SPONTANEOUS COMBUSTION AMONG CARGO

Otago Witness, Issue 2913, 12 January 1910, Page 11

SPONTANEOUS COMBUSTION AMONG CARGO

PAPER BY PROFESSOR PARK

Professor Park gave the members of the Nautical Court set up to inquire into the fire which broke out on board the s.s. Warrimoo the benefit of his observations and study of fires on shipboard by spontaneous combustion. The following is the paper read by Professor Park before the court: —

For a number of years I have devoted much time and consideration to the cause and prevention of fires on ships carrying wool and vegetable fibre as cargo. It is well known that all animal and vegetable fibre is liable to spontaneous heating, that may under certain conditions develop inito spontaneous ignition. I have directed my attention mainly to the larger question of fires on wool ships. I have followed the wool from the sheep's back to the dumping-press—that is, I have assisted in the yarding ami sheding of tho sheap in the shearing and baling of the wool at the shearing shed in three consecutive years, having handled' or assisted in the handling of over 25,000 sheep. I have watched the loading and draying of the wool-bales to the shipping port, and carefully scrutinised the dumping operations at Auckland, Wellington, and Lyttelton. As the result of my observations I have satisfied myself that the cause of the spontaneous ignition is to be found in the dumping process. Perhaps you will allow me to preface my remarks by saying that it has been overwhelmingly proved' by the evidence of shippers, shipmasters, and experts, that ship fires have been caused by spontaneous ignition in woo) carried as cargo. Notwithstanding the mass of practical and professional evidence it had before it, the New Zaland Royal Commission of 1906 frankly admitted that the cause of shipfires was still shrouded in mystery. It was proved by the evidence and recognised' by the commission that the factors of spontaneous ignition are- always present in wool, and perhaps in all vegetable fibres. But the commission, in summarising the evidence, expressed itself unable to discover what was tho trigger that accelerated the heating, and set the process of ignition in motion.

All wool in its natural state contains moisture and a fatty substance termed yolk, which sometimes constitutes as much as 30 per cent, of unwashed wool. In general the fat ranges from 16 to 26 per cent., and tho moisture from 10 to 20 percent. The standard of moisture allowed by the trade is said' to be 15 per cent. Scoured wool also contains a large percentage of fa.t and moisture. It is admitted by all European and

American authorities that the spontaneous heating and subsequent ignition are the

result of the oxidation, of the fat in the presence of moisture. Oxidation can oniV lake place in the presence of oxygen; and the atmosphere is the main source of supply of that gas. Now, the evidence before courts of inquiry has shown, that the seat of the fire was in all cases in the centre of the dumped bales or at the point, where the two packs in a dumped' bale touched ■each other.

To my mind this seemed to indicate that the inciting cause, or trigger, as it was very cntly termed by the commission of 1906, was inte.rne.l. And if internal, it was obvious that it must be an agency introduced into the bale during the durnrnng process. As I have already informed the court, I deliberately set myself the task of discovering the for obviously the cause had to be discovered before, a method of prevention could be devised. While watching the dumpinsr operations at Wellington and elsewhere, I was continually struck with the enormous pressure used to dump thr bales of wool. This prebsure generally ranges from 601 b to 1751 b per square inch on the end of the bale, but is commonly about 1201 b. As a result of considerable experience in the use of air-compressors and compressed air in connection with mining operations, and of the serious results resulting from the thermal effects of compressed air, I think I am warranted in believing that I have at last put my finger on the operative cause whioh leads to spontaneous ignition in dumpad wool. I believe the long-sought-for trigger is the heat generated in compressing the air entangled in the wool and contained in the bales during the dumping process; and in this view I am, I find, supports! by Sir James Hector, Professor Lewes (of the Royal Naval College, Greenwich), and Dr von Schwartz (of Hamburg). ' In his evidence before the Royal Commission of 1906, Sir James Hector, who, it would appear, narrowly escaped the discovery I have now made, said (H. 29; Exhibits, p 1): —" The compressing by dumping must raise the temperature of the bales »& a whole. This, if localised, might rais« the temperature to the incandescent point." Hector further emphasised the fact that ai l- is absolutely necessary for combustion. Professor Lewes, the eminent chemist, F.iid- ''The chemical co/nbination becomes mora and more rapid as the temperature ircreawjs, Until we leach the point at which ignition of the mass takes place."— (PL 29; Exhibits, p 14.) Dr von Schwartz, in 1904. in his report on " Fire and Explosion Risks," said:— " When fatty matter is present the tendency to spontaneous ignition is augmented by a low percentage of moisture, especially when pressure is also applied."—(H. 29; Exhibits, p 32.) The heat of compression due to a pressure of 601 b ner square inch, in accordance with a well-known phvsical law, amounts to 326dcg Fahr.. and of 1201 b to 499.5 deg. The temperature of air adiafcatically compressed, as the air would be in dumping at different pressures up to 4501 b per square inch, starting with free air at 60deg Fahr., are as follows : j J j j

The effect of the added -heat due to compression is twofold. In the first place the beat e&uses ■ m ° r ' e rapid oxidation of the

fatty substances contained .in the wool; and in the second place the compressed oxygen is chemically much more active than oxygen at atmospheric pressure.

