ASME B16.47 Series B Class 900 Weld Neck Flange — Dimensions, Bolting & Weights

A large diameter welding neck flange in the heaviest class ASME B16.47 tabulates, running NPS 26 through 48. Series B, formerly API 605, is the compact bolting pattern: smaller outside diameter and more, smaller bolts on a tight circle, making it lighter than the Series A counterpart at NPS 26-36; from NPS 38 to 48 the two series share identical Class 900 dimensions. The weld neck type carries a tapered hub butt-welded to the pipe, giving the best stress distribution and fatigue life of any flange type at these sizes.

They are tabulated in ASME B16.47-2020 and reproduced in full in the chart above: for each of the twelve sizes from NPS 26 to 48, the outer diameter, flange thickness, hub diameters at base and bevel, raised face diameter, length through hub, bolt circle, stud count, hole diameter and stud size — all in millimetres — plus the approximate weight. A 36 in flange, for example, is 1345 mm OD and 173.1 mm thick (plus the 7 mm face) on a 1200.2 mm bolt circle with twenty-four 3 in studs, at about 1143 kg.

Because beyond that, a conventional bolted flange at 150-plus bar stops making engineering sense. The 48 in flange already weighs about 2.26 tonnes on twenty-four 4 in studs; anything larger would demand bolting and ring sections outside sensible practice, so the standard ends the Class 900 table at 48 in while the lower classes run to 60. Larger ultra-high-pressure connections move to special designs — hub clamps or welded construction.

Yes — both series use the ASME B16.5 Table 2 Class 900 ratings: 153.2 bar at 38 °C for A105, derating with temperature. At NPS 26-36 Series B reaches that rating in a more compact, lighter flange; at NPS 38-48, B16.47 gives Series A the Series B dimensions, so the two series coincide there. The full bar table for carbon and stainless groups is on this page.

At the sizes where the two patterns differ, NPS 26 to 36, the saving is substantial: a 26 in Series B Class 900 weld neck weighs about 476 kg against 692 kg for Series A, and a 36 in about 1143 kg against 1540 kg. From NPS 38 upward the standard gives the two series identical Class 900 dimensions, so the saving applies to the 26-36 in range.

Match the mating equipment. Series B (ex API 605) is common in refineries, gas plants and offshore topsides; valves, pumps and older MSS SP-44 pipeline systems are usually Series A. When in doubt, compare the bolt count and bolt circle of the mating flange against the chart — at NPS 26-36 the two series never share a Class 900 drilling.

At NPS 26 to 36, no — different OD, bolt circle, bolt count and bolt size, so they can never bolt together there, nor to any B16.5 or B16.1 drilling. At NPS 38 to 48 the standard defines the two series with identical Class 900 dimensions. The series must still be stated on every enquiry and drawing, because at other sizes and classes the difference is real.

API 605 was the American Petroleum Institute's standard for large-diameter carbon steel flanges on refinery and tank-farm piping — compact, economical patterns for moderate pressures. When ASME consolidated large diameter flanges into B16.47 in 1990, API 605 became Series B, dimension for dimension. Drawings still call up API 605 regularly, and a flange made to our chart mates with it exactly.

No — at Class 400 and above the raised face is 7 mm (1/4 in), and the tabulated thickness C excludes it, while the length through hub V includes it. That is the reverse of the Class 75-300 convention, where the 2 mm face is included in the thickness — worth checking whenever you compare charts. The hub diameter at the bevel A equals the pipe outside diameter per ASME B36.10M.

Because ASME B16.47 deliberately leaves it to the purchaser. Large diameter pipe is ordered by wall thickness rather than schedule, so there is no standard bore to print — the flange is bored to match the pipe stated on the order. Give the pipe OD and wall thickness (or the exact ID) with your enquiry; our chart weights assume a bore for STD (9.53 mm) wall pipe.

From the weight column in our chart: a 26 inch weighs about 476 kg, a 36 inch about 1143 kg and a 48 inch about 2259 kg — every size above 32 in is more than a tonne, and the top of the chart is a 2.26-tonne forging on twenty-four 4 in studs. These are approximate carbon-steel weights bored for STD wall pipe; the exact weight varies with the bore and material.

The Series B Class 900 pattern balances bolt load size by size rather than in long runs, so count and stud size alternate as the chart climbs — and at one point the stud size even steps down: a 34 in flange carries twenty 3-1/4 in studs while a 36 in carries twenty-four 3 in studs. Always count holes and measure the bolt circle against the mating flange rather than assuming a trend.

Spiral wound gaskets to ASME B16.20 with inner ring on RF joints, or octagonal ring gaskets in RTJ grooves — common at this class — with stud bolts and heavy hex nuts: A193 B7 / A194 2H for carbon steel, B8M Class 2 for stainless flanges. Studs run 2-1/2 to 4 in on this chart, 20 to 24 per flange, and the holes are drilled 3.2 mm larger than the stud. Hydraulic tensioning is the only sensible make-up method at these stud sizes.

Carbon steel ASTM A105/A105N and A266 Cl.2 for large forgings, high-yield A694 F42 through F65 matched to API 5L X42-X65 pipeline pipe, low-temperature A350 LF2/LF3, stainless A182 F304/L, F316/L and F321, duplex F51 and super duplex F53, plus carbon-steel flanges with 316L, Inconel 625 or copper-nickel weld overlay on request. EN equivalents such as P250GH, P355NH and 1.4404 are available — all with EN 10204 3.1 certificates, 3.2 on request.

Because at 26 to 48 in and Class 900 pressures the piping loads are enormous and the joint lives on its fatigue performance. The weld neck's tapered hub feeds bending and pressure loads gradually from flange ring into pipe wall, and its full-penetration butt weld can be radiographed — which is why gas injection systems, compressor discharge headers and synthesis loops standardise on it at this class.

A 7 mm (1/4 in) raised face or a ring type joint — at Class 900 the RTJ is specified as often as the RF, with octagonal ring gaskets seating in machined grooves. Faces are serrated to 125-250 microinch Ra (3.2-6.3 micrometre) for spiral wound gaskets on RF joints. State the facing with the enquiry; RF and RTJ do not interchange in the field.

Forged from certified ingots or blooms, or rolled-ring forged at the larger diameters, then normalised, quenched-and-tempered or solution annealed per the material specification. Dimensional inspection follows ASME B16.47 tolerances with face finish per MSS SP-6, and marking per B16.47 and MSS SP-25 including "B16.47 SER. B" and "CL 900". Every batch ships with EN 10204 3.1 certificates; UT per ASTM A388, MPI/DPT, PMI, Charpy impact testing, NACE MR0175, PED and IBR compliance and 3.2 endorsement are available on request.

ANSI B16.47 Series B 900 lb WNRF flange, API 605 Class 900 weld neck flange, 900# compact large diameter welding neck flange and 26-48 in WN flange all describe the same product, and the rating sits at roughly PN 160 in metric terms. ANSI and ASME B16.47 are the same standard — custodianship moved to ASME but the old names, API 605 especially, persist on drawings.

Twelve sizes, NPS 26 through 48 — the Class 900 table ends at 48 in, unlike the lower classes which run to 60. Below 26 in, large flanges are B16.5 territory: our Class 900 weld neck dimensions page carries NPS 1/2 to 24 with the same style of chart, and the drilling families line up so systems transition cleanly at the 24/26 in boundary.