Pipe Flange Selection and Sourcing Guide: Types, Standards, and Mill-Direct Procurement

What a pipe flange actually does

A pipe flange is a forged or plate-cut ring with a hub, bolt holes and a machined sealing face, joined to a pipe end by welding, threading or a lap. Two flanges, a gasket and a set of stud bolts form a bolted joint. The flange carries the bolt preload, holds the gasket under the design seating stress, transfers pipe end loads into the vessel or header, and lets the joint be opened for inspection, cleaning or repair. In other words, the flange is the mechanical interface that turns loose pipe, fittings and equipment into a pressure-tight, maintainable system.

Because the flange is the joint's structural element, three numbers matter before any standard is named:

Once those are fixed, the structural family, the standard, the pressure class and the material can be selected. A mill-direct supplier that ships flanges in carbon & carbon alloy steel, stainless steel and copper & nickel alloy can hold a single RFQ to a single project schedule, which is why most EPC buyers stop listing flange as a one-line item and start treating it as a packaged supply item.

The seven structural families you will actually quote

A buyer's first decision is structural, because the standard, the price and the lead time all branch from it. The seven families below cover the vast majority of project flanges, and the eighth (custom) catches everything that does not fit.

Weld Neck (WN / WNRF)

A long tapered hub is butt-welded to the pipe. Stress flows smoothly from the flange to the pipe wall, so WN is the default for high pressure, high temperature, cyclic service and large sizes (NPS 6 and above). It is also the only ASME B16.5 family that is recommended for severe service such as hydrocracker reactors, ethylene furnaces and main steam lines.

Slip-On (SO / SORF)

The pipe slides into the bore and is fillet-welded on the outside and (optionally) inside. SO is faster to fit, easier to align and cheaper, so it dominates low-pressure firewater, cooling water, instrument air, utility and HVAC services. It is not the right choice for fatigue, high temperature or any service where a butt-welded joint is mandated by the piping class.

Blind (BL)

A solid disc with bolt holes, used to close a pipe end, a header, a manway or a pressure vessel nozzle. It is also used for hydrostatic and pneumatic testing. RFQ items must always include facing (RF / FF / RTJ), pressure class, material and any NDE requirement, because blinds carry the full design pressure on their own.

Socket Weld (SW)

The pipe sits in a socket and is fillet-welded around the hub. SW is used for small-bore (NPS 1/2 to NPS 2) high-pressure instrumentation and process lines, where a smooth bore and good alignment matter more than the cost saving of a threaded joint. It is not used where crevice corrosion or fatigue is a concern.

Threaded (TH / SCRD)

Internal tapered threads (NPT, BSPT, BSPP) attach the flange to the pipe without welding. TH is the right answer for non-flammable, non-toxic, low-pressure utility lines, for shop-fabricated skids that cannot be welded in the field, and for repair work in zones with hot-work permits. Threaded flanges are explicitly not used for severe service, and ASME B31.3 places them below 150# in the typical process plant piping class.

Lap Joint (LJ) with Stub End

A loose backing flange rotates around a butt-welded stub end. The stub end does the welding, the backing flange does the bolting. This gives full bolt-hole alignment freedom, which is why LJ is the standard for systems lined with rubber, PTFE or glass, for stainless pipe where the flange is plain carbon, and for systems that are dismantled often.

Loose Plate / Lapped Plate

A flat plate that bolts to a backing ring or a lapped stub. It is mostly used in waterworks, low-pressure drainage, and EN 1092-1 Type 02 / Type 04 plate flanges where cost dominates the spec. The trade-off is no hub, so plate flanges are not used for high pressure, high temperature or cyclic service.

Custom / Special

Anything outside the published standard tables. Long weld necks, oval manway flanges, anchor flanges, paddle spacers, spectacle blinds, spacer rings and orifice plates all fall here. These are quoted by drawing, with the same set of standard fields: material, bore, OD, thickness, drilling, facing, coating, inspection document and packing.

Standard map: which document governs which flange

The same physical flange can ship under five different standards depending on where the plant is built. A buyer's first call is to fix the standard; the rest of the RFQ (pressure class, material, facing) is then read directly from the standard's own tables.

Two practical points often missed in first-time RFQs:

Pressure class and pressure–temperature rating

Pressure class is not the same as design pressure. The class is a designation that maps to a table of allowable working pressures at temperatures defined in the standard. Class 150 ASME, PN 16 EN and 16K JIS are not the same number; they are different rating tables that, for carbon steel at room temperature, happen to land in the same ballpark.

The standard EN 1092-1 PN ladder

PN 2.5 / 6 / 10 / 16 / 25 / 40 / 63 / 100 / 160 / 250 / 400 covers low-pressure utility, building services, waterworks and chemical lines all the way up to high-pressure chemical and offshore. For a given PN, the maximum allowable pressure drops as temperature rises; the standard's own Table 9 (for 1.0460 / P250GH carbon steel) is the reference a mill works from.