I think I shall be able to show that the heat of compression is a factor to bo reckoned with. When air is compressed for mining purposes the final pressure rarely exceeds 751 b or 80lb per square inch. In most cases the working pressure is somewhere about 651 b per square inch, but even, at this comparatively low pressure the heat of compression is so great as to cause a i serious loss of efficiency at the air-driven | machines. In the first place the heat of compression causes a greater pressure against the advancing piston than the natural resistance of the air to compression. There is the resistance due to the effort of the heated air to expand, and this tendency i has to be overcome 'by utilising additional ' power in the compressor engine. I The compression heat also heats the sides > ot the cylinder, thus expanding the free air ' on entry, thereby reducing the weight of free air acted uioon by the cylinder at each stroke. A further loss of efficiency is caused by decrease of pressure that follows the loss "of temperature as the air passes ! from the air-receiver along the air main, ; for, according to a well-known law in. ' physics the pressure varies directly as the absolute temperature, if the volume is con- : Btant. and of course in the air receiver and ! air mains the volume is constant. ! Mr compressed and retaining its heat i is 'said to be compressed adiabatically; and ' with such compression the efficiency is very | low. The aim of the engineer is to keep the air at constant temperature by the I abstraction of the heat as fast as generated i In one type of air compressor the air i cylinder if provided with a water jaofcet, i through which a stream of cold watei I flows: in another type the air is comI pressed bv a column of water; and in yet i another a jet of cold water is discharged ! into the air cylinder. Even when these prei cautions are taken to abstract the heat of i compression, so imperfectly is this done on j account of the relatively low thermal oon- ! ductivity of water that, when the pressure ! required exceeds 651 b or 701 b per square | inch, the air is subjected to what is termed | two-stage compression. That is, the air is j compressed to 251 b or 301 b per square inch I in the low pressure cylinder, then passed i through a cooler, from which it passes to I the high pressure cylinder, in which it is i compressed to the final degree of «jmpresi sion .required. For pressure over 1001 b to | the square inch three-stage compression is ! employed. ~ , , . _ I Notwithstanding the precautions taken to I abstract the heat of compression generated at the comparatively low pressures at which air is used for industrial purposes, many disastrous explosions have taken place in air-compressor cylinders, caused by the valorising and ignition of the lubricating the oil is reaches. thafc t , n .„ • Enough has b**>n saia w> of heat generated during too ecu air is a «««wtaJartMJ pnn . operation ol a wen »j ciole. , . +1 dunming of wool. Coming back to the dumpi S The bale of wool Mi it come _ far H ß I g£ bale?ierSy weigh from & I SSn? They are commonly Bft tain a large amount two, In. the dumping Process on tub sometimes three, bales ar y d in Thi tat o? compression due to . prjj sure of 1201 b per rfraue inch is, as already * . » '. ta amount of the Mr t»»J£i,i„ in the dumped bale mainly entaneled in pockets at the centre of tho SSnal bnles. or at the point of contact b lrT a th weU-known fact that chemical *Jvity is accelerated by heat, and prac S Application of this, fact is in eve ry dav use in many industrial processes. The St generated durino- dummnar bv the compression, of the air entangled in th« W is not a theorv, but a fact based upon an unvarying physical law. _ It i« to this heat of rotrrareasKU) tha* I ascribe the snontaneow'i ignition that has I«d to tho shin-fires that have been "O prevalent in wool shins since hvorau ho dumping we« into use. It ir tha trigger °or incitins- cause that has been so long obscured in mystery. THE REMEDY IS VACUUM DUMPING. So long as wool is dumped by the present process it will always contain the latent elements of spontaneous heating and ignition. I therefore propose, in order to remove the inciting cause, that the dumping shall be performed in a vacuum cylinder or chamber—that is, the hydraulic press, whether single or double, is to be enclosed in a vacuum cylinder provided with an air-tight door. To the vacuum cylinder there is attached a vacuum pump provided with a vacuum gauge. The intention is to displace the air contained in the woolbales before dumping. More particularly described, the operation is to be carried on as hereunder. The bales of wool to be dumped are placed in the press, which stands in the vacuum-cylinder. The door of the vacuum cylinder its closed, and made air-tight, and tho air-pump started. When the vacuum gauge shows a vacuum of 26in. or thereabout, the bales are then dumped to the required degree of compression. The door of the vacuum cylinder is then opened, the binding hoops placed in position, and the dumped bale discharged. It is obvious that when the door of the vacuum chamber i.s opened there will be an inrush of cool, free air. which will slowly permeate the dumped bale, thus adding another factor of safety to the dumping process. Tine chief advantage claimed for tho

vacuum dumping- process, and to which all other advantages are subordinate, is the removal of the inciting cause to spontane-

ous ignition. Where a large amount of dumping is carried on the work can be conducted as expeditiously as at present by) providing duplicate vacuum presses. Thus, while the vacuum is being made in one press, the other can be charged and prepared for the air pumping. The vacuum pump can be placed in the engine-room ] under the eye of the engine-driver. The extra power required tor the vacuum pump : will be compensated by the less power re- ! quired to dump the bales in the presence : of a vacuum. To facilitate pumping two j or more presses can be connected with a vacuum cylinder, in which the vacuum is maintained by ihe air pump. I am satisfied that the adoption of vacuum dumping will remove a source of danger to life and property that is always imminent in woo! ships, and I am further satisfied that the time will come when vacuum dumping will be universally : adopted for all wcol, flax, tow, and all combustible animal and vegetable fibre. J

Pressure, Temp., Pressure, Temp., lb per sq. deg. lb per sq. deg. in. Falir. in. Pahr. 20 109.0 80 390.0 25 .147.0 85 405.0 30 186.0 90 420.0 35 209.5 95 434.5 40 235.5 100 447.5 45 259.5 150 562.0 50 282.0 200 649.3 55 307.0 250 723.7 60 326.0 300 788.2 65 340.5 350 852.6 70 358.5 400 900.0 75 375.0 450 944.0