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TANKERS.
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Page 88
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OR EVERY
LIQUIE OAD
By John S. Thorn,
F.R.S.A., A.I.R.T.E.
OST manufacturers of tanks for road vehicles are like tailors In who can supply ready-made articles, but cater largely for customers' individual preferences. Although there is at present a preponderance of work "to order," there is a trend towards the production of a general-purpose or allproducts tank by leading manufacturers, with the object of making something capable of carrying the maximum number of different commodities.
In many instances where the carriage of an unusual liquid is to be undertaken, the .tank maker must draw on the prospective customer's experience of that particular liquid and his knowledge of the provisions necessary to overcome any deleterious effect it may produce.
The transport of liquids in bulk is expanding steadily, but there are pitfalls for the unwary, as was testified by a leading operator (The Commercial Motor, April 13, 1956) when he related how the carriage of caustic soda in a mildsteel tank, whilst normally safe, had resulted in the light-alloy outlet valves disintegrating because their susceptibility to the liquor had been overlooked. Another point which should be borne in mind, is that the stainless-steel tanks used for carrying edible loads, such as milk, cannot always be employed for other liquids.
Look Before You Leap
There are over 30 different grades of stainless steel and a steel which may be eminently suitable for milk can suffer severe pitting corrosion by fruit juices containing sulphur-dioxide preservatives, for instance, if the necessary resistance is not incorporated originally. It is wise for operators to check with the tank manufacturer before embarking on the movement of a load new to them, to ensure that the tank material will not deteriorate through corrosive action.
Most tanks are provided with combined vacuum and pressure-relief valves to prevent distortion or buckling, but this appears to have been forgotten in a number of cases where tanks have buckled inward through tampering with the valves and/or closing up the manholes and so on, before the steam used for cleaning has cooled .down.
Because of the numerous tank materials D14 and linings necessary to cover a wide variety of loads, semi-trailer operation offers obvious advantages, but there are signs that road-rail interworking is growing, with the use of_ demountable tanks supported in their own sub-frame which can be secured to a platform body.
By far the greater proportion of bulk liquid transport is devoted to the movement of petroleum products, but many raw materials for industry and the food trade are handled in thisway. Although most loads can be carried at normal atmospheric pressure and temperature, there arc occasions when special provisions have to be made to avoid spoilage and evaporation losses.
Milk, for example, is best carried at about 34° F., and if loaded at this temperature it must be prevented from warming up appreciably by the use of suitable insulation. Insulation is also necessary where liquid oxygen or nitrogen is to be carried, because this cannot be sealed without giving rise to extremely high pressures, consequently, the transmission of heat to the load must be avoided as far as possible, if excessive evaporation is to be prevented. This naturally applies in some measure to other gases when carried in liquid form.
At the other end of the scale there are loads, such as bitumen and heavy coaltar fuels, which must be maintained at a temperature of 250° F'.-300' F. by employing lagging and providing steam or hot-oil coils for use at transit terminiif the desired fluidity is to be achieved.
Another load within this category is phenol, which must be kept above 105° F. to avoid solidification. Chocolate, now often carried fluid in bulk, is sensitive to temperature, for heat over 140° F. could result in burning, or a temperature below 90° F. in solidification, which is equally undesirable.
There are other loads which need to be insulated to prevent the transference of heat, but those I have quoted should . give some indication of the factors which must be taken into account_ • Some liquids need to be protected by pressurization in transit, two examples being certain types of beer and
acetaldehyde. Beer is usually _ trans_ ported under a pressure of up to 40 p.s.i. to prevent frothing and maintain aeration, whereas acetaldehyde is carried at a. pressure of 50 p.s.i. with a nitrogen blanket over the liquid to prevent the formation of an explosive mixture.
Yet another example of an awkward commodity to deal with is high-test peroxide, which forms part of some rocket fuels. This must be carried in an aluminium tank of 99.8 per cent. purity
• and completely devoidof ferrous Particles. A minute .quantity of such material is sufficient to disturb the chemical stability of the load and initiate a reaction resulting in evaporation and loss.
