Pressure–Temperature Derating for Instrumentation Tubing, Fittings & Valves

Reviewed by the Crestflo Engineering Team

Pressure-temperature derating cuts a component's allowable working pressure as service temperature climbs above ambient. Why? Because the alloy's allowable stress drops as it heats up. To find the derated rating, multiply the ambient (cold) allowable working pressure by a temperature derating factor: AWP(T) = AWP(ambient) × F(T). Here, F(T) = S(T) / S(ambient) is simply the ratio of allowable stress at temperature to allowable stress at ambient. This guide walks through the factor, the formula, charts for each alloy, and the rule for figuring out whether it's the tube, the fitting, or the valve that actually limits your rating.

On this page, pressure-temperature derating refers to the mechanical drop in allowable working pressure as temperature rises, for metal fluid-system components: instrumentation tubing, tube fittings, and valves. It's not electrical or cable ampacity derating (NEC). The metal-alloy factors used here aren't the same as those for plastic (PTFE/PFA) tubing.

Jump to: What it is · Why it happens · The factor & formula · Derating charts by alloy · Tube vs. Fitting vs. Valve · Valve P, T ratings · Seal & service limits · Why Crestflo · FAQ

What Pressure–Temperature Derating Is: AWP, MAWP & the Derating Factor

Every pressure-containing component has a cold working pressure (CWP), or an ambient allowable working pressure (AWP), the pressure it can safely hold at or near room temperature (typically rated at 70 °F / 21 °C or 100 °F / 37 °C). Heat the metal up, though, and its allowable stress drops. The safe pressure drops right along with it. So the maximum allowable working pressure (MAWP) at any given temperature is just the ambient rating scaled down by a temperature derating factor, a dimensionless multiplier of 1.0 or less.

Put simply: the number stamped on a tube, fitting, or valve is a cold rating. Your real, in-service rating is that cold number multiplied by the derating factor for your alloy at your operating temperature. Below ambient, most metal pressure ratings hold steady or even climb a bit, but impact toughness and seal limits take over as the real constraint, see cryogenic / LNG low-temperature selection.

Pressure–Temperature Derating: Also Known As

Engineers and buyers call this concept by a dozen different names, and this guide covers all of them: pressure, temperature derating, P-T derating, pressure, temperature rating, the temperature derating factor or temperature adjustment factor, elevated-temperature pressure rating, and working pressure versus temperature. It drives allowable working pressure (AWP), maximum allowable working pressure (MAWP), and the cold working pressure (CWP) baseline you start from. Underneath all of it sits allowable stress versus temperature, tabulated in ASME B31.3 for process piping and ASME B31.1 for power piping, then carried through to valves via ASME B16.34.

Why Pressure Ratings Drop With Temperature: Allowable Stress vs. Temperature

Pressure rating drops with temperature because the property behind it, allowable stress, drops too. At low and moderate temperatures, allowable stress is set by a fraction of the yield and tensile strength. Both fall as the metal heats up. At higher temperatures, a second limit kicks in: creep, the slow time-dependent deformation of metal under sustained load. Creep pushes allowable stress down even harder. Once allowable stress drops, the pressure the wall can safely hold drops right along with it, in direct proportion.

This is exactly what happens if you increase temperature at constant pressure margin: the safe working pressure decreases along the alloy's derating curve. The governing values come from the allowable-stress-versus-temperature tables of ASME B31.3 (process piping) and ASME B31.1 (power piping); tube product specifications such as ASTM A269 and ASTM A213 define the material those tables apply to. Because different alloys lose strength at different rates, each alloy family has its own derating curve, which is why alloy selection and rating are one coupled decision.

The Derating Factor and the Formula: How to Calculate a Rated Working Pressure

The derating factor is a ratio: allowable stress at your service temperature divided by allowable stress at ambient.

F(T) = S(T) / S(ambient), always ≤ 1.0.

