Views: 167 Author: Monica Publish Time: 2026-03-18 Origin: Site
Weld neck (WN) flange is the strongest and most inspectable flange type in ASME B16.5 because its long tapered hub is butt-welded to the pipe, giving a full-penetration joint that can be radiographed.
This guide gives the Class 150, 300 and 600 dimension tables from NPS 1/2 to NPS 12, the face and neck type selection rules, a **corrected** weight chart for NPS 1/2 to NPS 24, and the specification wording you need on a purchase order. It also flags an error that is carried by many flange weight charts on the web, including the one previously published on this page - see the weight section.
A WN flange is a flange with a long tapered hub that is butt-welded to the pipe to give a full-penetration, radiographable joint. Specify it whenever the service is high pressure, high temperature, cyclic or subject to radiographic examination - and accept its higher cost because it is the only common flange type whose weld can be fully verified.
The defining feature is the hub. Unlike a slip-on flange, which sits over the pipe and is held by two fillet welds, a weld neck flange has a hub whose outside diameter tapers from the flange base down to the pipe outside diameter. The bore is machined to match the pipe bore, so the finished joint has no step, no crevice and no sudden change in flow section. The pipe and the hub are then joined with a single full-penetration groove weld.
That geometry does two things at once. Mechanically, it moves the stress concentration away from the flange face and distributes it along the tapered hub and into the pipe, which is why weld neck flanges survive line surge, vibration and thermal cycling that would crack a fillet-welded joint. From a quality standpoint, the butt weld is a full-penetration groove weld, so it can be examined by radiography or ultrasonics against a defined acceptance criterion - which a fillet weld cannot be, in any meaningful way.
The practical consequence is that most codes and most end users require weld neck flanges above a certain service severity. It is the default for nickel alloy piping in chemical and offshore service, and it is what you will be asked for whenever the line class calls for RT or UT on the flange weld.
Because the load path runs through a tapered hub into the pipe rather than through a fillet weld into the flange face, and because the bore is matched to the pipe so there is no turbulence, crevice or erosion step at the joint. The result is roughly two to three times the fatigue life of a slip-on flange of the same size and class.
·Superior stress distribution. The taper moves the peak stress away from the flange face and into the pipe wall, which is exactly where the piping system is designed to carry it. This is what makes weld neck the right choice under line surge and mechanical vibration.
· High fatigue resistance. Because the bore is machined to match the pipe inside diameter, there is no step to generate turbulence or erosion at the joint. Fatigue life improves substantially over slip-on and socket weld construction.
· Verifiable weld. A full-penetration groove weld can be examined by radiographic testing or ultrasonic testing against a code acceptance criterion. Two fillet welds cannot be meaningfully volumetrically examined.
· Performance at temperature extremes. With the right material, a weld neck flange works from cryogenic service up to high-temperature process duty, because the hub accommodates the differential thermal expansion instead of loading a fillet weld in bending.
· Suitability for repeated make-up. The flange face is not distorted by the welding operation the way a fillet-welded flange can be, so the gasket seating surface stays flat.
The trade-off is cost and bulk. A weld neck flange uses more material than a slip-on of the same size and class, it costs more to forge and machine, and it takes more welding time to install. That is why it is specified where the service demands it rather than everywhere - see the selection comparison table further down.
ASME B16.5 Class 150 weld neck flanges run from NPS 1/2 at 90 mm outside diameter with four bolt holes up to NPS 24 at about 813 mm outside diameter with twenty bolt holes. The table below gives the full installation geometry from NPS 1/2 to NPS 12, which covers the overwhelming majority of process piping.
ASME B16.5 Class 150 weld neck flange dimensions, NPS 1/2 to NPS 12 (mm)
NPS | DN | Flange OD (O) | Thickness (tf) | Raised face dia. (R) | Hub dia. at base (X) | Length thru hub (Y) | Bolt circle (W) | No. of bolts | Bolt hole dia. (d) |
1/2 | 15 | 90 | 11.2 | 34.9 | 30 | 48 | 60.3 | 4 | 15.9 |
3/4 | 20 | 100 | 12.7 | 42.9 | 38 | 51 | 69.9 | 4 | 15.9 |
1 | 25 | 110 | 14.3 | 50.8 | 49 | 54 | 79.4 | 4 | 15.9 |
1 1/4 | 32 | 115 | 15.9 | 63.5 | 59 | 57 | 88.9 | 4 | 15.9 |
1 1/2 | 40 | 125 | 17.5 | 73.0 | 65 | 60 | 98.4 | 4 | 15.9 |
2 | 50 | 150 | 19.1 | 92.1 | 78 | 64 | 120.7 | 4 | 19.1 |
2 1/2 | 65 | 180 | 22.4 | 104.8 | 90 | 70 | 139.7 | 4 | 19.1 |
3 | 80 | 190 | 24.3 | 127.0 | 108 | 70 | 152.4 | 4 | 19.1 |
4 | 100 | 230 | 24.3 | 157.2 | 135 | 76 | 190.5 | 8 | 19.1 |
6 | 150 | 280 | 25.4 | 215.9 | 188 | 89 | 241.3 | 8 | 22.4 |
8 | 200 | 345 | 28.4 | 269.7 | 240 | 102 | 298.5 | 8 | 22.4 |
10 | 250 | 405 | 30.2 | 323.8 | 292 | 108 | 362.0 | 12 | 25.4 |
12 | 300 | 485 | 31.8 | 381.0 | 342 | 117 | 431.8 | 12 | 25.4 |
Class 150 raised face height is 1.6 mm (1/16 in). Do not confuse the raised face diameter (column R above) with the gasket inside diameter - the gasket is sized from the raised face diameter, not from the bore. Confirm the gasket dimensions against ASME B16.20 for spiral wound or B16.21 for non-metallic gaskets.
Note how little the flange thickness changes across this size range - from 11.2 mm at NPS 1/2 to only 31.8 mm at NPS 12. What grows is the outside diameter, the bolt circle and the number of bolts, because at low pressure the governing problem is not hoop stress in the flange ring but the bolt load needed to seat and hold the gasket over a larger sealing area.
