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. The carbon and low-alloy steel tables run from −29 °C to 538 °C, while the austenitic stainless groups extend to 816 °C. The six groups covered on this page span −29 °C to 538 °C. Selecting the right flange class means checking where your operating point sits within that 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 (°C)
covered on this page
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.
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.
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 bar, psi, or kPa.
Values from ASME B16.5-2020 reference tables. For official design calculations, always consult the current edition of the standard.
Interpolation
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.
Hydrostatic Test Pressure
Flanged joints are hydrotested at 1.5 × the 38 °C rating, rounded up to the next whole bar. A short-duration test pressure above the working rating is expected, not an error.
Group 1.1 carbon steel, bar gauge. Derived from the 38 °C ratings in the table above.
Worked Example — 18 bar Saturated Steam
A carbon steel line carries saturated steam at 18 bar g. Which flange class and gasket do we specify?
- 1. Saturated steam at 18 bar g runs at roughly 208 °C, so that becomes the design temperature rather than ambient.
- 2. Group 1.1 (carbon steel) Class 150 allows only 13.8 bar at 200 °C. That sits below 18 bar, so Class 150 is out.
- 3. Class 300 allows 43.8 bar at 200 °C and still 41.9 bar at 250 °C, a comfortable margin over 18 bar with headroom for transients.
- 4. Now the gasket. A graphite-filled spiral wound gasket (rated to 550 °C) suits comfortably, whereas a compressed-fibre sheet sits right at its 200 °C ceiling. We specify the spiral wound.
Pressures from ASME B16.5-2020 reference tables, Group 1.1 at 200 °C, bar gauge.
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 (Nominal Pressure) 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 DimensionsClass ↔ PN Quick Reference
European suppliers often quote ASME-pattern flanges under PN designations. EN 1759-1 assigns these nominal equivalents:
Nominal correspondence only — PN and Class ratings are derived from different reference stresses and derating rules, so never substitute one for the other without checking the governing standard. ISO 7005 rounds some designations (Class 400 → PN 63/64, Class 600 → PN 100, Class 1500 → PN 250).
Australian 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. AS 4331 (the local adoption of ISO 7005) is the formal Australian home of the PN system, while AS 2129 remains common for ambient water and general duty.
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).
Low-Carbon Steel / LCC
A216 WCC, A352 LCC, A350 LF6 Cl.2
Carbon steel grades selected for low-temperature toughness — the usual pick for cold-climate and LNG-adjacent duty where impact properties matter more than creep strength.
A352 LCC is limited to 340 °C and A350 LF6 to 260 °C; the upper rows of the table apply to A216 WCC.
C-½Mo Steel
A182 F1, A204 Gr.A/B
Low-alloy steel with improved creep resistance over plain carbon steel. Used for moderate high-temperature refinery and boiler duty, where it holds its rating noticeably better than Group 1.1 above 400 °C.
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.
Subject to sensitisation in the 425–815 °C range unless a stabilised or low-carbon grade is used.
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. Kept to 450 °C in practice — specify 316H for sustained service above this.
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.
Indicative continuous-service ranges — exact limits vary by grade and formulation.
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.
Common Questions
The questions engineers ask us most about PT ratings, answered from the tables on this page.
Allowable stress falls as temperature climbs, and the rating falls with it. Class 150 derates steepest: a Group 1.1 carbon steel flange drops from 19.6 bar at 38 °C to 1.4 bar at 538 °C, a loss of roughly 93%.
Between temperature steps, yes. ASME B16.5 permits linear interpolation on temperature, but never between pressure classes; round up to the next available class.
No. Flanged joints are hydrotested at 1.5 × the 38 °C rating, rounded up to the next whole bar, so a short-duration test pressure above the working rating is expected.
Standard carbon steel flanges stop at −29 °C. Below that, specify impact-tested grades such as A350 LF2 (to −46 °C) or A350 LF3 (to −101 °C). Austenitic stainless steels generally suit cryogenic duty.
Only nominally. EN 1759-1 pairs Class 150 with PN 20 and Class 300 with PN 50, but PN and Class ratings are derived from different reference stresses, so always check the governing standard rather than substituting one for the other.
No. The gasket must be checked independently against the same temperature, pressure, and medium. The gasket considerations section above covers the material limits.
Related Resources
Explore Further
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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.