ASME B16.5 Steel Flange Ratings & Standards Guide
Industrial steel flanges provide a bolted, demountable joint connection between pipes, valves, strainers, and pressure vessels. ASME B16.5 covers pipe flanges and flanged fittings from NPS 1/2 through NPS 24 metric/inch standard.
1. ASME Pressure Classes Explained
ASME B16.5 specifies seven pressure classes: **Class 150, 300, 400, 600, 900, 1500, and 2500**. The "Class" designation indicates the nominal pressure rating at elevated temperatures. As temperature rises, maximum allowable working pressure (MAWP) decreases according to material group.
| ASME Pressure Class | Max Working Pressure at 38°C (A105) | Max Working Pressure at 400°C (A105) | Primary Application |
|---|---|---|---|
| Class 150 | 19.6 bar (285 psi) | 6.5 bar (95 psi) | Low pressure utility, cooling water, gas distribution |
| Class 300 | 51.1 bar (740 psi) | 25.7 bar (373 psi) | Medium pressure process lines, chemical plants |
| Class 600 | 102.1 bar (1,480 psi) | 51.5 bar (747 psi) | High pressure steam, oil transmission pipelines |
| Class 1500 | 255.3 bar (3,705 psi) | 128.6 bar (1,865 psi) | Refinery hydrocracker units, offshore injection lines |
| Class 2500 | 425.5 bar (6,170 psi) | 214.4 bar (3,110 psi) | Ultra-high pressure subsea & power boiler piping |
2. Flange Facing Types (RF, FF, RTJ)
- **Raised Face (RF)**: The most common facing type. The gasket surface is raised 1.6mm (1/16") for Class 150/300, or 6.4mm (1/4") for Class 600+. Focuses bolt clamping pressure onto a smaller gasket area.
- **Flat Face (FF)**: The gasket surface is in the same plane as the bolting circle. Used when mating with cast iron or fragile equipment to prevent bending stress.
- **Ring Type Joint (RTJ)**: Features a precision-machined metallic ring groove. Uses a solid octagonal or oval metal gasket (soft iron or SS 316) for high pressure (>Class 900) or high temperature service.