1. Start with the structure, not the standard

The first fork in the road is the structure. Each type was designed around a specific piping problem — alignment, dismantling frequency, pressure class, cost ceiling — and choosing wrong here is what causes downstream rework. The seven structures that cover roughly 95% of industrial RFQs are:

• Weld Neck (WN / WNRF): butt-welded to the pipe, hub transfers stress into the pipe wall. The default choice above CL600 and for thermal cycling service.
• Slip-On (SO / SORF): fillet-welded outside and inside, easier to align, lower cost. The workhorse for CL150 / CL300 water, air, and low-pressure chemical service.
• Blind (BL / BLRF): solid disc to close line ends, vessel heads, or for hydrostatic testing. Always specify RF/FF/RTJ facing explicitly.
• Socket Weld (SW): pipe drops into a socket, single fillet weld. Used on small-bore (≤ NPS 2) high-pressure lines where crevice corrosion is managed by design.
• Threaded (TH / NPT): tapered thread engagement, no welding. Common in utility steam, instrument air, and plant air headers.
• Lap Joint (LJ): backing flange + stub end, allows bolt-hole rotation. The right call when the line is dismantled frequently for cleaning.
• Long Weld Neck (LWN): extended hub for vessel and column nozzles, often engineered to ASME B16.5 / B16.47 dimensions.

Notice that the structure tells you how the flange will be welded, aligned, and serviced — but it does not tell you what alloy to buy. That is a separate decision driven by the process fluid, temperature, and corrosion exposure.

2. The material question: carbon, stainless, or alloy

Once the structure is locked, the next conversation is metallurgy. A buyer who writes "WN, CL150, RF" on an RFQ without a material callout usually gets a carbon-steel quote by default — which is fine for water, steam, and air, but a maintenance risk for anything chloride-laden, sour, or above 400 °C. Below is the decision matrix our mill uses for internal project reviews.

A practical rule of thumb: if the line carries steam above 425 °C, hydrocarbon above 250 °C, or anything with chloride content, default the RFQ to F316L or F11/F22, and bring the mill into the conversation early. For a broader view of the stainless steel pipe system those flanges will be welded into, the mill cross-reference table in our catalog is the next stop.

3. Pressure class, facing, and the standard you actually need

Structure and material answer "how and of what"; the pressure class answers "how hard it can be pushed." The common global systems are:

• ASME B16.5 / B16.47 Class series: CL150, CL300, CL600, CL900, CL1500, CL2500 — used in oil & gas, power, and EPC projects worldwide.
• EN 1092-1 PN series: PN6, PN10, PN16, PN25, PN40, PN63, PN100 — the EU/ISO default for water, HVAC, and chemical plants.
• JIS B2220: 5K, 10K, 16K, 20K, 30K, 40K — domestic Japanese projects and Korean shipyards.
• GOST 33259 (Russia / CIS): PN1.6, PN2.5, PN4.0, etc., with Russian pressure-temperature tables.

Within a class, the facing type drives the gasket and bolt-up procedure. RF (Raised Face) is the default for ASME; FF (Flat Face) is mandatory when bolting to a flat-faced cast iron or fiberglass flange; RTJ (Ring Type Joint) is the right answer for CL600 and above hydrocarbon service. Mixing facing types is one of the most common errors we see on corrective RFQs — buyers ask for a CL600 WN with FF because the mating line is FF, then discover the pressure-temperature table downrates the joint.

4. Cross-reference your pipe schedule and the bolted joint

A flange is only as good as the joint it forms. Three spec lines must match the moment the flange leaves the quote:

1. Bore matches pipe schedule. The flange bore equals the pipe OD for CL150/300 WN, and the pipe ID for CL600 and above weld-neck flanges. A mismatch creates a step in the flow path and a corrosion pocket.
2. Bolt hole pattern matches the mating flange. PCD, number of holes, and hole diameter must be checked against the standard — not against the previous flange in the yard. This is a one-minute check that prevents three days of rework.
3. Gasket, stud bolt, and nut must travel together. A complete RFQ lists the flange, the gasket style (spiral wound, RTJ, compressed non-asbestos), and the stud bolt / nut grade (B7 / 2H, B8 / 8, B16, etc.). Our bundled approach treats gasket stud bolt nut and pipe flanges as one traceability chain, which is the same logic EPC clients use on refinery and LNG packages.

