Flange Pressure-Temperature Rating
Quick Answer: The flange pressure-temperature (PT) rating defines the maximum allowable working pressure a flange can withstand at a specific temperature. As temperature increases, the allowable pressure decreases. PT ratings are published in ASME B16.5 (for NPS 1/2 to 24) and ASME B16.47 (for NPS 26 to 60), organized by pressure class and material group. The ratings apply to the flange body only and assume a suitable gasket and properly torqued bolting.
How Flange PT Ratings Work
The ASME B16.5 PT rating system is based on three variables:
- Pressure class: 150, 300, 400, 600, 900, 1500, or 2500
- Material group: Defined by the flange material specification (e.g., ASTM A105 belongs to Group 1.1)
- Design temperature: The maximum sustained temperature of the pressure-containing fluid
The PT tables list the maximum working gauge pressure (psig) at each tabulated temperature. The rating is independent of flange size—a Class 300 NPS 2 flange has the same pressure rating as a Class 300 NPS 24 flange at any given temperature.
PT Rating Table: ASTM A105 / A350 LF2 (Material Group 1.1)
| Temperature | Class 150 | Class 300 | Class 600 | Class 900 | Class 1500 | Class 2500 |
|---|---|---|---|---|---|---|
| -20 to 100°F (-29 to 38°C) | 285 psig | 740 psig | 1,480 psig | 2,220 psig | 3,705 psig | 6,170 psig |
| 200°F (93°C) | 260 psig | 675 psig | 1,350 psig | 2,025 psig | 3,375 psig | 5,625 psig |
| 400°F (204°C) | 200 psig | 635 psig | 1,270 psig | 1,900 psig | 3,170 psig | 5,280 psig |
| 600°F (316°C) | 140 psig | 550 psig | 1,095 psig | 1,640 psig | 2,735 psig | 4,560 psig |
| 750°F (399°C) | 95 psig | 505 psig | 1,010 psig | 1,510 psig | 2,520 psig | 4,200 psig |
| 1,000°F (538°C) | 20 psig | 170 psig | 340 psig | 510 psig | 845 psig | 1,410 psig |
Values from ASME B16.5-2020, Table 2-1.1. For intermediate temperatures, linear interpolation is permitted.
Key Rules
Interpolation: For temperatures between tabulated values, linear interpolation of pressure is allowed. Extrapolation beyond the tabulated range is not permitted.
Material group assignment: Each ASTM material specification is assigned to a material group. For example, A105 and A350 LF2 belong to Group 1.1, A182 F304 to Group 2.1, and A182 F316 to Group 2.3. Different material groups have different PT rating tables.
Independence from size: The PT rating applies equally to all flange sizes within a given class and material group. The dimensional adequacy of the flange body is ensured by the size-dependent dimensions in ASME B16.5.
Bolting and gasket: The published PT ratings assume that the flange assembly uses the bolting material and gasket type specified in the applicable standard. Non-standard bolting or gaskets may reduce the effective rating of the assembly.
For a full explanation of all ASME pressure classes and material groups, see the flange pressure rating reference. For flange dimensional data by class, refer to ASME B16.5 flange types.
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