Class 900 Slip On Flange Dimensions — ASME B16.5 Chart, Bolting & Weights

A forged flange that slides over the pipe end — the bore is slightly larger than the pipe OD — and is secured with two fillet welds, one on the hub and one inside at the bore. No bevel preparation and easy alignment make it the lowest-cost welded flange for low- and medium-pressure process, water and utility piping. ASME B16.5 tabulates Class 900 slip-on flanges in NPS 1/2 to 24 (no 3-1/2).

They are tabulated in ASME B16.5-2020 Tables 16 and 17 and reproduced in full in the chart above: for each of the nineteen sizes, the flange OD, thickness, hub diameter, length through hub, bore, raised face diameter, bolt circle and bolting — all in millimetres — plus the approximate weight. A 6 in Class 900 slip-on flange, for example, is 380 mm OD and 55.6 mm thick (plus the 7 mm face) on a 317.5 mm bolt circle with twelve 1-1/8 in studs, at about 50 kg.

The rating depends on material and temperature. For Group 1.1 materials such as A105 carbon steel, ASME B16.5 allows 153.2 bar (roughly 2220 psig) at 38 °C, derating with temperature — the full bar table for carbon and stainless groups is on this page. Remember the two fillet welds must suit the same duty; always confirm the rating at your design temperature.

No — and that is one of the type's conveniences. The bore is sized to the pipe OD plus clearance, and the pipe OD is the same for every schedule, so one flange fits them all. The schedule matters only for sizing the fillet welds; state it for the welder, not for the flange.

Two fillet welds per ASME B31.3 Fig. 328.5.2B: slide the flange on, weld the hub fillet first with a leg at least 1.4 times the pipe wall, then the internal fillet at the bore. If the internal weld stands proud of the face, grind it flush so the gasket seats properly.

Set the pipe end back from the face by the pipe wall thickness plus about 3 mm before welding. The set-back leaves room for the internal fillet weld so it does not project beyond the face and interfere with the gasket — the slip-on equivalent of the socket weld's 1.6 mm gap rule.

Strength and fatigue life. A slip-on has roughly two-thirds the static strength of a weld neck of the same class and about one-third the fatigue life, because two fillet welds cannot match a full-penetration butt weld through a tapered hub. In exchange it costs less, needs no bevel and forgives misalignment — which is why utility and low-pressure process lines standardise on it, while critical and cyclic service goes weld neck.

Both take the pipe into the flange, but the socket weld has a machined shoulder that locates the pipe and takes one external fillet, leaving a smooth bore — standardised only in NPS 1/2 to 3. The slip-on has a plain through-bore, takes two fillet welds and runs the full size range. In small bore the socket weld is the stronger joint; above NPS 3 the slip-on is the fillet-welded option.

In severe cyclic service, where the fillet welds fatigue long before a butt weld would; in high-temperature or hydrogen service, where codes prefer full-penetration welds; and in corrosive or hygienic service, where the crevice between pipe and bore traps product. In all of those, a weld neck flange is the right answer.

No — at Class 400 and above the raised face is 7 mm, and the tabulated thickness B excludes it, while the length through hub H 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 from different sources.

From the weight column in our chart: a 2 in weighs about 11.5 kg, a 6 in about 50 kg, a 12 in about 148 kg and a 24 in about 608 kg. These are approximate carbon-steel weights; the exact weight varies slightly with material.

Carbon steel ASTM A105/A105N is the usual choice, with A350 LF2/LF3 for low temperature. Stainless A182 F304/L, F316/L, F321 and F347, alloy steel F5/F9/F11/F12/F22/F91 for high-temperature service, duplex F51 and super duplex F53/F55 — plus Monel 400, Inconel 600/625, Hastelloy C276 and copper-nickel 90/10 on request. EN equivalents such as P250GH, 1.4301 and 1.4404 are available — all with EN 10204 3.1 mill certificates.

A 7 mm raised face as standard, serrated to 125-250 microinch Ra (3.2-6.3 micrometre); flat face 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 above; 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, with the bore gauged on every piece. Dimensional inspection follows ASME B16.5 tolerances, face finish per MSS SP-6 and marking per MSS SP-25. Every batch ships with EN 10204 3.1 mill test certificates traceable by heat number; MPI/DPT, PMI, NACE MR0175, IBR and PED compliance and 3.2 endorsement are available on request.

ANSI B16.5 900 lb SO flange, 900# slip-on raised face flange, SORF Class 900 and slip-on welding flange all describe the same product, at roughly PN 160 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.

Because ASME B16.5 furnishes NPS 1/2 to 2-1/2 Class 900 flanges to the Class 1500 dimensions — the demand for true Class 900 small bore never justified its own table. The result is the chart's famous anomaly: the 2-1/2 in flange (245 mm OD, about 16.5 kg) is larger and heavier than the 3 in (240 mm OD, about 12 kg), which is a true Class 900 part. Order by the chart, not by intuition.