Standards Reference
ASME B16.5 and B16.47
These two standards cover the overwhelming majority of industrial steel flanges in service in North America. B16.5 handles the small and mid-bore end, B16.47 covers large diameter. Together they define class, dimension, tolerance, material, marking, and the pressure-temperature ratings that every spec sheet ultimately references.
ASME B16.5: Pipe Flanges and Flanged Fittings, 1/2" - 24"
B16.5 is the standard most often referenced when working with carbon steel, stainless steel, and alloy flanges in process piping. It covers nominal pipe sizes 1/2 inch through 24 inches in seven pressure classes (150, 300, 400, 600, 900, 1500, and 2500) and four common face types.
Pressure Classes
ASME B16.5 defines seven pressure classes: 150, 300, 400, 600, 900, 1500, and 2500. Class 400 is part of the standard but is uncommon in stock - it is normally substituted out to Class 600 unless a spec specifically requires it.
- 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 and hot hydrocarbon lines.
- Class 400 - an intermediate class that exists in the standard but is rarely specified. Class 600 typically fills the same duty with more inventory availability.
- Class 600 - higher pressure process service, including many refinery applications.
- Class 900 - high-pressure service, often specified for steam and chemical work.
- Class 1500 - severe service. Compact, heavy, expensive.
- Class 2500 - the top of the B16.5 range. Beyond this you move into API 6A territory.
Face Types
- Raised Face (RF) - the default for most process piping. Concentrates gasket load on the raised area. Class 150 and Class 300 carry a 1/16-inch raise height; Class 400 through Class 2500 carry a 1/4-inch raise. The two are dimensionally different and not interchangeable.
- Flat Face (FF) - used when bolting to a brittle flange (cast iron) where bending the cast flange must be avoided.
- Ring Type Joint (RTJ) - metallic ring gasket in a machined groove. Used on high-pressure and high-temperature service.
- Tongue and Groove / Male and Female - less common, used where gasket retention matters.
See the Complete Guide to Flanges for face-finish callouts and gasket selection guidance.
ASME B16.47: Large Diameter 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.
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.
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 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.
Materials Commonly Specified Under B16.5 and B16.47
B16.5 and B16.47 cover dimensional requirements. The material specification governs chemistry and mechanical properties. The combination is what produces a final pressure-temperature rating.
Carbon Steel
A105 (forged), A350 LF2 (low-temp impact tested), A516-70 (plate origin for blinds and large diameter). 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.
Alloy Steel
F11 (1.25Cr-0.5Mo), F22 (2.25Cr-1Mo), F91 (9Cr-1Mo-V) for elevated temperature service. 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.
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.
Need ASME Flanges for Your Project?
Texas Flange supplies B16.5 through 24 inches and sources B16.47 in both series to the size and class on the line list. Send the project and pricing comes back with material, class, and face finish confirmed line by line.