Flanges are the plain ol' workhorses of every industrial piping system, I tell you what. Y'all don't think on 'em when they're workin', and y'all sure can't ignore 'em when they ain't. They're the reason your refinery can swap out a valve without cuttin' pipe, the reason your water utility can pull a pump for service, and the reason a pressure vessel can get inspected and put back to work without rippin' out half a plant.
Well, this here's the complete guide. What flanges are, what they do, how they're built, what types are out there, what standards run the show, and how to pick the right one. Texas Flange been stockin' and shippin' flanges across ANSI, ASME, API, and AWWA standards since 1986, and the questions in this guide are the same ones our sales engineers answer ever' single day.
What in the World Is a Flange?
A flange is a forged or cast, disk-shaped fitting that lets y'all bolt two pipe sections together, or a pipe and a valve, or a pipe and a vessel, 'stead of weldin' 'em up for good. The connection comes right back apart. You can break a flanged joint, service the equipment, and bolt it back up without cuttin' pipe or grindin' welds. That one thing right there is what makes flanges mighty near indispensable in industrial piping.
Mechanically speakin', a flange is a ring of steel (or stainless, alloy, nickel, or duplex) with bolt holes all 'round the edge and a sealing face in the middle. That sealing face mates up against a gasket and a matchin' flange on the other side of the joint. Bolts squeeze down on the gasket. The squeezed gasket seals the joint. Pressure stays right where it belongs, in the pipe.
Flanges run under a whole stack of standards dependin' on the service. ASME B16.5 covers the most common ANSI-class flanges from 1/2 inch through 24 inches. ASME B16.47 takes care of the bigger sizes. API 6A covers wellhead and Christmas-tree flanges out in the oil and gas upstream patch. AWWA C207 covers steel waterworks flanges. Each standard sets the dimensions, materials, pressure ratings, marking conventions, and tolerances.
If y'all are shoppin' for flanges, start at our products page. If you need to look up dimensions or weights, the flange dimensions and weights reference has the tables right there for ya.
Flange Basics, Plain and Simple
A flange is an outside mechanical connection, that's all it is. Two flanges, one gasket, a set of bolts, and the joint is shut tight. Pull the bolts out and she comes apart. That comin'-apart business is the whole reason flanges exist 'stead of just weldin' everything end to end.
Most industrial flanges are forged carbon steel (ASME SA-105) or forged stainless steel (ASME SA-182, grades F304/L, F316/L). When the heat gets higher or the corrosion gets meaner, flanges are made from chrome-moly grades (F11, F22, F91), nickel alloys (Alloy 625, Alloy 825, Hastelloy), or duplex stainless (2205, 2507). Cast iron and ductile iron flanges turn up in waterworks and low-pressure utility service. Stainless and exotic alloys handle process, food, pharmaceutical, and offshore work.
A flange's job is to take the load of that bolt tension and turn it into even squeeze on the gasket, all while holdin' back the pressure inside the line. That's a structural problem and a sealin' problem both at once, yessir. The standards are there to make sure both get solved with parts you can count on and swap around.
What Flanges Do for a Livin'
Flanges do a whole lot more than just bolt pipe together. The full job description runs about a half-dozen related chores, and each one matters in its own corner of the industry.
Hook Up Pipes Good and Tight
That's the main job right there. A flanged joint, torqued up proper with the right gasket, holds full system pressure for as long as you need it. Flanges turn a piping system that's stuck for good into one you can service, and you don't give up a lick of seal integrity doin' it.
Keep a Tight Seal
Bolt tension puts an even squeeze across the whole gasket face. The gasket gives a little and fills up the machinin' marks and little surface flaws, blockin' off any way for a leak to sneak out. Different gasket materials and styles (spiral wound, ring-joint, full-face, soft cut) go with different flange face shapes and service conditions.
Make Inspection and Maintenance Easy
Bein' able to take 'em apart is the flange's secret power, and folks don't give it near enough credit. Need to pull a control valve, swap a strainer, replace a check, recalibrate an orifice plate, hydrotest a section of pipe, or pull a pressure relief valve for certification? Flanged connections make all that possible without cuttin' and re-weldin' pipe. Plants run on that fact, I tell you what.