The ASME B16.5 Class ladder

Class 150 / 300 / 400 / 600 / 900 / 1500 / 2500 covers utility steam (150, 300), process and refinery (300, 600), high-pressure steam and hydrocracker feed (600, 900), and high-pressure synthesis and reactor service (900, 1500, 2500). ASME B16.5 Table 2-1.1 for A105 carbon steel and Table 2-2.1 for A182 F304 stainless are the two tables that cover most plant work.

Derating — what really matters at temperature

A Class 600 flange is not 600 psi at 500 °C. A105 carbon steel derates from 600 psi at -29 °C / 38 °C down through 540 psi at 200 °C, 480 psi at 300 °C, 410 psi at 400 °C, 365 psi at 450 °C and 300 psi at 500 °C. Stainless 304 rises with temperature, peaks near 700 psi at 38 °C, and holds around 495 psi at 500 °C. The right way to read a rating table is in the temperature column of the service you actually have, not the ambient value printed on the brochure.

Material ladder: carbon, alloy, stainless, duplex, copper-nickel

Flange material follows the pipe material, with one rule: the flange in a higher alloy system is usually a forged equivalent of the pipe's plate or bar grade. The five most common groups are shown below.

Three rules that catch most RFQ errors:

Facing types: where the leak path lives

The facing is the machined surface that presses against the gasket. The leak path of a bolted joint runs through this surface, so the facing selection is a leak-prevention decision as much as a cost decision.

Raised Face (RF)

The most common facing. A 1.6 mm to 6.4 mm raised ring around the bore, serrated to 125–250 µin Ra for soft gaskets. RF is the default for Class 150 and Class 300 service, most process lines, and any system using a soft (compressed sheet, graphite, PTFE) or semi-metallic (spiral wound, kammprofile) gasket.

Flat Face (FF)

A flat face, no raised ring. FF is used when the flange is cast iron, when the mating flange is flat-faced (e.g., a glass-lined equipment nozzle), or where a full-face gasket is specified. Mixing an RF cast iron flange with an FF steel flange is a known field failure mode — the bolt load is concentrated on the small raised face and the cast iron cracks.

Ring Type Joint (RTJ)

A machined oval or octagonal groove holds a metal ring (R, RX, BX). The ring is the gasket. RTJ is used for Class 600 and above, high temperature, and any service where soft gaskets cannot survive (hydrocarbon above 200 °C, high-pressure steam, hydrogen, ethylene). RTJ flanges are typically used with RTJ gaskets matched to the standard (R for B16.5, RX and BX for B16.5 high pressure and API 6A).

Tongue & Groove (T&G) and Male & Female (M&F)

Precision-machined interlocking faces. Used for high-integrity chemical and pharmaceutical service where a self-aligning, self-centering joint is required. The trade-off is that the two halves must be bought as a matched pair from the same supplier, and gasket sizes are non-standard.

Facing finish

Serration is the standard finish for RF flanges using soft and semi-metallic gaskets — typically a 125–250 µin Ra spiral or concentric serration. For RTJ the groove is machined to the standard's hardness, depth and surface finish, and the matching ring has a softer hardness than the groove. Buyers who mix R and RX rings on the same groove almost always leak.

Bolt, gasket and stud selection — the joint is a system

A flange is only as good as the bolts, nuts and gasket that close it. The standard's pressure–temperature rating assumes a correctly selected gasket and a correctly tightened bolt.

Stud bolt and nut grade

ASME B16.5 pairs stud material with the flange class. A193 B7 / A194 2H is the workhorse for carbon and alloy service up to 425 °C. Above 425 °C, A193 B16 / A194 4 or A193 B8 CL2 / A194 8 is the standard high-temperature stud set. For sub-zero and stainless service, A320 L7 / A194 4 or A193 B8M / A194 8M is the answer. Mixing carbon B7 studs on stainless flanges is a corrosion risk and a hot-work risk in chloride service.

Gasket family

Compressed sheet (graphite, aramid, non-asbestos) is the default for utility and Class 150 / 300 service. Spiral wound with graphite or PTFE filler is the default for Class 300 and above, refinery, hydrocracker, and high-temperature steam. Ring-joint metal is reserved for the highest pressure and temperature. Kammprofile (grooved metal + soft filler) is used where blow-out resistance and re-torque capability matter, and where high-temperature cycling is a concern.

Bolt-up sequence

A four-pass cross pattern (12, 3, 6, 9 o'clock, then the gaps) is the standard. A torque wrench calibrated to the flange class, the stud grade and the gasket type is mandatory. Hand-tightening, slugging with a wrench, or skipping the re-torque after the first thermal cycle are the three field practices that most often lead to a joint that has to be re-made in the first 12 months of service.

Inspection documents buyers actually need

Flanges ship with documentation that has to match the project QA plan. The five most common documents and what each one proves:

For refinery, petrochemical and offshore work, EN 10204 3.2 is the most-requested document. For ASME-stamped pressure vessels, the equivalent is the MTC signed by an AISC / ASME QAI. For nuclear, RCC-M Section II applies on top of the ASME / EN certificate. A buyer who does not fix the document requirement in the RFQ is paying for a 3.1 in 70% of cases and re-issuing in the other 30%.