Class 300 weld neck flanges are thicker, heavier and more heavily bolted than Class 150 at the same size - at NPS 12 the flange thickness rises from 31.8 mm to 49.3 mm and the bolt count rises from 12 to 16. They are not interchangeable with Class 150: the bolt circle and bolt hole sizes differ, so a Class 300 flange will not bolt to a Class 150 flange.
ASME B16.5 Class 300 weld neck flange dimensions, NPS 1/2 to NPS 12 (mm)
NPS | DN | Flange OD (O) | Thickness (tf) | Raised face dia. (R) | Hub dia. at base (X) | Length thru hub (Y) | Bolt circle (W) | No. of bolts | Bolt hole dia. (d) |
1/2 | 15 | 95 | 12.7 | 34.9 | 38 | 51 | 66.7 | 4 | 15.9 |
3/4 | 20 | 115 | 14.3 | 42.9 | 48 | 56 | 82.6 | 4 | 19.1 |
1 | 25 | 125 | 15.9 | 50.8 | 54 | 60 | 88.9 | 4 | 19.1 |
1 1/4 | 32 | 135 | 17.5 | 63.5 | 64 | 64 | 98.4 | 4 | 19.1 |
1 1/2 | 40 | 155 | 19.1 | 73.0 | 70 | 67 | 114.3 | 4 | 22.2 |
2 | 50 | 165 | 20.7 | 92.1 | 84 | 68 | 127.0 | 8 | 19.1 |
2 1/2 | 65 | 190 | 23.9 | 104.8 | 100 | 75 | 149.2 | 8 | 22.2 |
3 | 80 | 210 | 27.0 | 127.0 | 117 | 78 | 168.3 | 8 | 22.2 |
4 | 100 | 255 | 30.2 | 157.2 | 146 | 84 | 200.0 | 8 | 22.2 |
6 | 150 | 320 | 35.0 | 215.9 | 206 | 97 | 269.9 | 12 | 22.2 |
8 | 200 | 380 | 39.7 | 269.9 | 260 | 110 | 330.2 | 12 | 25.4 |
10 | 250 | 445 | 46.1 | 323.8 | 321 | 116 | 387.4 | 16 | 28.6 |
12 | 300 | 520 | 49.3 | 381.0 | 375 | 129 | 450.8 | 16 | 31.8 |
The step from Class 150 to Class 300 at NPS 2 is instructive: outside diameter goes from 150 mm to 165 mm but the bolt circle barely moves, from 120.7 mm to 127.0 mm, while the bolt count doubles from four to eight. That is the standard absorbing a higher bolt load by adding bolts rather than by growing the flange, which is why you cannot mate the two classes even where their diameters look close.
Class 600 is where the raised face height steps up from 1.6 mm to 6.4 mm and the hub becomes substantially longer - at NPS 12 the length through hub reaches 200 mm against 117 mm for Class 150. Above this class, ring type joint faces become common and the flange mass rises steeply.
ASME B16.5 Class 600 weld neck flange dimensions, NPS 1/2 to NPS 12 (mm)
NPS | DN | Flange OD (O) | Thickness (tf) | Raised face dia. (R) | Hub dia. at base (X) | Length thru hub (Y) | Bolt circle (W) | No. of bolts | Bolt hole dia. (d) |
1/2 | 15 | 95 | 14.2 | 34.9 | 38 | 51 | 66.5 | 4 | 15.7 |
3/4 | 20 | 118 | 15.7 | 42.9 | 48 | 57 | 82.6 | 4 | 19.1 |
1 | 25 | 124 | 17.5 | 50.8 | 54 | 62 | 88.9 | 4 | 19.1 |
1 1/2 | 40 | 155 | 22.4 | 73.2 | 70 | 73 | 114.3 | 4 | 22.4 |
2 | 50 | 165 | 25.4 | 92.1 | 84 | 84 | 127.0 | 8 | 19.1 |
3 | 80 | 210 | 31.8 | 127.0 | 117 | 100 | 168.1 | 8 | 22.4 |
4 | 100 | 273 | 38.1 | 157.2 | 146 | 114 | 215.9 | 8 | 25.4 |
6 | 150 | 356 | 47.8 | 215.9 | 206 | 140 | 292.1 | 12 | 28.4 |
8 | 200 | 419 | 55.6 | 269.7 | 260 | 162 | 349.3 | 12 | 31.8 |
10 | 250 | 508 | 63.5 | 323.8 | 321 | 184 | 431.8 | 16 | 35.1 |
12 | 300 | 559 | 66.5 | 381.0 | 375 | 200 | 489.0 | 20 | 35.1 |
Raised face height changes with class: 1.6 mm (1/16 in) for Class 150 and 300, and 6.4 mm (1/4 in) for Class 400, 600, 900, 1500 and 2500. Getting this wrong is a common cause of a gasket that will not seat, because the compression stops against the raised face instead of against the gasket.
Thickness, length through hub, outside diameter and bolt circle all grow with class, but the length through hub grows fastest - which is the visible signature of a weld neck flange designed to carry bending into the pipe. At NPS 6, moving from Class 150 to Class 2500 roughly triples the thickness and more than triples the hub length.
