A corrugated bore is not a smooth bore. It carries flow perfectly well, but it resists that flow more than pipe does, and above certain velocities it begins to resonate — which is a fatigue problem, not a flow problem.
Velocity limits
The rule is short. In a corrugated metal hose the flow velocity should never exceed 45 m/s for gas or 22 m/s for liquid. Where the hose is installed in a bent condition those figures come down — by a quarter at 45°, by half at 90°.
| How the hose is routed | Braided — gas (m/s) | Braided — liquid (m/s) | Unbraided — gas (m/s) | Unbraided — liquid (m/s) |
|---|---|---|---|---|
| Straight run | 45 | 22 | 30.5 | 15.2 |
| 45° bend — reduce by 25 % | 33.8 | 16.5 | 22.9 | 11.4 |
| 90° bend — reduce by 50 % | 22.5 | 11 | 15.2 | 7.6 |
Braid changes the figure, and not for the reason people assume. The braid never touches the medium — it sits outside the bore and carries pressure, not flow. What it does is restrain the corrugations mechanically, and that is what raises the velocity at which flow-induced resonance starts to fatigue the crests. An unbraided hose of the same bore is free to respond to the flow, so its limit is lower: 30.5 m/s for gas and 15.2 m/s for liquid in a straight run, against 45 and 22 braided. The bend reductions apply the same way to both.
Bellows are a different rule
For an expansion joint the trigger comes from EJMA rather than from this table. Fit a flow sleeve when gas, steam or air exceeds 1.2 m/s per 25 mm of bore up to DN150 (or 7.6 m/s above it), when water or liquid exceeds 0.6 m/s per 25 mm up to DN150 (or 3 m/s above it), when there is an elbow, tee or valve within ten pipe diameters upstream — multiply the velocity by four first — or when the medium carries solids.
Pressure drop
A corrugated bore is not a smooth bore. Every convolution valley sheds a small vortex, and over a long run those add up. As a working rule, expect roughly three times the pressure drop of new welded steel pipe of the same nominal bore. Interlock hose and unlined bellows behave similarly. Short jumpers rarely matter; it is the long runs where it shows.
The practical consequence is a sizing rule rather than a calculation: where flow rate matters, increase the hose bore by twenty to forty per cent over the equivalent pipe. Sizing the hose to match the pipe is the reason a good many systems under-deliver.
Liner orientation
An internal liner is directional. It must be installed with the flow entering the liner, not against its free edge — a liner fitted backwards will chatter, wear and eventually detach. The flow direction is marked on every lined assembly we supply. If an assembly can be installed either way round, tell us at enquiry and we will fit liners at both ends.
Vacuum
Vacuum service is a separate load case and must be stated at enquiry. A hose or bellows designed only for internal pressure may collapse under external pressure. Corrugated hose handles vacuum well when it has been designed for it — the corrugation geometry resists collapse — but the ply count and any liner arrangement change.