BS 3293 Class 300 Weld Neck Flange — Large-Diameter Dimensions, Weights & Drilling

BS 3293 is the British Standard for large-diameter carbon-steel pipe flanges for the petroleum industry — sizes above NPS 24, where its companion standard BS 1560 stops. It uses the imperial pressure-class system (Class 150, 300, 400 and 600) and tabulates weld neck and slip-on flanges with dimensions in millimetres. This page covers the Class 300 weld neck series.

It is an old British standard that has been overtaken in new projects by ASME B16.47 for large-diameter flanges, but it is far from dead: refineries, terminals and pipelines engineered to British specifications across the UK, Middle East, Africa and Asia still carry BS 3293 flanges, and their drawings, spares lists and replacement orders cite the standard daily. We manufacture to it as cited.

They split the size range: BS 1560 covers petroleum flanges from NPS 1/2 to 24 (dimensionally aligned with ASME B16.5), and BS 3293 takes over above 24 inches. A British-specified plant of the era would use BS 1560 flanges on its process piping and BS 3293 on its large transfer, loading and pipeline connections.

All three sit in the same large-diameter lineage: MSS SP-44 pipeline flanges informed both BS 3293 and what is now ASME B16.47 Series A, so outside diameters, bolt circles and drilling generally correspond for common sizes and classes. That makes cross-standard replacement usually straightforward — but always confirm the drilling (bolt count, hole size, PCD) against the chart before mating flanges from different standards on a critical joint.

Class 300 is an ASME-style pressure-temperature rating class, not a fixed working pressure. For carbon steel at 38 °C it allows roughly 740 psig (about 51 bar), and the allowable pressure falls as temperature rises. Always confirm the rating for your material and design temperature against the class tables.

The Class 300 weld neck chart on this page covers NPS 28 to 36 (NPS 26 on request) — these are large-diameter flanges, produced as forged rings rather than standard forgings. For NPS 24 and below, see the BS 1560 / ASME B16.5 series.

NPS is the nominal pipe size in inches; all other values are millimetres. O is the flange outside diameter, C the flange thickness, Y2 the length through hub, A the weld-end outside diameter (it matches the pipe OD), X the hub diameter, R the raised face diameter, NBH the number of bolt holes, DBH the bolt hole diameter and D the bolt circle (PCD).

From the weight columns in our chart: an NPS 28 weighs about 340 kg (750 lbs), an NPS 32 weighs about 435 kg (959 lbs), an NPS 36 weighs about 560 kg (1235 lbs). These are carbon-steel weights with a standard bore — at these diameters, weight matters for handling, bolt tensioning equipment and freight, so check the chart before planning installation.

The NBH, DBH and D columns give the drilling per size: an NPS 28 takes 28 holes of 44.4 mm on a 939.8 mm bolt circle, and an NPS 36 takes 32 holes of 54.0 mm on a 1168.4 mm bolt circle. Imperial-series stud bolts are used; we supply matched fastener and gasket sets with the flanges on request.

Raised face is the standard — the R column in the chart gives its diameter per size. Ring type joint and special facings are machined on request. Surface finish is machined to suit the gasket specified, typically spiral wound at these diameters.

Spiral wound gaskets sized to the raised face are the usual choice at large diameters and petroleum duty; sheet gaskets serve benign low-pressure services. Large-diameter gaskets are fragile to handle — we can supply them with the flanges, sized to the chart, to avoid site mismatches.

The bore is machined to match the inside diameter of the mating pipe, so state the pipe wall thickness or schedule with your order. The A column in the chart is the weld-end outside diameter — it equals the pipe OD, confirming the weld prep matches your line pipe.

Carbon steel A105 and high-yield A694 F42–F65 are the petroleum classics; we also produce A350 LF2 for low temperature, stainless 304/316/321, duplex and super duplex, alloy steel F1–F91, Monel, Inconel, Hastelloy, titanium, copper nickel and clad construction — all as forged rings with EN 10204 3.1 certificates, PMI and heat-number traceability.

Above 24 inches the hydrostatic end force on a joint runs to hundreds of tonnes. The weld neck's tapered hub feeds that load smoothly into the pipe wall, the full-penetration butt weld can be examined by UT or radiography, and the matched bore avoids turbulence — slip-on construction is limited to the lower classes and less critical duty at these sizes.

They are produced as forged and rolled rings: a punched billet is ring-rolled to near size, heat treated, then machined all over — faces, bore, hub taper, weld bevel and drilling — on large vertical lathes and drilling centres. Every ring carries its heat number, and mechanical properties are certified per heat with EN 10204 3.1 documentation.

Dimensional inspection against the standard, marking with size, class, grade and heat number, and EN 10204 3.1 mill test certificates are standard. PMI verification, ultrasonic examination of the forging, magnetic-particle testing of machined surfaces, impact testing for low-temperature grades, NACE MR0175 compliance and third-party witness by LR, BV, DNV, SGS, TÜV or IBR are available on request.

Refineries and terminals with British-specification heritage running higher-pressure large-bore services — pump discharges, transfer mains and process headers ordered to the original BS 3293 drawings.

Size (NPS) and quantity, class (300), the standard as cited on your drawing (BS 3293 or ASME B16.47 Series A), bore schedule or pipe wall, facing, material grade, any impact-test or NACE requirement, and the certification and inspection you need. Tesco Steel & Engineering is an ISO 9001:2015 certified flange manufacturer and exporter in Mumbai, India, shipping worldwide with sea-worthy packing.