Handle Cyclic and Thermal Loads
A flanged joint designed right spreads out them cyclic mechanical loads (pump pulsations, compressor vibration, thermal cycling) through the bolt circle and gasket without leakin' a drop. The bolt preload and gasket pick are what carry that cyclic load. Get the spec right and the joint outlasts the equipment, yessir. Long pipe runs that grow and shrink with the temperature lean on flanged joints, expansion loops, and good anchorin' to soak up all that movement without overstressin' the line.
Stand Up to Extreme Service
Cryogenic service down past -320°F (liquid nitrogen) and high-temperature service up past 1,100°F (refinery cat-cracker, ultrasupercritical steam), and ever' bit in between. Flange materials and classes are rated across the full temperature range, with derating tables in ASME B16.5. The same standardized parts cover carbon steel general process, stainless food and pharma, duplex offshore, cast iron water utility, API 6A wellhead, and AWWA waterworks. Our catalog's mighty wide 'cause the service range is mighty wide.
Join Up Different Pipe Materials and Sizes
Reducing flanges, transition flanges, and flange-faced reducers let one diameter or material hook up to another. Carbon steel pipe to a stainless valve. Six-inch pipe to a four-inch fitting. Without flanges, ever' transition would be a custom weld with all the paperwork that comes along with it, and Lord knows nobody wants that.
How Flanges Hook Up
Folks search “flange connections” and “types of flange connections” often enough that the question earned its own section. The short answer: flanges connect by boltin' two matchin' faces together with a gasket in between, torqued down in a careful cross-pattern sequence.
Now the longer answer, that's all about what's boltin' up to what.
Flange to flange. This is the most common case. Two matchin' weld-neck or slip-on flanges, a gasket in between, bolts and nuts torqued in a cross-pattern.
Flange to valve. Most industrial valves come with flanged ends in standard ANSI classes. The valve face has the same bolt circle, raised face, and bore as a matchin' flange. Bolt that valve in place, torque them bolts, and the gasket seals up the joint.
Flange to vessel. Pressure vessels are often flanged at the nozzles, manways, and inspection ports. The vessel nozzle gets welded with a flange on the end, then the cover or piping bolts right to it. Pressure vessel design runs under ASME Section VIII Division 1, with custom flange design covered in Mandatory Appendix 2.
Flange to fitting. Flanged elbows, tees, reducers, and caps come in ever' standard size and class. Y'all use 'em in serviceable spool sections where you reckon you'll need maintenance down the road.
Torque sequencing matters. Flange bolts got to be tightened in a cross-pattern (some call it a star pattern) in several passes, bringin' all the bolts up to roughly 30 percent of final torque, then 60 percent, then 100 percent. That spreads the gasket squeeze out even. Crankin' one side of the bolt circle to full torque before you start on the other side warps the joint and causes leaks. If y'all are chasin' a leak on a fresh joint, check your torque procedure before blamin' the gasket. We got a full breakdown of flange leak causes and fixes if you want to dig deeper.
Flange Design Basics
Flange design's a balancin' act between three things pullin' against each other: structural strength, sealin' geometry, and machinability. ASME B16.5 standardizes the dimensions for the most common cases, but knowin' what's underneath that standard helps y'all spec smarter.
Flange Components
Ever' flange has the same basic anatomy, right quick:
- Hub. The thicker part of the flange that eases over from the bolt-circle face to the pipe bore. It carries the bendin' stress from the bolt load.
- Neck. The tapered part between the hub and the welded end (on a weld-neck flange). That taper passes the load smooth into the run pipe and cuts down on stress pilin' up at the weld.
- Bore. The inside diameter, machined to match the inside diameter of the matin' pipe schedule. Mighty important on weld-neck and socket-weld flanges.
- Raised face (or flat face / RTJ groove). The sealin' surface that touches the gasket. The shape depends on the face type.
- Bolt circle. The diameter through the centers of the bolt holes. Standardized per ASME B16.5 by size and class.
- Gasket surface finish. A controlled spiral or concentric serration (typically 125-250 microinch AARH) that grabs hold of the gasket and keeps it from blowin' out.
If y'all ever opened a flange catalog and wondered why the dimensions are so dadgum specific, well, them tolerances are there 'cause ever' dimension touches either bolt load distribution, gasket sealin', or weld geometry.