Marking, traceability and packaging

ASME B16.5 and EN 1092-1 both require a permanent mark on every flange. The minimum mark is manufacturer, material grade, size and pressure class. Heat number is required for alloy and stainless, and for any project under EN 10204 3.1 / 3.2 documentation.

Marking is not optional. A flange that ships without a legible heat number is a flange that cannot be tied back to a test certificate, and on a third-party-witnessed job it will be rejected at goods-in.

Packaging follows the route. Domestic European and Asian truck delivery uses plastic caps on the sealing face, foam separators, and wooden crates. Seafreight to a port-of-destination adds heat-treated wood (ISPM 15 compliant), reinforced bracing and a desiccant bag on stainless and alloy flanges to prevent surface oxidation during the sea voyage. For project shipments, a packing list that ties each crate to a line item in the project's pipe class is the only way to keep the warehouse and the field crew on the same page.

Common selection mistakes and how to avoid them

Five errors that show up in project RFQs more than any others:

Specifying ASME Class 150 where EN PN 16 is the project standard

Class 150 and PN 16 are dimensionally different. Bolts, gaskets, bolt circle, OD and facing all differ. The flange that fits one will not fit the other.

Substituting weld neck with slip-on on a Class 600 line

Slip-on is rated for the same class on paper, but ASME B31.1 and B31.3 do not permit SO above Class 300 on cyclic, high-temperature or fatigue service. The bolt-up, the alignment and the failure mode are different.

Forgetting the low-temperature impact test

Carbon steel flanges below -29 °C require impact testing per ASME B16.5 and EN 1092-1. A105 in the as-forged condition will not pass the Charpy test at the design minimum temperature without a normalization or a switch to A350 LF2.

Mixing RF and FF on the same joint

RF-Full Face Gasket mismatch and RF-FF flange pairing concentrate the bolt load on a small area of the FF face, cracking cast iron and distorting the bolt circle. The two facings are not interchangeable.

Quoting a forged flange as plate flange on the BOM

Plate and forged flanges share the OD and bolt pattern in published tables, but the pressure–temperature rating is different. Forged is always higher. Specifying plate on a header that should be forged cuts the safety margin by a factor of two or more in some classes.

Mill-direct procurement: what a buyer gains

Mill-direct supply consolidates flange, pipe fittings, pipe, gasket, stud bolt and nut, and valves into a single project package. EZ Steel Industrial ships a pipe flange RFQ alongside matching carbon & carbon alloy steel pipe, stainless steel pipe, copper & nickel alloy pipe, steel flanges and copper nickel flanges from three production sites — Cangzhou (alloy steel inventory, custom pipe fittings), Yangzhou (large-scale carbon and alloy steel pipe) and Lishui (stainless and copper-nickel). That is why a single RFQ, a single heat-traceable MTC and a single packing list cover a project that would otherwise split across three or four suppliers.

A practical mill-direct checklist

Frequently asked questions

Can ASME B16.5 and EN 1092-1 flanges be used on the same joint?

No. Bolt patterns, OD, gasket numbers and pressure-temperature ratings differ even when the nominal class is the same. A flange made to one standard does not fit a mating flange made to the other without a re-machine, and even then the rating no longer matches. The right way to handle a multi-standard project is to keep the standard on the line and the line on the standard.

What is the difference between a forged and a plate flange?

A forged flange is shaped from a single hot-worked billet; the grain flow follows the hub, so strength and impact resistance are higher. A plate flange is cut from plate and welded or rolled. Plate is cheaper, but its rating is lower than forged at the same class and material. For high-pressure, high-temperature or low-temperature service, forged is mandatory.

Which flange for seawater piping?

90/10 Cu-Ni (UNS C70600) weld neck or slip-on for general service; 70/30 Cu-Ni (C71500) for high-velocity water, polluted seawater and offshore firewater. Both are available with EEMUA 144 / 234 dimensions for offshore. The supporting stud set is typically B8M / 8M (A193 / A194) for chloride resistance.

How is the rating affected by gasket choice?

The standard's pressure-temperature table assumes a specific gasket type, facing finish and bolt grade. A spiral-wound gasket can develop a higher seating stress than a compressed sheet, but a flat soft gasket cannot. The published rating is for the standard combination; deviating from that combination without re-derating invalidates the standard's guarantee.

What is the minimum order and typical lead time for a mill-direct flange package?

Standard ASME B16.5 Class 150 / 300 carbon steel flanges are usually in stock and ship in 2–3 weeks. Class 600 and above, alloy, stainless, duplex and copper-nickel flanges are usually made to order with a 4–10 week lead time depending on size, material and inspection requirement. Mill-direct supply in a single project batch (covering flange, pipe fittings, stud bolt and gasket) typically shortens the overall project lead time by 2–4 weeks compared to multi-supplier sourcing, because the entire package is heat-traceable, inspected and packed on one schedule.