Class 900 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 of the nineteen sizes 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 900 weld neck flange, for example, is 290 mm OD, 44.5 mm thick plus the 7 mm face, on a 235 mm PCD, at about 24 kg.

The rating depends on material and temperature. For Group 1.1 materials such as A105 carbon steel, ASME B16.5 allows 153.2 bar (roughly 2220 psig) at 38 °C, easing to about 120 bar at 300 °C and 86 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 900 (also written #900 or 900 lb) is a pressure-temperature rating class, not a working pressure of 900 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 900 A105 flange is good for roughly 2220 psig cold, nearly two and a half times the number in its name, and less when hot.

No — 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. That is the reverse of the Class 150 and 300 convention, where 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.

Look closely at the chart and the anomaly is real: the 2-1/2 in flange is 245 mm OD, 41.3 mm thick, on heavier bolting than the 3 in at 240 mm OD and 38.1 mm — the smaller bore outweighs the larger, 16.3 kg against 14.1. The reason is B16.5's small-bore rule: Class 900 flanges in NPS 1/2 through 2-1/2 are furnished to Class 1500 dimensions, and the 1500 pattern at 2-1/2 in happens to be bigger than Class 900's own pattern at 3 in. It looks like a misprint on every chart that carries it; it is the standard working as written.

Two gaps are worth knowing. NPS 3-1/2 does not exist at Class 900 — the standard drops the size entirely at this rating — and neither does NPS 22, so the chart runs nineteen sizes from 1/2 to 24 rather than the twenty-one of the lighter classes. A take-off that carries a 3-1/2 in line into Class 900 territory has to move to 4 in (or drop to 3 in) at the spec break.

From the weight columns in our chart: a 2 inch Class 900 weld neck flange weighs about 11.3 kg (25 lbs), a 6 inch about 50 kg (110 lbs), a 12 inch about 147 kg (324 lbs) and a 24 inch about 680 kg (1500 lbs) — nearly double Class 600, size for size, and at the big end these are crane-handled components. These are published carbon-steel weights with a standard bore; the exact weight varies with the bore schedule and material density.

7 mm (nominally 1/4 inch) — the tall face shared by Class 400 and above, against the 2 mm face of Class 150 and 300. In practice, though, many Class 900 hydrocarbon and wellhead line classes specify RTJ facings instead, where an octagonal ring gasket seats in a machined groove — state RF or RTJ with the enquiry, because the two are not interchangeable in the field.

With Class 600, never — every size differs in OD, bolt circle and bolting. With Class 1500 the identity runs small-bore only: NPS 1/2 through 2-1/2 Class 900 flanges are furnished to Class 1500 dimensions, so those sizes mate perfectly with their 1500 counterparts — the stamped rating still governs the joint. From NPS 3 upward, 900 and 1500 are different flanges and do not interchange.

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 900 pressures the mating pipe runs Sch 80, 120, 160 or heavier, and the match matters more than ever: 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 900 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, always — and at this class the bolting is serious hardware. A 2 inch flange takes 8 studs of 7/8 inch UNC; a 12 inch takes 20 of 1-3/8 inch; a 24 inch takes 20 studs of 2-1/2 inch UNC at 440 mm long, joints that are routinely made up with hydraulic tensioners rather than wrenches. A193 B7 / A194 2H remain the standard grades on carbon-steel joints, with B16 for high-temperature service.

Two cultures share this class. Spiral wound gaskets to ASME B16.20 with inner and outer rings serve RF joints — graphite-filled as the default. But much of Class 900 hydrocarbon and wellhead work specifies RTJ facings outright, with octagonal ring gaskets (soft iron or low-carbon steel for A105 flanges, stainless for stainless) seating in machined grooves. Our RTJ gasket size chart on this site covers the ring numbers per flange size.

Because at 150-plus bar the joint must be as strong as the pipe, and only the weld neck's tapered hub and full-penetration butt weld deliver that — with a weld geometry open to full radiography, which line classes at this rating demand. Slip-on and threaded flanges effectively disappear from B16.5 practice at Class 900; blinds and spectacle blinds remain for isolation, and their Class 900 charts are on this site.

For A105 at 38 °C the allowable working pressures are roughly 2220 and 1480 psig. Dimensionally the step is severe: a 6 inch flange grows from 355 to 380 mm OD and from 47.7 to 55.6 mm thickness, bolting steps from 1 inch to 1-1/8 inch studs, and weight rises by a third or more. Class 900 is also where RTJ facings and hydraulic bolt tensioning become routine — the class belongs to hydrocracking, gas injection and wellhead work, not general process piping.

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 — a frequent requirement at this class — and third-party inspection by LR, BV, DNV, SGS, TÜV or IBR on request.

Yes — blind, slip-on, lapped joint and spectacle blind dimension charts for Class 900 are on this site (linked in the sidebar), alongside the weld neck charts for every class from 150 to 2500. Special bores, facings, RTJ grooves and flanges to customer drawings are quoted case-by-case.

Hydrocracking and hydroprocessing units, high-pressure steam and boiler circuits, gas injection and gas lift systems, wellhead and christmas-tree piping, high-pressure pipelines and pump discharge — the heavy end of oil, gas and power, where Class 600 has run out and Class 1500 is not yet required.

Yes — "ANSI 900", "ANSI B16.5" and "900 lb" all refer to what is today the ASME B16.5 Class 900 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.