5. Three case studies from the mill floor

Case A — Seawater cooling for a 2×600 MW coastal power plant

The buyer originally specified WNRF CL150 in A105 with RF facing for the cooling water return headers. Service was once-through seawater at 32 °C, 4 m/s, with intermittent chlorination. The mill pre-sales team flagged two issues: (a) A105 RF facing would pit in chlorinated seawater within 24 months; (b) the PCD matched EN 1092-1 PN16, not ASME CL150. The corrected RFQ moved to Cu-Ni 90/10 backing flanges with B151 stub ends in CL150, EEMUA 234 dimensions, plus rubber gaskets. Service life projection extended from < 3 years to > 25 years, and bolt-up was simplified because the entire train ran on the same PCD.

Case B — Refinery hydrocracker feed line, 320 °C, 11 MPa

The original specification called for A105 WN flanges. Service temperature was above the A105 limit, and the hydrogen partial pressure demanded a Cr-Mo grade to avoid high-temperature hydrogen attack. The mill re-issued the quotation with ASTM A182 F22 CL600 WN with RTJ facing, paired with F22/F11 butt-weld fittings, B16 stud bolts, and 316SS ring joint gaskets. Pre-heat and PWHT were noted in the MTC and on the dispatch documents, so site QA closed the file in a single review.

Case C — Cross-country gas transmission, 8.0 MPa, ambient

A 36-inch gas line running across mixed climate. The line-pipe spec was API 5L X65 PSL2, and the original drawing called for ASME B16.47 Series A CL300 flanges in A105. Two issues: (a) the matching pipeline works valve and fitting were in A234 WPB, which is acceptable; (b) low-temperature impact at -30 °C required A350 LF2 CL1 instead of A105. The mill re-issued the quote with A350 LF2 CL1, full Charpy impact test at -46 °C, and the same facing. Total material cost up 6%, but the line passed the hydraulic test in a single attempt and avoided a winter shutdown on the third valve station.

6. Mill-direct checklist before you send the RFQ

Run this 10-line list before issuing the inquiry. Anything missing should be filled in by the mill pre-sales team before the offer is released — a 15-minute call at this stage is worth a 15-day RFQ cycle later.

• Flange structure (WN / SO / BL / SW / TH / LJ / LWN)
• Standard + pressure class (ASME B16.5 CL? / B16.47 Series A or B / EN 1092-1 PN? / JIS B2220 ?K / GOST 33259 PN?)
• Size (NPS / DN) and bore (to match pipe schedule)
• Material grade (A105 / A350 LF2 / A182 F304/F316/F11/F22/F51 etc.)
• Facing (RF / FF / RTJ) and facing finish (stock finish, 125-250 AARH, 63 AARH for RTJ)
• Gasket, stud bolt, and nut grade — bundled or split
• Inspection document: EN 10204 3.1 / 3.2, NACE MR0175, PMI, UT, MT, PT as applicable
• Coating: bare / hot-dip galvanized / painted / FBE / PTFE
• Packaging: bare / wooden crate / seaworthy for ocean freight
• Delivery basis: FOB / CFR / CIF and target port

7. Why a mill-direct partner changes the conversation

Trading houses can move a flange on the same day the PO arrives. Mills move the entire package — pipe, fittings, flanges, gaskets, stud bolts, and valves — under one heat number, one MTC chain, one production lot. For an EPC or plant owner, that consolidation is the difference between a four-week RFQ cycle and a six-week RFQ cycle, and it removes the failure mode of "Mismatched MTC, package rejected at site."

At EZ Steel Industrial, the flange line is produced alongside the carbon, stainless, alloy, and copper-nickel pipe programs, the pipe fittings program, and the industrial valve program. The 480,000+ ton annual capacity, the API 5L / EN / ASME certifications, and the in-house ISO 9001 lab let one mill heat trace an entire skid — from steel flanges to industrial valves — back to the same steel pour. That is the only way to ship a fully bundled, fully traceable piping package on a tight EPC schedule.