The rated (derated) working pressure works out to the ambient rating times that factor.

AWP(T) = AWP(ambient) × F(T)

To calculate a temperature-derated working pressure in four steps:

1. Find the ambient AWP. For tubing, it comes from the tube OD, wall thickness, and the alloy's ambient allowable stress. Work it out with the tube pressure & wall-thickness calculator, or read tube sizing and wall thickness for the method. A fitting's number comes straight from its catalog CWP. For a valve, use the ASME B16.34 class rating instead.
2. Find the derating factor for your alloy at your service temperature (charts below).
3. Multiply: AWP(T) = AWP(ambient) × F(T).
4. Check the lowest-rated component and the seal ceiling, the system's only as strong as its weakest link (see below).

Worked example. Take a 316L instrumentation tube with an ambient allowable working pressure of 5,000 psi for its wall and OD, pulled straight from the calculator rather than assumed here. At 600 °F (316 °C), the 316/316L factor is 0.85, so the derated rating works out to 5,000 × 0.85 = 4,250 psi. Push things to 1000 °F (538 °C) and the factor falls to 0.76, which gives 5,000 × 0.76 = 3,800 psi.

The factors themselves are fixed by the standard. Your ambient AWP, though, depends entirely on the tube geometry you're working with. Values follow the applicable standard and can be adjusted to fit requirements.

Derating Factor Charts by Alloy: 316/316L Stainless, Duplex & Nickel Alloys

316/316L Stainless Steel Tubing Temperature Derating Factors

Multiply the ambient allowable working pressure by the factor for your service temperature to get the derated value. The temperature derating factors below, for 316 / 316L stainless steel, follow the ASME B31.3 allowable-stress basis. They're the same figures published in standard instrumentation-tubing data, just presented here as a table you can pull straight from HTML instead of hunting through a locked PDF. This is what answers the common question, "316 stainless steel tubing pressure rating vs temperature."

Factors follow the ASME B31.3 allowable-stress basis, and they're customizable to your requirement. Look at the pressure-versus-temperature graph in this tubing pressure rating chart: the curve stays flat up to about 300 °F, then drops steadily as allowable stress declines. 316 stainless tubing tops out at roughly 1000 °F (538 °C) for pressure service, though it can run higher when oxidation is the limiting factor. Underlying grade data lives at 316/316L and 304/304L.

Nickel, Duplex & CRA Alloys: Derating Behavior and Maximum Service Temperature

Alloy families don't all lose pressure capacity at the same rate, and their upper-temperature ceilings differ too. Austenitic and nickel alloys derate slowly and hold up well under high heat. Duplex and super-duplex are another matter. Their ceiling sits much lower, because sustained exposure between 600 °F and 885 °F (315 °C to 475 °C) embrittles the material. Continuous service caps them at roughly 600 °F (315 °C) as a result. The table lists ceilings commonly cited for continuous service. Still, the actual derating factor for each alloy at each temperature comes from that alloy's allowable-stress row in ASME B31.3.

Ceilings are the limits most often cited for continuous service. But the actual allowable-pressure factor for a given alloy and temperature comes from the ASME B31.3 allowable-stress table for that grade. Need the alloy-specific derating set for your service? Request your derating reference from the engineering team. Corrosion behavior deserves its own comparison: check the PREN comparison table, then match alloy to service using material selection by service.

Tubing vs. Fitting vs. Valve: Which Component Limits Your Rating

A fluid system's rating comes down to its lowest-rated component at the service temperature. Each element gets its rating from a different basis, so each one has to be derated on its own terms, and you take the minimum of the set. Stainless steel tubing's pressure rating, the instrument tubing pressure rating in most instrumentation systems, often isn't the limiting factor. The fitting or valve usually governs instead.

Values follow the applicable standard, though they can be adjusted to your requirement. The tube, the fitting, and the valve each derate along its own curve, so the assembly's rated pressure at temperature comes down to whichever of the three is lowest. Size the tube using the tube pressure & wall-thickness calculator, then match it to instrumentation tubing in the same alloy. For that, check the tubing size chart.