Expensive metals for tanks, and the nickel, tin, zinc, rubber and similar protective films hitherto used, are gradually being replaced by plastics preparations which have a wider coverage and are cheaper both to purchase and apply. One such coating is Epikote, a resin
' amine cure of white appearance which can be applied by spraying. At least Iwo of the larger manufacturers of tanks are installing or have installed equipment for carrying out this -process..
An added advantage of this material, apart from its toughness and resistance to abrasion, is that it can be used in paste form to fill-in the re-entrants formed between baffles or divisions and the inner periphery of the tank where case of cleaning and hygiene are prime considerations.
Elliptical tanks may be used only for pump, suction or gravity discharge, because the section offers little resistane
to pressure across the minor axis of the ellipse. From this it follows that the round tank is essential for the carriage of 'liquids under pressure or where gas pressure discharge is used.., Where the method of load discharge has no influence on the choice of section, the elliptical tank is preferable because it affords a lower centre of gravity..
Cylindrical tanks can be Mounted at an angle to obtain a centre of gravity height •comparable with that of an elliptical tank, but there are other yea: sons for this inclination. If the excellent hoop stress distribution of the circular section is to he enjoyed to the full, weldedon sumps for drainage are best avoided and some degree of slope is required to achieve the same result. Another reason for adopting the cylindrical tank, apart from the pressure factor, lies in semitrailer work, where pressure discharge is much more necessary because of mechanical complications with pumps.
If the turntable of the coupling hitch is relatively narrow the necessary lateral stability must be imparted by lowering the back end of the tank as much as possible, and it is much easier to get the cylindrical tank down between the wheels of the rear axle or bogie.
Elliptical tanks as used in semi-trailer work are mostly itepped to compensate for the height of the coupling hitch and provide a level tank line. With this type of construction, drainage sumps are the order of the day, but their incorporation does not materially affect the strength of the tank section, as they are normally welded on what is the weakest part. About 90 per cent, of semi-trailer tanks are frameless, whereas only about 10 per cent. of drawbar trailers are constructed in this way.
A drawback of the stepped-elliptical frameless semi-trailer tank is that the location of the support cradles for the running gear is somewhat critical if undesirable stress concentrations arc to be avoided. This is not so with the frameless round semi-trailer tank, where the running gear and cradles can be mounted almost anywhere from a strength consideration, the principal factor determining the position of the rear axle or axles being their loading limit.
Round Tank Stronger
Another point in favour of the round tank is that it lends itself to frameless use more readily than its elliptical counterpart, because it normally has a section modulus far greater than that of any sub-frame that could be provided to support it.
If a wide turntable is used for semitrailer operation, to impart lateral stability in conjunction with a level stepped-elliptical tank or a cylindrical tank, severe torsional stresses cart be imposed on the, tank. shell. The method of overcoming this adopted by Carrimore and other concerns, is to employ a gimbal-type turntable. This has, in addition to its normal transverse pivot, a longitudinal pivot to prevent. the transmission of cross-racking stresses. However, to retain the stability of the . design, this movement is limited by stops.
Methods of mounting for rigid vehicles, semi-trailers and framed trailers vary somewhat from manufacturer to manufacturer and are influenced by the preferences of individual customers. Nevertheless, there is an underlying similarity of approach. The usual practice with frameless semi-trailers is to weld cradles to the tank at predetermined points and bolt coupling gear, landing gear and rear. axle or -axles to them in ' a convendonal manner.
When a tank of this type is insulated, however: there is some divergence of opinion as to whether the running gear should be insulated from the tank shell because of thermal losses by radiation.
Carrimore, for example, favour the use of angle members welded to the tank shell and protruding just beyond the level of the insulant and outer skin, so that the running gear can be bolted to them, with interposed insulating washers or packing pieces.
Welded Cradle
• Other manufacturers, such as W. P. Butterfield, Ltd.,' normally regard this as an unnecessary complication and employ a straightforward welded cradle and bolted running gear, with the insulant packed close to the cradles at their points of junction with the tank shell. _
A method of bolting running gear to semi-trailers and avoiding heat losses used by Thompson Bros, (11ilston), Ltd.,, is to fabricate double channel members in the form of a box section and secure them at strategic points on the tank shell. Captive nuts are provided on the inner face of the box section, so that the mounting cradles may be bolted to them through the outer skin, which is separated from the support face by asbestos or similar packing. A normal insulant is used between the shell and outer skin of the tank, whilst excessive heat transference through the box section is prevented by insulant which is trapped within.
