What ASME B16.5 Covers
ASME B16.5 standardizes flanges and flanged fittings in sizes NPS 1/2 to NPS 24 so that a flange, gasket and bolting set made by any conforming manufacturer will mate and seal. Larger sizes, NPS 26 and above, move to ASME B16.47. The standard fixes the bolt circle, bolt holes, flange thickness, hub dimensions, facing geometry and the pressure-temperature ratings for each combination of class and material group.
Every flange we supply to B16.5 is traceable to its heat and ships with an EN 10204 3.1 or 3.2 mill test certificate, so the material group used in the rating calculation is documented on the paperwork, not assumed.
Pressure Classes
The class number is a rating index, not a direct pressure. A Class 150 flange does not hold 150 psi at every temperature; its allowable working pressure falls as temperature rises and depends on the material group. The table below shows the range each class spans.
| Class | Typical use | Facing commonly specified |
|---|---|---|
| 150 | Low-pressure utility, water, general process | Raised face or flat face |
| 300 | General process and chemical lines | Raised face |
| 400 | Higher-pressure process service | Raised face |
| 600 | Hydrocarbon and steam service | Raised face or ring-type joint |
| 900 | High-pressure oil and gas | Ring-type joint |
| 1500 | High-pressure, high-temperature | Ring-type joint |
| 2500 | Severe high-pressure duty | Ring-type joint |
Flange Types
B16.5 defines the common flange forms, each matched to a joining method and inspection level.
| Flange type | How it joins the pipe | Where it fits |
|---|---|---|
| Weld neck | Butt-welded to pipe through a tapered hub | High-pressure, cyclic and critical service; full radiography possible |
| Slip-on | Pipe slides in, filleted inside and out | Lower pressure, easier alignment |
| Socket weld | Pipe seats in a socket, fillet welded | Small-bore high-pressure lines |
| Threaded | Screwed onto threaded pipe | No-weld or hazardous-area small bore |
| Lap joint | Used with a stub end, flange floats free | Frequent dismantling; flange can be a cheaper grade |
| Blind | Solid plate, bolts to close a flanged opening | Line termination and inspection points |
Facing Types and Surface Finish
The facing is the sealing surface machined into the flange face. B16.5 specifies several, and the facing must match on both flanges in a joint.
| Facing | Abbreviation | Notes |
|---|---|---|
| Raised face | RF | Most common; concentrates gasket load on a raised ring |
| Flat face | FF | Full-face gasket; used against cast-iron or brittle mating parts |
| Ring-type joint | RTJ | Metal ring gasket in a machined groove; high pressure and temperature |
| Tongue and groove | T&G | Mating male and female profile; precise gasket location |
| Male and female | M&F | Recess locates the gasket; limits blow-out |
| Lap joint | LJ | Sealing handled by the stub-end face |
Raised-face flanges are usually supplied with a serrated finish, either a concentric or spiral groove pattern, which grips a soft gasket. A smooth finish is specified where metal or PTFE-faced gaskets are used. State the facing and the surface finish on the enquiry so the groove pattern matches your gasket.
Materials for B16.5 Flanges
B16.5 lists material groups and each group drives the pressure-temperature rating. Forged stainless and nickel-alloy flanges are typically ordered to ASTM A182, which covers the F-grade forgings used for flanges, fittings and valve parts. We hold flange material across stainless, nickel-alloy and alloy-steel families.
| Grade | UNS / spec family | PREN (approx.) | Why it is chosen |
|---|---|---|---|
| 304 / 304L | S30400 / S30403, A182 F304/F304L | ~18 | General process; L grade resists sensitization in welded joints |
| 316 / 316L | S31600 / S31603, A182 F316/F316L | ~24 | 2 to 3% molybdenum lifts pitting resistance over 304 |
| 904L | N08904, super-austenitic | ~34 | Sulfuric acid and chloride service; copper-bearing chemistry |
| Incoloy 825 | N08825, Ni-Fe-Cr | ~31-40 | Sour service and mixed reducing and oxidizing acids |
| Inconel 625 | N06625, Ni-Cr-Mo | high | High-temperature strength and severe corrosion resistance |
The most common grade error on flange orders is defaulting to 304 for chloride-adjacent or coastal duty, where it pits. Confirm the service chlorides before choosing 304 over 316, and read our note on pitting corrosion and the PREN pitting index if the medium is aggressive. For welded stainless flanges held in the sensitization range, specify the low-carbon L grade to keep the heat-affected zone corrosion-resistant, as covered in our guide on preventing sensitization.
Bolting and Gaskets
B16.5 ratings assume the correct bolting. Flange bolting is normally ordered to ASTM A193 / A194, for example B8 or B8M studs with matching nuts for stainless service. The gasket style must suit the facing: spiral-wound or soft-faced for raised face, and a ring gasket of the specified ring number for RTJ grooves.
How B16.5 Relates to Other Flange and Fitting Standards
B16.5 stops at NPS 24. Above that, large-diameter flanges follow ASME B16.47. Flanges are commonly paired with buttweld fittings to ASME B16.9 and, in small bore, with forged socket-weld and threaded fittings to ASME B16.11. The flange forging material itself is governed by ASTM A182 for stainless and alloy grades. Our full flange range spans stainless steel, inconel, hastelloy and alloy steel.
Documentation and Traceability
Every B16.5 flange ships with an EN 10204 3.1 or 3.2 mill test certificate tying the part to its heat chemistry and mechanical results. If you need to confirm the grade, dimensions or facing on receipt, our guides on reading a mill test certificate and material traceability walk through what to check.
Frequently Asked Questions
What size range does ASME B16.5 cover?
What are the pressure classes in ASME B16.5?
What is the difference between raised face and RTJ facing?
Which materials can B16.5 flanges be made from?
Does ASME B16.5 specify bolting and gaskets?
Request a Quote
Nandini Steel stocks ASME B16.5 flanges across stainless, nickel-alloy and alloy-steel grades, ex-stock for immediate dispatch, each with an EN 10204 mill test certificate. Send your class, size, facing and grade to reachus@nandinisteel.com, WhatsApp +91 99202 09184, or use our contact page for live pricing.