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

They are tabulated in ASME B16.5 and reproduced in full in the chart above: for each size from NPS 1/2 to 24, the outer diameter, flange thickness, hub and weld-neck diameters, length through hub, raised face diameter and height (2 mm at this class), bolt circle and weld face — all in millimetres — plus the published weight. A 4 in Class 300 weld neck flange, for example, is 255 mm OD, 31.8 mm thick, on a 200 mm PCD, at about 11.3 kg.

The rating depends on material and temperature. For Group 1.1 materials such as A105 carbon steel, ASME B16.5 allows roughly 740 psig (about 51 bar) at 38 °C — some two and a half times Class 150 — and the class holds its rating far better as temperature rises, still offering roughly 550 psig around 300 °C where Class 150 has fallen to about 140. Stainless and alloy grades have their own tables. Always confirm the rating at your design temperature.

Class 300 (also written #300 or 300 lb) is a pressure-temperature rating class, not a working pressure of 300 psi. Each class maps to a table in ASME B16.5 giving the maximum allowable working pressure at each temperature for each material group — a Class 300 A105 flange is good for roughly 740 psig cold, comfortably more than the number in its name, and less when hot.

Because ASME B16.5 tabulates minimum flange thickness including the 2 mm raised face for Class 150 and 300, and that is how our chart lists it — a 2 in Class 300 flange reads 22.4 mm. Many trade charts print the body thickness with the face deducted (20.7 mm for the same flange) and show the face height in a separate column, which makes two correct charts look 1.6-2 mm apart. Check which convention a chart uses before comparing; ours follows the standard.

From the weight columns in our chart: a 2 inch Class 300 weld neck flange weighs about 4.1 kg (9 lbs), a 6 inch about 19 kg (42 lbs), a 12 inch about 64 kg (140 lbs) and a 24 inch about 263 kg (580 lbs) — roughly 50 to 100 percent heavier than Class 150, size for size. These are published carbon-steel weights with a standard bore; the exact weight varies with the bore schedule and material density.

Class 150 and 300 share the light raised face — 2 mm (nominally 1/16 inch), shown in the RF Height column of our chart — whereas Class 400 and above step up to the 6.35 mm (1/4 inch) face. So a take-off moving from 150 to 300 keeps its facing assumptions, but one moving from 300 to 400 or 600 does not — the taller face needs longer studs, and it is a classic estimating miss.

No. For every size the Class 300 flange carries a larger outer diameter, a larger bolt circle and usually more or bigger bolts than its Class 150 counterpart — a 4 in flange, for instance, is 230 mm OD on a 190.5 mm PCD at Class 150 but 255 mm OD on a 200 mm PCD at Class 300. A Class 300 flange will never bolt to a Class 150 flange of the same size. When a line changes class at a spec break, the joint needs matching flanges on both sides.

ASME B16.5 covers pipe flanges from NPS 1/2 to NPS 24 (DN 15 to DN 600) in classes 150, 300, 400, 600, 900, 1500 and 2500. Flanges larger than 24 inches are covered by ASME B16.47 (Series A and B). Our Class 300 weld neck chart on this page lists twenty-one sizes with dimensions in millimetres, NPS 22 included.

The bore of a weld neck flange is machined to match the inside diameter of the pipe it will be welded to, so state the pipe schedule with your order. At Class 300 pressures the mating pipe is commonly Sch 40 or Sch 80 — and the match matters more here than at Class 150, because a bore step at the butt weld concentrates turbulence and erosion exactly where the joint works hardest. That is why the Bore column in our chart reads "as per pipe schedule".

We manufacture Class 300 weld neck flanges in carbon steel A105, low-temperature A350 LF2/LF3, stainless steel A182 F304/L, F316/L, F310, F321, F347 and F410, 254 SMO (F44), 904L, duplex 2205 (F51), super duplex 2507 (F53/F55), alloy steel F1/F5/F9/F11/F22/F91, Monel 400/K500, Inconel 600/625/825, Hastelloy C276/C22, titanium, copper nickel 90/10 and 70/30, and bimetal/clad — each with EN 10204 3.1 mill certificates.

Stud bolts with two heavy hex nuts are the standard practice at Class 300, and that is what the RF Stud Length column is for; machine bolts remain acceptable on mild utility duty, so our chart gives both lengths. A 2 inch flange takes 8 bolts of 5/8 inch UNC — note the jump to an 8-bolt pattern already at 2 inches, where Class 150 still uses 4 — a 12 inch takes 16 of 1-1/8 inch, and a 24 inch takes 24 of 1-1/2 inch.

Spiral wound gaskets to ASME B16.20 with an outer centring ring are the default at Class 300 bolt loads, graphite-filled for steam and hydrocarbons and PTFE-filled for chemical duty; quality sheet gaskets remain acceptable on milder services. RTJ facings with oval or octagonal ring gaskets are a routine request at this class for hydrocarbon service — state RF or RTJ with the enquiry.

At Class 300 pressures and the temperatures that usually come with them, the weld neck's tapered hub and full-penetration butt weld give the joint the strength and fatigue life of the pipe itself, with a smooth bore, no crevice and a weld that radiography can examine. Slip-on flanges are still permitted at this class, but the case for them weakens as pressure and cycling rise — on steam, hot oil and hydrocarbon lines the weld neck is the standard choice.

For A105 at 38 °C the allowable working pressures are roughly 740 and 285 psig respectively, and the gap widens dramatically with temperature — around 300 °C a Class 300 flange still carries roughly 550 psig where Class 150 has fallen to about 140. Dimensionally the Class 300 flange is thicker, wider and heavier on a larger bolt circle, on the same light 2 mm raised face. When a service sits near the top of the Class 150 envelope, or sees real temperature, Class 300 is the standard step.

Every batch is dimensionally inspected against ASME B16.5, marked per MSS SP-25 and supplied with EN 10204 3.1 mill test certificates traceable by heat number. We offer PMI, ultrasonic and magnetic particle examination, impact testing for low-temperature grades, NACE MR0175 compliance for sour service, and third-party inspection by LR, BV, DNV, SGS, TÜV or IBR on request.

Yes — slip-on, blind, socket weld, lapped joint, long weld neck and spectacle blind dimension charts for Class 300 are all on this site (linked in the sidebar), alongside the weld neck charts for Class 150 and for classes 400 through 2500. Special bores, facings and flanges to customer drawings are quoted case-by-case.

Class 300 is the workhorse of lines with real pressure or temperature: steam and condensate systems in power plants, refinery and petrochemical process piping, hot oil and thermic fluid circuits, boiler feed and blowdown, compressed gas systems, and offshore service — the territory where Class 150's fast derating rules it out but the heavy classes would be overkill.

Yes — "ANSI 300", "ANSI B16.5" and "300 lb" all refer to what is today the ASME B16.5 Class 300 table; the standard transferred from ANSI to ASME custodianship decades ago but the old name persists on drawings and enquiries. The dimensions on this page are the current ASME B16.5 values.