System for Rigids ,
On rigid vehicles, the strap and cradle are widely used. One of the principal supporters of this layout is the Steel Barrel Co., Ltd. With this system it is, of course, necessary for the straps to be placed at points where the tank is supported by internal divisions and this does impose some limitations on the positions of cradles and support feet.
With some uninsulated tanks, hard rubber is inserted between the cradle and underside of the tank and between the mounting feet and chassis frame. The use of rubber for the cradle is, however, largely a matter of customers' preference and is not regarded as essential by certain manufacturers.
An alternative material is flexible packing, such as Balata, which could also be employed beneath the mounting feet
D18
to avoid metal-to-metal contact and allow the provision of dimples to clear chassis-frame rivet-heads and so on, and to compensate for slight dimensional variations.
If a tank is insulated, rubber on the supporting arch of the cradle is considered more necessary to avoid the tank resting on only one or two points in each cradle. Wooden hoops are usually placed round the tank between the shell and outer skin at strapping points, providing a physical separator and preventing distortion. It should be appreciated, however, that the use of wood imposes a restriction on the upper limit of load temperature.
Even with the rigid vehicle there are concerns, such as W. P. Butterfield, Ltd., who prefer welded-on cradles with longitudinal stiffening webs and integral feet, on the ground that the welds ensure complete support of the tank. In some cases, to provide a certain amount of resilience and to absorb chassis movement, the mounting feet are attached to
the cradles through bonded rubber-metal bushes, there being the usual packing between the feet and the chassis-frame side members.
The use of rubber-metal bushes between the mounting feet and cradles, especially in the case of aluminium tanks, is favoured also by the A.P.V. Co., Ltd., who use this system for a large proportion of their tanks for road vehicles. The cradles are secured by strapping, bolting or welding.
The usual method of securing mounting feet to the frame side members of rigid vehicles is by U-bolts, but the methods of supporting the channel-seelion flanges differ Slightly. Some makers prefer to use hardwood packing and others corrugated steel sheet on edge.
Some manufacturers believe that the upper and lower flanges of the chassis side members should be allowed to move relative to one another if the frame is to perform its function to the full. In consequence, they adopt the technique of bolting angle brackets to the vertical face of the side members to form a 'platform with the side-member upper flange. To this the mounting feet are bolted with interposing hard rubber,or other packing material. Completely flexible mountings for rigid vehicles and framed trailer work are used, and an example of this is the Metacone unit produced by Metalastik, Ltd.
This incorporates a rubber cone in shear and compression, supported in a housing on the chassis-frame side members, with a central" bush bolted to the underside of a bracket welded to the tank. The lower end of the bolt normally carries a metal-backed rubber washer to limit movement on rebound by contacting part of the housing.
A method of mounting with virtually universal action, which also includes Metalastik bushes, comprises two bushes at the sear earricd in trunnions and pivoted transversely, and a longitudinally pivoted front mounting bush supported in a transverse member also carried . in rubber bushes to . afford a universal action.
Whilst eminently suitable for off-theroad work or unmade roads, this layout tends to be costly and is usually restricted to tanks of under 2,000-gal. capacity if high unit loading of the mountings is to be avoided.
With rigid vehicles it is preferable to arrange the tank mountings over the spring hanger brackets. Forward mountings should be located as close to the cab as possible.
This arrangement reduees the load on the mounting itself, and the bending moment acting on the frame side members between the mounting and the front-spring rear hanger bracket by shortening the distance between them. These comments apply to fourand six-wheelers rather than eight-wheelers, where the tank front mounting usually lies between the-hanger brackets of the springs of the second front axle.
One vehicle manufacturer believes that if the foregoing requirements have to be sacrificed in favour of satisfactory weight distribution, a mounting with adequate flexibility is essential. If this course cannot be adopted it may be -necessary to increase the size of the front tyres to afford the required carrying capacity, although this does away with the interchangeability between front and rear tyres and may lead to heavy steering.