Flange

What is a Flange

A flange is a mechanical component that forms a detachable, pressure-tight joint between two pipe runs, or between a pipe and a valve, pump, instrument, or pressure vessel. The joint is made by clamping two flange faces together with a gasket compressed between them, the clamping force supplied by a ring of bolts. Because the joint can be unbolted, flanges are the standard way to build a piping system that must be maintained: a section can be isolated, a valve replaced, or a filter element changed without cutting and re-welding pipe.

A flanged joint is best understood as a system of four interacting parts, not a single product. The two flanges provide the structural ring and the gasket seating surface. The gasket is the soft or semi-soft element that fills surface imperfections and seals the gap. The bolting (stud bolts and nuts) provides and holds the seating load. Leak tightness is a function of all four: a perfect flange will still leak if the gasket is wrong, the bolts are under-tightened, or the faces are misaligned. This is why flange engineering is as much about assembly procedure as about the casting or forging.

The industrial history of the bolted flange runs alongside the history of steam and pressure piping. As boiler and process pressures rose through the late 19th and early 20th centuries, ad hoc flange dimensions caused chronic interchangeability problems between makers. Standardization brought order: in the United States the ASME B16.5 standard for pipe flanges and flanged fittings became the dominant reference for sizes NPS 1/2 through NPS 24, while in Europe the DIN series, later consolidated into EN 1092-1, fixed the PN-rated metric flange. Large-diameter and waterworks flanges were standardized separately under ASME B16.47 and AWWA C207.

Functionally, a flange must do three things at once. It must carry the longitudinal and bending loads of the pipe like any structural member. It must hold the internal pressure without the bolts yielding or the body distorting. And it must keep the gasket compressed within a narrow band of stress, high enough to seal but not so high that the gasket is crushed or extruded. These three duties are why a flange is far thicker and heavier than the pipe it joins, and why a flange of a higher pressure class is dimensionally larger even at the same nominal pipe size.

The scale of flange service is wide. The same component family spans low-pressure waterworks rated under 20 bar, general process piping at tens of bar, and refinery and oilfield wellhead service into the hundreds of bar, with API 6A flanged wellhead equipment rated to 103 MPa (15,000 psi) and beyond. As with most piping components, there is no universal flange: selection is the act of mapping a specific pressure, temperature, medium, and pipe size onto the right standard, class, type, face, and material.

Flange Types by Connection

The first axis of flange classification is how the flange attaches to the pipe. ASME B16.5 defines six standard structural types: weld neck, slip-on, socket weld, threaded, lap joint, and blind. Each represents a different trade-off among pressure capability, fatigue resistance, cost, alignment ease, and inspectability. Choosing the wrong type is a common and expensive error: a slip-on flange specified for a cyclic high pressure line will fatigue at the fillet weld, while a weld neck flange specified for a low pressure utility line wastes material and welding time. The table below summarizes the six.

Weld neck flanges have a long tapered hub that is butt-welded to the pipe so that the flange bore matches the pipe inner diameter. The smooth transition reduces stress concentration and turbulence, making the weld neck the only type recommended for severe service: high pressure (Class 600 and above), thermal cycling, vibration, and cryogenic or elevated temperature duty. It is the most expensive structural type and requires full pipe-grade butt welding and radiography, but it is the reference against which other types are judged. For any line where joint failure carries safety or shutdown cost, the weld neck is the default.

Slip-on flanges slide over the outside of the pipe and are welded with two fillet welds, one inside and one outside. They are cheaper than weld necks, more forgiving of small length errors during fit-up, and need less welder skill, which is why they dominate general utility and lower pressure process piping. Their weakness is fatigue: the fillet welds carry the load through a discontinuity, so slip-ons are not used for cyclic, high pressure, or high temperature service. The internal weld bore must also be ground smooth where flow erosion is a concern.

Socket weld and threaded flanges serve small-bore lines. A socket weld flange seats the pipe end in a recessed bore and is fillet-welded on the outside only, giving good strength for small high pressure instrument and steam lines, typically NPS 3 and below; its drawback is an internal crevice that can trap fluid and corrode. A threaded flange uses a tapered NPT thread and requires no welding at all, which suits galvanized pipe, non-weldable materials, and locations where hot work is forbidden, but it is limited to low pressure, small bore, non-cyclic duty.

