Understanding Pressure-Temperature Ratings
A flange's pressure class — Class 150, 300, 600, and so on — is not a fixed pressure limit. It is a designation that defines the maximum allowable working pressure at a given temperature for a specific material group. As temperature increases, the allowable pressure decreases. In our experience, this is the single most common source of confusion when an engineer first encounters PT envelopes — they assume the class number is the pressure rating.
This inverse relationship is documented in ASME B16.5 across 49 material-specific tables, covering temperatures from −29 °C to 538 °C. Selecting the right flange class means checking where your operating point sits within the pressure-temperature envelope — not just reading a single number.
The gasket and bolting must also be independently rated for the same service conditions. A Class 600 flange paired with a gasket material that fails at your operating temperature is still a joint integrity risk.
7
Pressure Classes
150 through 2500
49
Material Tables
in ASME B16.5
−29 to 538
Temperature Range
degrees Celsius
Important
PT ratings define the flange's capability, not the gasket's. The gasket must independently survive the same temperature, pressure, and chemical environment. A Class 600 flange with the wrong gasket material is still a failure waiting to happen.
Interactive PT Envelope Explorer
Use this tool to visualise the operating envelope for different ASME B16.5 flange classes and material groups. You can also select a gasket material to see how its temperature limit interacts with the flange rating.
Operating at and .
Flange Max:
Gasket Max:
Understanding the Design Envelope
Safe Zone
Your operating point is well within the ASME B16.5 rated limits and the gasket's temperature capability. Standard design practices apply.
Review Zone
You are within 10% of the maximum rated pressure or near the gasket's temperature limit. Engineering review is recommended.
Critical Limit
Operating point exceeds the flange's pressure rating or the gasket's temperature limit. System failure risk is high.
Interpolation Rules
ASME B16.5 permits linear interpolation between listed temperature steps to determine intermediate pressure ratings. However, interpolation between pressure classes is not permitted — you must always round up to the next available class.
Workshop Note
If your operating point falls in the review zone, consider upgrading to the next pressure class, specifying a higher-grade material group, or consulting our engineering team for application-specific advice.
ASME B16.5 Maximum Pressure by Temperature
The tables below show maximum allowable working pressures for common material groups at each temperature step. Select your material group and choose between bar and psi units.
Values from ASME B16.5-2025 reference tables. For official design calculations, always consult the current edition of the standard.
Australian & International Standards
Different standards handle pressure-temperature ratings in different ways. Understanding which standard governs your application is the first step in determining the correct flange class.
ASME B16.5
Pipe Flanges & Flanged Fittings
The primary standard for flanges NPS (Nominal Pipe Size) ½ to 24. Provides full PT derating curves across 49 material tables and 7 pressure classes.
View DimensionsAS 2129
Flanges for Pipes, Valves & Fittings
The legacy Australian standard (Table D, E, etc.). While it contains derating tables, it is primarily used for ambient-temperature service in modern projects.
View DimensionsEN 1092-1
European Flange Standard
Uses PN (Pressure Nominale) designations like PN 10, 16, 25, 40, which function similarly to ASME pressure classes. EN 1092 includes material-specific PT tables similar to ASME B16.5.
View DimensionsAustralian Practice
For elevated-temperature or high-pressure service in Australia, engineers typically specify ASME B16.5 or EN 1092-1 due to their more rigorous material-specific derating curves.
Material Groups Explained
The same pressure class can have very different pressure ratings depending on the flange material. ASME B16.5 organises materials into numbered groups, each with its own PT table.
Carbon Steel
A105, A216 WCB, A515/A516 Gr.70
The most common and cost-effective flange material for general industrial service. Suitable for temperatures up to 538 °C, though ratings decrease significantly above 400 °C.
Caution: prolonged exposure above 425 °C risks graphitisation (carbide conversion).
Type 304 Stainless
A182 F304, A351 CF8
Standard austenitic stainless steel with strong corrosion resistance. Lower pressure ratings than carbon steel at ambient temperature, but retains strength better at elevated temperatures.
Type 316 Stainless
A182 F316, A351 CF8M
Molybdenum-bearing austenitic stainless with superior corrosion resistance in chloride environments. Higher PT ratings than Type 304 across most temperature steps.
Type 316L (Low Carbon)
A182 F316L, A351 CF3M
Low-carbon variant of 316 for welded construction. Limited to 450 °C maximum per ASME B16.5.
Lower pressure ratings than standard 316 at elevated temperatures.
Gasket Considerations
The flange's PT rating sets the upper bound of the joint's capability — but the gasket must independently survive the same conditions.
Gasketing Rules of Thumb
- 01. Pressure limit: keep non-asbestos sheet gaskets (CNAF) at Class 300 and below for critical services.
- 02. Mating flanges: never mate a Raised Face (RF) flange to a Flat Face (FF) flange if one is brittle (cast iron).
- 03. Graphite oxidation: flexible graphite is rated to 550 °C in steam but begins to oxidise in air above 450 °C.
- 04. Thinner is better: use the thinnest gasket the joint will tolerate to reduce leak paths and creep relaxation.
Soft-Cut Gaskets
Rubber and fibre gaskets are typically limited to lower temperatures. Your flange may be rated to 300 °C, but an EPDM gasket fails above 150 °C.
Browse RangeSpiral Wound Gaskets
A wider PT envelope, depending on the filler. Graphite filler extends to 550 °C; PTFE filler is limited to 260 °C.
Explore GuideMetal Gaskets (RTJ)
Ring joint gaskets match or exceed the flange's PT rating. They are the default choice for high-pressure, high-temperature connections.
Browse RangeCommon Mistakes
Four specification mistakes account for most flange joint failures.
Assuming Class = Fixed Pressure
Class 150 does not mean 150 psi at all temperatures. At 400 °C, a carbon steel flange drops to just 6.5 bar (94 psi).
Ignoring Material Groups
Carbon steel (Group 1.1) and Type 316 stainless (Group 2.2) have different PT curves for the same class.
Exceeding Gasket Limits
A high-class flange does not upgrade the gasket's material limit. The joint is only as strong as its weakest component.
Forgetting Transients
Steam hammer and thermal cycling can push conditions above steady-state design values.
Related Resources
Explore Further
Need Help Selecting the Right Flange Class?
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Disclaimer
This guide is provided for general engineering reference only and does not constitute professional advice, specification, or guarantee of performance. Actual results depend on specific application conditions. Universal Gaskets Pty Ltd accepts no responsibility or liability for decisions made based on this information. For full terms, see our Terms & Conditions.