Metal loses strength as it gets hotter. The working pressure printed against a hose or bellows is a figure at ambient temperature, and it must be reduced — derated — for service at any temperature above that. Skipping this step is one of the most common specification errors in the industry.
How to apply it
pT = permissible working pressure at temperature T p20 = catalogue working pressure at 20 °C Ct = derating factor for the material at temperature T
Where an assembly contains more than one material in the pressure boundary — a 321 hose with 304 braid, say — use the lowest factor of the materials present. The assembly is limited by its weakest component at temperature, not by its best one.
Derating factors
These factors are the ratio of the 1 % proof stress at temperature to the proof stress at 20 °C, which is the basis ISO 10380 uses. It is the more conservative of the two conventions in this industry — the other, used mainly in North America, works from ASTM allowable stresses and gives higher numbers for the same steel at the same temperature. If you are comparing our figures against a supplier whose 300 °C factor is around 0.8 rather than 0.5, that is the difference, and neither number is wrong. Ask which basis you are being quoted on.
We rate on the lower basis, deliberately
It costs us catalogue numbers. A DN50 assembly in 316L throughout reads 15.1 kg/cm² at 300 °C on our basis and would read around 22 kg/cm² on the other one, for the identical hose. We publish the lower figure because it is the one that leaves margin in a plant where the temperature is not as steady as the datasheet says, and because a rating you can hand to a third-party inspector without a footnote is worth more to us than a bigger number. If a quotation shows a materially higher hot rating for the same construction, the steel has not changed — the convention has. Ask which one it was worked on.
| Medium temperature | SS 304L | SS 316L | SS 321 (to order) | SS 904L (to order) |
|---|---|---|---|---|
| 20 °C | 1.00 | 1.00 | 1.00 | 1.00 |
| 100 °C | 0.72 | 0.74 | 0.83 | 0.87 |
| 200 °C | 0.59 | 0.62 | 0.74 | 0.76 |
| 300 °C | 0.51 | 0.54 | 0.67 | 0.65 |
| 400 °C | 0.46 | 0.50 | 0.62 | 0.57 |
| 450 °C | 0.45 | 0.48 | 0.61 | — |
| 500 °C | 0.44 | 0.47 | 0.60 | — |
| 550 °C | 0.43 | 0.47 | 0.59 | — |
The other end of the scale
Austenitic stainless does not become brittle at low temperature the way carbon and ferritic steels do — it keeps its toughness down to cryogenic conditions. Our stocked grades are rated to −196 °C, and material is available for service below that.
What does need checking at low temperature is everything that is not the hose: gaskets, liners, coatings and, above all, the carbon steel components in any flange or hardware set. Those govern the low-temperature limit of the assembly far more often than the stainless does.
A worked example
Given. A DN50 single-braid assembly in 316L, hose and braid. From the pressure table it is rated 28 kg/cm² at 20 °C. The line runs at 300 °C.
Factor. 316L at 300 °C is 0.54.
Result. 28 × 0.54 = 15.1 kg/cm² at 300 °C. If the line runs at 20 kg/cm² hot, this assembly will not do — go to double braid, which is rated 44 kg/cm² cold and 23.8 kg/cm² at 300 °C.
Related
Material grades · Media compatibility · Standards we build to