What are the pressure-temperature ratings for steel flanges?

Every piping system has a point where metal components must be joined, sealed, and held against internal pressure. pipe flanges are the components that make those joints possible, and their pressure–temperature (P–T) rating is the single most important number that determines whether a connection is safe for a given service. A rating that is too low can leak or rupture; a rating that is far above the actual need adds cost, weight, and unnecessary bolting stress. Understanding how P–T ratings are derived, where to find them in the standard, and how to apply them to the material of construction is the foundation of sound piping design.

This guide walks through how steel flanges are rated, what the tables in ASME B16.5 actually tell you, and how the same principles apply to EN 1092-1 PN designations used in European projects. It also explains how the values relate to the pipe, fitting, and gasket packages typically delivered as a complete project bundle.

1. What Is a Pressure–Temperature Rating?

A P–T rating is the maximum allowable working pressure of a flange at a specific metal temperature. The values are not constant: as temperature rises, the yield and tensile strength of the flange material fall, so the allowable pressure is reduced to keep the calculated stress within safe limits. The relationship is shown in the standard’s rating tables as a series of pressure values for each pressure class across a temperature range, typically from −20 °F (−29 °C) up to 1500 °F (816 °C).

In practice, the rating answers two questions at once:

At design temperature, what is the highest pressure the flange can hold?
At a given operating pressure, what is the highest temperature the flange can withstand?

Both views come from the same table; the user simply moves down the temperature column or across the pressure row to the class they have selected.

2. The Governing Standard: ASME B16.5

For most refineries, chemical plants, and power stations built to American practice, ASME B16.5 “Pipe Flanges and Flanged Fittings (NPS 1/2 through NPS 24)” is the reference. The current edition defines:

Seven pressure classes: 150, 300, 400, 600, 900, 1500, and 2500.
Pressure–temperature tables in Table 2-1.1 (and 2-1.2 for flanged fittings) for each material group.
Material groups (numbered 1.1 through 4.3) that bundle together metals with similar high-temperature strength behavior.
Dimensional data (bolt circle, number and size of bolts, facing type, hub dimensions) for every nominal pipe size and class combination.

A useful clarification: Class 150 is not 150 psi. The class number is a dimensionless identifier. The actual allowable pressure at any given temperature is read from the table, and it changes with both the material group and the temperature row. The same Class 150 flange in carbon steel and in stainless steel will have different P–T values at 800 °F, for example, because the underlying material strength curves differ.

For larger sizes, ASME B16.47 covers Series A (MSS SP-44) and Series B (API 605) large-diameter flanges from NPS 26 through NPS 60, with its own rating tables. For wellhead and Christmas tree equipment in oil and gas, API 6A is used instead. The selection rule of thumb is: NPS ≤ 24 → B16.5; NPS 26–60 → B16.47; wellhead service → API 6A.

3. How Materials Are Grouped in B16.5

The standard does not publish a unique table for every alloy. Instead, it groups materials with comparable high-temperature strength into numbered groups, and each group shares one common P–T table. The most commonly referenced groups for industrial service are:

Group 1.1 – Carbon and low-alloy steels: ASTM A105, A350 LF2, A216 WCB, A352 LCC. The workhorse for general hydrocarbon, water, steam, and air service.
Group 1.2 – Same family with higher allowable stress: A350 LF3, A352 LCB for low-temperature service.
Group 1.5 – High-yield carbon steel for pipeline: A694 F42, F46, F52, F56, F60, F65, F70 for transmission pipelines.
Group 2.1 – Austenitic stainless steels: A182 F304, F304L, F310, F316, F316L, F321, F347. Used in corrosive and cryogenic services.
Group 2.2 – Same family, higher alloy: A182 F309, F310.
Group 3.1 – Nickel alloys: B564 N04400 (Monel 400), N06600 (Inconel 600), N08800 (Incoloy 800).
Group 4.1 – High-alloy and Cr-Mo steels: A182 F5, F9, F11, F22, F91 for high-temperature steam and refinery service.

Because each group shares one table, the design engineer does not need to recalculate allowable stress for every individual alloy. The trade-off is that a stronger alloy in the same group is not exploited; you pay for the higher grade but use the same rating. This is one reason why moving from a 304 stainless flange to a 316 stainless flange in the same service does not allow an increase in design pressure.

4. Reading a P–T Table – A Worked Example

Suppose the line is steam at 400 °F (204 °C), design pressure 285 psig, NPS 6, welded to the pipe. A weld neck flange in ASTM A105 (Group 1.1) is being considered.

Look up Group 1.1 in Table 2-1.1, ASME B16.5.
Find the row for 400 °F. For Class 300, the allowable pressure is approximately 635 psig; for Class 150, approximately 200 psig.
ｓｅｌｅｃｔ the lowest class that still exceeds 285 psig with a sensible margin: Class 300 at 635 psig is the next standard step up and provides roughly 2.2× design margin, which is appropriate for steam service.
Confirm that the pipe schedule and the matching weld neck hub can also withstand 400 °F at the same stress level – otherwise the flange will out-rate the pipe and the joint remains pipe-limited.

