The one-seventh rule: how fast you can draw acetylene from a cylinder and why

Acetylene suppliers give a withdrawal limit for their cylinders, and almost none of them say why. The limit is a rate: the long-standing benchmark is one-fifth of the available gas per hour, and KIGL specifies one-seventh. Behind it sit two separate pieces of physics, both quantified in the solubility literature, and both of them turn the rule from an inconvenience into a safety measure.

As compiled by S. A. Miller (1965), the standard way to assess how well a cylinder gives up its gas was to hold it in a water bath at 15 °C and see how long it could maintain a discharge rate of one-fifth of its available acetylene per hour. The extent varied from 60 % to 90 % of the total available gas according to the quality of the porous mass. The same figure appears as a legal limit in the Dutch Factory Inspector's instruction leaflet on dissolved acetylene (1950), which forbade withdrawal at more than 20 % of the rated capacity per hour. The benchmark and the regulation are the same number, and that is the origin of the rule the whole industry repeats.

The C.P.I., Safety in the Production and Use of Acetylene (Paris 1968), stated the problem without a figure: in cold weather the gas output slows because the solubility of acetylene in the solvent rises as temperature falls, and heating the container with a flame is forbidden. No Indian Standard sets a customer withdrawal rate. CGA G-1 cl 6.2 does: one-seventh of cylinder capacity per hour for welding and cutting, one-tenth for solvent-sensitive intermittent use and one-fifteenth for continuous use.

The second mechanism is why the rule is about rate rather than pressure. Hölemann and Hasselmann's solubility isotherms for acetylene in acetone (Forschungsberichte des Landes Nordrhein-Westfalen, 1952–1959), as compiled by Miller, give the dissolved load at 20 °C as 689 g per kg of acetone at 20 atm total, 472 g at 15 atm and 295 g at 10 atm. A fast draw takes the cylinder down that curve faster than the gas can diffuse out of the bulk liquid. The liquid supersaturates and releases gas as bubbles throughout its volume rather than at the surface.

That liquid is already swollen. Each gram of acetylene dissolved adds about 1.6 ml to the solution volume, so at 15 °C and 15 atm the loaded acetone occupies 1.84 times its original volume. The foam reaches the valve and acetone leaves the cylinder as liquid droplets, not vapour. Miller's porous-mass requirement (vii), that the mass should discharge its acetylene without entraining solvent in liquid form, is a statement that acetone retention is a property of the mass, and that it is a property that fails first under a fast draw.

Both mechanisms, chilling and frothing, act together: a cold, frothing cylinder delivers low pressure and loses solvent in the same event.

Most sources present the withdrawal rule as a matter of delivery pressure. Miller's data make it a safety rule as well, and this is the point worth stating plainly. Every gram of acetone that leaves the cylinder as liquid shifts the acetylene-to-solvent ratio in the direction that makes the cylinder more sensitive to a backfire. The Gas Cylinders Rules 2016 put it in the Explanation to Rule 37: a shortage of solvent makes the cylinder less resistant to decomposition from flashback. Over many cycles a cylinder that is habitually over-drawn arrives at the filling station with a persistent solvent deficiency, and the two published cylinder-fire investigations cited on this site, in Great Britain in 2010 and Taiwan in 2005, both found too little acetone together with a defective porous mass, although neither attributed the shortfall to over-drawing by users. Solvent make-up at filling, covered under inspection at filling, exists to reverse exactly this drift.

Take an illustrative cylinder with a nominal content of 7 m³ (about 8 kg) of acetylene:

Because the rule is a fraction of the contents, the permitted rate falls as the cylinder empties. A large cutting torch or a multi-burner heating job that needs 2–3 m³/h therefore needs three or more cylinders on a manifold, each drawing within its own limit, with a non-return valve per cylinder and a flashback arrestor after the regulator. The manifold guide covers the arrangement.

KIGL's figure is 28.6 % below the benchmark. Three considerations support the margin. Miller's own observation is that at 20 % per hour even a good mass sustains the rate for only 60–90 % of the charge; a lower rate stays sustainable for more of it. Indian ambient temperatures swing widely, and in the cold season there is less heat available in the surroundings to replace what desorption removes. And a lower draw keeps the frothing mechanism further away, which protects both the solvent inventory and the weld or cut, since liquid acetone at the torch disturbs the flame. The companion page on acetone carry-over shows that the vapour component of solvent loss is governed by pressure and temperature, whereas the liquid component is governed by rate; the withdrawal limit addresses the second.

IS 16271:2014 cl 3.3.1 gives a typical emptying rate of one-eighth of the maximum acetylene content per hour, and AIGA 037/16 gives one-eighth to one-tenth. These are controlled blow-down rates used by the filler when emptying a cylinder before measuring its solvent content, so that the measurement is not distorted by cooling or by acetone carry-over. They are not customer withdrawal rates, although they are limited by the same physics. AIGA 022/26 notes that a large full cylinder takes 8 to 10 hours to blow down at that rate.