Lap joint and blind flanges complete the set. A lap joint flange is a loose backing ring that rotates freely over a butt-welded stub end, so the bolt holes can be aligned without rotating the pipe; it is favored where joints are opened frequently or where the pipe is an expensive alloy and only the stub end need be the alloy grade while the loose ring is carbon steel. A blind flange is a solid disc with no bore, used to close the end of a pipe, a vessel nozzle, or a valve, and to provide a future tie-in or inspection access point. Blind flanges are rated for the full pressure class and are often the highest-stressed item in a set.

Pressure Classes and PN Ratings

The second axis of classification is the pressure rating, and here two parallel systems coexist worldwide. The American ASME B16.5 system uses dimensionless class numbers; the European EN 1092-1 system uses PN numbers. Both are pressure-temperature systems, meaning the printed number is only a label for a full derating curve, not a single allowable pressure. Understanding the distinction is the single most common gap in flange procurement, because a flange that is "300 class" or "PN 40" still has a different allowable pressure at 200 degrees Celsius than at room temperature.

In ASME B16.5 the classes are 150, 300, 400, 600, 900, 1500, and 2500, covering sizes NPS 1/2 through NPS 24 (Class 2500 stops at NPS 12). The class number is not a pressure; it is a rating group. The actual allowable working pressure depends on the material group and the temperature. For ASTM A105 carbon steel, a Class 150 flange is rated about 285 psi (19.6 bar) at 38 degrees Celsius but falls steeply as temperature rises. The table below shows the published B16.5 pressure-temperature ratings for A105 carbon steel at three reference temperatures.

Two things stand out from the table. First, Class 150 derates far more aggressively with temperature than Class 300 and Class 600, losing more than 60 percent of its rating by 371 degrees Celsius, which is why Class 150 is rarely used for hot service even at modest pressures. Second, the numerical class does not equal the allowable pressure: a Class 600 flange is not "good for 600 psi" but for roughly 1,480 psi cold in carbon steel. The class is a family, and the real number must be read from the standard's rating table for the specific material group.

EN 1092-1 uses the PN system, where the number is the nominal pressure in bar at room temperature. The standard defines PN 2.5, 6, 10, 16, 25, 40, 63, 100, 160, 250, 320, and 400, with PN 10, PN 16, PN 25, and PN 40 being the workhorses of European industrial piping. A PN 16 flange is nominally rated 16 bar at ambient and, like the ASME system, derates with temperature through the standard's material-group p/T tables. EN 1092-1 also numbers its flange forms (for example Type 11 is the weld neck flange, equivalent in function to the ASME weld neck), so a complete EN designation names both the type and the PN.

The ASME class and EN PN systems are not interchangeable. A Class 150 and a PN 16 flange are roughly comparable in pressure but have different outside diameters, bolt circle diameters, bolt counts, and thicknesses, so they cannot be bolted directly to each other. Mating an ASME system to an EN system requires a transition or adapter flange machined to both patterns, or a spool drilled to the mating standard. The table below gives a rough comparison of where the two systems overlap; it is a guide for planning, not a substitute for the dimensional tables.

Beyond these two systems sit the large-diameter and waterworks standards. For pipe above NPS 24, ASME B16.47 applies, split into Series A (derived from MSS SP-44, thicker and heavier with fewer, larger bolts) and Series B (derived from the former API 605, thinner with more, smaller bolts). The two series of the same size and class are dimensionally different and must be specified explicitly. For potable water and waterworks, AWWA C207 defines steel ring flanges in classes B, D, E, and F, rated approximately 86, 150 to 175, 275, and 300 psi respectively, sized from 4 inch up to 144 inch.

Face Types, Gaskets and Bolting

The third axis of flange selection is the face: the geometry of the sealing surface where the gasket sits. The face type must be matched between the two mating flanges and to the gasket, and it sets the surface finish, the gasket family, and the sealing mechanism. ASME B16.5 recognizes several face types; the three encountered most often are flat face (FF), raised face (RF), and ring-type joint (RTJ). The less common tongue-and-groove and male-and-female faces appear on specialized equipment.

