An instrument air hose is a small-bore assembly feeding an actuator, a positioner or an analyser. The pressure is a few bar and the flow is negligible. It is fitted for one reason: a rigid tube run to a component that moves or vibrates will work-harden and snap, usually at the compression fitting, and usually at three in the morning.
What this service does to a hose
Fatigue is the whole point. A stainless tube taken from a rigid header to a valve actuator that moves with the valve, or to an instrument on a vibrating line, sees a bending cycle every stroke. Tube fails by fatigue at the ferrule of the compression fitting. A short flexible assembly with a proper live length distributes that bend over many convolutions instead of concentrating it at one point. That is the entire justification for the part, and it means the assembly has to be long enough to have a live length — a very short hose between two rigid points is no better than the tube it replaced.
Water in the air. Instrument air is meant to be dry to a specified dewpoint. Systems that are meant to be dry are frequently not, and liquid water in a small-bore line will sit in the corrugation valleys of a horizontal run. In an unheated outdoor location that water freezes, and a frozen instrument air line is a stuck actuator.
Small bore is unforgiving on bend radius. A DN6 assembly has a flexing bend radius of 140 mm — over twenty times its own bore. Fitters routinely bend small hoses far tighter than that because they feel flexible in the hand. They are flexible; they are not undamaged.
Vibration transmission to the instrument. A braided hose is stiffer than it looks. On a sensitive transmitter the hose can carry enough vibration to affect the reading, which is an argument for a longer, slacker assembly rather than a tight one.
What we would fit
| Element | What we would supply | Why |
|---|---|---|
| Hose | 316L annular corrugated, DN6 to DN12 | Bore is set by the actuator's stroke speed, not by pressure drop |
| Braid | 304 or 316 wire, single braid | Instrument air pressures are far below any small-bore rating |
| Ends | Compression, NPT or BSP to match the existing tube fittings and the instrument port | It has to mate with what the instrument manufacturer supplied |
| Length | Long enough to give a real live length and stay above the flexing bend radius | A hose with no live length is a rigid tube with extra joints |
| Routing | Arranged so the assembly falls to a drain point where the air may be wet | Water in a valley in an outdoor location freezes |
What we would not fit, and what happens
A very short assembly between two rigid points. It concentrates the bend and fails the same way the tube did.
Interlock hose. Not pressure-tight, in any bore.
A hose routed to trap water on an outdoor line. The trap freezes and the actuator stops.
A generic fitting adapted to the instrument port. Adaptors leak, and on a small bore at low flow a small leak is a permanently drooping actuator.
What we need to size it
| We need | Because |
|---|---|
| Supply pressure and the instrument's required flow | Small bore at low flow can still throttle a fast-stroking actuator |
| Whether the air is dry to a dewpoint, and the minimum ambient temperature | It decides whether freezing is a credible failure |
| How much the connection moves, and in which direction | It sets the length and the live length required |
| Fitting type and thread form at each end | It has to mate with the instrument port as supplied |
| Whether the location is indoor, outdoor or coastal | It decides 304 or 316 on the braid |
| Whether vibration at the instrument is a measurement concern | It argues for a longer, slacker assembly |