NPS 6 weld neck flange across all ASME B16.5 pressure classes (mm)
Class | Outside diameter (O) | Thickness (tf) | Raised face dia. (R) | Bolt circle (W) | Length thru hub (Y) | Bolts | Bolt hole dia. (d) | Stud bolt size x length |
150 | 280 | 25.5 | 215.9 | 241.3 | 88.6 | 8 | 22.2 | 3/4 in x 100 mm |
300 | 320 | 36.6 | 215.9 | 269.9 | 98.6 | 12 | 22.2 | 3/4 in x 120 mm |
600 | 355 | 54.3 | 215.9 | 292.1 | 123.4 | 12 | 28.6 | 1 in x 170 mm |
900 | 380 | 62.0 | 215.9 | 317.5 | 146.4 | 12 | 31.8 | 1 1/8 in x 190 mm |
1500 | 395 | 89.0 | 215.9 | 320.0 | 177.4 | 12 | 38.1 | 1 3/8 in x 260 mm |
2500 | 485 | 114.4 | 215.9 | 368.0 | 279.4 | 8 | 54.0 | 2 in x 345 mm |
Two details in that table repay attention. The raised face diameter stays constant at 215.9 mm across every class - the sealing geometry does not change, only the metal around it. And the Class 2500 flange drops back to eight bolts, because at that point the bolt hole diameter has grown to 54 mm and eight very large studs deliver more clamping load than twelve smaller ones. Stud bolt lengths above are for a raised face joint with a spiral wound gasket and exclude the chamfered ends.
For carbon steel weld neck flanges to ASME B16.5, expect roughly 7.4 kg at NPS 4 Class 150, 11.9 kg at Class 300 and 18.9 kg at Class 600 - rising to about 39.6 kg, 63.9 kg and 101.7 kg respectively at NPS 12. The full corrected chart for NPS 1/2 to NPS 24 is below.
Corrected weld neck flange weight chart, carbon steel (kg) - Class 150, 300 and 600
NPS | DN | Class 150 (kg) | Class 300 (kg) | Class 600 (kg) |
1/2 | 15 | 0.9 | 0.9 | 1.4 |
3/4 | 20 | 0.9 | 1.4 | 1.8 |
1 | 25 | 1.4 | 1.8 | 1.8 |
1 1/4 | 32 | 1.4 | 2.3 | 2.7 |
1 1/2 | 40 | 1.8 | 3.2 | 3.6 |
2 | 50 | 2.7 | 4.1 | 5.4 |
2 1/2 | 65 | 4.5 | 5.4 | 8.1 |
3 | 80 | 5.2 | 8.1 | 10.4 |
3 1/2 | 90 | 5.4 | 9.0 | 11.7 |
4 | 100 | 7.4 | 11.9 | 18.9 |
5 | 125 | 9.5 | 16.2 | 30.6 |
6 | 150 | 11.7 | 20.3 | 36.5 |
8 | 200 | 18.9 | 31.1 | 54.0 |
10 | 250 | 24.3 | 45.0 | 85.5 |
12 | 300 | 39.6 | 63.9 | 101.7 |
14 | 350 | 51.3 | 92.7 | 156.2 |
16 | 400 | 63.0 | 112.5 | 216.5 |
18 | 450 | 74.3 | 144.0 | 249.8 |
20 | 500 | 88.7 | 180.0 | 310.5 |
22 | 550 | 101.3 | 209.3 | 324.0 |
24 | 600 | 120.6 | 261.0 | 439.7 |
These weights are for forged carbon steel at 7.85 g/cm3 and are approximate - actual weight varies by manufacturer, by bore schedule and by machining practice. Use them for planning, freight estimating and support design, and ask your supplier for the certified actual weight before you commit to a shipping or lifting plan.
Because several widely circulated charts understate weld neck flange mass by roughly 40 to 50 per cent. A common error gives NPS 4 Class 150 as 3.9 kg when the correct figure is about 7.4 kg - and that mistake propagates into freight quotes, lifting plans and pipe support designs. If a chart you are using disagrees with the table above, check it against a second source before relying on it.
Cross-check: understated figures versus corrected weld neck weights (Class 150, kg)
NPS | Commonly published (understated) | Corrected | Understatement |
2 | 1.6 kg | 2.7 kg | about 41% low |
4 | 3.9 kg | 7.4 kg | about 47% low |
6 | 6.8 kg | 11.7 kg | about 42% low |
8 | 10.5 kg | 18.9 kg | about 44% low |
12 | 21.8 kg | 39.6 kg | about 45% low |
16 | 35.0 kg | 63.0 kg | about 44% low |
24 | 78.5 kg | 120.6 kg | about 35% low |
It is worth showing how the error is caught, because the method is more useful than any single number. Model the flange as a solid disc of outside diameter O and thickness tf with the pipe bore removed, plus a hub cylinder of diameter X and length (Y - tf), then multiply by density. For NPS 4 Class 150 that gives roughly 8.8 kg against the tabulated 7.4 kg - about 19 per cent high, exactly as expected, because the real hub is a taper, not a cylinder. The understated chart's 3.9 kg is not a rounding difference; it is a different component.
We publish this correction openly because it is the kind of error that costs money downstream. An understated flange weight produces an underdesigned pipe support, a lifting plan based on the wrong load, and a freight quote that gets revised after the truck is loaded. For nickel alloy flanges the error is larger in absolute terms still, because the material is worth several times more per kilogram - see the density conversion table below.
Multiply the carbon steel weight by the ratio of the alloy density to 7.85 g/cm3. The correction is small for stainless - about 2 per cent - but reaches about 13 per cent for Hastelloy C276, which matters on a heavy flange and matters much more on the invoice than on the crane.
Density conversion factors for weld neck flange weight (relative to carbon steel at 7.85)
Material | Typical density (g/cm3) | Multiply carbon steel weight by | NPS 6 Class 150 example (kg) |
Carbon steel A105 | 7.85 | 1.000 | 11.7 |
Duplex 2205 / F51 | 7.80 | 0.994 | 11.6 |
Stainless 304 / F304 | 7.93 | 1.010 | 11.8 |
Stainless 316 / F316 | 7.98 | 1.017 | 11.9 |
Inconel 625 / N06625 | 8.22 | 1.047 | 12.2 |
Monel 400 / N04400 | 8.44 | 1.075 | 12.6 |
Hastelloy C276 / N10276 | 8.89 | 1.133 | 13.3 |
The weight difference between a carbon steel and a nickel alloy flange is therefore modest - but the price difference is not. On a large nickel alloy flange the material dominates the cost, which is why C276 flanges and Inconel 625 flanges are quoted against a certified weight rather than a nominal one.