Raised face (RF) is the most common face in process piping. A small raised platform, typically 1.6 mm high for Class 150 and 300 and 6.4 mm for Class 400 and above, lifts the gasket seating area above the bolting plane, concentrating the bolt load onto a smaller annular area and improving sealing. RF flanges use ring gaskets, most often spiral-wound, and are specified with a serrated surface finish. The raised face is the default choice wherever both mating components are steel and the pressure class is within the soft-gasket range.

Flat face (FF) places the gasket surface flush with the bolting plane and uses a full-face gasket that extends to and is pierced by the bolt holes. Its main role is to mate against brittle-bodied or cast equipment, such as cast iron pumps, valves, and strainers, where a raised face would impose a bending moment that could crack the casting. Pairing a flat face against a raised face is generally avoided for the same reason. FF joints are common in low pressure water, fire protection, and equipment connections.

Ring-type joint (RTJ) achieves a metal-to-metal seal. A precision groove is machined into each flange face, and a soft metal ring, oval or octagonal in section, is seated in the groove and crushed by bolt load until it seals by line contact against the groove walls. RTJ is the standard for high pressure and high temperature service, typically Class 600 and above in oil, gas, and refinery duty, where a soft gasket would extrude or burn. RTJ grooves demand a much finer surface finish than RF faces and a harder-than-ring groove material. The table below summarizes the three faces and their finish and gasket requirements.

Gaskets are governed by their own standards. ASME B16.20 covers metallic gaskets, spiral-wound, ring-joint, and metal-jacketed, and dimensionally matches them to B16.5, B16.47, and API 6A flanges. ASME B16.21 covers non-metallic flat gaskets. Spiral-wound gaskets, the workhorse for raised faces, alternate a metal winding (often 304 or 316 stainless) with a soft filler (graphite or PTFE) and add inner and outer guide rings to control compression. Ring-joint gaskets for RTJ grooves are solid soft-iron, low-carbon, or stainless rings in R, RX, or BX styles. The gasket material, not just the type, must be checked against the medium and temperature.

Bolting is the load-supplying element and is fixed by the flange standard: B16.5 specifies the bolt circle diameter, the number of bolts, and the bolt size for every size and class. The most common bolting set is ASTM A193 Grade B7 alloy-steel stud bolts with ASTM A194 Grade 2H heavy hex nuts, suited to service to about 425 degrees Celsius. For low temperature, B7M or L7 grades with Charpy impact testing are used; for high temperature, B16 or chrome-moly grades. Correct assembly per ASME PCC-1, cross-pattern tightening in passes to a calculated target torque with lubricated threads, is what actually delivers the gasket seating stress the joint was designed for.

Materials and Spec-Sheet Parameters

The fourth axis is material, which must match the pipe grade, the medium, and the temperature window. Forged flanges follow ASTM material specifications that deliberately parallel the pipe specs, so that a flange and its pipe share chemistry, strength, and weldability. Selecting a flange material that does not match the pipe creates a welding and code-compliance problem even before corrosion is considered. The table below lists the most common forged flange grades and the pipe families they match.

ASTM A105 is forged carbon steel for ambient and elevated temperature service, the default flange material for water, steam, air, and hydrocarbon piping up to about 425 degrees Celsius, and it matches the common A53, A106, and API 5L pipe grades. It is economical and widely stocked but is not suitable for low temperature impact-toughness service or for corrosive media. For service below freezing, ASTM A350 LF2 is the standard substitute: it is a low-temperature carbon steel qualified with Charpy V-notch impact testing, commonly down to minus 46 degrees Celsius, matching A333 low-temperature pipe.

ASTM A182 F304/F304L and F316/F316L are the forged austenitic stainless grades for corrosion resistance, matching A312 stainless pipe. F316 adds molybdenum for better resistance to chlorides and reducing acids than F304, and the "L" low-carbon variants reduce sensitization and intergranular attack after welding. Compared with an equivalent A105 carbon forging, A182 stainless forgings typically cost on the order of 70 to 80 percent more, so stainless is specified where the medium requires it, not as a default. For high-temperature power and refinery service, A182 F11, F22, and F91 chrome-moly grades match A335 alloy pipe; for seawater and sour service, duplex grades such as F51 and F53 are used.

