Starting stopping and purging an acetylene plant: getting the air out and keeping it out

Almost every acetylene plant procedure is, underneath, a rule about air. Acetylene alone can decompose; acetylene with air or oxygen is flammable from a few per cent upward and needs only about 0.02 mJ to ignite. Start-up exists to get the air out of every vessel before the gas goes in, and shutdown exists to stop the air getting back. The steps below are those of the KIGL plant operation manual for its carbide-to-water plants, with the reasons attached.

The 1968 C.P.I. code put the three rules most clearly. Before any apparatus is put into circuit, scavenge it with acetylene until the emergent gas contains at least 98 % acetylene, and vent the scavenge outside the building. Keep a slight excess pressure of more than 25 mm water everywhere on the low-pressure side. Stop the compressor automatically before suction can fall far enough to draw air in. German regulations, as compiled by S. A. Miller (1965), add the same requirement from the other end: within a short time of starting up there must be no more than 3 % oxygen in any operational part of the generator.

Nitrogen inerting comes first where it is available, because it means the acetylene that follows is flushing an inert gas rather than air. The acetylene purge cycles then remove the nitrogen and any pockets the first purge missed.

The generator is filled to level before any carbide is in a hopper. Carbide is charged with the hopper vented and, where available, nitrogen purged; the fill cap gasket is moistened and the vent closed over it.

1. Hook prepared cylinders to the manifold, close the manifold inlet, open cylinder and manifold valves and vent old gas through the arrestor vent valves.
2. Start the water supply; open water to the generator through the manual bypass to the required level; open water to the cooler, compressor tank and cylinder sprays.
3. Open the generator and arrestor vent valves with the arrestor outlet valve closed.
4. If nitrogen is available, purge the generator for 5 minutes through the high-level check valve.
5. Start the agitator, then hopper No. 1 feed.
6. Purge by building a little pressure and releasing it through the vent valves: 8 to 10 times at commissioning or after cleaning, at least twice at normal start-up. On the panel this is the repeated AUX ON, check pressure rise, AUX OFF and vent.
7. Close the arrestor and generator vents; purge downstream equipment with nitrogen if available.
8. Open the arrestor discharge valve and purge with acetylene at the scrubber, low pressure drier and purifier drains.
9. Start the compressor with all drain valves open, close each stage separator drain in turn, purge at the high-pressure drier drains, then close all purge and vent valves.
10. Build to the 14 kg/cm²g back pressure and open the manifold inlet valves gradually.

All valves are opened and closed slowly. The C.P.I. code notes the risk of ignition by friction where flow is too rapid, and a slow valve also avoids a pressure shock that can lift a safety valve.

For a plant that supplies a pipeline rather than cylinders, the line is purged with nitrogen from the scrubber to every consumption point until the outgoing gas contains not more than 2.5 % oxygen, the end-point set by IS 15190 (Part 1):2002 cl 12.6.3, using nitrogen of at least 97.5 % purity (cl 12.6.2); it is then purged with acetylene until the gas is smelt at the consumption points. Older KIGL manuals give 2.8 %; the governing standard is 2.5 %. Nitrogen is the only purge gas: EIGA Doc 237 cl 6.1 prohibits carbon dioxide because of static. The pipeline rules guide covers the design side.

The 2.5 % limit assumes the nitrogen is nitrogen. KIGL has investigated an event in which purge gas, delivered with oxygen more than ten times the acceptance limit and backed by a supplier certificate claiming it was in specification, created a detonable atmosphere in a hopper that friction from tramp metal then ignited. The wet arrestors and relief valves contained it. The lesson: test every purge-gas supply at the point of use with your own calibrated analyser, sign it per batch, and do not charge carbide or start the compressor until it passes.

Two related rules from audits of other plants: purge through the dedicated vent valves, never from the main gas line, which leaves air-acetylene pockets in dead legs; and keep vent and nitrogen-purge lines separate rather than teed together.

Carbide is not left in a hopper over a long shutdown. Any moisture that gets in will generate acetylene in the hopper.