Why wet acetylene is safer than dry acetylene and where the hazard rises in a DA plant

Acetylene saturated with water vapour is harder to decompose than dry acetylene at the same pressure, and the advantage grows as the gas gets hotter. That is one of the few favourable physical facts about acetylene, and it shapes how a carbide-based dissolved acetylene (DA) plant is laid out: the gas is kept wet for as long as the process allows, and the section after the driers is treated as the most demanding part of the plant.

The classic data are Rimarski's, measured on pure acetylene in identical 20-litre vessels with ignition by an incandescent platinum wire, and reported by the C.P.I. (Safety in the Production and Use of Acetylene, Paris 1968). The figure measured is the lowest effective pressure at which a decomposition, once started, spreads through the whole vessel.

Two things stand out. The dry limit falls steadily with temperature, from 0.60 kg/cm² at 10 °C to 0.10 kg/cm² at 140 °C. And at 80 °C, saturation lifts the limit from 0.40 to 1.4 kg/cm², three and a half times higher in effective pressure. In absolute terms that is a rise from about 1.39 to about 2.36 atm, roughly 1.7 times, which is still a substantial margin.

The water contents are physically consistent. Water's saturation vapour pressure is about 12 kPa at 50 °C and about 47 kPa at 80 °C (NIST steam data), and those figures divided by the limiting total pressures give very nearly the 7.1 % and 19.4 % that Rimarski recorded.

The compilation by S. A. Miller (1965) presents the same physics as two graphs that run in opposite directions. For dry acetylene, the lines of Rimarski and Boesler fall from about 1.6 to about 1.15 atm absolute between 0 and 200 °C. For water-saturated acetylene the limiting total pressure, water included, rises with temperature: about 1.74 atm absolute at 50 °C and 2.63 atm absolute at 80 °C.

The decomposition of acetylene to carbon and hydrogen is self-sustaining only above a critical pressure, because the carbon-laden flame loses heat by radiation. Any inert gas in the mixture absorbs part of the heat released without adding any, so the flame needs more acetylene per unit volume, and therefore a higher pressure, to survive. That mechanism is covered in detail on the acetylene decomposition page.

Water vapour is a particularly useful diluent in a carbide plant for one reason: its share of saturated gas rises much faster with temperature than the acetylene limit falls. A hotter generator makes dry gas more dangerous but makes saturated gas safer. The C.P.I. records the same point qualitatively, describing water vapour as playing "a highly moderating role" at high temperatures. This also explains why incandescent carbide inside a generator is a smaller decomposition risk than might be feared: the gas around it is always steam-laden. The same moderating effect is one reason generator temperature control works.

The C.P.I. lists the conditions that increase decomposition risk as higher pressure, a larger container, purer acetylene and drier acetylene. Purity and dryness are quality virtues for the customer; for decomposition they are aggravating factors.

Moisture also helps in wide pipe, but by less. The compilation by S. A. Miller (1965) gathers the results of Rimarski, Boesler and Jones on limiting pressure against vessel diameter: about 2.2–2.4 atm absolute at 50 mm, falling by 250–300 mm bore to roughly 1.8 atm absolute for wet gas and 1.4 atm absolute for dry. The sources disagree on how strong the diameter effect is: Sargent's long-pipeline data show a much steeper fall with diameter than these curves, and Ivanov and Kogarko's closed-tube work agrees with Sargent. Ivanov and Kogarko also showed that an arrangement simulating an open tube gives less favourable results than closed-tube tests, so the wide-pipe figures above should not be relied on as a margin. The diameter effect itself is the subject of pipe diameter and detonation.