ASME B16.5 vs B16.47: A Practical Guide for Piping Engineers

We get this question periodically from engineers: “Do I specify B16.5 or B16.47 for my flanges?”

The answer matters. Specifying the wrong standard creates ordering confusion, can land you with flanges that don’t fit, and leads to rework nobody budgeted for. But the choice between B16.5 and B16.47 isn’t complicated once you understand what each standard actually covers.

After four decades in the flange business, here’s how I walk people through it.

The Quick Answer

ASME B16.5 covers small-bore flanges, 1/2 inch through 24 inch nominal pipe size (NPS).

ASME B16.47 covers large-bore flanges, 26 inch through 60 inch NPS.

If your piping system runs 1/2 inch to 24 inch, you specify B16.5. If it runs 26 inch and larger, you specify B16.47. If you have a mixed system, you use B16.5 on the small side and B16.47 on the large side. Each standard owns its size range.

That’s the executive summary. The rest of this is the practical detail.

One thing to get straight first: “Class” is not “PSI”

Before we go further, a point that trips up newer engineers and shows up in a lot of sloppy spec sheets. The pressure ratings in these standards are Classes, not pounds per square inch. Class 150, Class 300, Class 600, and so on are rating designations, not a literal pressure.

A Class 150 carbon steel flange is not limited to 150 psi. At ambient temperature it’s good for roughly 285 psig, and that allowable working pressure drops as temperature climbs. The actual number comes from the pressure-temperature rating tables in the standard, based on your material group and design temperature. So write “Class 150,” not “150#” meaning 150 psi, and never assume the class number is your working pressure. I’ll use Class designations throughout.

ASME B16.5: The Small-Bore Standard

B16.5 is the flange standard you’ll meet most often. It covers:

• Pressure classes: 150, 300, 400, 600, 900, 1500, and 2500
• Nominal pipe sizes: 1/2 inch through 24 inch
• Flange types: weld neck, slip-on, blind, threaded, socket weld, and lap joint
• Facing types: flat face, raised face, ring joint (RTJ), and tongue-and-groove
• Materials: carbon steel (A105, A350 LF2), stainless (A182 F304L, F316L), chrome-moly (F11, F22, F91), and a range of specialty alloys
• Full dimensional data: bolt circles, bolt hole sizes, flange thicknesses, and face heights

B16.5 is comprehensive. Once you pick your size, class, type, and material, the standard tells you exactly what the flange should be. No ambiguity.

(Orifice flanges, used for flow measurement, follow the companion standard ASME B16.36. They share the B16.5 class and bolt patterns but carry their own spec, so call them out as B16.36 when you order them.)

ASME B16.47: The Large-Bore Standard

B16.47 shows up less often, unless you live in large pipeline, power generation, or water utility work, but it’s just as important.

B16.47 has two series, A and B. Both cover 26 inch through 60 inch NPS and share the same core pressure classes: 150, 300, 400, 600, and 900. The one difference in the class lineup is at the bottom: Series B carries a lighter Class 75 that Series A does not. Series A starts at Class 150.

So why two series? This is where the original version of this explanation usually goes wrong, so let me be precise. The main difference between Series A and Series B is not pressure range. They share the same classes from 150 up. The difference is dimensional, and it traces to their lineage. Series A flanges (the heritage of the old MSS SP-44 line) are generally heavier, with a larger bolt circle and fewer, larger bolts, and they start at Class 150. Series B flanges (derived from the old API 605 line) are more compact and lighter, with more bolts of smaller diameter on a tighter pattern, and they add the lighter Class 75 at the bottom of the range.

The practical consequence is the part you cannot ignore: Series A and Series B flanges of the same size and class are not interchangeable. Different bolt counts, different bolt circles, different patterns. They will not mate.

B16.47 covers the same core flange types as B16.5 (weld neck, slip-on, blind, lap joint) but at a scale that changes how you handle them. A 48 inch Class 300 weld neck flange can weigh several thousand pounds and needs rigging and lifting equipment to move. These are not parts you set by hand.

