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

A large diameter welding neck flange made to ASME B16.47, the standard that takes over where B16.5 stops — and Class 900 is the heaviest class B16.47 tabulates, running NPS 26 through 48. Series A, formerly the MSS SP-44 standard, is the heavier of the two bolting patterns, with larger outside diameters, thicker flanges and fewer but larger bolts than Series B. The weld neck type carries a long 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 1460 mm OD and 171.5 mm thick (plus the 7 mm face) on a 1289 mm bolt circle with twenty 3-1/2 in studs, at about 1540 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.

From NPS 38 upward, B16.47 defines Series A Class 900 with the Series B (API 605) dimensions — the two series converge. At this class the step looks subtler than at Class 300-600 but is just as real: the 38 in flange shares the 36 in flange's 1460 mm OD and 1289 mm bolt circle, yet is 19 mm thicker with a different hub, and the two are different flanges. Always confirm the mating flange's bolt circle, count and thickness in this range.

Match the mating equipment. Series A (ex MSS SP-44) is the heavier pattern and the usual choice for pipelines, large valves, pumps, vessel nozzles and any system built around older MSS SP-44 piping; Series B (ex API 605) is the compact pattern, seen on tanks and some process plants. At Class 900 the question only bites at NPS 26-36, where the two differ — from 38 up the standard gives them identical dimensions.

MSS SP-44 was the Manufacturers Standardization Society's steel pipeline flange standard, and when ASME consolidated large diameter flanges into B16.47 it became Series A, dimension for dimension. Drawings and valve catalogues still call up MSS SP-44 regularly, and a flange made to our chart mates with it exactly — the two names describe the same flange.

B16.47 adopts the ASME B16.5 Table 2 ratings, so Class 900 here rates the same as Class 900 in B16.5: for Group 1.1 materials such as A105, 153.2 bar at 38 °C, easing to 119.5 bar at 300 °C — the full bar table for carbon and stainless groups is on this page. A694 high-yield grades are rated per Group 1.1 unless otherwise agreed. Always confirm the rating at your design temperature.

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 150 and 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 692 kg, a 36 inch about 1540 kg and a 48 inch about 2259 kg — every flange 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 rating steps from 102.1 to 153.2 bar at 38 °C for A105 — half as much again — and the hardware steps with it: a 36 in flange grows from 1315 to 1460 mm OD, from 123.9 to 171.5 mm thickness and from about 773 to 1540 kg, twice the weight, with studs stepping from 2-1/2 to 3-1/2 in. The size range also shrinks: Class 600 runs to NPS 60 where Class 900 stops at 48. Our Series A Class 600 page carries the companion chart.

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 2-3/4 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 long 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. A". 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 A 900 lb WNRF flange, MSS SP-44 Class 900 weld neck flange, large diameter 900# 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 name persists 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.