Pressure Vessel Flange Design
Standard ASME B16.5 flanges cover most piping work, but pressure vessel nozzles, custom heat exchanger heads, and specialty closures often call for custom-designed flanges. ASME Section VIII Division 1, Mandatory Appendix 2, governs the design of custom pressure vessel flanges usin' the Taylor Forge method.
The design process takes all this into account:
- Operating pressure and temperature
- Bolt-up condition (gasket seating)
- Operating condition (internal pressure plus residual gasket load)
- Hub stress, radial stress, and tangential stress in the flange ring
- Bolt area and bolt material
- Gasket factor (m) and minimum seating stress (y)
A properly engineered Appendix 2 flange will out-spec any standard B16.5 flange of the same nominal size, but it costs more and takes longer to make, I reckon. Use a B16.5 flange if it meets the service. Only move on up to Appendix 2 when your geometry, pressure, or material needs push past what the standard offers.
Texas Flange regular supplies pressure vessel flanges built to customer drawings and ASME Section VIII requirements. If your project's got custom pressure vessel work, send us them drawings.
Who Uses Flanges, Anyhow
Flanges turn up anywhere pipe's carryin' somethin' and somebody's gonna need to take it apart later.
Oil and gas. Upstream wellhead (API 6A), midstream pipelines (ASME B31.4, B31.8), downstream refining (ASME B31.3). Down here on the Gulf, that's one of the biggest flange users by volume, no doubt.
Chemical and petrochemical. Stainless and alloy flanges in process service. ASME B31.3 runs the piping code. NACE MR0175 runs the material selection for sour service.
Power generation. Steam piping, boiler flanges, condensate, feedwater. F11, F22, and F91 chrome-moly grades rule the high-temperature side.
Water and wastewater. AWWA C207 steel flanges in Class B (most common in distribution), Class D (transmission and pump-station service), and Class E. They got different bolt circles than ASME B16.5. Often paired up with ductile iron pipe per AWWA C110 / C153.
Food and beverage. Sanitary service. F304L and F316L stainless. Specialty flanges with smooth inside bores and high surface finishes so they work with clean-in-place.
Pharmaceutical. Much like food and beverage, only with even tighter cleanliness and paperwork requirements.
Pulp and paper. Heavy-duty service in a mighty corrosive process. Stainless and duplex are common.
Mining and metals. Slurry service, abrasion-resistant materials, and often oversized flanges to handle all that grit.
General manufacturing. Air, water, hydraulic, and steam utility lines in ever' kind of plant. Carbon steel B16.5 is what you'll mostly see here.
What Flanges Cost in Today's Market
Flange pricin' today is a whole different conversation than it was three years back. Tariffs, raw material ups and downs, and the AIS / BABA premium on federally funded infrastructure have all moved what it costs to get a flange out to a job site.
For a current breakdown of how trade policy, domestic content rules, and material costs are shapin' flange pricin' right now, take a look at our flange market and regulations update.
As for Texas Flange, we keep our stock deep across A105, A350 LF2, F304L, and F316L in standard ANSI classes, plus AWWA C207 Class B and selected API 6A sizes. For AIS-required projects we quote domestic-melt-and-manufacture material project by project, with lead times that hold up against anybody's. Send us the line item and we'll tell y'all what's available and when, right quick.
Different Kinds of Flanges for Different Jobs
Six main flange types cover the big majority of industrial jobs, with two specialty types fillin' out the catalog.
Weld Neck. Tapered hub butt-welded to the run pipe. Best for high-pressure, high-temperature, cyclic, and critical service. Most common in oil and gas, refining, and pressure vessels. Check out weld neck flange dimensions and specs.
Slip-On. Slips over the pipe end and gets fillet-welded inside and out. Easier to line up than a weld neck, lower pressure rating, used in lower-pressure service and waterworks. Have a look at our forged steel slip-on flanges.
Blind. Solid disc with no bore. Shuts off the end of a flanged pipe run. Used at terminations, on inspection ports, and in pressure vessel manways. Have a look at our ASME B16.5 blind flanges.
Socket Weld. Pipe slips down into a socket bore and gets fillet-welded on the outside. Limited to smaller sizes and lower pressures. Common in process piping for instrument lines and small-bore service.
Threaded. Inside NPT threads. No weldin' required. Used in low-pressure, low-temperature, non-cyclic service. Common in utility lines and small-bore hydraulic and pneumatic. Take a look at threaded (NPT) flange dimensions.
