Most steel flange catalogs end at 60 inch. ASME B16.5 covers 1/2 inch through 24 inch. ASME B16.47 Series A and B cover 26 inch through 60 inch. AWWA C207 covers ring flanges through 144 inch for waterworks. Beyond those limits, the flange is custom work and the catalog is no longer the document. The drawing is the document. Getting that drawing right requires understanding what changes as the diameter climbs.
Custom large-diameter steel flanges show up in tank fabrication, vacuum and pressure vessel construction, hydroelectric and water transmission, dredge and marine work, and on the largest pieces of mechanical equipment built. A 96 inch blind flange on a vacuum chamber, a 144 inch ring flange on a tank, a 203 inch transition flange between a steel pipe and a concrete cylinder pipe segment, all live outside the standard reference tables. This walks through what has to be on the drawing, where the metallurgical decisions matter, and what to leave to the supplier to confirm.
Plate Selection Drives the Design
Custom large-diameter flanges are almost always cut from rolled steel plate. The plate grade sets the strength, toughness, and weldability of the finished flange. ASTM A516 Grade 70 is the workhorse for pressure-rated plate. It has a minimum yield of 38 ksi, minimum tensile of 70 ksi, and standard practice includes normalizing for sections over 1.5 inch thickness. A516-70 is qualified for pressure service under ASME Section VIII and is the default plate for a pressure-rated blind or ring flange.
ASTM A36 carbon plate is the structural workhorse. It has a minimum yield of 36 ksi and is not pressure-qualified by default. A36 is the right plate for structural ring flanges, tank flanges, and AWWA C207 flanges where the working pressure is bounded by the standard rather than by an ASME pressure code calculation. A516-70 and A36 are not interchangeable. A36 should never substitute for A516-70 on a pressure-rated drawing, and A516-70 is expensive overkill on a structural drawing.
ASTM A350 LF2 in plate form is available for low-temperature pressure service down to minus 50 degrees F with charpy impact testing. A516-70 with supplementary impact testing per ASME Section VIII covers similar territory. For sour service, A516-70 modified with reduced sulfur and supplementary NACE hardness testing is the qualified product. For cryogenic service below minus 50 degrees F, the plate options shift to ASTM A203 nickel-bearing grades, with A203 Grade D at 3.5 percent nickel for service down to minus 150 degrees F. The grade decision on a custom flange is made before the dimensional drawing is finalized.
Cutting, Machining, and Drilling at Scale
A 96 inch flange is laid out on plate from the centerline of the bore. The outside diameter is scribed, the bolt circle is scribed, the bolt holes are spotted, and the bore is scribed. Modern shops use CNC plasma or waterjet for the rough cut and CNC mill for the bore, bolt holes, and facing. The cut precision and the machining precision both fall under fabrication tolerance. AWWA C207 allows plus or minus 1/8 inch on outside diameter for flanges over 30 inch, and bolt circle tolerance follows the standard band. ASME B16.47 has tighter dimensional tolerances even on Series B and the standard sets explicit values for hub bore, hub thickness, and facing flatness.
Custom flanges that fall outside any published standard live by the drawing tolerance. A common project requirement is to drill the bolt pattern on the flange and on the mating component as a matched set to ensure alignment. For a 144 inch flange with a 48-bolt or 60-bolt pattern, a matched-set drill is not optional. Drift pin holes are commonly added at four to eight equally spaced locations to allow field alignment before the bolts are tensioned. The drift pin diameter and tolerance is specified on the drawing and is typically one to two sizes larger than the bolt to allow for thermal differential and fabrication drift.
Facing is the next dimensional question. A raised face on a 96 inch flange is typically 1/4 inch tall at Class 150 equivalent rating and 1/2 inch tall at higher equivalents. A flat face is used where the mating component is rubber-lined or where a full-face gasket is required. A tongue-and-groove or ring-type-joint (RTJ) face is uncommon at this scale but available where the service pressure justifies it. Weld neck hubs on plate flanges are typically rolled from plate, formed to the hub profile, and welded to the flange face. Cast hub options exist for the largest sizes but are project-specific.