Reading a flange spec sheet is a discipline of its own. Beyond the headline standard, class, type, face, and material, a complete flange callout fixes the following parameters, each of which can void interchangeability if mismatched:

• Standard and edition: ASME B16.5, ASME B16.47 (with Series A or B named), or EN 1092-1 with the form type. The series and edition must be explicit.
• Nominal size and schedule: NPS and pipe schedule (which sets the bore of a weld neck), or DN for metric flanges. The bore must match the connecting pipe wall.
• Pressure rating: ASME class or EN PN, read against the material group and design temperature in the standard's p/T table, not taken as a fixed pressure.
• Face type and finish: RF, FF, or RTJ, with the surface finish in microinch or micrometre Ra and, for RTJ, the groove number.
• Material grade and certification: ASTM grade, heat traceability, and material test report (EN 10204 3.1 or 3.2), with impact testing where low temperature applies.
• Bolting and gasket reference: the matching bolt grade (A193 B7 / A194 2H) and gasket standard (B16.20 / B16.21), and the gasket style and material.
• Special requirements: NACE MR0175/ISO 15156 for sour service, PED 2014/68/EU or ASME for the design code, and any fugitive-emission or fire-safe requirement.

One subtlety worth flagging is that the bore of a weld neck flange is tied to the pipe schedule, not just the nominal size. Two Class 300 NPS 6 weld neck flanges with different schedule bores will not mate cleanly to pipes of different wall thickness without a counterbore or a transition. Ordering the flange bore to match the actual pipe schedule, often noted as "bore to suit Sch 40" or similar, avoids a mismatch that is invisible on the outside face but defeats the flow path on the inside.

Selection Decision Factors

To turn the preceding five chapters into a specific purchase, follow the decision sequence below. Most flange selection errors are not arithmetic mistakes but ordering errors: the wrong series, an unstated bore, a face mismatch, or a material that does not match the pipe. Working the steps in order, and fixing each on the RFQ, removes the common failure modes. The eight steps can serve as a fixed flange specification template.

1. Standard and size: Decide the governing standard first, ASME B16.5 (NPS 1/2 to 24), ASME B16.47 (above NPS 24, naming Series A or B), EN 1092-1 (DN, PN), or AWWA C207 for waterworks. Then fix the nominal size and, for weld necks, the matching pipe schedule that sets the bore.
2. Pressure class or PN: Select the class or PN from the design pressure and temperature using the standard's pressure-temperature table for the chosen material group, not from the bare design pressure. Allow margin for surge, water hammer, and upset conditions.
3. Flange type: Choose weld neck for high pressure, cyclic, or high temperature lines; slip-on for general utility; socket weld or threaded for small bore; lap joint for frequently dismantled or costly-alloy lines; blind for closures and future tie-ins.
4. Face type and finish: Raised face for steel-to-steel process piping, flat face to mate cast iron or brittle equipment, ring-type joint for high pressure and high temperature service. State the surface finish in Ra and, for RTJ, the groove style.
5. Material grade: Match the flange ASTM grade to the pipe and medium: A105 for general carbon steel, A350 LF2 for low temperature, A182 F304/F316 for corrosion, chrome-moly or duplex for high temperature or aggressive service. Require the material test report.
6. Bolting and gasket: Specify the bolt grade (commonly A193 B7 / A194 2H, or low-temperature L7/B7M) and the gasket per ASME B16.20 or B16.21, matched to the face type. Confirm gasket material against the medium and temperature.
7. Codes and certification: Name the design code (ASME or PED 2014/68/EU), sour-service requirements (NACE MR0175 / ISO 15156), fugitive-emission or fire-safe needs, and the certification level (EN 10204 3.1 or 3.2) with heat traceability.
8. Assembly and test: Plan the joint as an assembly: controlled bolt-up per ASME PCC-1, hydrostatic test, and, where required, leak-tightness testing. The best flange will still leak under a poor torque sequence.

One last dimension is commonly underweighted at the purchasing stage: traceability and serviceability over the asset life. Heat-number stamping and a retained material test report let a flange be re-qualified or replaced years later without guesswork; a known, stocked standard size eases the eventual gasket or bolt replacement; and a flange ordered to a clearly named series and bore avoids the field surprise of a part that will not bolt up. Established forging makers such as those certified to PED and supplying full EN 10204 3.1/3.2 documentation, available across China and internationally, are the reliable choice for projects where the joint must be maintained for decades rather than discarded.