There is no such thing as an acid hose. There is a hydrochloric acid hose, a sulphuric acid hose and a nitric acid hose, and they are not the same part. Which acid, at what concentration, at what temperature, with what aeration — those four answers decide whether bare 316L is comfortable, whether it will be destroyed in a week, or whether the only honest answer is a PTFE liner with the metal carrying nothing but the pressure.
What this service does to a hose
Hydrochloric acid attacks stainless at every concentration. Chloride is the ion austenitic stainless is least able to live with, and hydrochloric acid is chloride in its most aggressive form. There is no grade in our range that handles it. The answer is a PTFE liner carrying the medium, with 316L outside carrying only pressure and mechanical load — the same construction we use for wet chlorine.
Sulphuric acid is concentration-dependent and counter-intuitive. Concentrated sulphuric above roughly 90 % passivates stainless and is handled in it routinely. Dilute sulphuric, below about 20 %, is strongly reducing and attacks it. In between, and at raised temperature, the behaviour is complicated enough that the concentration and temperature must be stated and checked rather than assumed. A dilution that happens inside the pipe — a water flush that was not fully cleared, condensation in a vent line — moves the duty from the safe end of that curve to the dangerous end without anybody changing the process.
Nitric acid is the one stainless likes. It is strongly oxidising and it maintains the passive film. 304L is often preferable to 316L here, because molybdenum is less resistant in hot concentrated nitric — one of the few duties where the more expensive grade is the wrong choice.
Everywhere, it is velocity and stagnation that finish the job. High velocity strips the passive film mechanically; stagnant liquid under a deposit lets the chemistry under it go its own way. A corrugated bore does both: it accelerates flow over the crests and holds it still in the valleys.
What we would fit
| Element | What we would supply | Why |
|---|---|---|
| Hydrochloric, any concentration | PTFE-lined 316L, liner carrying the medium | No grade in our range resists HCl; the liner is not optional |
| Sulphuric, concentrated above 90 % | 316L annular corrugated, subject to temperature | Concentrated sulphuric passivates stainless; dilution is the hazard |
| Sulphuric, dilute or of variable strength | PTFE-lined | Dilute sulphuric is reducing and attacks 316L |
| Nitric | 304L annular corrugated in preference to 316L | Nitric is oxidising and maintains the film; molybdenum is a liability in hot concentrated nitric |
| Ends on any lined assembly | PTFE flared over the flange face, with a stainless backing flange | The medium must never see the joint metal |
What we would not fit, and what happens
Bare stainless on hydrochloric acid. It will pit through, and it will do it fast enough that the assembly is scrap before the first invoice is paid.
316L on hot concentrated nitric, because it is “the better grade”. It is not, on this duty.
An assembly that cannot be flushed and drained. Residual acid in a corrugation valley over a shutdown does more damage than the process does.
Interlock hose. Not pressure-tight, and the seam is a crevice that cannot be cleaned.
What we need to size it
| We need | Because |
|---|---|
| The acid, by name | There is no generic answer to this question |
| Concentration, and the full range it can see including flushes | The safe window is often narrow, and dilution moves you out of it |
| Maximum temperature | Almost every acid gets more aggressive with temperature |
| Whether the line is aerated, and whether it stands full | Aeration and stagnation are the two multipliers |
| Flow velocity, or bore and flow rate | High velocity strips the passive film mechanically |
| Whether the line is flushed after use, and with what | The flush is sometimes more damaging than the process |