Class 300 Long Weld Neck Flange Dimensions — ASME B16.5 Chart, Neck & Weights

A weld neck flange with an extended straight neck that serves as the nozzle itself: the flange ring bolts to the piping and the neck end is welded directly into a vessel, column, drum or exchanger shell. One forging and one weld replace a pipe stub plus a weld neck flange — a stronger, more fatigue-resistant nozzle with full material traceability. The ring is a standard ASME B16.5 Class 300 flange; the neck is made to order.

The flange ring — OD, thickness, facing, drilling — is per ASME B16.5-2020 Tables 10 and 11 and is reproduced in the chart above, together with the industry-standard neck OD, bore and length for the standard LWN. A 6 in Class 300 LWN flange, for example, has a 320 mm OD ring, 36.6 mm thick, a 206.2 mm neck OD, a 152.4 mm bore and a standard length of 305 mm, at about 48 kg. Neck dimensions are not part of B16.5 and are made to order.

The flange is rated as an ASME B16.5 Class 300 flange: 51.1 bar (roughly 740 psig) at 38 °C for Group 1.1 materials such as A105, derating with temperature — the full bar table is on this page. The neck is designed separately as a nozzle under the vessel code (ASME VIII UG-37 / UG-45), so both checks must pass.

Only the flange portion — OD, thickness, facing, drilling and pressure rating. Neck OD, bore and length are not standardised; they follow industry convention (the values in our chart) or the vessel drawing, and the neck is designed as a nozzle under the vessel code. That is why every LWN order states the length and bore explicitly.

Standard for general nozzles, with the bore equal to the nominal size. Heavy barrel (HB) carries a thicker neck that provides integral opening reinforcement per ASME VIII UG-37, eliminating the repad. Equal barrel (E) has a pipe-schedule bore so the neck can be butt-welded to pipe of the same size and schedule. State the style with the order.

Standard lengths are 229, 305 and 406 mm (9, 12 and 16 in), and any length can be supplied. Specify the overall length H from the raised face to the neck end — long enough to clear the vessel insulation and give the welder access, which is usually what sets it.

The standard LWN bore equals the nominal size — a 6 in flange has a 152.4 mm bore — which is larger than any pipe-schedule bore. Where the neck must butt-weld to pipe, order the equal-barrel style with the bore matched to the pipe schedule instead.

The LWN wins on integrity: one forging and one shell weld instead of a forging, a pipe piece and two welds. Fewer welds means fewer examinations, better fatigue life and cleaner traceability — which is why vessel codes and critical-service nozzle specifications prefer it, and why the premium over a stub-plus-flange fabrication usually pays for itself in NDE alone.

Plain (square cut) as standard, for set-in or set-on nozzle welds to the shell; a 37.5 degree bevel for butt welds on request, and nut stops or reliefs where the nozzle design calls for them. State the end preparation with the order.

Yes — for Class 300 the tabulated flange thickness B includes the 2 mm raised face, and the overall length H is measured from the raised face to the neck end. Our charts follow the ASME B16.5 convention on every page.

From the weight column in our chart: a 2 in weighs about 9 kg, a 6 in about 48 kg and a 24 in about 353 kg at the standard lengths shown. The neck dominates the weight, so a longer or heavy-barrel neck weighs proportionally more; exact weights are confirmed on order.

Carbon steel ASTM A105/A105N — supplied as SA-105 for ASME VIII vessels — with A350 LF2/LF3 for low temperature and A266 Cl.2 for large necks. Stainless A182 F304/L, F316/L, F321 and F347, alloy F5-F91, duplex F51 and super duplex F53/F55 — plus Monel, Inconel, Hastelloy and copper-nickel on request. EN equivalents available — all with EN 10204 3.1 mill certificates.

A 2 mm raised face as standard, serrated to 125-250 microinch Ra (3.2-6.3 micrometre); flat face and RTJ are available on request. State the facing with the enquiry.

Spiral wound gaskets to ASME B16.20 or non-metallic gaskets to B16.21, with stud bolts and heavy hex nuts — A193 B7 / A194 2H for carbon steel, B8M Class 2 for stainless flanges. Counts, hole sizes and stud sizes per flange size are in the chart; each hole is drilled 3 mm larger than its stud.

Forged from certified bars and billets, normalised, quenched-and-tempered or solution annealed per the material specification. The flange is inspected to ASME B16.5 tolerances and the neck to the drawing; face finish per MSS SP-6, marking per MSS SP-25, UT per ASTM A388 on request. Every batch ships with EN 10204 3.1 certificates; MPI/DPT, PMI, Charpy, HIC/NACE MR0175, IBR, PED and ASME VIII U-stamp vessel documentation on request.

ANSI B16.5 300 lb LWN flange, 300# long welding neck, nozzle flange and LWNRF Class 300 all describe the same product, at roughly PN 50 in metric terms. ANSI B16.5 and ASME B16.5 are also the same standard — custodianship moved from ANSI to ASME decades ago but the old name persists on drawings.

At NPS 1/2 to 3-1/2 the two classes share the same ring OD and bolt circle at every size, so side by side they are hard to tell apart. The differences are the ring thickness, the raised face (2 mm at Class 300, 7 mm at Class 600) and of course the rating — 102.1 against 51.1 bar at 38 °C for A105. Check the marking, not the silhouette.