What is ASME B16.5?
ASME B16.5 is the American dimensional standard for pipe flanges and flanged fittings from NPS 1/2 inch through NPS 24 inch. It defines seven pressure classes — 150, 300, 400, 600, 900, 1500 and 2500 — together with bolt circles, facing types, thicknesses and pressure-temperature ratings by material group.
Key Takeaways
- ASME B16.5 covers NPS 1/2 inch to 24 inch; above 24 inch the governing standard is ASME B16.47 Series A or Series B.
- The seven classes are 150, 300, 400, 600, 900, 1500 and 2500 — a class is a rating designation, not a pressure in psi.
- A Class 150 flange in ASTM A105 is rated 285 psig at ambient but only about 170 psig at 400 °F; the rating falls as temperature rises.
- Facing options are raised face (RF), flat face (FF) and ring-type joint (RTJ), with RTJ standard from Class 900 upward on hydrocarbon service.
- Kalp Alloys manufactures the full B16.5 type and class range in carbon, stainless, alloy, duplex, nickel, titanium and copper grades.
- Class 400 is rarely specified in practice; most projects step from Class 300 straight to Class 600.
What the standard actually specifies
ASME B16.5 is a dimensional and rating standard. It does not tell an engineer which flange to choose; it fixes what a flange of a given type, size and class must measure and what pressure it may carry at a given temperature in a given material group.
For each combination of size and class the standard fixes outside diameter, thickness, hub dimensions, raised face diameter and height, bolt circle diameter, number of bolts and bolt hole diameter. Two flanges of the same size and class from different manufacturers therefore bolt together, which is the entire commercial point of the document.
It also fixes the pressure-temperature rating tables. Those are organised by material group — Group 1.1 covers ASTM A105 carbon steel, Group 2.2 covers 316 stainless, and so on — because the allowable stress of the material, not the geometry alone, sets the working pressure.
Pressure classes and what they mean
The single most common misreading in this category is treating the class number as a pressure in psi. It is not. Class 150 does not mean 150 psi. The class is a rating designation whose allowable pressure depends on material group and temperature, and it falls as the metal gets hotter.
| Class | At 100 °F | At 400 °F | At 600 °F | At 800 °F |
|---|---|---|---|---|
| 150 | 285 | 170 | 140 | 75 |
| 300 | 740 | 635 | 550 | 410 |
| 600 | 1,480 | 1,270 | 1,095 | 825 |
| 900 | 2,220 | 1,900 | 1,640 | 1,235 |
| 1500 | 3,705 | 3,170 | 2,735 | 2,060 |
| 2500 | 6,170 | 5,280 | 4,560 | 3,430 |
Two practical consequences follow. First, a line designed at 600 °F cannot be rated on the ambient figure. Second, changing material changes the rating even when the geometry is identical — a 316 stainless flange and an A105 carbon flange of the same class and size do not carry the same pressure at temperature. The working method is set out on the pressure-temperature rating chart.
ASME B16.5 versus ASME B16.47
The size boundary is the thing to get right at the enquiry stage, because ordering the wrong standard produces a flange that will not mate with the counter-flange on site.
| ASME B16.5 | ASME B16.47 Series A | ASME B16.47 Series B | |
|---|---|---|---|
| Size range | NPS 1/2" to 24" | NPS 26" to 60" | NPS 26" to 60" |
| Classes | 150 to 2500 | 75, 150, 300, 400, 600, 900 | 75, 150, 300, 400, 600, 900 |
| Predecessor standard | — | MSS SP-44 | API 605 |
| Relative size and weight | — | Heavier, larger bolt circle | Lighter, more compact |
| Interchangeable with the other series | — | No | No |
Series A and Series B are not interchangeable with each other. A 30 inch Series A flange will not bolt to a 30 inch Series B flange, so the series must be stated on the purchase order and never assumed. Both series are manufactured at Ahmedabad — see ASME B16.47 flanges.
Facing types and finish
The facing is the sealing surface, and specifying it correctly matters as much as the class.
- Raised face (RF) — the default across Class 150 to 2500. The gasket seats on a small raised area, concentrating bolt load. Standard serrated finish is 125 to 250 AARH.
- Flat face (FF) — the whole face is flat, used when mating to a cast-iron or non-metallic flange that would crack under the concentrated load of a raised face.
- Ring-type joint (RTJ) — a machined groove takes a metal ring gasket, type R, RX or BX. Standard on high-pressure hydrocarbon service, generally from Class 900 upward, and on sour-service lines where a metal-to-metal seal is required.
Gasket selection follows from the facing, not the other way round. A spiral wound gasket suits a raised face; an octagonal or oval ring suits an RTJ groove.
Materials and what we manufacture to B16.5
ASME B16.5 governs dimensions and ratings; the material specification is a separate ASTM document, and both appear on the purchase order. A typical line item reads: weld neck flange, 6 inch, Class 300, RF, ASTM A105N, ASME B16.5, SCH 40 bore.
- Carbon steel — ASTM A105 / A105N, and ASTM A350 LF2 for low-temperature service.
- Stainless steel — ASTM A182 F304, F304L, F316, F316L, F321, F347.
- Alloy steel — ASTM A182 F5, F9, F11, F22, F91 for elevated temperature.
- Duplex and super duplex — ASTM A182 F51 (2205), F53 (2507), F55.
- Nickel alloys — ASTM B564 Inconel 625, Incoloy 800 and 825, Monel 400, Hastelloy C276.
- Titanium — ASTM B381 Grade 2 and Grade 5.
Every flange is supplied with an EN 10204 3.1 or 3.2 mill test certificate stating the ordered standard, class, material grade and heat number.
Frequently Asked Questions
No. The class is a rating designation, not a working pressure. A Class 150 flange in ASTM A105 carbon steel is rated about 285 psig at 100 degrees Fahrenheit and roughly 170 psig at 400 degrees Fahrenheit, because the allowable stress of the material falls as temperature rises.
NPS 1/2 inch through NPS 24 inch. Flanges from NPS 26 inch to 60 inch fall under ASME B16.47 Series A or Series B instead.
No. The two series have different bolt circles, thicknesses and weights, so a Series A flange will not mate with a Series B flange of the same size and class. The series must be stated on the purchase order.
They refer to the same standard. It originated as an ANSI document and is now maintained by ASME, so older drawings and specifications often still say ANSI B16.5.
Ring-type joint (RTJ) is normal from Class 900 upward and on sour-service lines, because the metal ring gasket seals metal to metal. Raised face with a spiral wound gasket is standard at Class 150 to 600.
Yes. Inspection by SGS, BV, TUV or Lloyds can be witnessed at our Ahmedabad plant, and the release note is issued together with the EN 10204 3.2 certificate.