ASA Flanges Explained: Dimensions, Pressure Classes and Material Grades
Ask three engineers on an Indian project site what an ASA flange is and you will get three answers. Ask them whether it is different from an ANSI flange and the confusion deepens.
The short answer is that they are the same thing, and the terminology is a historical artefact. But the practical consequences of getting the pressure class or bolt circle wrong are real, expensive, and discovered at the worst possible moment — when two flanges will not bolt together on site.
This guide covers what ASA flanges are, how the pressure classes differ, which dimensions actually matter when ordering, and how to specify grade and documentation correctly.
Asiad Steel Industries has supplied stainless steel and alloy flanges from Pune since 1991, to oil and gas, petrochemical, chemical, paper mill and general process projects across India and export markets.
What Are ASA Flanges?
ASA flanges are pipe flanges manufactured to the dimensional standard now published as ASME B16.5. “ASA” refers to the American Standards Association, the original publishing body. The standard was later renamed ANSI and then ASME B16.5 in 1996. ASA, ANSI and ASME B16.5 flanges of the same class and size are dimensionally identical and fully interchangeable.
The naming history explains the confusion:
| Era | Designation | Status |
|---|---|---|
| 1932 | ASA B16E | Original publication |
| Mid-century | ANSI B16.5 | Renamed body |
| 1996 onward | ASME B16.5 | Current designation |
In Indian procurement and design documents, “ASA 150” and “ANSI 150” are still used interchangeably. Both mean ASME B16.5 Class 150. You do not need to change your terminology — you need to make sure the class and size are unambiguous on the purchase order.
Scope of the standard: ASME B16.5 covers flanges from NPS 1/2 inch to NPS 24 inch (DN 15 to DN 600). For NPS 26 inch and above, the governing standard is ASME B16.47.
ASA Flange Pressure Classes
ASME B16.5 defines seven pressure classes:
| Class | PN equivalent | Typical use |
|---|---|---|
| 150 | PN 20 | General low-pressure service, most common class |
| 300 | PN 50 | Medium pressure, chemical and process plant |
| 400 | — | Uncommon; some refinery designs |
| 600 | PN 110 | Higher pressure process and refinery lines |
| 900 | PN 150 | High pressure |
| 1500 | PN 260 | Very high pressure |
| 2500 | PN 420 | Extreme pressure, available up to 12 inch |
The critical point about pressure class: the class number is not a pressure in psi. It is a rating designation. The actual allowable pressure depends on both the material group and the operating temperature, and it drops as temperature rises. A Class 150 flange in a common material group is rated around 285 psi at ambient temperature, and considerably less at elevated temperature.
Always check the pressure-temperature rating table for your specific material group. Specifying by class alone, without confirming service temperature, is a frequent source of under-rated installations.
Two useful anomalies
Two overlaps in the standard save money if you know about them:
- Class 400 flanges in sizes up to NPS 3½ inch are identical to Class 600 flanges in all respects
- Class 900 flanges are identical to Class 1500 for sizes NPS 1/2 inch through 2½ inch
In these size ranges you can often source the more widely stocked class.
Why Class Determines Dimensions, Not Just Pressure
This is the point most buyers miss, and it causes real site problems.
Two flanges of the same nominal size but different classes have different bolt circles, different bolt counts and different thicknesses. They will not mate.
A worked example: a 3 inch Class 150 flange has four bolt holes. A 3 inch Class 300 flange has eight. Both are “3 inch flanges” and neither will bolt to the other.
Raised face height also varies by class. At 1 inch nominal size, Class 150 and Class 300 flanges carry a 1/16 inch raised face, while Class 600 uses a 1/4 inch raised face. Gasket selection has to account for this.
Practical rule: a purchase order reading “3 inch SS flange” specifies almost nothing. A complete specification needs:
- Nominal size (NPS or DN)
- Pressure class (150, 300, 600 and so on)
- Flange type (weld neck, slip-on, blind, lap joint, socket weld, threaded)
- Facing (raised face, flat face, ring type joint)
- Material grade (304, 316, 316L, A105 and so on)
- Bore and matching pipe schedule, for weld neck flanges
Item six matters specifically for weld neck flanges, because the bore must match the connected pipe wall thickness. Slip-on, socket weld and threaded flanges are unaffected by wall thickness and only require the outside diameter.
ASA Flange Types
| Type | Description | Best for |
|---|---|---|
| Weld neck | Long tapered hub, butt welded to pipe | High pressure, cyclic loading, critical service |
| Slip-on | Slides over pipe, fillet welded inside and out | Lower cost, easier alignment, moderate pressure |
| Blind | Solid disc, no bore | Closing off a line or vessel opening |
| Lap joint | Used with a stub end, flange rotates freely | Systems needing frequent dismantling; flange can be a cheaper material |
| Socket weld | Pipe inserts into a socket, fillet welded | Small bore, high pressure |
| Threaded | Screwed connection, no welding | Where welding is prohibited or impractical |
Browse our range: ASA flanges, weld neck flanges, socket weld flanges, lap joint flanges, threaded flanges and table flanges.
