Nitrogen Purging in an Acetylene Plant

The short answer

Every acetylene plant line and vessel must be purged of air, air-acetylene mixtures and air-inert mixtures with dry nitrogen before the acetylene compressor is started. Air is never used as a purge medium, and acetylene mixed with air must never be compressed — that mixture is hazardous at any pressure, not merely at high pressure.

The purge is verified, not assumed. Sample the nitrogen at the plant outlet and test it for oxygen before charging carbide or starting the compressor. The controlling acceptance limit is an oxygen reading below 2.8%; IIGAS recommends working to a tighter target of 0.5% or less.

Why nitrogen and never air

Acetylene has an exceptionally wide flammable range — roughly 2.5% to 100% in oxygen — which means that almost any proportion of acetylene mixed with air inside a plant is within range. There is no dilution that makes the mixture safe by accident.

Compression makes it worse in two independent ways. Raising the pressure of an acetylene-air mixture raises both its sensitivity to ignition and the energy released if it does ignite. And acetylene is thermodynamically unstable in its own right: under pressure and heat it can decompose without any oxygen present at all, with a confined pressure rise of ten to eleven times the initial pressure. A compressor is precisely a source of pressure and heat.

Nitrogen displaces the air without introducing either hazard. It must be dry — moisture carried in with the purge gas simply loads the drying train and, further downstream, the cylinder solvent.

When the plant must be purged

The purge procedure

1. 1
   1 · Isolate and vent down

   Isolate the section to be purged and vent it to atmospheric pressure through the proper vent point. For a carbide hopper, vent to below 0.03 kg/cm²g before anything is opened.
2. 2
   2 · Confirm the nitrogen supply

   Check the nitrogen itself before you rely on it — cylinder identity, batch certificate and, where any doubt exists, an oxygen reading taken at the bank. A purge is only as good as the gas doing it.
3. 3
   3 · Admit dry nitrogen

   Purge from the nitrogen bank through the plant’s purge and flush points. A bank of four to six nitrogen cylinders of about 6 m³ each serves the purge points on a typical plant.
4. 4
   4 · Repeat the cycle

   Pressurise and vent repeatedly rather than relying on a single sweep. On commissioning, run the generator through 8–10 cycles; on a routine start-up, at least half that.
5. 5
   5 · Sample at the outlet

   Take the sample at the plant or pipeline outlet — the last point to clear — not close to the nitrogen inlet, where it will read clean long before the far end has.
6. 6
   6 · Test for oxygen and accept

   The oxygen content of the outlet sample must read below 2.8%. Work to 0.5% or lower where you can. Only when the sample passes may carbide be charged or the compressor started.
7. 7
   7 · Record it

   Log the purge, the cycles, the sample point and the oxygen reading. On a commissioning or a post-incident restart, this record is the evidence that the plant was handed over gas-free.

Verify the nitrogen, not just the purge

The step most often skipped is checking the purge gas itself, and it is the one that has caused real harm. In a 2025 field investigation on an installed acetylene plant, cylinders supplied and labelled as purge nitrogen were found on analysis to contain roughly 19% and 48% oxygen. In other words, one was effectively air in a nitrogen cylinder and the other was oxygen-enriched beyond it.

A purge performed faithfully with that gas does not make the plant safe — it fills the plant with the exact atmosphere the procedure exists to exclude, while every step in the logbook reads as complete. The written procedure was not the failure point; the unverified assumption about the cylinder contents was.

Two controls close this gap, and neither is expensive: require a certificate of analysis for every batch of purge nitrogen, and take an oxygen reading on site at the bank before the purge begins. The outlet sample in step 6 will normally catch a bad supply as well — which is exactly why that acceptance test must never be treated as paperwork.

Common purging mistakes

• ✓Using compressed air because the nitrogen bank is empty. There is no version of this that is acceptable — stop the job instead.
• ✓Sampling near the nitrogen inlet rather than at the far outlet, which reads clean while the far end of the system is still full of air.
• ✓One long sweep instead of repeated pressurise-and-vent cycles. Dead legs, instrument tappings and vessel domes do not clear on a single pass.
• ✓Treating the oxygen test as a formality to be recorded rather than a gate that can stop the start-up.
• ✓Forgetting the sections isolated during maintenance — a purged main line with an unpurged branch is an unpurged plant.
• ✓Using wet or industrial-grade nitrogen of unverified quality, which loads the driers and may not be what the label says.
• ✓Opening a carbide hopper before it has been vented below 0.03 kg/cm²g and purged.

Purging and the compressor

The compressor is the reason the acceptance limit exists. Everything upstream of it operates near atmospheric pressure, where an air-acetylene mixture is dangerous but bounded. The compressor is where that mixture would be raised through three stages of pressure and heat, and it is the point at which a purge failure stops being recoverable.

This is also why the interlocks matter and must never be defeated to get a plant running. On an IIGAS plant the fail-safe condition stops all motors except the agitator, and the plant is designed so that a lost instrument-air supply, a high generator temperature or a high pressure takes the carbide feed out rather than pressing on.