LPG does nothing to stainless steel. Propane and butane are not corrosive, they carry no water to speak of, and the pressures involved — roughly 7 bar for propane at 20 °C, under 2 bar for butane — are trivial against any assembly we would build. The difficulties are all about what happens when liquid LPG stops being liquid, and about the fact that a leak on this duty finds an ignition source.
What this service does to a hose
Liquid offtake flashes. Draw liquid propane through a restriction and part of it boils. Boiling takes latent heat out of the remaining liquid and out of the pipe wall, and a line that was at ambient can drop towards −42 °C, which is propane’s atmospheric boiling point. Austenitic stainless is entirely comfortable there — it is one of the few materials that gets tougher as it gets colder — but the assembly gets shorter, and if both ends are rigidly anchored that contraction has to go somewhere. On a short pigtail it is nothing. On a ten metre run it is millimetres of pull at the collar, repeated every time the tank is drawn down hard.
Vapour offtake is the easy case and is where most burner and kitchen duty sits: low pressure, ambient temperature, no flashing. Here the hose is doing an ordinary job and the specification is driven by the fittings and by how often it is handled.
Static is a real hazard and a dry one. LPG has very low electrical conductivity. Flowing product generates charge, and a hose assembly that is not bonded end to end can hold it. A braided metal hose with welded end fittings is continuous metal from one flange face to the other, so it bonds itself — but only if the fittings are welded. That is one more reason we do not crimp.
Sour supply. Some LPG streams carry hydrogen sulphide. Where the supply is sour, say so, because it changes the conversation from a routine hose to a materials one.
What we would fit
| Element | What we would supply | Why |
|---|---|---|
| Hose | 316L annular corrugated for liquid and vapour alike | The medium is benign; 316L is chosen for the fittings environment, not the LPG |
| Braid | 304 or 316 wire, single braid on vapour, double on liquid offtake and tanker duty | Double braid where the assembly is handled, dragged or fitted to a road tanker |
| Ends | Flanged, NPT or BSP as the installation requires; POL and cylinder connections to order | Cylinder fittings are a defined family and must match the regulator, not the pipe |
| Attachment | TIG welded, which also makes the assembly electrically continuous | Bonding and pressure integrity come from the same weld |
| Length | Sized with contraction included where liquid is drawn | Flashing shortens the run; the hose must not be the thing that takes the strain |
What we would not fit, and what happens
Interlock hose. Not pressure-tight, and on a flammable gas that ends the discussion.
A crimped assembly. It relies on mechanical grip for both the seal and the earth path, and it relaxes with thermal cycling. On this duty the thermal cycling is guaranteed.
A liquid line sized without the contraction. The hose survives the cold easily; the anchors either side may not. Size the run, not just the hose.
An assembly with an unbonded joint anywhere in it. A single non-conducting coupling in the run defeats the whole earth path.
What we need to size it
| We need | Because |
|---|---|
| Vapour or liquid offtake | Liquid flashes; vapour does not. This is the first question |
| Working pressure and the relief valve setting | The relief setting is the design pressure |
| Minimum temperature the line can reach on draw-down | It sets the contraction allowance |
| Whether the supply is sour — any H₂S at all | It changes the material discussion completely |
| Which code the installation is being built to | It decides what we can certify and what we must decline |
| End connections, including any cylinder or POL fitting | These are defined families and cannot be improvised |