Three faces matter in practice: raised face (RF) for general process service, flat face (FF) when bolting to cast iron or brittle equipment, and ring type joint (RTJ) for Class 600 and above or where a metal-to-metal seal is required. Choose on the mating item and the service, not on habit - a flat face against a raised face is a guaranteed leak and a cracked flange.
WN flange face types and selection rules
Face | Seal mechanism | Raised face height | Typical classes | Use when |
Raised face (RF) | Soft or semi-metallic gasket compressed on the raised annulus | 1.6 mm for Class 150/300; 6.4 mm for Class 400 and above | 150 to 2500 | General process service - the default for most piping |
Flat face (FF) | Full-face gasket; no bending moment from the raised annulus | None - face is flat across the full diameter | 150 to 300 | Mating to cast iron, glass-lined, plastic or other brittle equipment |
Ring type joint (RTJ) | Metal ring gasket in a machined groove, metal-to-metal seal | Groove machined into the face | 600 and above | High-pressure gas, oilfield, volatile or high-temperature service |
Tongue and groove | Male-female interlock confining the gasket | Machined | 150 to 1500 | Where gasket blowout resistance is needed |
Male and female | Recessed gasket seat | Machined | 150 to 1500 | Legacy and specialist services |
Never bolt a raised face flange to a flat face flange. The raised annulus loads the flat flange in bending across its full width and will crack it, and the gasket will not compress uniformly. If you must transition, use a raised face to flat face adapter or specify the mating equipment with a raised face.
Raised face finish matters as much as the face type. ASME B16.5 calls for a serrated finish, concentric or spiral, in the range of about 3.2 to 6.3 micrometres Ra (125 to 250 microinch). A mirror-smooth face will not grip a soft gasket, and a face that is too rough will leak through the serration valleys.
Three: the standard weld neck for pipe-to-pipe and pipe-to-valve joints, the long weld neck (LWN) whose extended barrel replaces the first section of a vessel or tank nozzle, and the reducing weld neck whose bore and weld end are a smaller size than the flange, used to change line size at the flange itself.
WN flange neck variations
Variation | Geometry | Weld end | Typical use | Dimension note |
Standard weld neck | Tapered hub, bore matched to pipe | Butt weld to matching pipe size | Pipe to pipe, pipe to valve or equipment | Length through hub Y per ASME B16.5 |
Long weld neck (LWN) | Extended straight barrel before the flange | Butt weld at the far end of the barrel | Vessel, tank and column nozzles | Barrel length is specified by the purchaser, not by B16.5 |
Reducing weld neck | Full-size flange, reduced bore and weld end | Butt weld to the smaller pipe size | Changing line size without a separate reducer | State both sizes: NPS 4 x NPS 2, for example |
A long weld neck is often the right answer on a vessel nozzle because it removes one weld from the joint. Instead of welding a short nozzle stub to a flange and then to the shell, the LWN barrel is welded directly into the shell or head, so the number of welds and the number of examinations both drop. The barrel length is a purchaser-specified dimension - ASME B16.5 tabulates the flange end, not the barrel.
A reducing weld neck is economical only where the size change is needed at that exact location. If you have room, a standard flange plus a buttwelding reducer is usually cheaper and easier to replace later.
Choose on pressure, inspection requirement and dismantling frequency: weld neck for critical, high-pressure or radiographed joints; slip-on for low-cost non-critical lines; socket weld for small bore; threaded where welding is not permitted; blind to close an end; and lap joint where the line is dismantled often or bolt hole alignment is difficult.
Flange type selection comparison
Flange type | Connection | Pressure / temperature suitability | Relative cost | RT / UT inspectable | Best application |
Weld neck (WN) | Butt weld, full penetration | High pressure and temperature, cyclic load | High | Excellent | Critical, high-pressure, inspectable piping |
Slip-on (SO) | Two fillet welds | Low to moderate pressure | Low | Poor | Low-cost, non-critical lines |
Socket weld (SW) | Single fillet weld | Small bore, moderate to high pressure | Low to medium | Poor | Small-bore high-pressure utility lines |
Threaded | Screwed connection | Low pressure, no welding | Low | Not applicable | Non-welded, non-flammable, non-toxic areas |
Blind | No bore - closes the end | Matches the line rating | Medium | Not applicable | Line and vessel blanks, hydrotest isolation |
Lap joint (LJ) | Slips over a stub end, bolted | Low to moderate pressure | Medium | Not applicable | Frequent dismantling, alignment-critical or expensive piping |
The cost comparison is often read backwards. A slip-on flange is cheaper to buy, but the installed cost is closer than the purchase price suggests, because a slip-on needs two fillet welds against one groove weld, and the fillet welds cannot be volumetrically examined. Where the line class requires radiography, the weld neck is the only compliant choice and the comparison is not really a comparison at all.
· **Specify weld neck when** the design pressure or temperature is high, the service cycles, the line class calls for RT or UT, or the consequence of a leak is severe.
· **Specify slip-on when** the line is low pressure, non-hazardous and cost-driven - see the slip-on flange guide.
· **Specify socket weld when** the bore is small and welding on the inside of the pipe is impractical - see what SW flange means.
· **Specify threaded where** hot work is not permitted - see the threaded flange guide.
· **Specify blind to** close a line end or isolate for hydrotest - see the blind flange guide.
· **Specify lap joint when** the line is dismantled frequently or when bolt hole alignment must be adjustable - see what a lap joint flange is.
The class number is not a pressure. Class 150 gives about 19.6 bar (285 psi) at ambient for Group 1.1 carbon steel, and it de-rates continuously with temperature - to about 1.4 bar at 1000 °F. You must read the allowable pressure from the ASME B16.5 pressure-temperature table for the specific material group shown on the flange.