Valve & Double Block-and-Bleed Pressure–Temperature Ratings (ASME B16.34)

Valves don't get their pressure-temperature rating the way tubing and fittings do. They qualify to ASME B16.34, along with the applicable API standards such as API 6D / 598 / 602, which set pressure-temperature ratings by pressure class and material group instead of a single derating factor. Each material group has its own class-rating table, and that table already accounts for the allowable-stress drop that comes with rising temperature. A valve's rated pressure at temperature comes straight from the Class 800, 1500, or 2500 table for its body material, the class designations forged instrumentation valves are commonly built to. Flange pressure-temperature ratings work on that same class-and-material-group principle.

Crestflo double block-and-bleed valves are built to ASME B16.34, rated for working pressure up to 3,000 psi. Every manifold gets a full seat- and shell-leak test up to 10,000 psi. Temperature caps valve ratings too, always set by whatever the seat and seal material can handle. Details below. Take a look at rated instrumentation valves, the double block-and-bleed valve, and high-pressure fittings & valves.

Seal & Service Limits That Override the Metal Rating

A correctly derated metal body can still get capped by two things the alloy factor never accounts for. The first is the seat and seal material. A valve or fitting is only rated to the temperature ceiling of its PTFE, PEEK, Viton (FKM), or Kalrez (FFKM) seal, and that ceiling usually sits well below the metal's own limit. More detail is available at seal material temperature selection. The second factor is the media and environment. Sour service caps both hardness and alloy choice under NACE MR0175 / ISO 15156; chloride content drives pitting resistance; and cryogenic service comes down to impact toughness. Take the lowest of the three limits every time: metal derating, seal ceiling, and media limit. For matching service to alloy, see material selection by service and sour-service selection (NACE).

Why Crestflo Builds the Rated Part

Crestflo is the instrumentation arm of an export house with four decades behind it, recognized by the Government of India and carrying a solid delivery record across the Middle East. It's a new brand, but the manufacturing heritage underneath it runs deep. The company builds instrumentation tubing, tube fittings, and valves across the full range of stainless, duplex, super-austenitic, and high-nickel/CRA materials, in both imperial and metric sizes. That means a part gets rated for your actual temperature and pressure conditions, not sold at a cold rating with the derating math left to you.

Compression fittings hold ASTM F1387 performance qualification, so drop-in interchange is straightforward. Valves qualify to ASME B16.34 and the relevant API standards. The company is ISO 9001, 14001, and 45001 certified, plus PED certified. Every part ships with an EN 10204 3.1 material test certificate, 3.2 available on request, and each one carries a heat or lot number. Testing covers hydraulic proof, burst, impulse, and thermal cycling, with 100% seat-and-shell leak checks built in.

The commercial picture is straightforward too: standard items sit in ready stock, specials are made to order, and delivery runs six to eight weeks. Origin is Indian, though US or European melt can be arranged on request, and full material traceability comes standard throughout. We Engineer Confidence.

Get a Temperature-Rated Build

Send us your service conditions, media, design pressure, and operating temperature, and our engineering team will spec the alloy, wall, and component set rated for it. End users can request a quote or request a sample. OEMs and distributors should talk to an engineer about private-label and line-card programs. Datasheets and CAD are available to customers on request.

Frequently Asked Questions

Related Guides & Resources

• Tube sizing and wall thickness · Tube pressure & wall-thickness calculator
• Choosing materials by service · Sour-service selection (NACE) · Cryogenic / LNG selection
• Picking seal material by temperature · PREN comparison table
• ASME B31.3 allowable-stress basis · ASME B16.34 valve ratings · ASTM A269 · ASTM A213 · ASTM F1387
• Instrumentation tubing · Instrumentation valves · High-pressure fittings & valves · Materials