Why Two Standards At All?

Why didn’t ASME just write one standard from 1/2 inch to 60 inch?

Engineering practicality. The bolt spacing, hub geometry, and thickness relationships that work for a 1/2 inch flange don’t scale cleanly to a 60 inch flange. And the applications are different. B16.5 lives in process piping, equipment connections, and pressure vessels. B16.47 lives in large transmission lines, power plant circulating systems, and high-flow water mains. Splitting the standards let ASME optimize each for its range instead of forcing one set of compromises across the whole spread.

Size and Class at a Glance

| Standard | Size Range | Pressure Classes | Typical Applications | | — -| — -| — -| — -| | B16.5 | 1/2 in to 24 in | 150, 300, 400, 600, 900, 1500, 2500 | Process piping, equipment, pressure vessels | | B16.47 Series A | 26 in to 60 in | 150, 300, 400, 600, 900 | Large piping, heavier/legacy bolt pattern (MSS SP-44) | | B16.47 Series B | 26 in to 60 in | 75, 150, 300, 400, 600, 900 | Large piping, compact/lighter bolt pattern (API 605) |

Flange Types: What Lives in Each Standard

Both standards cover the primary types, with a few practical differences:

• Weld neck: in both standards. The workhorse for high-pressure and critical service. The tapered hub butt-welds to the pipe, bore matches the pipe bore, and the weld can be radiographed.
• Slip-on: in both. Easier to fit (slide over the pipe, weld inside and out) but lower-rated than weld neck. Common in lower-pressure, non-critical lines.
• Blind: in both. Caps off the end of a line, nozzle, or valve.
• Lap joint: in both. Used with a stub end and a loose backing flange, handy where you want to rotate the flange for bolt alignment or save on alloy.
• Threaded: in B16.5, mostly smaller sizes. Rare in large bore, where threading that diameter isn’t practical.
• Socket weld: B16.5 only, small sizes (up to about 3 inch). Not part of B16.47.

Facings and Ring Joints

Both standards cover several facing types:

• Flat face (FF): flat surface, used with full-face soft gaskets, typically in lower-pressure service and against cast-iron equipment.
• Raised face (RF): the most common facing. The raised height is not constant. In B16.5 it’s 1/16 inch for Classes 150 and 300, and 1/4 inch for Class 400 and above. B16.47’s raised-face dimensions are taller and are set in that standard’s tables for each class. Don’t assume one number across the board.
• Ring joint (RTJ): the high-pressure choice. The face carries a machined groove, and a solid metal ring sits in it to form a metal-to-metal seal. RTJ is standard on Class 600 and above in a lot of services, and mandatory in many offshore and high-pressure applications. Covered in both standards. When you spec RTJ, you also spec the ring profile (oval or octagonal) and the groove number, because the ring has to match the groove exactly or it won’t seal.
• Tongue-and-groove: in both standards, used with confined gaskets. Less common today since RTJ took over most of the high-integrity work.

The Mistakes That Cost Time and Money

Mistake 1: Not specifying Series A or B on a B16.47 flange If your line says “26 inch B16.47 flange” with no series, the order is ambiguous and the two series don’t interchange. Best case, the vendor calls to ask and you lose a day. Worst case, the wrong pattern shows up. Always write “B16.47 Series B” (or Series A) explicitly.

Mistake 2: Specifying RTJ without the ring profile RTJ comes as oval or octagonal, with specific groove (R) numbers. Leave it off and the seal is a coin toss. Always state the profile and groove number from the standard.

Mistake 3: Treating the standard as the material decision The standard gives you dimensions. It does not pick your metallurgy. A105 carbon steel is wrong for cryogenic service (you want A350 LF2). Standard 316 struggles in high-chloride service (you want duplex or better). Service conditions drive material, and that’s on you.

