ASME B16.47 governs large-diameter steel flanges from NPS 26 through NPS 60. The standard splits into two series with different dimensional origins:
Series A traces back to MSS SP-44. Flanges tend to be larger and heavier, with more robust bolt patterns. This is the series most commonly specified in Middle East oil and gas projects, particularly for offshore and subsea tie-ins where structural margin matters.
Series B derives from API 605. Dimensions are more compact, using fewer bolts with smaller diameters. You see this series more in European district heating and petrochemical retrofits where footprint and weight savings are prioritized.
Class 400 sits in the middle-to-upper pressure range within B16.47. Available classes run from 150 to 900, and Class 400 is rated for moderate-to-high pressure service. For context, a Class 400 blind flange at ambient temperature handles working pressures roughly 2.5 times what a Class 150 flange of the same size can take.
How Each Dimension Drives Engineering Decisions
Flange Outside Diameter (O)
The OD sets the footprint. For a NPS 60 Class 400 blind flange, that footprint is 74.25 inches, or just under 1.9 meters across. In compact European skid-mounted units, this diameter can interfere with structural steelwork and walkway clearances. In Middle East onshore installations where space is less constrained, the OD is less of a layout issue but still affects crane reach planning during installation.
Minimum Thickness (T)
Thickness is the primary pressure-retaining dimension. At Class 400, the NPS 60 blind flange requires 7.44 inches (189 mm) of minimum forged thickness. Add the 1/4-inch raised face and you are looking at a 7.69-inch (195 mm) thick plate of metal.
Thickness drives two things: pressure capability and forging complexity. Thicker flanges require larger forging presses and longer heat treatment cycles. For nickel alloy materials like Inconel 625 or Hastelloy C-276, which are increasingly specified in Middle East sour service applications, the thickness directly impacts lead time and cost. A NPS 48 Class 400 blind flange in Inconel 625 can take 16-20 weeks to forge and machine, compared to 6-8 weeks for the same piece in A105 carbon steel.
Raised Face Diameter (R)
The raised face provides the gasket seating surface. Its diameter determines the gasket's outer boundary. For raised face flanges, the gasket ID sits just inside the raised face diameter, and the OD extends to roughly the bolt circle inner edge.
Get this wrong and you get either gasket blowout (gasket too narrow) or insufficient gasket compression (gasket too wide). We recommend specifying gasket dimensions explicitly in purchase orders rather than relying on "standard" sizing, particularly for sizes above NPS 36 where gasket availability becomes spotty.
Bolt Circle and Hole Count
The bolt circle diameter and hole count together define the flange's sealing force distribution. Series A Class 400 flanges carry between 28 and 36 bolt holes. The NPS 46 is the only size that uses 36 holes; most others use 28 or 32.
Why does hole count vary non-monotonically? The bolt pattern is optimized so that bolt stress stays within acceptable limits at the rated pressure. At NPS 38, the pressure envelope allows a reduction to 1.88-inch bolt holes (smaller than the adjacent NPS 36 and NPS 40 sizes). At NPS 46, the geometry requires an additional four bolts to maintain the pressure boundary.
Procurement tip for Gulf region buyers: Many Saudi and UAE projects follow Saudi Aramco SAES standards, which sometimes override B16.47 bolt specifications. Verify whether your project requires Aramco-supplementary bolt requirements before placing orders.
Pressure-Temperature Ratings at Class 400
Class 400 does not mean 400 psi. The class number is a dimensionless reference point. Actual working pressure depends on material group and operating temperature.
For Group 1.1 materials (A105, LF2 carbon steel), the rated working pressure at ambient temperature is approximately 400 psi for smaller sizes, but B16.47 does not publish a single flat rating. The pressure rating scales with flange size and material strength at temperature.
At elevated temperatures, the picture changes. Carbon steel Group 1.1 at 400°C (750°F) retains roughly 65% of its ambient pressure rating. At 475°C (887°F), the B16.47 Series A Class 400 blind flange rating drops to about 50% of ambient. For Middle East crude processing units running at 350-420°C, this derating is critical. A flange selected at ambient rating without temperature correction will leak or fail under operating conditions.
For high-temperature applications above 450°C, consider Cr-Mo alloy steels (F11, F22, F91) or stainless grades (F304H, F316H). European refineries in Germany and Italy frequently specify F22 for Class 400 service in the 400-510°C range.
Series A vs Series B: Which One Should You Specify?
The choice between Series A and Series B is often driven by client specification rather than engineering preference. But understanding the trade-offs helps when writing deviation requests or alternative proposals.
Series A advantages:
More bolts mean better load distribution and lower individual bolt stress
Larger flange OD provides more gasket seating area for better seal reliability
Better suited for cyclic service and thermal transients
Preferred by most Middle East national oil companies (Saudi Aramco, ADNOC, QatarEnergy)
Series B advantages:
Smaller, lighter flanges reduce structural loading and support requirements
Fewer bolts speed up assembly and disassembly during turnarounds
Lower material cost (less steel per flange)
More common in European district heating and lower-pressure petrochemical service
