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Outdated Thinking on Alloy Tanks

9th August 1963, Page 58
9th August 1963
Page 58
Page 59
Page 58, 9th August 1963 — Outdated Thinking on Alloy Tanks
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L. E. DRAISEY ALTHOUGH it can hardly be said that the manufacture of aluminium alloy road tanks is a new trend, it is nevertheless surprising (in my view) how many misconceptions still exist about the uses of alloys in this specialized field, and about the special properties of this material. Certainly, the marked• increase in the number of such tankers coming into service should indicate the reverse; but in talking to operators I repeatedly find this is not so.

Yet, as stated, alloy tanks comprise no upstart newcomer to bulk work, as the first were built in 1932. Their particular properties, which are frequently reported, have largely remained unchanged. Basically they are threefold: light weight, giving less wear and tear when running empty; ability (again because of low weight) legally to accommodate higher payloads; and their resistance to corrosion.

Some of the misconceptions, and even antagonisms, seem to arise because operators do not realize the developments which have taken place in this field, so it is useful to skim briefly over the past three decades of tank building.

The design of earlier tanks generally followed that of vessels constructed in mild steel, but the material used was almost invariably commercial purity aluminium. The smaller tanks were 10 s.w.g. thickness and larger tanks 8 s.w.g. Most were mounted on rigid chassis frames, usually through the medium

of a resilient bearer block, and the tank bulkheads were almost without exception of the conventional dished type.

They were welded by the oxyacetylene process and records show that the greatest danger in this type of construction was for the commercial purity aluminium to fatigue. This was due to the detail design and the method of welding, associated with the internal dished bulkhead. Despite these problems many of the tanks are still in use after 25 or more years of service, having outlived several chassis.

The immediate post-war years saw little change in the design of the alumininum tank and, whilst large numbers of tank vehicles were being purchased, only a small proportion employed aluminium tanks.

It was not, in fact, until the early 1950s that two developments took place which really sped the progress of these tanks. These were new high-strength magnesium alloys, and the method of welding them by the argon arc inert gas process. The tank design was fairly conventional. Some of the earliest were put into service in the Middle East by a subsidiary of British Petroleum, and are operating extremely successfully.

From this time on construction in ordinary purity aluminium became less favoured and increased use was made of aluminium alloys, which gave tensile strengths that compare with mild steel. Armed with this new material and improved welding techniques, the bodybuilding industry focused its attention on the development of an articulated, frameless tanker.

It was felt that the tendency to fatigue in areas of weld and stress concentration was due in great part to a combination of torsion and local stresses in the chassis frame. Although resilient mountings were often employed, it could not be said with certainty how effective this form of damping was in reducing stress levels due to the inter-relationship between tank and chassis.

The frameless shell itself possessed amazingly high strength and, in order to transfer dynamic loads over as large an area of the tank wall as possible, a suitable bulkhead had to be built into the shell. Thus it was that manufacturers carried out considerable development work on various types of bulkhead and, in many cases, the dished bulkhead gave way to a flat member with local reinforcement.

Another form of bulkhead, employed by Armstrong Whitworth Equipment Ltd., was the corrugated one. This is pressed from a flat plate to form vertical corrugations.

The initial cost of an aluminium alloy tank body is probably 25/30 per cent higher than a tank of equivalent size in mild steel. How, therefore, is this outlay recovered? A 24-ton-gross unit will carry as much as 400 gallons more of product than the mild steel tank. Thus, in approximately 10 journeys, the equivalent of a.full tank load extra will have been carried for no increase in running cost_

It can be seen that the higher initial capital outlay can be quickly recovered. It is not unreasonable to assume (dependent upon the mileage covered) that the whole of the initial extra cost can be offset in a period of between one and two years.

e difference in payload will, of course, vary with the let carried. This, and a variation of running costs by tors, means that as an example it is necessary to take a il case, although the figures below are based on fact.

e haulier in question was carrying 4,000 gal. of produce ling 8 lb. per gal. on a journey of 80 miles daily in a mild tank. This was replaced by an alloy tank weighing ximately I ton less, which meant that 300 gal. more could rried on each journey.

e operator's running costs wcrc based on 0.5d. per gal. over cage of 80 per day on 11 journeys a week, and the followicts therefore apply: tra payload carried weekly=300 x 11 ... 3,000 gal.

.tekly saving at 0.5d. per gal. ... £6 17 6d.

ring per annum =6 17s. 6d. x 52 £357 10s. Od.

ring on licensing per annum by reducing inladen weight ... £20 Os. Od.

tat saving per year . . £377 10s. Od. It was found that the alloy tank initially cost approximately 30 per cent more than the steel tank with suitable lining, which represented a basic cost difference of approximately £500. It can be seen that the difference in the higher capital cost would be completely recovered in approximately 16 months, which is a very small proportion of the tank life.

Road tank manufacturers have so drastically reduced the weight of their equipment by the use of aluminium alloy that it is reasonable to assume they may have reached the ultimate in lightweight construction. It seems, therefore, that further reductions in the unladen weight of tanker vehicles must mainly be achieved on the motive units, running gear and ancillary equipment. To this end, many chassis manufacturers have realized the necessity for producing lightweight equipment for use with this special type of vehicle.

There is a marked tendency by tank manufacturers to discard the use of a chassis frame, not only on articulated vehicles but on rigid vehicles too. The rigid frameless vehicle has been exhibited in this country but, with close liaison by tank and chassis manufacturers, may soon be commonplace on British roads.

New materials and techniques have enabled the designers to usc a scientific approach to their design. The extra payloads they offer, their extreme versatility and resistance to corrosion will, it seems, guarantee them a permanent place in the field of bulk liquid transport.

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