A dry or pre-action sprinkler system spends its whole life full of gas, and if that gas is compressed air it is also full of oxygen. Oxygen is what drives corrosion inside sprinkler pipe, so replacing the air with nitrogen strips out most of the oxygen the reaction depends on.
Why corrosion happens in sprinkler pipe
Corrosion needs three things at once: unprotected metal, oxygen and water. A dry-pipe system supplies all three. The pipe is bare steel inside. The compressed air used to supervise it is an endless supply of oxygen. And the water is already there — a system is never fully drained after hydrostatic testing, and compressed air carries more in as condensate.
The pipe has to stay steel, and residual water cannot be drained out of a dry system in practice. Oxygen is the only one of the three that can practically be reduced in a system already in service, which is what nitrogen supervision does.
For scale: dry and pre-action systems are involved in 59% of fire losses caused by corrosion-related obstructions to sprinkler flow (FM Global), and 73% of dry and pre-action systems inspected had significant corrosion after 12.5 years of normal service (VdS). Wet systems, by comparison, account for 13% of those obstruction losses.
| What corrosion does | Why it matters on a riser |
|---|---|
| Thins the pipe wall | Black steel thins uniformly; galvanized pits. |
| Opens pinhole leaks | Supervisory pressure falls, the generator runs more and more to keep up, and eventually the panel reports a low-pressure trouble. |
| Obstructs sprinkler flow | Corrosion product breaks loose and collects at heads and in branch lines. |
Why nitrogen works
Nitrogen is inert. Fill the pipe with it and most of the oxygen that was doing the damage is displaced out. Dissolved oxygen is the major factor controlling how fast steel sprinkler pipe corrodes, and corrosion slows sharply as the oxygen inside a pipe falls.
What it does to each corrosion mechanism
| Mechanism | What nitrogen supervision does to it |
|---|---|
| Generalized oxygen corrosion | Generalized oxygen corrosion is the dominant mechanism, and the one nitrogen targets directly. Uniform wall thinning wherever air and water meet steel. |
| Attack on the galvanizing layer | The zinc layer is consumed first, in localized pits rather than evenly. Inerting the pipe slows that attack, which is why galvanized gains most from nitrogen. |
| Microbiologically influenced corrosion (MIC) | Inerting does not eliminate it — some of the organisms involved do not need oxygen. MIC is assessed and treated on its own terms. |
| Pinhole leaks and repeat repairs | Fewer leaks, so less supervisory pressure loss, fewer trouble signals and fewer impairments to chase them. |
Nitrogen against compressed air
Long-term coupon testing has run for more than 3,000 days on Schedule 10 black and galvanized pipe, half full of water, under compressed air and under 98% nitrogen. Black steel's projected service life goes from about 20 years on air to about 63 years on nitrogen, and galvanized from about 10 years to about 176 years.
On air, the galvanizing layer is the fastest-failing part of the system. Inerted, it is effectively no longer the limiting factor, and black steel under nitrogen supervision can outlast galvanized pipe under air supervision.
| Characteristic | Compressed air | N2-BLAST® nitrogen |
|---|---|---|
| Oxygen | About 21% | 1.5% at the published 98.5% purity |
| Moisture | Carries water vapor, which condenses in cold pipe | Dry, so it adds far less condensate |
Purity is worth the last half-point. Every current N2-BLAST® generator is specified to 98.5% nitrogen or better. At 98% the pipe still holds 2% oxygen; at 98.5% it holds 1.5% — a quarter less oxygen available to drive the reaction.
Does nitrogen help with ice plugs?
Yes — it reduces the risk, in two ways:
- Less water goes in. The generator's product gas is dry, so it displaces moisture-laden shop air rather than adding to it, and residual moisture is more likely to stay as vapor at low temperature.
- The pipe stays smoother. Corrosion roughens and pits the bore, and those pockets are where water sits and freezes.
Where nitrogen supervision is used
Anywhere the pipe sits full of gas rather than water between activations, and air is the traditional supervisory medium:
| System type | How nitrogen fits |
|---|---|
| Dry pipe | Nitrogen replaces air as the supervisory gas. The dry-pipe valve and the air maintenance arrangement work the same way; only the gas changes. |
| Pre-action | Same substitution. A pressure-supervised system spends its whole life pressurized but not activated, which is when the corrosion happens. |
How the generator makes the nitrogen
N2-BLAST® generators make their nitrogen on site by pressure swing adsorption. Ambient air is roughly 78% nitrogen and 21% oxygen. Filtered compressed air passes through one of two beds of carbon molecular sieve: under pressure the sieve holds the oxygen back and lets nitrogen through to the storage tank. Before the sieve saturates the beds switch, and the vented bed releases the captured oxygen to atmosphere as an oxygen-enriched stream. The beds alternate, so one is always producing while the other regenerates and the output is continuous.
On-site generation tops the system up on demand, with no cylinder deliveries to schedule.
Related articles
- N2-BLAST Fire Protection Systems Overview
- FPS-500 and FPS-900 Overview
- FPS-1650, FPS-3250, and FPS-5000 Overview
- FPS-10000, FPS-16500, and FPS-22500 Overview
- Nitrogen Supervision vs. Vapor Corrosion Inhibitors (VCI) in Fire Protection Systems