Bolt Patterns at the Largest Sizes
Bolt count climbs with diameter. A 24 inch Class 150 flange has 20 bolts. A 48 inch flange in equivalent rating has 44 bolts. A 96 inch ring flange in AWWA C207 Class D has 92 bolts. A 144 inch ring flange has 132 bolts. The bolt size also climbs, from 1 inch on the smaller flanges to 1-1/2 inch or 1-3/4 inch on the largest. Bolt tensioning at this scale uses hydraulic tensioners with simultaneous multi-stud operation. Hand-torquing is not practical on the larger sizes and the bolt-up procedure on the drawing should specify the tensioning method.
Stud grade is part of the bolt pattern decision. ASTM A193 B7 carbon-alloy studs are the default for non-corrosive service. A193 B7M is the supplementary hardness-controlled variant for sour service. A193 B8 and B8M are austenitic stainless variants for corrosive or temperature-cycling service. The stud diameter and grade combination determines the achievable preload. On a 96 inch Class 300 equivalent flange, the bolt preload required to seat a 1/8 inch thick spiral wound gasket runs into the hundreds of thousands of pounds total across all studs. The preload calculation is part of the flange design package and should be reviewed during procurement.
Nut and washer selection follows the stud. ASTM A194 Grade 2H heavy hex nuts mate to B7 studs. A194 Grade 7 mates to B8M. Hardened washers per ASTM F436 are used where the bolt-up pressure approaches the bearing yield of the flange surface. For the largest flanges, the bolt material specification, the nut material specification, the washer specification, and the gasket specification should all appear on the drawing or in the bill of materials.
NDE and Documentation
Non-destructive examination of custom large-diameter flanges scales with the service. For ASME Section VIII pressure-rated flanges, ultrasonic testing (UT) of the plate at the bolt circle and hub regions is standard, and magnetic particle testing (MT) or liquid penetrant testing (PT) of the machined surfaces is common. The acceptance criteria are typically called out by the ASME Section VIII division applicable to the equipment. AWWA C207 ring flanges in standard service typically only require visual examination per the standard, although some owners specify supplementary UT or MT at additional cost.
Welded hub configurations require additional NDE at the hub-to-flange weld. Full penetration weld with 100 percent radiographic testing or 100 percent UT is the standard for pressure-rated weld-neck plate flanges. The weld procedure specification, the welder qualification record, and the NDE reports become part of the documentation package that ships with the flange. Material test reports (MTRs) on the plate are non-negotiable. For domestic-melt requirements under Buy American, AIS, BABA, or ARRA funding, the MTR must show domestic melt and pour and the mill certification must trace to a domestic mill. That documentation requirement should be on the purchase order and reflected on the drawing.
For sour service under NACE MR0175 or NACE MR0103, hardness mapping of the flange after final heat treatment and after any post-weld treatment is required. The map shows hardness readings at specified locations across the flange face, hub, and bolt circle, with acceptance criteria from the governing NACE annex. A flange that passes hardness on initial inspection can fail after field welding if the weld procedure introduces hardness above the limit, so the procedure for any field welding should also be NACE-qualified.
What to Put on the Purchase Order
A complete custom large-diameter flange purchase order includes: outside diameter and tolerance, inside diameter (bore) and tolerance, thickness and tolerance, bolt circle diameter and tolerance, bolt hole quantity and diameter and tolerance, bolt hole positioning (straddle or off-center relative to the principal axis), facing type and dimensions, hub configuration if any (weld neck, slip-on, blind), plate material specification including grade and supplementary requirements, heat treatment condition, NDE requirements at the plate stage and the finished part stage, MTR requirements including domestic-melt callout if applicable, and any project-specific certifications including NACE compliance, charpy impact testing, or third-party inspection.
Drawing approval is typically a two-step process for the largest sizes. A preliminary drawing goes from the supplier to the engineer for dimensional and metallurgical approval before the plate is cut. A final drawing including any field-fit revisions accompanies the finished part on delivery. Both drawings become part of the project record and stay with the flange through its service life. The engineering hours that go into a custom 144 inch flange drawing run well into the dozens, and that engineering is part of what the purchase order is buying.
Schedule on custom large-diameter work is driven by plate availability, machining capacity, and NDE turnaround. Stock plate in standard grades is usually available within a few weeks. Special grades, low-sulfur heats, or supplementary impact-tested plate run longer. The machining and NDE steps are predictable once the plate is in the shop. For projects with hard mechanical completion dates, the right move is to engage the supplier early, lock the drawing, and order the plate against the drawing rather than against a placeholder. Call (281) 484-8325 with the application, dimensions, and service conditions and we will walk through the drawing, the plate selection, and the documentation package that the project will need.