If the same line were retuned to operate at 750 °F, the Class 300 A105 rating drops to roughly 320 psig – still above 285 psig, but the margin has tightened considerably. At 800 °F the rating is down to roughly 235 psig, which is below the design pressure; at that point the engineer must either ｄｒｏｐ to a lower pressure class, switch to a Cr-Mo material such as A182 F11 (Group 4.1), or add a derate.

5. The European Counterpart: EN 1092-1

For projects built to European practice, the parallel system is EN 1092-1, which uses a PN designation (Pression Nominale). PN is the maximum allowable pressure in bar at a reference temperature of 20 °C; as with ASME, the allowable pressure at elevated temperature is reduced and is read from the per-material table in the standard.

Common PN designations for industrial service are PN 6, PN 10, PN 16, PN 25, PN 40, PN 63, PN 100, and PN 160, with higher PN 250 and PN 400 available for special applications. The values are not directly interchangeable with ASME class numbers; a Class 150 ASME flange is similar in capacity to a PN 20 EN flange, but the bolt patterns, facing finishes, and dimensional standards are different. Mixing components from the two systems is not permitted.

6. Why Temperature Always Lowers the Rating

The downward slope of every P–T table is not arbitrary – it is the direct reflection of how metals lose strength as they get hotter. Two physical effects are at work:

Decreasing yield and tensile strength: the dislocation movement that allows a metal to deform becomes easier at elevated temperature, so the stress that the flange can carry without permanent deformation falls.
Creep: above roughly 0.4 of the metal’s melting temperature (in absolute terms), the material continues to deform slowly under steady load. For carbon steel, this becomes significant above about 800 °F (425 °C); for austenitic stainless, the curve is flatter, which is one of the reasons stainless retains higher ratings at high temperature than carbon steel of the same class.

The ASME B16.5 tables already account for both effects. The engineer’s job is to read the table at the actual design metal temperature, not at the normal operating temperature. For lines that go through a periodic regeneration or sterilisation-in-place cycle, the rating must be checked against the highest temperature seen during the cycle, not the average.

7. Selecting a Flange for a Real Project

Putting the numbers into a real selection flow looks like this:

Confirm the design code (ASME B31.1 for power piping, B31.3 for process piping, B31.4 for pipelines, EN 13480 for European pressure piping, etc.).
List the design pressure, design temperature, and the most severe coincident operating case (start-up, shut-down, blocked discharge, fire case).
Match the material to the corrosion and temperature service first, then look up its group in B16.5.
Read the P–T table at the design temperature. Pick the lowest class that gives an acceptable safety margin – typically 1.5× to 2× the design pressure for non-corrosive service.
Verify that the facing, gasket, and bolting are consistent with the class, the fluid, and the temperature. A Class 600 flange with a low-pressure gasket is a mismatch and will not deliver the rated performance.
Check dimensional standards: NPS ≤ 24 use B16.5; NPS 26–60 use B16.47 Series A or B; wellhead service uses API 6A.

Doing the steps in this order avoids the common mistake of choosing a class from the pipe schedule and then discovering that the flange material cannot support the temperature.

8. How Manufacturing Affects the Rating

The P–T rating on paper assumes a properly manufactured flange that has been forged or cast to the relevant material specification, heat-treated as required, and inspected. In practice, three manufacturing factors directly influence whether the rated value is achieved:

Forging versus casting. Forged flanges (ASTM A105, A182) have a finer grain structure and higher mechanical properties than cast flanges (A216 WCB). The rating is the same in the table, but a casting with a hidden shrinkage porosity will fail well below the rating.
Heat treatment. Normalising, quenching and tempering, or solution annealing (for stainless) restores the microstructure after forging. Skipping the required heat treatment can leave residual stress that lowers the effective rating at temperature.
Machining of the sealing face. The raised face, tongue, groove, or ring joint must meet the surface finish (typically 125–250 RMS for RF, 63 RMS or better for RTJ). A rough face will not seal at the rated pressure, even if the flange body itself is sound.

For this reason, sourcing flanges from a mill that holds ISO 9001, API 5L/API 5CT, and PED compliance, and that issues a mill test certificate (MTC) traceable to the heat, is non-negotiable on any service where the rating actually matters. Hydrostatic testing at 1.5× the cold rating and non-destructive testing (UT or RT) on the weld prep are the standard final checks.