Maximum allowable pressure by class, Group 1.1 carbon steel at ambient
Class | At -20 to 100 °F | In bar | Approximate PN equivalent | Typical service |
150 | 285 psi | 19.6 | PN 20 | Water, air, low-pressure process |
300 | 740 psi | 51.0 | PN 50 | General process piping |
400 | 990 psi | 68.3 | PN 68 | Legacy systems |
600 | 1480 psi | 102.0 | PN 100 | High-pressure process |
900 | 2220 psi | 153.1 | PN 150 | High-pressure steam |
1500 | 3705 psi | 255.5 | PN 250 | Critical high pressure |
2500 | 6170 psi | 425.4 | PN 420 | Ultra-high pressure |
Class 150 de-rating with temperature, Group 1.1 carbon steel (A105)
Temperature (°F) | Allowable pressure (psi) | Allowable pressure (bar) |
-20 to 100 | 285 | 19.6 |
200 | 260 | 17.9 |
300 | 230 | 15.9 |
400 | 200 | 13.8 |
500 | 170 | 11.7 |
600 | 140 | 9.7 |
650 | 125 | 8.6 |
700 | 110 | 7.6 |
800 | 80 | 5.5 |
900 | 50 | 3.4 |
1000 | 20 | 1.4 |
The two tables above are for ASME B16.5 material Group 1.1 (A105 and equivalent carbon steel). ASME B16.5 Table 2 publishes a separate pressure-temperature table for every material group, and the group number is not the same as the class. Common mappings: Group 1.1 carbon steel, Group 2.1 austenitic 304 and 304L, Group 2.2 austenitic 316 and 316L, Group 2.3 stabilised 321 and 347. Read the rating from the table for the group printed on the flange.
This is the single most expensive misunderstanding in flange specification. A Class 300 stainless flange does not have the same allowable pressure as a Class 300 carbon steel flange at elevated temperature, because austenitic stainless loses strength with temperature on a different curve. On nickel alloy piping - see nickel alloy flanges for subsea and deepwater service - the margin is different again, and the rating must come from the correct table, not from the class number.
ASTM A105 for carbon steel forgings, A350 LF2 for low-temperature service, A182 for stainless and alloy steel forgings, A694 for high-yield transmission flanges, and ASTM B564 for nickel alloy forgings. ASME B16.5 governs the dimensions regardless of material; the ASTM specification governs what the metal is.
Material specifications for weld neck flanges
Specification | Material family | Typical grades | Service |
ASTM A105 | Carbon steel forgings | A105 | Ambient and elevated temperature, general service |
ASTM A350 | Low-temperature carbon steel | LF2, LF3 | Low-temperature and cryogenic service requiring notch toughness |
ASTM A182 | Stainless and alloy steel forgings | F304, F304L, F316, F316L, F321, F347, F51, F53 | Corrosive service, high temperature, duplex for chlorides |
ASTM A694 | High-yield carbon and alloy steel | F42 to F70 | High-pressure transmission and gathering lines |
ASTM B564 | Nickel alloy forgings | N02200, N02201, N04400, N06600, N06625, N08825, N10276, N06022, N06200 | Severe corrosion, sour service, high temperature |
Nickel alloy weld neck flanges are the case where material and geometry interact most. The dimensions come from ASME B16.5 and are identical to a carbon steel flange of the same size and class - but the allowable pressure comes from a different material group table, the welding procedure is entirely different, and the price is several times higher. See C276 flange dimensions, Inconel 625 flanges, Monel 400 flanges, Incoloy 825 flanges and Nickel 201 flanges for the grade-specific pages, and the C276 welding guide and the Inconel 625 welding guide for the weld procedure.
Thickness carries a plus-only tolerance - +3.2 mm / -0 for NPS up to 18 and +4.8 mm / -0 for NPS 20 and larger - because thickness is a minimum. Bolt circle is +/- 1.6 mm, bolt hole diameter is +0.8 mm / -0, and outside diameter is +/- 1.6 mm for NPS up to 24.
ASME B16.5 dimensional tolerances for weld neck flanges
Dimension | Size range | Tolerance | Practical meaning |
Outside diameter (O) | NPS up to 24 | +/- 1.6 mm | Affects clearance and wrench access |
Outside diameter (O) | Above NPS 24 | +/- 3.2 mm | Large diameter flanges are permitted more variation |
Thickness (tf) | NPS up to 18 | +3.2 mm / -0 | Thickness is a minimum - a thicker flange is compliant |
Thickness (tf) | NPS 20 and larger | +4.8 mm / -0 | Same principle, larger band |
Bolt circle (W) | All sizes | +/- 1.6 mm | Drives bolt fit-up |
Bolt hole spacing | All sizes | +/- 0.8 mm | Cumulative fit-up error |
Bolt hole diameter | All sizes | +0.8 mm / -0 | Holes may be larger, never smaller |
Bolt circle eccentricity | NPS up to 2 1/2 | 0.8 mm max | Concentricity to the facing |
Bolt circle eccentricity | NPS 3 and larger | 1.6 mm max | Concentricity to the facing |
Bore | NPS up to 10 | +/- 0.8 mm | Must match the pipe bore |
Bore | NPS 12 to 18 | +/- 1.6 mm | Must match the pipe bore |
Length through hub (Y) | NPS up to 10 | +/- 1.6 mm | Affects the weld end take-off |
Length through hub (Y) | NPS 12 and larger | +/- 3.2 mm | Affects the weld end take-off |
Raised face height | 1.6 mm face | +/- 0.8 mm | Gasket compression |
Raised face height | 6.4 mm face | +/- 0.4 mm | Gasket compression |
Tolerance values differ slightly between editions of ASME B16.5 and between published summaries - some sources round the same limits to 1.5 mm or 3.0 mm. Always quote against the edition named on your project specification, and treat the table above as planning guidance rather than as a Code extract.
The plus-only thickness tolerance is the one that surprises buyers. Flange thickness is a minimum dimension in ASME B16.5, so a flange that arrives thicker than the tabulated value is compliant and is not a defect. Rejecting one on those grounds is a specification error, and it is a common one.
Stud bolts sized to the bolt hole diameter of the class and size, two nuts per bolt, and a gasket sized to the raised face diameter - not to the bore. Use ASME B16.20 spiral wound or ring joint gaskets for metallic service and ASME B16.21 for non-metallic full-face or ring gaskets, and size the stud length from the combined flange thickness, gasket thickness and nut height.
