Bitumen is the duty where the hose has to be kept hotter than the product, not cooler. Asphalt binder is pumped at 150–190 °C and it sets solid on the way down. A hose that is allowed to cool becomes a length of stiff pipe with product welded into every corrugation valley, and the next time it is bent, it cracks.
What this service does to a hose
The corrugation is a trap. An annular corrugated bore is a series of pockets. On a low-viscosity medium that does not matter. On bitumen it matters enormously — the pockets never fully clear, the residue in them cools first, and over a season the effective bore closes in and the hose loses its flexibility from the inside out. A stiffened hose being flexed at its original design radius is being worked far harder than the design intended.
Heat is the specification. At 180 °C, 316L holds about 0.63 of its 20 °C rating; at 200 °C, 0.62. Pressures on a bitumen transfer line are modest, so this is rarely the binding constraint, but it must still be applied — and if the tracing is steam at 10 barg the jacket is itself a pressure part at 184 °C.
Thermal cycling. Every loading cycle takes the assembly from ambient to 180 °C and back. That is a full thermal cycle on a part that is also being bent, and it is the reason a stabilised grade earns its place here where it might not elsewhere.
What we would fit
| Element | What we would supply | Why |
|---|---|---|
| Bore | PTFE-lined smooth bore inside the corrugation, wherever the duty allows it | A smooth bore drains. A corrugated bore holds product, and held product is what stiffens the hose |
| Hose | 321 annular corrugated where the line cycles hot daily; 316L where it is held hot continuously | 321 is stabilised and tolerates sustained time in the sensitisation range |
| Braid | Matching grade, sized on the derated pressure | The rating that matters at 180 °C is not the catalogue one |
| Tracing | Steam or hot-oil jacket over the assembly, or electric trace with the hose lagged | The hose has to be brought up before product enters it, not after |
| Ends | Flanged, with the jacket terminated clear of the flange face | A jacket that runs onto the gasket face makes the joint impossible to seal |
What we would not fit, and what happens
An unlined corrugated bore on cyclic bitumen duty, unless the line is genuinely cleared hot every time. It will stiffen, and then it will crack at the root of a convolution.
Interlock hose. Not pressure-tight, and the seam would pack solid with binder.
A jacketed assembly where nobody has sized the jacket. The jacket is a pressure part in its own right, with its own rating, its own relief and its own certificate.
A hose sized on the cold pressure table. At 180 °C roughly a third of the rating has gone before the line is even commissioned.
What we need to size it
| We need | Because |
|---|---|
| Product temperature at the hose, and the tracing medium and its pressure | Two pressure parts, two ratings, two certificates |
| Working pressure and the pump type | A gear pump deadheaded against a set line makes far more than its rating |
| Whether the line is cleared hot after every use | It decides whether an unlined bore is viable at all |
| How many load cycles a day, and the seasonal pattern | It decides stabilised grade against plain 316L |
| Movement at the connection — is this a loading arm, a swing joint or a fixed run | It sets the length and the bend radius that governs |
| End connections and whether the jacket must terminate short of the flange | It is a fabrication detail that has to be agreed before we cut |