road and workshop
Page 37
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by Handyman
Benchwise: repair risks (1)
THE MISTAKEN BELIEF that these days more liberties can be taken with the repair of fuel tanks seems to be on the increase. It's a dangerous view and one, I think, to some extent promoted by the more common use of diesel engines for road transport. The consequence has been a slackening in the precautions that must always be taken in order to avoid burns and other accidents that can follow any careless application of heat to certain equipment—not only fuel tanks.
In fact you cannot be too careful. An example of one case where great danger can occur is unskilled labour extracting usable spares with a gas cutter from vehicles in a scrap yard. A petrol tank may have been empty for years and yet gases can be set off by the approach of a gas flame which will cause a lethal explosion.
I know of a case where a First World War tank (armoured vehicle) was being cut up and removed from the pedestal on which it had stood since the early 1920s. The contractor had, of course, examined the vehicle before setting his gang to work; the large petrol tank was bone dry and there was no trace of smell and he looked to see whether there might be a live cartridge, fuse, grenade, etc, tucked away in some odd corner.
On the face of things it seemed that the vehicle, after standing out in the open in all weathers for so many years, would be quite safe to cut up. The gas-cutting operation then went ahead for some days. But there came a time when a man was either working on the fuel tank or very near it, when a violent explosion occurred. There was still sufficient heavy metal around to shield most of the flying debris, but nevertheless the man lost an eye and his forearm.
The lesson The lesson to be learned is that if a fuel tank has once held inflammable liquid, then time standing empty means nothing: enough gases may be retained in the pores and crevices of the metal to mix with the air content and form mixture which even the first heat, yet alone spark or open flame, can explode.
Therefore I emphasize that heat, friction or an open flame is dangerous where any container of oils, liquid fats, petrol or fuel oil is concerned. And to this list must be added other forms of cylinder or container—in fact the most unlikely seeming items on a vehicle, or garage equipment can prove lethal when flame is played in or around them and gas forms.
I will quote two examples from experience, the first being one where I agree it could be considered a possible risk, namely a 22 cu ft garage air receiver which needed a drainage connection refitting after insurance inspection.
Vents opened To make sure that it was risk free, all pipes and other vents were opened up and a high-pressure flow of air was passed through for 30 minutes, after which it was sluiced out with cold water and again blown dry. It was then considered safe and the faulty drain plug area was drilled out, countersunk and a new screw seating positioned, needing only a run of weld to complete.
Midway through the welding operation, there was a roar rather than a bang, the vessel jumped and flame shot from the three vents to a distance of over 12ft, lasting two to three seconds, enough time to set fire to the operator's overalls and singe the arms and eyebrows of the man steadying the vessel. What had occurred was that an air receiver had, through the years of operation, collected an inside skin of grease from the pump, and whatever else passed the input filter. This coating had been left undisturbed by either air blasting or cold water, needing only the heat of welding to loosen it, change a portion of it to a gas, which ignited, and in turn warmed and released the rest of the layer or coating. The long flame which resulted was in a way the saving factor in this case, being the reason for there being no instantaneous and heavier blast.
The other incident took all of us by surprise. It occurred during the Second World War in the heyday of vac-servo brakes when quite large cast alloy cylinders were used, carrying a leather seal for the piston. One of these cylinders had been bruised in an accident and in view of the shortages at this time a serious attempt was made to recover this valuable cylinder, which also carried the valve gear and relay levers, to bring it back into line.
A dummy piston was turned to cylinder size, and with the aid of a mallet this was driven steadily into the barrel towards the sealed end (except for a fin, plug hole) and as flame was moved over the crushed-in area, the dummy piston gradually brought the lower places back to size. This process was going well when there was a sudden crack, a little flame, and the piston struck the operator in the face, damaging his jaw.