A compressor discharge hose is asked to do three things at once: contain hot gas, absorb the machine’s vibration, and survive a pressure pulse that arrives on every revolution. On a reciprocating machine the peak pressure at the hose can be well above the gauge reading, and the gauge is what everybody sizes from.
What this service does to a hose
Pulsation is not pressure. A reciprocating compressor delivers gas in discrete pulses, and the pressure wave that leaves the cylinder reflects off every discontinuity in the pipework. Where a reflection returns in phase with the next pulse, the two add. The peak at that point can be considerably above the mean discharge pressure that the gauge and the datasheet both show. A hose sized on the mean is being cycled to something higher, tens of times a second, for its whole life. That is a fatigue duty and the fatigue curve is what governs, not the burst rating.
The gas is hot. Compression heats gas. A single-stage machine can put out 150–180 °C at the discharge, and interstage temperatures on a multi-stage set are similar. At 180 °C, 316L holds roughly 0.63 of its 20 °C rating — so a DN40 assembly rated 32 bar cold is a 20 bar assembly at the discharge nozzle. This is one of the few duties where the derating and the working pressure are genuinely close together, and it must be checked.
Oil carryover. Lubricated compressors carry oil into the discharge. Hot oil in a corrugation valley cokes over time, and coked deposits reduce the effective bore and stiffen the hose. On an oil-flooded screw machine downstream of the separator this is minor; on a reciprocating machine it is not.
Vibration at the nozzle. The reason the hose is there. Sized and installed correctly it isolates the pipework from the machine. Sized on the static bend radius, or installed along the line of movement rather than across it, it becomes the shortest path for the vibration instead of the barrier to it — see pump connector hose for the geometry.
What we would fit
| Element | What we would supply | Why |
|---|---|---|
| Hose | 316L annular corrugated; 321 where interstage temperatures are sustained and high | Hot cycling is where the stabilised grade earns its cost |
| Braid | Double braid as standard on reciprocating discharge | The peak is above the mean and it arrives many times a second |
| Liner | Fitted on lubricated machines and where velocity is high | It keeps oil out of the valleys and flow off the convolutions |
| Ends | Flanged, with one end swivel so the pair can be aligned without twisting the hose | Bolting up misaligned flanges builds torsion in permanently |
| Length | Sized on live length and the flexing bend radius | It flexes continuously; the static figure does not apply |
What we would not fit, and what happens
A hose sized on mean discharge pressure with no pulsation allowance. The failure is fatigue at the collar and it arrives long before anyone expects it.
A hose sized on the cold pressure table. A third of the rating is gone at the discharge temperature.
A hose installed in line with the machine's movement. It transmits the vibration instead of isolating it.
Interlock hose. Not pressure-tight.
What we need to size it
| We need | Because |
|---|---|
| Machine type — reciprocating, screw, centrifugal — and the number of cylinders | It decides how much pulsation allowance we build in |
| Discharge pressure and discharge temperature | Two separate sizing inputs, both required |
| Peak pressure from a pulsation study if one exists | It replaces our conservative assumption with your real number |
| Whether the machine is lubricated and whether there is a separator | It decides whether a liner is needed |
| Amplitude and direction of nozzle movement | It sets the length and the orientation |
| Flange standard and rating at both ends | It has to bolt to the machine and the pipework as built |