A heat exchanger moves. A shell-and-tube unit with a fixed tubesheet grows against its own shell every time it is brought up to temperature, and a plate pack shifts every time it is opened and retightened. The nozzles have to accept that movement from somewhere, and if the pipework is rigid the exchanger takes it instead — as a rolled tube joint that starts to weep, or a gasket that will not hold after the third shutdown.
What this service does to a hose
Differential expansion is the whole problem. In a fixed-tubesheet shell-and-tube exchanger the tubes and the shell run at different temperatures, so they try to be different lengths. Austenitic stainless expands roughly 17 micrometres per metre per degree — a three metre bundle running 60 °C hotter than its shell wants to be about three millimetres longer. That load goes into the tube-to-tubesheet joints unless something in the circuit yields. A hose across each nozzle is the cheapest thing in the system that can yield.
Then the whole exchanger moves. Beyond the internal differential, the unit itself sits on saddles with one end usually free to slide. Piping connected rigidly to a sliding nozzle is piping in tension. The hose is there to let the nozzle go where the thermal design intends it to go.
And it runs hot while it does it. This is the point most often missed. The hose takes the movement at temperature, so it is being flexed on its derated rating and at its flexing bend radius, not its static one. A DN50 assembly rated 30 bar cold with a 490 mm flexing radius is a 18.6 bar assembly at 200 °C, and the 490 is the number that applies — not the 160 mm static figure that a fitter measuring the space will find first.
What we would fit
| Element | What we would supply | Why |
|---|---|---|
| Hose | 316L annular corrugated below 300 °C; 321 where the unit cycles hot daily | 321 is stabilised and tolerates repeated time in the sensitisation range |
| Braid | Matching grade, sized on the derated pressure at the operating temperature | The exchanger's design pressure is a hot pressure |
| Ends | Flanged to the nozzle standard, one end swivel where alignment is tight | Bolting up a fixed pair of flanges is how torsion gets built into the assembly |
| Length | Sized on live length with the calculated movement, plus margin | Live length is what flexes; the ferrules are dead length and do nothing |
| Orientation | Installed across the direction of movement, never along it | A hose takes bending and offset. It does not take axial pull or torsion |
What we would not fit, and what happens
A hose asked to take axial movement in line with itself. That is an expansion joint's job, not a hose's. If the movement is purely axial along the nozzle centreline, you want an axial expansion joint with the anchors and guides that go with it, and we will tell you so rather than sell you a hose.
A hose sized on the static bend radius. The static figure applies to a hose bent once on installation and left. An exchanger nozzle moves every cycle, so the flexing figure applies — on DN50 that is 490 mm against 160.
A hose sized on the cold pressure. Roughly a third of the rating is gone at 200 °C.
Two rigid flanges bolted up out of line. The twist stays in the assembly for its whole life. Use a swivel flange at one end.
What we need to size it
| We need | Because |
|---|---|
| Exchanger type — fixed tubesheet, floating head, U-tube or plate | It decides how much movement there is and in which direction |
| Shell-side and tube-side temperatures, and the ambient at start-up | The differential is what drives the growth |
| Design pressure on the side the hose is on | Derated at the operating temperature, this is the sizing pressure |
| Direction and magnitude of the nozzle movement, or the run lengths to calculate it from | Axial movement may mean an expansion joint rather than a hose |
| Nozzle size, flange standard and rating | It has to bolt to what is there |
| Whether the unit cycles daily or runs continuously | Daily cycling is a fatigue duty and changes the grade |