9. Beyond the Flange: Rating the Whole Joint

A flange is only as strong as the joint around it. Three companion items must be rated to the same level for the connection to deliver the published P–T value:

Bolting. ASTM A193 B7 studs with A194 2H nuts are the default for carbon and low-alloy service. For high temperature (above 800 °F), B16 studs are specified. For stainless service, B8 class 1 or 2 studs with B8 nuts are common. The stud material must maintain its allowable stress at the design temperature; using carbon steel studs on a 1000 °F austenitic stainless flange is a classic mistake.
Gasket. Spiral-wound gaskets (with graphite or PTFE filler) cover the bulk of refinery and chemical service. Ring joint (RTJ) gaskets are used at Class 600 and above for high pressure. Soft gaskets are limited by temperature; compressed non-asbestos sheets cap out at roughly 750 °F, while flexible graphite extends to about 1000 °F in steam.
Pipe and fitting compatibility. The pipe or fitting welded to the flange must have its own P–T rating that equals or exceeds the flange rating. A pressure tube in ASTM A335 P11 can carry a Class 600 joint; an ASTM A106 Grade B pipe cannot reach the same temperature and must be derated accordingly.

10. Common Mistakes When Reading P–T Ratings

Even experienced engineers make the same handful of errors. Watch for these on every review:

Using the Class 150 number (≈285 psig at 100 °F, the cold reference temperature) as the rating of the same flange at 800 °F. It is not – the value drops sharply with temperature.
Mixing the ASME and EN tables. A PN 16 flange is not interchangeable with a Class 150 flange even though the cold pressure values are close.
Ignoring the coincident case. A line may run at 200 psig and 200 °F in normal service but see 350 psig and 600 °F during a steam-out. The flange must be rated for the worst case.
Using the flange rating for the gasket. The gasket has its own temperature and pressure limits defined by the manufacturer and by ASME Section VIII.
Forgetting the corrosion and erosion allowance. A flange that is rated for 600 psig today may be at 540 psig after ten years of wall loss – the rating does not change, but the effective safety margin does.

11. Practical Checklist for Procurement

Before releasing a purchase order for flanges, confirm that the data sheet covers the following items – they are the minimum needed to convert a P–T table value into a working joint on site:

Standard and edition (for example, ASME B16.5-2017, EN 1092-1:2018).
Pressure class or PN designation.
Material grade and UNS number, plus the heat-treatment condition.
Flange type (Weld Neck, Slip-On, Blind, Socket Weld, Lap Joint, Threaded, Ring Type Joint).
Facing (RF, FF, RTJ, tongue and groove, male and female) and surface finish.
Bore size (standard or reduced) to match the pipe schedule.
Dimensional standard for the size range (B16.5, B16.47 Series A, B16.47 Series B).
Required testing and certification: MTC EN 10204 3.1, hydrotest, PMI, UT, RT, dye-pen, magnetic particle.
Third-party inspection requirement, if any (customer, TUV, BV, SGS, Lloyd’s).

A flange order that omits any of these items almost always generates a clarification request that delays delivery, and a flange that arrives without the right MTC cannot be released for pressure service in most plants.

12. A Note on Sourcing and Project Bundling

In real procurement, the flange is rarely bought alone. The same enquiry typically covers the pipe, fittings, gaskets, stud bolts, and sometimes the valves that make up a complete spool. A supplier that can hold inventory in carbon steel, stainless steel, and alloy steel flanges across the full ASME and EN class range, and that can deliver matching pipes, fittings, and fasteners from the same quality system, simplifies the project considerably. Multi-location production with mill test traceability, NDT on every heat, and a documented MTC for each component reduces the risk of a mismatch on site.

For bundled industrial piping packages that pair pipe fittings with steel flanges to the same pressure class and material group, the project documentation reads as one consistent specification rather than three separate ones. That single-spec approach is the easiest way to make sure the published P–T rating of the flange is the rating the joint actually delivers when it is in service.

Frequently Asked Questions

Is a Class 150 flange rated for 150 psi?

No. The class number is a label, not a pressure. A Class 150 carbon steel flange is rated at about 285 psig at 100 °F and falls to about 95 psig at 800 °F. The actual allowable pressure is always read from the P–T table for the material group at the design temperature.

Can a stainless steel flange be rated higher than a carbon steel flange of the same class?

In some temperature ranges, yes. Austenitic stainless steels retain their strength better than carbon steel above about 600 °F, so at high temperature the stainless flange can carry more pressure. At low temperature the difference is small.

Do EN 1092-1 PN flanges use the same dimensions as ASME B16.5?

No. PN and Class use different bolt patterns, different facing standards, and different materials groupings. A PN 40 flange and a Class 300 flange are not interchangeable, even when the cold pressure rating is similar.

What is the difference between ASME B16.5 and B16.47?

B16.5 covers NPS 1/2 through NPS 24. B16.47 covers NPS 26 through NPS 60 in two series – Series A (MSS SP-44) and Series B (API 605). Above NPS 24, B16.47 is the standard to use.

How is the rating affected by a raised face versus a flat face?

The pressure–temperature rating of the flange body does not change. The facing affects the gasket selection and the bolt load required to seal, but the allowable working pressure of the flange itself is the same.