Polyethylene or stainless steel for the instrument-air run is usually argued as a price question, and that is the wrong axis. The real variable is quieter: how much slow, invisible leakage you will tolerate between header and device. PE leaks in three ways metal does not — permeation through the wall, aging seals at the fitting, temperature ceilings that arrive without warning — while stainless leaks the old-fashioned, audible way at joints you can fix. Pick the material by deciding which failure to own.
Key takeaways
- PE is permeable to air: gases diffuse slowly through the wall, worst on long, thin-wall runs. No joint is needed for this leak.
- Plastic lines seal at elastomeric O-rings in push-in or barbed fittings that age, harden and relax. Metal seals at ferrules and threads you can retighten.
- DOE’s compressed-air figures put a 1/16-inch hole at 6.5 cfm and about $523 a year in electricity at $0.05 per kWh — a 1/8-inch hole at 26 cfm and $2,095. Leaks eat 20–30% of output in a poorly maintained plant.
- LDPE serves to roughly 70–80 °C, softening at 85–95 °C; HDPE runs 60–80 °C continuous, melting near 130 °C. In a fire, PE melts and vents the circuit by design; stainless holds pressure and the valve holds position.
- PE is the right answer for low-pressure, non-critical, temporary or vibrating service — fairly used, not a compromise.
The hidden cost of plastic air lines
The leak you cannot hear has three sources, and only one is a fitting. First, permeation: gases diffuse through the wall — the oxygen diffusion rates are published in resin data — so a long, thin-walled run leaks air through intact plastic. Second, the fittings: push-in and barbed connections seal with elastomeric O-rings that take compression set, harden with heat, wear at every re-insertion. Third, the economics: the DOE Compressed Air Challenge prices a 1/16-inch hole at 6.5 cfm and about $523 a year at $0.05/kWh on continuous duty, a 1/8-inch hole at 26 cfm and $2,095 — double both at $0.10/kWh. The sourcebook puts leakage at 20–30% of output in a poorly maintained plant. None of this makes plastic dishonest; it moves the invoice into the electricity column.
Temperature and fire
The ceilings are lower than shop habit assumes. LDPE’s continuous service tops out near 70–80 °C, Vicat softens at 85–95 °C, melting at 105–115 °C. HDPE runs 60–80 °C continuous — about 60 °C for pressure pipe — melting near 130 °C. Above a compressor the failure mode is softening and creep, not drama.
Fire behavior is a design trade-off, not a defect. Fire-safety practice routes plastic tubing to shutdown valves so it melts in a fire: the signal vents, the spring-return actuator drives the valve to its fail-safe position. Stainless does the opposite — it keeps pressure and holds the valve’s last position, a virtue only when holding is the safe state; for fail-close fire service the melting plastic line is sometimes the intended mechanism. A valve that must stay operable through a fire gets hard piping plus fireproofing. Stainless has its own caution list too — chloride stress corrosion among it — mapped in chloride SCC and 316 fittings.
Vibration and fatigue
Here flexibility genuinely helps. Plastic damps vibration instead of transmitting it, and forgives relative motion between equipment and frame — the motion that work-hardens metal. Rigid stainless does not — unsupported length amplifies vibration until fatigue cracks nucleate at the fitting. The remedy is clamping to rigid structure at intervals, with spacing tables by size; the logic is in tube clamp spacing. A flexible loop takes motion at moving boundaries instead of the end fitting. This is the axis where plastic legitimately beats metal, and why OEM equipment ships with it.
Where PE is the right answer
PE is not a failed metal. For low-pressure pneumatics and instrument-air drops — published ratings sit in the roughly 75–125 psi range by wall — it is light, cheap, flexible and chemically resistant across a wide media list. It is the default for potable and condensate lines, chemical transfer where its corrosion immunity shines, temporary runs that coil up and vanish, flexible connections on moving equipment. Specify with the ceiling in view: protect from heat, support where it sags, count fittings as future maintenance. For long, permanent, exposed distribution, the engineering standards default to metal — copper, PVC-coated copper or 316 stainless — for the permeation, aging and fire reasons above.
The cost math
The honest comparison is a framework. One side: the PE line’s price plus fittings, plus the electricity of its leaks — the DOE table gives the per-hole figures, the plant’s leak-audit percentage the fleet number. Other side: the stainless line’s price plus clamps and labor, essentially zero permeation loss, minus rework avoided on runs that would have softened, sagged or burned. Then the non-energy terms each plant prices differently: fire behavior by valve, remake cost by connection, the cost of a failure nobody hears. Stainless coil tubing often wins the long-run case on installation labor alone. Run the numbers and the material picks itself.
RFQ lines
service and pressure— medium, working pressure: instrument air, potable, chemical or condensate.temperature exposure— normal and upset, ambient and nearby equipment; often decisive alone.run length and routing— meters, exposed or protected, vibration sources.fire expectations— which valves fail safe, which stay operable; material follows.connection standard— fitting family and remake expectations, so spares arrive matching.- One line list, one quote: our instrumentation tubing range covers both materials with the fittings to terminate them.
FAQ
Is PE tubing OK for instrument air?
Conditionally — for low-pressure drops and short runs, inside published pressure and temperature ratings, fittings treated as maintenance items. For permanent, exposed distribution the engineering standards default to metal: permeation, seal aging and fire behavior all argue for it. PE feeds the device; metal owns the plant.
How much air does a small leak waste?
The DOE Compressed Air Challenge figures: a 1/16-inch hole at 100 psig leaks about 6.5 cfm, costing roughly $523 a year at $0.05 per kWh; a 1/8-inch hole leaks about 26 cfm and roughly $2,095 — continuous duty, so double at $0.10/kWh. Fleet-wide, leakage runs 20–30% of output in a poorly maintained plant.
What temperature can PE tubing take?
LDPE serves to roughly 70–80 °C continuous, softens at 85–95 °C, melts at 105–115 °C; HDPE runs 60–80 °C — about 60 °C for pressure pipe — melting near 130 °C. Treat the published ceiling as a hard wall: creep starts below any visible failure.
Does plastic tubing melt in a fire — is that bad?
Sometimes it is the design: fire-safety practice routes plastic tubing to shutdown valves so it melts, venting the signal and driving a spring-return actuator to its fail-safe position. Stainless holds pressure and keeps the valve where it was. The choice depends on the valve — fail-safe service may prefer plastic, operable-through-fire service requires hard piping with fireproofing.
When should I use stainless instrument tubing?
For permanent, exposed, long or safety-relevant runs — headers, lines near heat, valves that must hold position or stay operable in a fire. Add it where vibration is clamped rather than absorbed, remakes must stay flame-free, and the leak audit says the plastic fleet pays for itself in electricity.
