Class 400 Weld Neck Flange Dimensions — ASME B16.5 Chart, Weights & Bolting

They are tabulated in ASME B16.5 and reproduced in full in the chart above: for each size from NPS 1/2 to 24, the outer diameter, flange thickness, hub diameters at base and bevel, length through hub, raised face diameter and height (7 mm at this class), bolt circle and weld face — all in millimetres — plus the published weight. A 4 in Class 400 weld neck flange, for example, is 255 mm OD, 35 mm thick plus the 7 mm face, on a 200 mm PCD, at about 15.9 kg.

The rating depends on material and temperature. For Group 1.1 materials such as A105 carbon steel, ASME B16.5 allows 68.1 bar (roughly 990 psig) at 38 °C, easing to about 53 bar at 300 °C and 38 bar at 450 °C — the full bar table for carbon steel, 304 and 316 groups is reproduced on this page. Stainless and alloy grades have their own columns. Always confirm the rating at your design temperature.

Class 400 (also written #400 or 400 lb) is a pressure-temperature rating class, not a working pressure of 400 psi. Each class maps to a table in ASME B16.5 giving the maximum allowable working pressure at each temperature for each material group — a Class 400 A105 flange is good for roughly 990 psig cold, well over twice the number in its name, and less when hot.

Because ASME B16.5 says so: in NPS 1/2 through 3-1/2, Class 400 flanges are furnished to Class 600 dimensions. At small bores the metal saved by a dedicated Class 400 pattern is negligible, so the standard simply deletes it — which is why the first nine rows of our chart are identical to the Class 600 chart, and why a small-bore "Class 400" order is dimensionally a Class 600 flange. From NPS 4 upward Class 400 has its own, lighter table, and that is where the class earns its keep.

No — and this is the reverse of the Class 150 and 300 convention, so it catches people. For Class 400 and above, ASME B16.5 tabulates flange thickness C excluding the 7 mm raised face, while the length through hub V includes it. For Class 150 and 300 the standard includes their 2 mm face in the thickness. Our charts follow the standard on every page and say so under each table — worth checking whenever you compare charts from different sources.

From the weight columns in our chart: a 2 inch Class 400 weld neck flange weighs about 5.4 kg (12 lbs), a 6 inch about 26 kg (57 lbs), a 12 inch about 79 kg (175 lbs) and a 24 inch about 281 kg (620 lbs) — a little heavier than Class 300, noticeably lighter than Class 600 in the larger bores. These are published carbon-steel weights with a standard bore; the exact weight varies with the bore schedule and material density.

Class 400 is where the tall face begins: 7 mm (nominally 1/4 inch), against the 2 mm face of Class 150 and 300. The taller face adds to every stud length, which is why a take-off that steps from 300 to 400 must not carry its bolting assumptions across — the classic estimating miss. Our stud chart below already allows for the 7 mm face.

With Class 300, never — every size differs in OD, bolt circle and bolting. With Class 600 the honest answer is "only where B16.5 makes them identical": in NPS 1/2 through 3-1/2 a Class 400 flange is furnished to Class 600 dimensions, so the two mate perfectly — but the rating stamped on the flange still governs what the joint may carry. From NPS 4 upward, Class 400 and 600 are different flanges on different bolt circles and do not interchange.

ASME B16.5 covers pipe flanges from NPS 1/2 to NPS 24 (DN 15 to DN 600) in classes 150, 300, 400, 600, 900, 1500 and 2500. Flanges larger than 24 inches are covered by ASME B16.47 (Series A and B). Our Class 400 weld neck chart on this page lists twenty-one sizes with dimensions in millimetres, NPS 22 included.

The bore of a weld neck flange is machined to match the inside diameter of the pipe it will be welded to, so state the pipe schedule with your order. At Class 400 pressures the mating pipe is commonly Sch 80 or heavier, and the match matters: a bore step at the butt weld concentrates turbulence and erosion exactly where the joint works hardest. That is why the Bore column in our chart reads "as per pipe schedule".

We manufacture Class 400 weld neck flanges in carbon steel A105, low-temperature A350 LF2/LF3, stainless steel A182 F304/L, F316/L, F310, F321, F347 and F410, 254 SMO (F44), 904L, duplex 2205 (F51), super duplex 2507 (F53/F55), alloy steel F1/F5/F9/F11/F22/F91, Monel 400/K500, Inconel 600/625/825, Hastelloy C276/C22, titanium, copper nickel 90/10 and 70/30, and bimetal/clad — each with EN 10204 3.1 mill certificates.

Stud bolts with two heavy hex nuts, as a rule — by Class 400 the bolt loads and the 7 mm face have taken machine bolts out of ordinary practice, which is why our chart lists stud lengths only. A 2 inch flange takes 8 studs of 5/8 inch UNC, a 12 inch takes 16 of 1-1/4 inch, and a 24 inch takes 24 of 1-3/4 inch, with A193 B7 / A194 2H the standard stud and nut grades on carbon-steel joints.

Spiral wound gaskets to ASME B16.20 with an outer centring ring are the standard at Class 400 bolt loads — graphite-filled for steam and hydrocarbons, PTFE-filled for chemical duty. Sheet gaskets have no real place at this class. RTJ facings with oval or octagonal ring gaskets are available on request for hydrocarbon service — state RF or RTJ with the enquiry.

At Class 400 pressures the question has largely answered itself: the weld neck's tapered hub and full-penetration butt weld give the joint the strength and fatigue life of the pipe itself, with a smooth bore and a weld open to radiography — which is what most line classes at this rating demand. Slip-on flanges exist at Class 400 but are uncommon; on steam, hot oil and process duty the weld neck is the standard choice.

In the larger bores. Below NPS 4 the question is moot — B16.5 furnishes small Class 400 flanges to Class 600 dimensions anyway. From NPS 4 up, Class 400 buys a genuine weight and cost saving over Class 600 (a 24 inch weld neck saves roughly 70 kg per flange) for services that sit between the Class 300 and 600 envelopes. The honest caveat: many stockists carry 300 and 600 deep and 400 thin, so for small quantities the schedule may favour the step to 600 — for project quantities, manufactured to order, Class 400 earns its place.

Every batch is dimensionally inspected against ASME B16.5, marked per MSS SP-25 and supplied with EN 10204 3.1 mill test certificates traceable by heat number. We offer PMI, ultrasonic and magnetic particle examination, impact testing for low-temperature grades, NACE MR0175 compliance for sour service, and third-party inspection by LR, BV, DNV, SGS, TÜV or IBR on request.

Medium-pressure steam and feedwater circuits in power plants, refinery and petrochemical process units, hot oil systems and gas processing — services that sit above the Class 300 envelope but do not justify Class 600 metal. Because the class is stocked thinly, it appears most often on engineered projects where the take-off is manufactured to order.

Yes — "ANSI 400", "ANSI B16.5" and "400 lb" all refer to what is today the ASME B16.5 Class 400 table; the standard transferred from ANSI to ASME custodianship decades ago but the old name persists on drawings and enquiries. The dimensions on this page are the current ASME B16.5 values.