Material Grades for ASA Flanges
Flange dimensions do not change with material. The outside diameter, bolt circle, thickness and drilling are fixed by the standard whether the flange is carbon steel, stainless steel or galvanised. Only the pressure-temperature rating changes.
| Material | Specification | Typical application |
|---|---|---|
| Carbon steel | ASTM A105 (forged) | General service, oil and gas, water |
| SS 304 / 304L | ASTM A182 F304 / F304L | General corrosion resistance, food, chemical |
| SS 316 / 316L | ASTM A182 F316 / F316L | Chloride exposure, marine, aggressive chemical |
| Alloy steel | ASTM A182 F5, F11, F22 | Elevated temperature service |
| Nickel alloys | Hastelloy, Inconel, Monel | Severe corrosion duty |
304 versus 316 for flanges: the deciding factor is chloride exposure. SS 316 contains 2 to 3 percent molybdenum, which substantially improves resistance to pitting and crevice corrosion in chloride-bearing environments — seawater, many process chemicals, and coastal atmospheres. For inland non-chloride service, 304 is usually adequate and meaningfully cheaper.
Marking and Documentation
Every ASME B16.5 flange must carry permanent marking per MSS SP-25, including:
- Manufacturer identification
- Material designation, for example ASTM A105 or F316
- Pressure class, for example CL150
- Nominal size
- Heat number for traceability
Request EN 10204 3.1 mill test certificates with every order. Without documentation tying the flange to a specific heat number, you cannot demonstrate material compliance during a third-party inspection or project audit. On oil and gas or export projects this is not optional.
Common ASA Flange Ordering Mistakes
- Specifying size and material only. Omitting the class is the single most common error and produces flanges that will not mate.
- Assuming ASA and ANSI differ. They do not. Do not order both for the same line thinking you are hedging.
- Ignoring the pressure-temperature table. Class 150 at ambient is not Class 150 at 300°C.
- Wrong bore on weld neck flanges. The bore must match the pipe schedule, not just the nominal size.
- Mismatched gasket to facing. A raised face flange needs a gasket sized to the raised face, not the full flange face.
- Skipping the MTC. Discovered during inspection, this delays project handover.
Conclusion
ASA flanges are ASME B16.5 flanges under an older name, and the terminology is harmless. What is not harmless is treating pressure class as a pressure rating rather than a dimensional specification.
Class determines bolt circle, bolt count, thickness and raised face height. Two flanges of the same nominal size in different classes are different components. Specify size, class, type, facing, grade and bore together, every time, and most flange problems disappear before they reach site.
Asiad Steel Industries supplies ASA and ASME B16.5 flanges in stainless steel, carbon steel and special alloys from Pune, in Classes 150 through 2500 and sizes NPS 1/2 inch to 24 inch, with full mill test certification. We also manufacture non-standard flanges to drawing where existing plant requires them.
Need flanges specified correctly? Send your line details or drawings and our technical team will confirm class, type and grade before quoting.
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FAQs
What is the difference between ASA and ANSI flanges?
There is no dimensional difference. ASA refers to the American Standards Association, the body that originally published the standard as ASA B16E in 1932. It was later renamed ANSI, and the standard became ASME B16.5 in 1996. ASA 150 and ANSI 150 flanges of the same size are identical and fully interchangeable. Both terms remain in common use in Indian procurement documents.
What pressure can an ASA 150 flange handle?
Class 150 corresponds to PN 20 and is rated at approximately 285 psi at ambient temperature for common material groups. The allowable pressure decreases as operating temperature rises, so the pressure-temperature rating table for your specific material group must be checked. For higher pressures, specify Class 300, 600 or above.
Can I bolt a Class 150 flange to a Class 300 flange?
No. Flanges of different pressure classes have different bolt circle diameters, different numbers of bolt holes and different thicknesses. A 3 inch Class 150 flange has four bolt holes while a 3 inch Class 300 flange has eight. They cannot be mated. Both flanges in a joint must be the same size, class and facing.
What sizes do ASA flanges come in?
ASME B16.5 covers nominal pipe sizes from 1/2 inch to 24 inch, equivalent to DN 15 to DN 600, across seven pressure classes. For sizes of 26 inch and above, the applicable standard is ASME B16.47, which is a separate specification with its own dimension tables.
Should I use 304 or 316 for stainless steel flanges?
Use SS 316 or 316L wherever chlorides are present — marine and coastal environments, seawater service, and many process chemicals. Its molybdenum content resists pitting and crevice corrosion. SS 304 is adequate and more economical for inland, non-chloride general service. For welded assemblies, the low carbon 304L and 316L variants resist sensitisation at the heat-affected zone.
What information do I need to order an ASA flange?
Six items: nominal size, pressure class, flange type (weld neck, slip-on, blind and so on), facing (raised face, flat face or ring type joint), material grade, and for weld neck flanges the bore matching the connected pipe schedule. Also specify whether EN 10204 3.1 mill test certification is required, which it usually is for project work.
