Thermic fluid is hot oil circulated at 250–320 °C and almost no pressure — a few bar to move it round the loop. It is the inverse of a steam system: the temperature is higher and the pressure is trivial. That combination catches people out, because the pressure looks so easy that nobody applies the derating, and 316L at 300 °C has lost nearly half its rating.
What this service does to a hose
Temperature, applied continuously for years. Our published factor for 316L is 0.54 at 300 °C. A DN50 assembly rated 30 bar cold is a 16 bar assembly on a thermic fluid header. That is still far above the four or five bar the pump makes, so the margin is real — but it has to be checked, because it is the same arithmetic that fails on a smaller bore at a higher circuit pressure.
Sustained time in the sensitisation range. Between roughly 450 and 850 °C, unstabilised austenitic stainless precipitates chromium carbide at the grain boundaries and loses corrosion resistance there. A thermic fluid loop does not reach that range in normal running, but a hose in a fired-heater bay can, and the argument for a stabilised grade like 321 gets stronger the closer the hose sits to the firing.
The fluid finds every joint. Hot heat-transfer oil has very low surface tension at operating temperature. It will pass through a joint that would hold water, hold steam and hold compressed air. This is the single most common complaint on thermic fluid systems and it is a joint problem, not a hose problem — which is exactly why we weld rather than crimp, and why we would push you towards flanged or welded ends and away from threads.
Degradation products. Heat transfer fluids crack over time into light ends and heavy ends. The light ends lower the flash point of what is in your pipe. The heavy ends form carbon that deposits in low-velocity regions — and the corrugation valleys of an unlined hose are a low-velocity region.
What we would fit
| Element | What we would supply | Why |
|---|---|---|
| Hose | 321 annular corrugated above 300 °C or in a fired-heater bay; 316L below it | 321 is stabilised; the closer to the firing, the stronger that argument |
| Braid | Matching grade, sized on the derated pressure | At 300 °C only 54 % of the catalogue figure is available |
| Liner | Fitted where velocity is high or the fluid is old and carbonising | It keeps flow off the convolutions and reduces the low-velocity pockets |
| Ends | Flanged or butt weld. We would avoid threaded joints entirely | Hot heat-transfer oil passes threads that hold everything else |
| Length | Sized on live length with the hot growth of the run included | The pipework grows before the hose is asked to flex |
What we would not fit, and what happens
Threaded ends. They will weep, and on this fluid weeping is the failure.
A PTFE-lined hose at 300 °C. The liner sets a ceiling well below the stainless. Where a smooth bore is genuinely needed, give us the temperature first and we will say honestly whether it can be done.
Interlock hose. Not pressure-tight, on a flammable fluid.
A hose sized on the cold table. Nearly half the rating has gone before the loop is commissioned.
What we need to size it
| We need | Because |
|---|---|
| Fluid name and grade, bulk temperature and film temperature | Film temperature is what the metal actually sees at the wall |
| Circuit pressure and the relief setting | The relief setting is the design pressure |
| Where the hose sits relative to the heater | It decides stabilised grade against plain 316L |
| Whether the loop runs continuously or is cycled | Cycling adds a fatigue duty to a thermal one |
| Movement at the connection, and the run lengths either side | Hot pipework grows and the hose has to allow it |
| End connections — and whether threads are being proposed anywhere | If so we will argue about it before we quote |