A full flange specification contains more information than most people initially realize. Each element tells you something specific about the flange — and getting any one of them wrong means the flange either won’t fit, won’t seal, or won’t hold the pressure. Here’s how to decode a flange spec from start to finish.
Anatomy of a Flange Specification
A complete flange spec looks something like this:
4" 150# Weld Neck Flange, A105, Raised Face, ASME B16.5
Breaking that down element by element:
- 4" — Nominal Pipe Size (NPS). The flange bore and bolt circle are sized for 4" pipe.
- 150# — Pressure class. Defines the pressure-temperature rating.
- Weld Neck — Flange type. Defines how it connects to the pipe.
- A105 — Material grade. Carbon steel forging.
- Raised Face — Facing type. Defines the gasket seating surface.
- ASME B16.5 — Dimensional standard. Governs all dimensions and ratings.
Pressure Class — What the Number Actually Means
The class number (150, 300, 600, 900, 1500, 2500) is a designation — not a direct PSI rating. The actual allowable working pressure depends on the material and temperature.
| Class | A105 Carbon Steel at 100°F | A105 at 500°F | Common Use |
|---|---|---|---|
| 150# | 285 PSI | 170 PSI | Water, steam, low-pressure process |
| 300# | 740 PSI | 510 PSI | Medium-pressure steam, process |
| 600# | 1,480 PSI | 1,015 PSI | High-pressure process and power |
| 900# | 2,220 PSI | 1,530 PSI | High-pressure steam and gas |
| 1500# | 3,705 PSI | 2,550 PSI | Very high pressure service |
| 2500# | 6,170 PSI | 4,255 PSI | Extreme pressure service |
Pressure-temperature ratings per ASME B16.5 Table 2-1.1 for Group 1.1 materials (A105, A516 Gr. 70, etc.). Always verify against the current edition of B16.5 for your specific material and temperature.
Flange Types — Which Connects How
- Weld Neck (WN): Long tapered hub, full-penetration butt weld to pipe. Strongest type. Required for high-pressure, high-temperature, and cyclic service. Radiographically inspectable.
- Slip-On (SO): Slides over pipe, fillet welded inside and outside. Easier alignment, lower cost. For lower-pressure service where weld neck strength isn’t required.
- Blind (BL): Solid plate, no bore. Used to blank off pipe ends, valve bodies, or pressure vessel nozzles. Available in all classes.
- Threaded (TH): Threaded bore, no welding required. For low-pressure service or where welding is prohibited. Not for cyclic or high-temp service.
- Socket Weld (SW): Recessed socket, pipe inserts and is fillet welded externally. Small bore (2" and under) high-pressure service.
- Lap Joint (LJ): Used with stub end. Flange rotates freely for easy bolt hole alignment. For systems requiring frequent disassembly.
Facing Types — What the Gasket Seats Against
- Raised Face (RF): A small raised ring on the gasket seating surface. Standard for most steel flanges in ASME B16.5. Concentrates bolt load on a smaller gasket area for better sealing.
- Flat Face (FF): Seating surface is flush with the flange face. Required when mating to cast iron flanges (ASME B16.1) to prevent cracking from uneven bolt load.
- Ring Type Joint (RTJ): Oval or octagonal metal ring seated in a groove. For Class 600 and above in high-pressure and high-temperature service where soft gaskets would extrude.
- Tongue and Groove (T&G): Mating male/female surfaces that locate the gasket precisely. Used on pressure vessels and specialty equipment.
Critical rule: Raised face flanges must never be bolted directly to flat face cast iron flanges. The raised face concentrates load at the center of the cast iron flange, which can crack it. If connecting steel to cast iron, use a full-face gasket and consider converting to flat face on the steel flange side.
Material Grades — What’s on the Mill Cert
| ASTM Grade | Description | Common Use |
|---|---|---|
| A105 | Forged carbon steel | Standard for most industrial flanges |
| A350 LF2 | Low-temp forged carbon steel, impact tested to -50°F | Cold service piping |
| A694 F52/F60/F65 | High-yield forged steel for pipeline service | Pipeline flanges |
| A182 F304/F316 | Stainless steel (304 or 316) | Corrosive service |
| A182 F11/F22 | Alloy steel (1.25Cr-0.5Mo / 2.25Cr-1Mo) | High-temperature service |
| A182 F51 | Duplex 2205 stainless | Highly corrosive or chloride service |
Raised Face Height — A Detail That Trips People Up
For Class 150 and 300 flanges, the raised face height is 1/16". For Class 400 and above, it’s 1/4". This matters when calculating flange-to-flange dimensions for piping isometrics and when selecting gaskets. The gasket sits on the raised face — its outside diameter should not extend beyond the raised face for RF gaskets.
Carbon Steel Flanges in Stock
PVFPro stocks A105 carbon steel flanges in Class 150 through 1500. Weld neck, slip-on, blind, threaded, socket weld. Mill certs available.
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