Standards Reference
ASME B16.5 and B16.47 Steel Flanges
These two standards cover the overwhelming majority of forged and plate-cut steel flanges in service in North America. B16.5 handles the small and mid-bore end. B16.47 takes over on large diameter. Together they define class, dimension, tolerance, material, marking, and the pressure-temperature ratings that every steel flange spec sheet ultimately references.
ASME B16.5: Steel Pipe Flanges and Flanged Fittings, 1/2" - 24"
B16.5 is the standard you bump into first when working with carbon steel, stainless steel, and chrome-moly alloy flanges in process piping. It covers nominal pipe sizes 1/2 inch through 24 inches in seven pressure classes and four face types. Almost every B16.5 flange we ship is forged, though plate-cut is permitted for blinds and certain slip-on configurations where the standard allows.
Pressure Classes
- Class 150 the workhorse for moderate-pressure service. Roughly 285 psi at ambient with Group 1.1 materials, derating as temperature climbs.
- Class 300 the next step up, common on steam, hot hydrocarbon lines, and most refinery service.
- Class 600 higher-pressure process service. Many cracker and reformer headers land here.
- Class 900 high-pressure service, often specified for steam, chemical, and gas processing.
- Class 1500 severe service. Compact, heavy, expensive. F22 and F91 forgings are common here.
- Class 2500 the top of the B16.5 range. Beyond this you move into API 6A wellhead territory.
Face Types
- Raised Face (RF) the default for most steel process piping. Concentrated gasket load on a 1/16 inch raised ring through Class 300, 1/4 inch above.
- Flat Face (FF) used when bolting steel to a brittle flange (cast iron) where bending the cast flange must be avoided. Common on B16.1 retrofit work.
- Ring Type Joint (RTJ) metallic ring gasket in a machined groove. Used on high-pressure and high-temperature service, especially Class 600 and above on hydrocarbon lines.
- Tongue and Groove / Male and Female less common, used where gasket retention matters and the flange pair is engineered together.
See the Complete Guide to Flanges for face-finish callouts and gasket selection guidance.
ASME B16.47: Large Diameter Steel Flanges, 26" - 60"
B16.47 picks up where B16.5 stops. It covers nominal sizes 26 inches through 60 inches, but unlike B16.5 it offers two distinct series with different bolt patterns. The series you specify matters as much as the class. On custom work we supply B16.47 geometry well past 60 inches up to 203 inches when the project requires it.
Series A
Derived from the older MSS SP-44 standard for steel pipeline flanges. Series A flanges are heavier overall, with fewer but larger-diameter bolts on a wider bolt circle. Designed to handle higher bolt loads and the bending forces seen on large transmission pipelines.
Common where pipeline operators and process plants need extra margin on large lines. Often plate-cut from A516-70 at the upper sizes.
Series B
Derived from the older API 605 standard. Series B flanges are lighter, with more bolts of smaller diameter on a tighter bolt circle. The geometry favors compactness and reduced material cost where the application allows.
Common on process vessels, tank nozzles, and applications where weight and outside dimensions matter.
Series A and Series B are not interchangeable.
A 36-inch Class 300 Series A flange has a different bolt circle, different bolt count, and different OD than a 36-inch Class 300 Series B flange. Mating them is not possible without a transition piece. Always confirm the series on the spec sheet, the requisition, and the mill cert before the flange ships.
Steel Materials We Commonly Supply
B16.5 and B16.47 cover dimensional requirements. The material specification governs chemistry and mechanical properties. The combination is what gives you a final pressure-temperature rating. Below are the steel grades we move every week, from A105 carbon at one end to F91 chrome-moly at the other.
Carbon Steel
A105 (forged), A350 LF2 (low-temp impact tested), A516-70 (plate origin for blinds and large diameter). The carbon workhorses. See carbon steel materials.
Stainless Steel
F304 / 304L and F316 / 316L are the everyday austenitics. F321, F347, and duplex grades for specific service. See stainless steel materials.
Chrome-Moly Alloy
F11 (1.25Cr-0.5Mo), F22 (2.25Cr-1Mo), F91 (9Cr-1Mo-V) for elevated temperature service. F91 is the standard for ultra-supercritical steam. See alloy steel materials.
Line Pipe Grades
A694 F42, F52, F60, F65, and F70 for pipeline service that requires matched yield with the connecting pipe. Carbon steel with controlled chemistry and impact testing.
Forged or Plate-Cut: Both Routes
B16.5 and B16.47 are dimensional standards. They define the finished geometry, not the production method. Forged flanges are the default below 24 inches because the forging press economics work in that range. Above 24 inches, and especially on blinds and large slip-on rings, plate-cut from A36 or A516-70 becomes cost-effective and often delivers a shorter lead time. We supply both routes on the same project when sizes and pressures justify it. The mill cert paperwork makes the production method explicit either way.
Common Applications
B16.5 shows up everywhere process piping does: refineries, petrochemical plants, gas processing, power generation, chemical production, pulp and paper, and food and beverage. B16.47 takes over on large-bore transmission, separator vessels, large heat-exchanger nozzles, and oversized process headers. If the line is over 24 inches, B16.47 is almost always the right reference, and beyond 60 inches we work custom against the same geometric rules.
Need ASME Steel Flanges for Your Project?
We carry B16.5 through 24 inches and source B16.47 in both series from the size and class that matches your spec. Forged and plate-cut both available. Send the line list and we will confirm material, class, and face finish on every line item.