Lap Joint. Two-piece design: a stub end (welded to the pipe) and a backing flange (free to spin 'round). Used where linin' up the bolt holes is tricky, or where the line material is exotic and a carbon steel backing flange saves some money. Take a look at lap joint flanges and stub ends.
Specialty types include Reducing flanges (one nominal size down to another in a single flange) and Spectacle Blind / Paddle Blind (positive isolation of a section of line: the spectacle is a permanent figure-eight, the paddle comes right out).
For the deeper breakdown on what fits which job, our flange selection buyer’s guide walks y'all through the decision step by step.
Different Kinds of Flange Face
Face type is the shape of the sealin' surface. The matin' flange has got to have the same face type, or that joint ain't gonna seal.
Raised Face (RF). The default, plain and simple. The face stands proud of the bolt circle by 1/16 inch (Class 150 and 300) or 1/4 inch (Class 400 and above). Puts the bolt load right on the gasket. Most common across ASME B16.5 service.
Flat Face (FF). Full-face gasket, no raised area. Y'all use it when boltin' to brittle housings (cast iron, fiberglass, low-strength bronze) that would crack under a raised-face bolt-up.
Ring Type Joint (RTJ). Metal-to-metal seal usin' a soft iron, soft steel, or alloy ring in a machined groove. Standard for ASME B16.5 600# and above in high-pressure / high-temperature service, and required for API 6A. Gasket ring per ASME B16.20.
Tongue and Groove (TG), Male and Female (MF). Specialty service where the gasket's got to be locked in by the shape of the thing. Lethal service, hydrogen, and certain process applications.
For the full technical breakdown on raised-face geometry, gasket math, and when to step away from RF, take a look at the raised-face flange guide.
Flange Classes and Service Ratings
Three pressure-class systems run the flange market the world over.
ASME pressure classes. 150#, 300#, 600#, 900#, 1500#, 2500#. These here are pound classes, not literal pressure ratings. A 300# flange in A105 carbon steel handles 740 psig at 100°F and 550 psig at 600°F. The pressure-temperature relationship is governed by the ASME B16.5 P-T tables for sizes 1/2″ through 24″, and ASME B16.47 for sizes 26″ through 60″. The P-T tables are published in the standards themselves; ASME don't permit republication. For bore, bolt circle, and weight by size, take a look at our Class 150 dimension tables.
API 6A pressure classes. 2,000, 3,000, 5,000, 10,000, 15,000, and 20,000 psi. These are literal working pressures for wellhead and Christmas tree equipment. API 6A flanges look a lot like B16.5 flanges but they got different bolt circles, different bore tolerances, and required RTJ faces. They ain't interchangeable with B16.5, no sir.
AWWA classes. Class B, Class D, Class E, Class F. Lower-pressure waterworks service, ductile iron pipe interface, large bolt patterns. Class B per AWWA C207 is rated for 86 psi cold working pressure in sizes 4″ through 24″, and 75 psi in sizes 26″ through 144″. They got different bolt circles than ASME B16.5 of the same nominal size. They ain't interchangeable.
The number one rule: pressure class has got to match the system class exactly, or be specifically engineered for the transition. Y'all cannot bolt a 150# flange to a 300# flange and expect a sealed joint at any pressure. The bolt circles match up in some sizes, but the geometry, bolt load, and gasket squeeze are designed for different workin' pressures.
Flange Standards
Flanges are some of the most standards-heavy products in industrial supply. Standards are there 'cause the parts got to swap out across mills, distributors, and contractors. Here are the active revisions y'all'll see called out on a current spec sheet.
- ASME B16.5-2025 – Pipe Flanges and Flanged Fittings: NPS 1/2 through 24, Classes 150 through 2500.
- ASME B16.47-2025 – Large Diameter Steel Flanges: NPS 26 through 60, Series A and Series B.
- ASME B16.20-2023 – Metallic Gaskets for Pipe Flanges: Ring-Joint, Spiral Wound, and Jacketed.
- ASME B16.21-2021 – Nonmetallic Flat Gaskets for Pipe Flanges.
- ASME B16.36-2015 (R2020) – Orifice Flanges.
- ASME SA-105 / SA-182 / SA-350 – Material specifications for forged carbon, stainless, and low-temperature carbon flanges.
- ASTM A961/A961M-24 – Standard Specification for Common Requirements for Steel Flanges, Forged Fittings, Valves, and Parts for Piping Applications.
