ANSI B16.1 Class 250 Weld Neck Flange — Dimensions, Drilling Chart & Weights

A forged steel welding neck flange — not cast iron — with a raised face, drilled to the ASME B16.1 Class 250 bolt pattern used by cast-iron flanges. B16.1 itself is the grey cast iron standard, and cast iron cannot be butt-welded, so when a line needs a weld neck that bolts to a Class 250 cast-iron valve, pump or fitting, the flange is produced in steel with the cast-iron drilling. Body dimensions follow ASME B16.5 Class 300 up to NPS 24 and B16.47 Series A at 30 and 36 in.

They are tabulated in full in the two charts above: NPS 1 to 24 uses the ASME B16.5 Class 300 weld neck body, whose drilling is identical to B16.1 Class 250, and NPS 30 and 36 use the B16.47 Series A body. A 6 in flange, for example, is 320 mm OD and 36.6 mm thick on a 269.9 mm bolt circle with twelve 3/4 in bolts, at about 19 kg; a 24 in is 915 mm OD on an 812.8 mm circle with twenty-four 1-1/2 in bolts, at about 263 kg.

The bolt pattern is — for NPS 1 to 24 the bolt circle, hole count and hole size coincide exactly, so the two bolt together. The differences are everything else: B16.1 Class 250 covers cast iron with a 1.6 mm raised face and much lower ratings, while B16.5 Class 300 covers steel with a 2 mm raised face. In practice a steel Class 300 weld neck is what the market calls a Class 250 weld neck — which is exactly what this page describes.

Because cast iron is brittle. With raised faces bearing on raised faces, over-torqued high-strength studs put a bending moment on the unsupported cast-iron rim and can crack it — so ASME B16.5 restricts RF-to-RF joints against cast iron to low-strength ASTM A307 Grade B bolting with a thin ring gasket. The steel flange tolerates far more than the joint is allowed to see; the limits exist to protect the iron side.

Raised face to raised face: a ring gasket no thicker than 1.6 mm, extending to the bolt holes, with ASTM A307 Grade B machine bolts. If the piping class demands A193 B7 studs, order the steel flange flat-faced instead and use a full-face gasket against a flat-faced cast-iron flange — that configuration spreads the bolt load and permits the stronger bolting. Hole counts, hole sizes and bolt sizes per flange size are in the charts above.

The joint is limited by the cast-iron side, not the steel flange. For ASTM A126 Class B cast iron, B16.1 Class 250 allows 500 psig (34.5 bar) up to 66 °C for NPS 1-12 and 300 psig (20.7 bar) for NPS 14-24 — the larger sizes rated for liquid service, flanges only — derating with temperature per the table on this page. The steel weld neck itself carries its B16.5 Class 300 rating (A105: about 51.1 bar at 38 °C), so it is never the weak link in the joint.

ASME B16.1 tabulates Class 250 in NPS 1 to 24 continuously, then 30, 36, 42 and 48. We supply NPS 1-24, 30 and 36 from the charts on this page. NPS 26 and 28 are not B16.1 sizes and are supplied to B16.47 Series A Class 300, the same drilling family; NPS 42 and 48 (B16.1: 36 holes for 2 in bolts on a 1340 mm circle, and 40 on 1543 mm) have no matching steel-flange standard and are made against a drawing.

Yes. Class 250 cast iron carries a 1.6 mm raised face and the steel flange a 2 mm one; both are simply gasket seats on the same bolt pattern, and the 0.4 mm difference is absorbed by the gasket. Note that our chart values for thickness C and length through hub V include the 2 mm raised face, following the B16.5 Class 300 convention for these bodies.

A genuine quirk: ASME B16.1 Class 250 specifies 2 in (50.8 mm) holes at NPS 30 and 2-1/4 in (57.2 mm) at NPS 36, while B16.47 Series A — whose body the flange uses — drills 47.6 and 54.0 mm for the same bolts. We drill per B16.47 by default, which suits both patterns, and drill the full B16.1 holes when the order says so. It is the one place in the range where the two standards disagree.

Wherever steel pipe meets the heavier class of cast-iron equipment: Class 250 cast-iron gate, globe, check and butterfly valves, pump casings, strainers and expansion joints, AWWA C207 Class E and F and C110 250 psi ductile-iron fittings, and the high-pressure fire-water, HVAC, cooling-water and process-water lines built around them.

Carbon steel ASTM A105/A105N is the usual choice, with A694 F42-F65 for pipeline service and A350 LF2 for low temperature. Stainless A182 F304/L, F316/L and F321, duplex F51 and super duplex F53 cover corrosive duty, and copper-nickel 90/10 composite or solid flanges are made for seawater on request. EN equivalents such as P250GH, 1.4301 and 1.4404 are available — all with EN 10204 3.1 mill certificates.

The bore of a weld neck flange is machined to match the inside diameter of the pipe it will be welded to. The chart shows the bore for standard-weight (STD) pipe, but we bore to whatever schedule you order — state the schedule or the exact ID with the enquiry so the butt weld lines up without a step.

From the weight column in our charts: a 6 inch weighs about 19 kg, a 12 inch about 64 kg, a 24 inch about 263 kg and a 36 inch about 578 kg. These are approximate carbon-steel weights with a standard bore; the exact weight varies with the bore schedule and material.

Yes — flat face is available on request, and it is the right choice whenever the piping class requires A193 B7 studs: a flat-faced steel flange with a full-face gasket against a flat-faced cast-iron flange spreads the bolt load and permits the stronger bolting. With the standard raised face, keep to the A307 Grade B ring-gasket rule.

Class is pressure family: Class 125 matches the B16.5 Class 150 pattern, runs flat-faced and tops out at 200 psig on the cast-iron side, while Class 250 matches the Class 300 pattern, carries a raised face and reaches 500 psig. The two are not interchangeable — different bolt circles, counts and hole sizes at every size. Our B16.1 Class 125 weld neck page carries the full companion chart.

Forged from certified billets, normalised or solution annealed per the material specification, dimensionally inspected to B16.5/B16.47 tolerances with the drilling template-checked against B16.1 Class 250, face finish per MSS SP-6 and marking per MSS SP-25 including "B16.1 CL250 DRILLING". Every batch ships with EN 10204 3.1 mill test certificates traceable by heat number; UT/MPI/DPT, PMI, NACE MR0175 and 3.2 third-party endorsement are available on request.

ANSI B16.1 250 lb RF weld neck, Class 250 WN, 250# WNRF and "steel flange with Class 250 cast-iron drilling" all describe the same product — a steel welding neck flange carrying the Class 250 cast-iron bolt pattern. ANSI B16.1 and ASME B16.1 are also the same standard; custodianship moved from ANSI to ASME decades ago but the old name persists on drawings.