Bolting and gasket selection by flange class
Class | Typical gasket | Gasket standard | Bolting note |
150 | Non-metallic ring gasket or spiral wound with a soft filler | ASME B16.21 / B16.20 | Lowest bolt load; easiest to seal |
300 | Spiral wound with graphite or PTFE filler | ASME B16.20 | Higher bolt load; controlled tightening required |
600 | Spiral wound, or ring joint where RTJ faces are specified | ASME B16.20 | 6.4 mm raised face; check stud length against the taller face |
900 and above | Ring joint (RTJ) for RTJ faces; spiral wound for RF | ASME B16.20 | Controlled bolt preload and procedure are mandatory |
Stud length is the dimension most often ordered wrong. It must cover both flange thicknesses, the compressed gasket thickness, two nuts and the protrusion allowance - and the tabulated length excludes the chamfered ends. The NPS 6 table earlier in this guide gives the ASME B16.5 tabulated stud lengths for a raised face joint with a spiral wound gasket, class by class, and shows how steeply they grow: 100 mm at Class 150 against 345 mm at Class 2500.
On nickel alloy piping, specify bolting material compatible with the flange and with the environment. A carbon steel stud on a C276 flange in a corrosive atmosphere is the weak link in the joint, and it is a common one.
A compliant ASME B16.5 flange is marked with the manufacturer's name or trademark, the ASTM material specification and grade, the pressure class, the size, and the B16.5 designation. If any of those five is missing, the flange is not traceable and should be rejected.
Required and expected flange marking elements
Marking element | Example | Why it matters |
Manufacturer | JN or a registered trademark | Traceability to the forging source |
ASTM material specification and grade | A182 F316 / B564 N10276 | Confirms the chemistry and mechanical properties |
Pressure class | 300 | Confirms the rating - it is not the same as pressure |
Nominal pipe size | 4 or NPS 4 | Confirms bore and drilling |
Dimensional standard | B16.5 | Confirms which dimension table applies |
Heat number or heat code | H-xxxxx | Links the flange to its material test report |
Additional purchaser requirements | RTJ, NACE, PMI | Project-specific acceptance conditions |
Marking is where a material test report and a physical flange are joined together. If the heat code on the flange does not appear on the certificate, you do not have traceability - you have two unrelated documents. On nickel alloy flanges, where the material value is high and substitution is a real risk, positive material identification is worth specifying.
ASME B16.5 covers NPS 1/2 to NPS 24 in classes 150 through 2500. From NPS 26 to NPS 60 the governing standard is ASME B16.47, which has two non-interchangeable series - Series A, derived from MSS SP-44, and Series B, derived from API 605.
ASME B16.5 versus ASME B16.47
Standard | Size range | Pressure classes | Origin | Interchangeable? |
ASME B16.5 | NPS 1/2 to NPS 24 | 150, 300, 400, 600, 900, 1500, 2500 | Original US flange standard | Reference standard |
ASME B16.47 Series A | NPS 26 to NPS 60 | 150, 300, 400, 600, 900 | Derived from MSS SP-44 | No - heavier, more bolts |
ASME B16.47 Series B | NPS 26 to NPS 60 | 75, 150, 300, 400, 600, 900 | Derived from API 605 | No - lighter, fewer bolts |
Series A and Series B flanges of the same size and class will not bolt together. They have different outside diameters, different bolt circles and different bolt counts. If you are replacing a single large-diameter flange, identify which series the existing line uses before ordering - this is a routine and expensive error. See B16.47 Series A dimensions, Series B dimensions and large-diameter flange dimensions.
State the size, class, face type, material specification and grade, bore schedule, dimensional standard with edition, gasket and bolting if supplied, certification level and marking requirements. ASME B16.5 does not fix the bore - the purchaser must - so a flange order without a schedule on it is incomplete.
Step 1. Fix the pipe size, schedule and bore: The flange bore must match the pipe you are welding to. Take the pipe outside diameter and wall thickness from ASME B36.19M and state the schedule on the order, because ASME B16.5 does not fix the bore - the purchaser specifies it.
Step 2. Choose the pressure class from the design condition: Take the design pressure and design temperature, then read the allowable pressure from the ASME B16.5 pressure-temperature table for your material group. Class 150 is not 150 psi; it is about 19.6 bar at ambient for carbon steel and far less at high temperature.
Step 3. Pick the material specification and grade: Carbon steel to ASTM A105, low temperature to A350 LF2, stainless to A182 F304 or F316, duplex to A182 F51 or F53, and nickel alloys to ASTM B564. Match the flange material to the pipe material so the weld procedure and the corrosion behaviour are consistent.
Step 4. Select the face type: Raised face for general process service, flat face when the mating item is cast iron or another brittle material, and ring type joint for Class 600 and above or where a metal-to-metal seal is required.
Step 5. Select the neck type: Standard weld neck for pipe-to-pipe and pipe-to-valve, long weld neck for vessel and tank nozzles where the barrel replaces the first section of the nozzle, and reducing weld neck where the line size changes at the flange.
Step 6. Read off the dimensions you need to install: Outside diameter and bolt circle for the bolting layout, thickness and length through hub for the weld end and the overall take-off, and raised face diameter for the gasket. Use the dimension tables in this guide for Class 150, 300 and 600.
Step 7. Calculate or request the weight: Use the corrected weight chart in this guide for an estimate, adjust for material density, and ask your supplier for the certified actual weight before booking freight or designing pipe supports.
Step 8. Write the full specification onto the purchase order: NPS, class, face type, material specification and grade, bore schedule, the dimensional standard (ASME B16.5 with edition), gasket and bolting if supplied, certification level, and marking requirements.
