Inspecting a dissolved acetylene cylinder at filling under IS 16271:2014: before during and after the charge

Every time a dissolved acetylene cylinder comes back to the filling station it has to be proved fit before it is charged again. IS 16271:2014, drawn from ISO 11372:2011, sets out the minimum inspection for single cylinders in three stages, before, during and after filling, and its central task is the one that cannot be done by eye: confirming that the right amount of solvent is in the cylinder so that the right amount of acetylene can go in. Rule 39 of the Gas Cylinders Rules 2016 adds a thorough visual examination to IS 8433 at every charge.

For an oxygen cylinder, filling inspection is about the shell and the valve. For a DA cylinder it is also about the contents system. The porous mass and the acetone are what stop a decomposition propagating, as set out on inside a dissolved acetylene cylinder, and the acetone is consumed a little at every use. The Explanation to Rule 37 of the Gas Cylinders Rules 2016 puts the stakes in one sentence: excess solvent can make a cylinder hydraulically full and a shortage makes it less resistant to decomposition from flashback, so solvent replenishment before refilling is essential.

Two published investigations show what happens when it is not done. The SAHTECH investigation of a September 2005 fire at a filling plant in Taiwan, presented at the AIGA acetylene safety seminar in 2009, found excessive settling of the porous mass and acetone not topped up, with poor residual-gas measurement hiding the shortage; about 90 cylinders were fire-affected and 8 exploded. The HSE investigation of the January 2010 explosion in Great Britain (Hooker, Hodges, Geary and Wakefield, IChemE Hazards 26, 2016) concluded that too little acetone and/or too much acetylene, with an undetected void in the mass, were the most likely causes, and identified manual acetone dosing, thermometers mounted away from the cylinders, gauge charts in 5 °C steps and chalk marking on racks of about 110 cylinders as weaknesses. Both point at the filling stage.

This is the step that IS 16271 added beyond a visual check. Clause 3.3.1 requires:

Annex B gives the method. The actual acetylene-to-solvent ratio F is obtained from the cylinder's residual pressure in bar and its temperature in °C through a vapour-pressure-type equation for acetone, and the solvent content then follows from the measured weight, the stamped tare, the specified solvent content and the saturation acetylene. The tare definition matters here: IS 16271 cl 2.14 makes the tare include the solvent and the acetylene that saturates it at atmospheric pressure and 15 °C, so a cylinder below tare after deducting residual gas has lost acetone. The user-side version of this arithmetic is on checking contents by weight.

The slow emptying rate and the 3-hour wait exist because desorption cools the cylinder. AIGA 022/26 records that rapid blow-down gives a false empty reading and that acetone carry-over is greatest during rapid blow-down. AIGA 037/16 sets scale tolerances for this measurement at ±20 g up to 6 litres water capacity, ±50 g to 20 litres and ±100 g above, with scales checked daily and calibrated annually.

Clause 3.4.1 requires the filler to verify that the valve is not blocked, for example by checking surface temperature, since a charging cylinder warms from the heat of solution, and that the valve and fusible plugs do not leak externally. On any suspicion the leak check extends to the valve gland nut, and filling stops and recommences only after rectification. Clause 3.4.2 requires cylinders filled simultaneously to have the same solvent type unless backflow is prevented by a technical device, with special care where porous materials differ or residual gas is high.

The charging rack itself carries one flashback arrestor per rack between compressor and rack and a non-return device at each charging station, as recorded in the C.P.I.'s Safety in the Production and Use of Acetylene (Paris 1968); the non-return valve page covers why both are needed.

Clause 3.5.1 requires leak tests with a leak-detection fluid at three places: the valve outlet seat, the valve-to-cylinder interface, and each fusible plug including its interface with the cylinder or valve. Clause 3.5.2 requires verification by weighing that the maximum acetylene content is not exceeded. Its Note gives the diagnostic that makes pressure useful once weight is known: at ambient temperature and correct total weight, a pressure that is too high indicates a solvent deficiency, solvent contaminated with water, or a high concentration of insoluble gases; a pressure that is too low indicates excess solvent. Clause 3.5.3 checks the valve protection. The pressure-and-contents page explains why pressure is otherwise misleading.

Rule 41 of the Gas Cylinders Rules 2016 requires a record of each charge: date, empty weight without gas, weight of solvent charged before gas charging, full weight, dates solvent was added and the results of Rule 39 examinations. Rule 42 puts the date of last filling, the weight of gas filled and the full cylinder weight on the label.

A freshly charged cylinder is not uniform inside. Strizhevskii (Zhur. Prikl. Khim., 1963, 36, 2320) reported that a DA cylinder behaves like a gas–liquid chromatography column: air, being poorly soluble in acetone, concentrated at the bottom, where it was 14.5 % by volume on the acetylene against 0.7 % at the top, with rack gas of 0.5–1 % air; equilibrium by diffusion took 10 to 14 days, and phosphine in unpurified gas distributed itself the same way. IS 16271 sets no holding period, and nor does any other standard KIGL has reviewed. A settling period before despatch is nonetheless a reasonable practice for consistent delivered gas, and the cylinder must in any case settle thermally before its pressure means anything.