Mistake 4: Assuming classes are interchangeable A Class 300 flange does not cover Class 600 service. Wall thickness, hub, and bolting are all different. Size the system to design pressure and temperature, then pick the class off the rating tables.

Mistake 5: Changing the gasket without rechecking the joint B16.5 and B16.47 joints are designed around a bolt load and gasket combination. Swap an RTJ metal ring for a soft sheet gasket and you’ve changed the seating load and the whole bolt-up calculation. If you change gasket type, recheck the assembly.

Worked Examples

Example 1 A refinery is building a crude unit with 12 inch piping at 50 psig operating pressure and needs flanges to tie piping to equipment.

Spec: ASME B16.5, 12 inch, Class 150, weld neck, RTJ, A105. Size and pressure sit squarely in B16.5. Weld neck and RTJ are standard, and A105 suits the service.

Example 2 A power plant is running a 36 inch condensate return loop at about 150 psig.

Spec: ASME B16.47 Series B, 36 inch, Class 150, weld neck, raised face, A105. The 36 inch size is past B16.5’s 24 inch ceiling, so B16.47 applies. Series B is the call here because it matches the project’s flange standard and the mating equipment.

Example 3 A water utility is installing a 48 inch transmission main at roughly 75 psig and needs flanges at sectional isolation valves.

Spec: ASME B16.47 Series B, 48 inch, Class 75, weld neck, flat face, A105. Again past B16.5’s range. Note that Class 75 only exists in Series B, so the class itself pins the series here. Where both series carry the class you need, pick the series to match the mating flanges and the rest of the system, not by guessing.

Notice the pattern in those examples: the series choice follows the project specification and the mating components, not the pressure level. The one exception is Class 75, which is a Series B-only class. That series-follows-the-project habit is the part people get backwards.

Bolt Patterns and Ordering

Both standards fix the bolt circle diameter, bolt count, and bolt hole size for every size and class. That standardization is what lets you buy from different mills and still bolt up:

1. Any two B16.5 flanges of the same size and class share a bolt pattern. Interchangeable across manufacturers.
2. Any two B16.47 Series B flanges of the same size and class share a pattern.
3. Series A and Series B do not. Different patterns, not interchangeable.

Bolt diameters run from about 1/2 inch on the smallest B16.5 flanges up to 1.5 to 2 inch (and larger) on big B16.47 flanges. The standard also sets flange thickness for each class to keep the joint strong.

When you order, specify all of it:

• Standard (B16.5 or B16.47)
• Series (B16.47 only: A or B)
• Size (NPS)
• Class (150, 300, 600, and so on)
• Flange type (weld neck, slip-on, and so on)
• Facing (flat, raised, RTJ, tongue-and-groove)
• Material grade (A105, A182 F316L, and so on)
• If RTJ: ring profile and groove number

Fill all of that in and there’s nothing left to interpret. Leave a field blank and you’ve created a question that has to be answered before anything ships.

Why This Matters

As a piping engineer, B16.5 and B16.47 are the language you use to order a finished part. Get the spec right and the flanges fit, the bolt holes line up, the pressure rating matches, the gasket seals, and the install is uneventful. Get it wrong and you’re looking at parts that don’t fit, inadequate ratings, rework, and schedule slip.

That’s why I push on specification clarity every time. The vendor builds exactly what the spec says. The spec is where the engineering either holds up or falls apart.

Bottom Line

B16.5 and B16.47 aren’t competitors. They’re complementary, each tuned to its size range. B16.5 for 1/2 inch to 24 inch, B16.47 for 26 inch to 60 inch. Identify your range, use the right standard, and specify completely: standard, series if it’s B16.47, size, class, type, facing, material, and ring details for RTJ.

If you’re not sure which standard applies or how to nail down the spec, that’s exactly the kind of question we field at Texas Flange every day. A good supplier should be able to walk you through it and catch the gaps before they become rework, not after.