- API 6A, 21st Edition – Specification for Wellhead and Christmas Tree Equipment.
- AWWA C207 – Steel Pipe Flanges for Waterworks Service, Sizes 4 In. Through 144 In.
- NACE MR0175 / ISO 15156 – Materials for Use in H2S-Containing Environments in Oil and Gas Production.
- AWWA C110 / C153 – Ductile-Iron and Compact Ductile-Iron Fittings.
A quick note on names: most of these standards used to be ANSI standards. Over the last twenty-five years, ANSI handed administration over to ASME. ANSI B16.5 is now ASME B16.5. Same standard, different administrator, that's all.
Mill markings on ever' legitimate flange ought to include the heat number, size, class, material grade, manufacturer, and standard reference. If a flange shows up unmarked, that's a red flag, not a bargain. For the full breakdown on readin' mill paperwork, take a look at the mill test report guide.
Wrappin' It Up
Flanges are the connection point that lets industrial piping get built, serviced, expanded, inspected, and rebuilt without tearin' the whole system apart. They come in a half-dozen face types, six common body types plus specialty variants, three pressure-class systems, and dozens of material grades, all run by an interlockin' set of ASME, ASTM, API, AWWA, and NACE standards.
The right flange for any project is the one that matches the service medium, the operatin' pressure and temperature, the code of construction, the matin' component, and the paperwork requirements. Pick the wrong one and y'all are buyin' twice. Pick the right one and that joint outlasts the equipment.
Texas Flange been answerin' “what flange do I need” questions since 1986, with deep stock across ANSI, ASME, API, and AWWA standards, in 50+ material grades, and AIS-compliant material in stock for federal projects. If your spec's a mess, send it on anyway. If it's clean, request a quote and we'll holler back at you right quick. Y'all can also pull 3D CAD flange drawings for any size and class.
Questions Folks Ask Us
What's a flange used for?
A flange is used to hook up two pipe sections, or a pipe and a valve, or a pipe and a vessel, in a way that y'all can unbolt and put back together. Bein' able to take it apart is what makes industrial piping serviceable.
What's the difference between a flange and a fitting?
A flange is specifically a bolted connection point with a gasket. A fitting is a catch-all word for any pipe component (elbow, tee, reducer, cap) used to change direction, branch off, or close off a line. Flanges and fittings sometimes get combined (a flanged elbow, for example), but the two words ain't interchangeable.
What are flanges made from?
The most common are forged carbon steel (ASME SA-105) and forged stainless (SA-182, F304/L, F316/L). Past those: A350 LF2 for low-temperature carbon, F11 / F22 / F91 chrome-moly for high temperature, Alloy 625 / 825 / Hastelloy for corrosive service, and 2205 / 2507 duplex for high-strength corrosion-resistant work. The flange selection guide walks y'all through which material fits which service.
What's the highest-pressure flange y'all can buy?
Per ASME B16.5, the top class is 2500#. For higher pressures, y'all move on up to API 6A wellhead flanges, rated up to 20,000 psi. Past 20K, you're in custom-engineered territory and you need a sales engineer on the phone before the order goes through.
Can y'all swap RF and FF flanges?
No sir. Boltin' a raised-face flange to a flat-face component will crack that FF component if it's brittle (cast iron, fiberglass). Match the face type to the matin' component, or use a transition spool.
Do I need an MTR for my flange order?
If the project spec calls for one, yes. ASME B31.3 process piping, API 6A wellhead, and most government-funded infrastructure require MTRs. Texas Flange supplies MTRs on request at no extra charge for stock material. Ask for it on the PO, not after that flange is already welded into the line.
What's the difference between ANSI and ASME flange standards?
Same standards, different administrators. ANSI handed administration of the flange standards over to ASME in the last twenty-five years. A current ASME B16.5 flange is what folks still casually call an “ANSI flange.” The dimensions are identical.
How do I know what gasket goes with my flange?
Gasket pick follows from face type, pressure class, temperature, and service. Spiral wound (per ASME B16.20) for raised face. Ring joint (also B16.20) for RTJ. Soft cut or full-face for flat face. That gasket's got to be rated for the service temperature and pressure, not just match the bolt circle.”]}
📞 Holler at our team today for expert guidance on your next flange order.
Last updated: May 7, 2026.