Purchase order line specification for a weld neck flange
Field | Example entry | Why it is required |
Type | Weld neck flange (WN) | Fixes the hub geometry and the weld end |
Size | NPS 6 (DN 150) | Fixes bore, drilling and dimensions |
Pressure class | Class 300 | Fixes thickness, drilling and rating |
Face | Raised face (RF), serrated finish | Fixes gasket type and sealing |
Dimensional standard | ASME B16.5 (state the edition) | Fixes which dimension table applies |
Material | ASTM A182 F316 / ASTM B564 UNS N10276 | Fixes chemistry and properties |
Bore | Sch 40S (state wall thickness in mm) | ASME B16.5 does not fix the bore - the purchaser does |
Quantity | 24 pieces | Commercial |
Certification | EN 10204 Type 3.1 material test report | Traceability to the heat |
Marking | Manufacturer, ASTM spec and grade, class, size, B16.5, heat code | Required by ASME B16.5 |
Supplementary | PMI testing, NACE MR0175 compliance, hydrotest | Project-specific acceptance |
The two fields most often left off are the bore schedule and the edition of B16.5. Leaving off the bore means the supplier will ship a standard bore that may not match your pipe schedule, which turns into a welding problem at the fit-up. Leaving off the edition means a dimension that changed between editions can be supplied in good faith and still be wrong for your project.
Six errors account for most of the problems: treating the class as a pressure, omitting the bore schedule, mismatching the raised face height with the class, mating a raised face to a flat face, mixing B16.47 Series A with Series B, and relying on an understated weight chart. Every one of them is prevented by a single extra line on the purchase order.
WN flange specification mistakes and their consequences
Mistake | What happens | Prevention |
Treating class as pressure | A Class 150 flange specified for 150 psi at 600 °F fails - the true allowable is about 9.7 bar at 600 °F, not 19.6 | Read the pressure-temperature table for the material group and temperature |
Omitting the bore schedule | The flange bore does not match the pipe, producing a step at the weld and a fit-up problem | State the schedule or the bore diameter in millimetres on the order |
Wrong raised face height | The gasket bottoms out on the raised face instead of compressing | Confirm 1.6 mm for Class 150/300 and 6.4 mm for Class 400 and above |
Raised face mated to flat face | The flat flange is loaded in bending and cracks; the joint leaks | Specify matching faces, or use an approved adapter |
Mixing B16.47 Series A and Series B | Large-diameter flanges will not bolt up - different OD, bolt circle and bolt count | Identify the existing series before ordering a replacement |
Using an understated weight chart | Undersized pipe supports, wrong lifting plan, revised freight quote | Cross-check against a second published source before designing or booking |
The class-as-pressure error deserves emphasis because it is genuinely dangerous, not just inconvenient. Class 150 does not mean 150 psi. At 600 °F the allowable pressure for Group 1.1 carbon steel is about 140 psi, and at 1000 °F it is about 20 psi. A flange selected on the class number alone is a flange selected without a rating.
Verify the marking against the purchase order and the material test report, confirm the heat number links the two, measure the critical dimensions - outside diameter, bolt circle, thickness and bore - and check the face for damage and correct finish. Reject anything whose marking and certificate do not reconcile.
· Marking versus certificate. The manufacturer, ASTM specification and grade, class, size and heat code on the flange must all appear on the material test report. If they do not, you do not have traceability.
· Dimensional check. Measure outside diameter, bolt circle diameter, bolt hole diameter and bolt count, flange thickness and bore. Confirm each against the ASME B16.5 table for the stated class and size, allowing the tolerances listed earlier in this guide.
· Face condition. Check the raised face for scoring, dents and corrosion, and confirm the serrated finish is present. A damaged sealing surface will leak regardless of bolt load.
· Bore match. Confirm the bore matches the pipe schedule you are welding to. A mismatch here produces a weld misalignment that is expensive to correct after the fact.
· Material verification. On nickel alloy and stainless flanges, consider positive material identification, especially where a substitution risk exists or where the consequence of a mix-up is severe.
· Segregation. Keep stainless and nickel alloy flanges away from carbon steel handling equipment, slings and storage. Embedded free iron rusts and will show up later as corrosion on a flange you paid a premium to protect.
Start with the flange hub for the full product range, then the weld neck product page for purchasing, then the grade-specific pages for nickel alloys, then the ASME B16.5 and B16.47 dimension pages for the tables themselves. The list below links every page that carries detail this guide only summarises.
· Flanges - the JN Alloy flange hub covering the full product range
· Weld neck flanges - product page for sizes, materials and supply
· Know about weld neck flanges - functions, types and applications
· ASME B16.5 flange dimension chart - all flange types across every pressure class
· ASME B16.47 Series A and Series B - large diameter flange dimensions from NPS 26 to NPS 60
· Large diameter flange dimensions - the NPS 26 and above range
· Hastelloy C276 flanges and the complete C276 flange guide
· Inconel 625 flanges, Inconel 625 flange supply and Inconel 718 flanges
· Monel 400 flanges and Monel 400 flange dimensions
· Incoloy 825 flanges, Nickel 201 flanges and Alloy C2000 flanges
· Inconel 600 flanges, Incoloy 800 flanges and Hastelloy G30 flanges
· 904L flanges, 254 SMO flanges, duplex 2205 flanges and 316H flanges
· Nickel alloy flanges for subsea and deepwater service
· Slip-on flanges, blind flanges, socket weld flanges, threaded flanges, lap joint flanges, spectacle blinds, orifice flanges and plate flanges
· ASME B36.19M pipe dimensions and Schedule 40 versus Schedule 80 - for matching the flange bore to the pipe
· C276 welding guide, Inconel 625 welding guide and pipe spool fabrication - what happens after the flange arrives
· Nickel alloy pipe and prefabrication services - the rest of the line class
What does WN flange mean?
WN stands for weld neck. A WN flange has a long tapered hub that is butt-welded to the pipe, producing a full-penetration joint that can be examined by radiography or ultrasonics. It is the strongest and most inspectable flange type covered by ASME B16.5.
What is the difference between a weld neck flange and a slip-on flange?
