ASME B16.47 Series B Class 75 Weld Neck Flange — Dimensions, Bolting & Weights

A large diameter welding neck flange — NPS 26 through 60 — in the lightest class ASME B16.47 tabulates. Series B, formerly API 605, is the compact bolting pattern: smaller OD, thinner flange and many small bolts on a tight circle. Class 75 exists only in Series B, developed for large-bore, low-pressure lines such as water, air, vent, flare and low-pressure gas, where a Class 150 flange would be heavier and costlier than the service needs.

They are tabulated in ASME B16.47-2020 and reproduced in full in the chart above: for each of the eighteen sizes from NPS 26 to 60, 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 1034 mm OD and 35.1 mm thick on a 992.1 mm bolt circle with forty 3/4 in studs, at about 66 kg.

Class 75 is the lowest pressure class in ASME B16.47 and exists only in Series B. It was carried over from API 605, the American Petroleum Institute standard written for large-diameter, low-pressure carbon steel flanges on refinery and tank-farm piping, which ASME adopted into B16.47 as Series B in 1990. There is no Series A Class 75 and no B16.5 equivalent — it is a class of its own.

Only if two conditions hold: the design pressure and temperature must fall within the Class 75 rating — 9.6 bar (140 psig) at ambient for carbon steel — and the mating flange must also be Series B Class 75, because the drilling does not match Class 150 of either series. Where both hold, the saving is dramatic: a 36 in Class 75 flange weighs about 66 kg against roughly 290 kg for its Series A Class 150 counterpart.

Class 75 is rated in ASME B16.47 Table 3, not the B16.5 tables the other classes borrow: for Group 1.1 carbon steel such as A105, 9.6 bar (140 psig) from -29 to 38 °C, decreasing with temperature. It is not an ASME B16.5 class and has no B16.5 equivalent. Refer to B16.47 Table 3 for the full curve and other material groups, and confirm for your bolting and gasket.

Weight and money. Large-bore utility lines — cooling water, air, vent, flare — rarely see more than a few bar, and a Class 150 flange there carries steel the service never uses. The Class 75 chart cuts the flange weight by half or more at every size: 36 kg instead of 136 kg at 26 in, 215 kg instead of 928 kg at 60 in, against Series A Class 150. Over a circulating-water system with dozens of joints, the saving in material, handling and bolting is substantial.

Match the mating equipment. Series B (ex API 605) is the compact pattern with many small bolts, common on storage tanks and low-pressure process plant; Series A (ex MSS SP-44) is the heavier pipeline pattern. For Class 75 the question answers itself — the class exists only in Series B — but the series must still be stated on the enquiry, because nothing in Series B interchanges with Series A or with B16.5/B16.1 drilling.

No. Class 75 has no Series A counterpart, and its bolt circles and hole patterns match neither Class 150 of either series nor any B16.5 or B16.1 flange. A Class 75 flange mates only with another Series B Class 75 flange — which is why the class and series must appear on every drawing and enquiry.

Yes — for Class 75 the tabulated thickness C includes the 2 mm raised face, and the length through hub V is measured from the raised face. The hub diameter at the bevel A equals the pipe outside diameter per ASME B36.10M, which is how the flange lines up with the pipe for the butt weld.

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 36 kg, a 36 inch about 66 kg, a 48 inch about 122 kg and a 60 inch about 215 kg — light enough that most sizes are a two-man or small-crane lift rather than a rigging operation. These are approximate carbon-steel weights bored for STD wall pipe; the exact weight varies with the bore and material.

Because the stud size steps up at those sizes — from 5/8 to 3/4 in at NPS 36, to 7/8 in at 44 and to 1 in at 56 — and fewer, larger studs replace many smaller ones. A 34 in flange carries fifty-two 5/8 in studs while a 36 in carries forty 3/4 in studs. It is a normal feature of the Series B pattern, but worth knowing when counting holes against a mating flange.

Spiral wound gaskets to ASME B16.20 or non-metallic full-face gaskets to B16.21 — the natural choice on flat-faced low-pressure joints — with stud bolts and heavy hex nuts: A193 B7 / A194 2H for carbon steel, B8M Class 2 for stainless flanges. Studs run 5/8 to 1 in on this chart, 36 to 52 per flange, and the holes are drilled 3.2 mm larger than the stud — ordinary hand and air tooling makes these joints up.

Carbon steel ASTM A105/A105N and A266 Cl.2 for large forgings, high-yield A694 F42/F52/F60 matched to API 5L line 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.

A 2 mm raised face as standard, serrated to 125-250 microinch Ra (3.2-6.3 micrometre), with flat face on request — common on Class 75 duty where full-face gaskets and soft rubber sheet are the gasket culture. State RF or FF with the enquiry.

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 75". 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 Class 75 WNRF flange, API 605 Class 75 weld neck flange, 75 lb large diameter welding neck flange and 26-60 in low-pressure WN flange all describe the same product, and the rating sits at roughly PN 10 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.

Eighteen sizes, NPS 26 through 60 — the full B16.47 range. When the design pressure outgrows the 9.6 bar Class 75 rating, the next step is Series B Class 150, which keeps the compact pattern, or Series A Class 150 where the mating equipment demands the heavier MSS SP-44 drilling — both charts are on this site, linked below.