Oxy-acetylene regulators and working pressures: the acetylene ceiling oxygen settings by job, and how a regulator fails

An oxy-acetylene set runs on two pressures that are set by hand at the regulators and are almost never questioned once the flame looks right. One of them, the acetylene pressure, has a hard legal and physical ceiling. The other, the oxygen pressure, is where nearly all the adjustment for the job happens. This page gives both with their sources, explains the two regulator types, and sets out the failures that matter.

IS 14814:2023 clause 7.2.1 is short. Acetylene in the free state outside the cylinder is normally permissible at a pressure of 0.612 atm. Under special circumstances, with adequate safety measures and after clearance from the Chief Controller of Explosives, it may be used at a maximum of 1.021 atm. These are limits on free-state use at the point of use; IS 15190 (Part 1):2002 separately sets pipeline design classes up to 155 kPa gauge, which is a pipe-design figure, not a working pressure for the torch.

The reason is that acetylene decomposes without oxygen. Jones, Scott, Kennedy and Huff of the US Bureau of Mines found in 1944 (RI 3755) that a decomposition flame would propagate through pure acetylene above about 1.4 atm absolute in their apparatus, and later work showed the limit falling in wider pipes and at higher temperatures. The regulatory ceilings keep free acetylene close to atmospheric pressure, but they are not a guarantee: 1.4 atm absolute is only about 0.4 atm gauge, and with strong ignition decomposition is possible at lower pressures still. The history and the numbers are on where the 1.5 bar rule comes from. The cylinder itself holds acetylene at up to 16 kgf/cm² at 15 °C (Gas Cylinders Rules 2016, Rule 37(vi)) only because the gas is dissolved in acetone in a porous mass; the regulator is the boundary between that protected state and free gas.

The most useful fact in the old machine-cutting tables is how little the acetylene setting changes. The British Oxygen Co.'s 1960 booklet Oxygen Machine Cutting gives these conditions for mild steel with straight-bore nozzles:

Acetylene stays at 2 psi from 25 mm to 300 mm. The preheat oxygen rises with the size of the preheat flames, and the cutting oxygen with the depth of kerf it must clear. The nozzle size changes too, but the acetylene regulator, once set, is rarely touched. An operator who finds they are winding the acetylene up to 10 or 12 psi to get a cut has the wrong nozzle, a starved cylinder or a blocked line, and is drawing the supply towards the ceiling for no gain. Nozzle choice is on flame types and cutting tip selection.

Welding blowpipes run lower. The 1965 welding literature, as compiled by S. A. Miller, distinguished two families. In the equal-pressure or mixing type, both gases enter at approximately equal pressures above atmospheric. In the injector type, oxygen at 10 to 40 psi passes through a venturi and aspirates acetylene supplied at 1 psi or less; the C.P.I. 1968 code gives the injector oxygen as about 1.0 to 3.0 kg/cm². Either type works on dissolved acetylene cylinders. IS 1323:1982, the Indian code for oxy-acetylene structural welding, adds the practical rule that flame size is regulated preferably by changing the nozzle rather than by altering the gas pressure, with pressures as the nozzle maker recommends.

A regulator is a valve held open by a spring and closed by the pressure it produces on a diaphragm. The 1965 literature describes the two arrangements: the stem or inverse type, where inlet pressure tends to close the seat, and the nozzle or direct type, where inlet pressure tends to open it. A two-stage regulator is two single-stage units in series in one body; the first stage drops cylinder pressure to an intermediate value and the second regulates finely from a nearly constant input.

The difference matters for acetylene more than for oxygen. An oxygen cylinder's pressure falls steadily as it is used. An acetylene cylinder's pressure moves with temperature as well as contents; CGA G-1 records about 2.5 psi of change per degree Fahrenheit, and a cylinder in the morning sun climbs while one in a cold yard sags. A single-stage regulator lets some of that swing through to the torch, so the flame drifts from neutral. A two-stage regulator, or a single-stage one that the operator re-sets as conditions change, holds the flame. For machine cutting and long welding runs the two-stage regulator is worth its cost.