A weld neck flange is butt-welded to the pipe with a single full-penetration groove weld and has a tapered hub that transfers stress into the pipe. A slip-on flange slides over the pipe and is held by two fillet welds, is cheaper, and cannot be meaningfully radiographed. Use weld neck for critical service and slip-on only for low-pressure non-critical lines.
What are the dimensions of a Class 150 weld neck flange?
At NPS 4, a Class 150 weld neck flange is 230 mm outside diameter, 24.3 mm thick, with a 157.2 mm raised face, 135 mm hub diameter at the base, 76 mm length through hub, a 190.5 mm bolt circle, 8 bolt holes of 19.1 mm. The full NPS 1/2 to NPS 12 table is in this guide.
How much does a weld neck flange weigh?
For carbon steel to ASME B16.5, roughly 7.4 kg at NPS 4 Class 150, 11.9 kg at Class 300 and 18.9 kg at Class 600. At NPS 12 the same classes weigh about 39.6 kg, 63.9 kg and 101.7 kg. Multiply by the density ratio for stainless or nickel alloy - about 1.02 for 316 and about 1.13 for Hastelloy C276.
Why do flange weight charts disagree with each other?
Because several widely circulated charts understate weld neck flange mass by 40 to 50 per cent. If one chart gives NPS 4 Class 150 as 3.9 kg and another gives 7.4 kg, the second is correct for carbon steel. Cross-check against a second source before using any weight chart for support design, lifting plans or freight.
Is Class 150 the same as 150 psi?
No. Class 150 is a pressure class designation, not a pressure rating. For Group 1.1 carbon steel it is about 19.6 bar (285 psi) from -20 to 100 °F, but it de-rates to about 9.7 bar at 600 °F and about 1.4 bar at 1000 °F. The allowable pressure also changes with material group.
What is the raised face height on a flange?
1.6 mm (1/16 in) for Class 150 and Class 300, and 6.4 mm (1/4 in) for Class 400, 600, 900, 1500 and 2500. Getting this wrong is a common cause of a gasket that bottoms out on the raised face instead of compressing.
Can I bolt a raised face flange to a flat face flange?
No. The raised annulus loads the flat flange in bending across its full width and will crack it, and the gasket will not compress uniformly. Specify matching faces, or use an approved raised face to flat face adapter.
What is a long weld neck flange used for?
A long weld neck (LWN) flange has an extended straight barrel instead of a short hub, and is used for vessel, tank and column nozzles. It replaces the first section of the nozzle, so the joint needs one weld instead of two. The barrel length is specified by the purchaser, not by ASME B16.5.
What is a reducing weld neck flange?
A reducing weld neck flange has a full-size flange with a smaller bore and weld end, so the line size changes at the flange itself. It is specified as two sizes, for example NPS 4 x NPS 2. Where there is space, a standard flange plus a separate reducer is usually cheaper.
What sizes does ASME B16.5 cover?
NPS 1/2 to NPS 24, in pressure classes 150, 300, 400, 600, 900, 1500 and 2500. Sizes from NPS 26 to NPS 60 are covered by ASME B16.47 Series A or Series B.
Are ASME B16.47 Series A and Series B flanges interchangeable?
No. They have different outside diameters, bolt circles and bolt counts, so a Series A flange will not bolt to a Series B flange of the same size and class. Identify the existing series before ordering a replacement - see Series A and Series B.
What material specifications cover weld neck flanges?
ASTM A105 for carbon steel, A350 LF2 for low-temperature service, A182 for stainless and alloy steel forgings such as F304, F316, F321 and F347, A694 for high-yield transmission service, and ASTM B564 for nickel alloys including N10276, N06625, N04400 and N08825.
Does ASME B16.5 specify the flange bore?
No. ASME B16.5 tabulates the outside diameter, thickness, hub, drilling and raised face, but the bore must be specified by the purchaser to match the pipe schedule. An order without a bore schedule is incomplete and a frequent cause of weld misalignment.
What are ASME B16.5 flange thickness tolerances?
+3.2 mm / -0 for NPS up to 18 and +4.8 mm / -0 for NPS 20 and larger. Flange thickness is a minimum dimension, so a flange that arrives thicker than the tabulated value is compliant and should not be rejected.
What is the bolt circle tolerance on a flange?
About +/- 1.6 mm for the bolt circle diameter, with bolt hole spacing at +/- 0.8 mm and bolt hole diameter at +0.8 mm / -0. Bolt holes may be larger than tabulated but never smaller.
Which face type should I choose - RF, FF or RTJ?
Raised face for general process service, flat face when bolting to cast iron, glass-lined or other brittle equipment, and ring type joint for Class 600 and above or where a metal-to-metal seal is required. Never mate a raised face to a flat face.
How long should the stud bolts be for a flange joint?
Long enough to pass through both flanges plus the compressed gasket and two nuts, with protrusion allowance. The tabulated length excludes the chamfered ends. For NPS 6 raised face with a spiral wound gasket, ASME B16.5 gives 100 mm at Class 150 rising to 345 mm at Class 2500.
What markings must a weld neck flange carry?
The manufacturer's name or trademark, the ASTM material specification and grade, the pressure class, the size and the ASME B16.5 designation. A heat code linking the flange to its material test report is expected and should be verified on receipt.
How do I check a flange on receipt?
Reconcile the marking against the purchase order and the material test report, confirm the heat number links the two, measure outside diameter, bolt circle, thickness and bore against the ASME B16.5 table for the stated class, and check the raised face for damage and correct serrated finish.
What is the bore of a weld neck flange matched to?
The pipe it is welded to. The bore is machined to match the pipe inside diameter so there is no step, crevice or turbulence at the joint - which is a large part of why weld neck flanges have better fatigue life than slip-on or socket weld construction. State the pipe schedule on the order.
Are nickel alloy weld neck flanges dimensionally different from carbon steel ones?
No. The ASME B16.5 dimensions are identical for the same size and class regardless of material. What changes is the pressure-temperature rating, which comes from a different material group table, the welding procedure, the weight by about 2 to 13 per cent depending on density, and the price by a large multiple.
