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DURING my many years with Scammell Lorries Ltd. of Watford,
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the interest of the staff in their job was vocational. We specialized in specials and variety conditioned our lives.
We pioneered the production and application of articulated vehicles in this country and we consider that we were the leaders if not the pioneers on the production of large-capacity bulk liquid vehicles. Today bulk tankers are playing a major role in the economic growth of the country and this article highlights some of the milestones in the development of bulk liquid transport over the years. It also indicates some of the more important types of development that could influence future design.
The first road tanker produced by Scammell was built in 1920. This was a six-wheel articulated vehicle with a stepped rectangularsection tank having wooden bearers and mounted rigidly on a drop-frame. It was designed for carrying 2,200gal of heavy fuel oil and was fitted with a Kelvin single-cylinder engine driving a Fueherd pump for loading and discharging the oil, the viscosity of the liquid being reduced by the heat from the exhaust gases of the tractor engine which were piped through the tank. This early vehicle is depicted in illustration No. 1.
The rectangular-section tank was fabricated from flat mild-steel sheet with edge-welded corner seams, which fractured and were rewelded with unfailing regularity after every few journeys. To overcome this weakness, tanks of stepped cylindrical construction (illustration No. 2) were employed in place of rectangular tanks and experiments were made with other types of tank of differing section mounted on straight frames (illustration No. 3).
With the stepped elliptioel tanks weld failures still occurred in areas where there were abrupt changes of section, so subsequent dropframe vehicles had two independent tanks.
In 1926 Scammell filed the first patent for a frameless tank vehicle and this was the basic prototype of the modern frameless tanker attic used for the transport of liquids, powders, granular and flocculent solids and gases.
When first introduced the frameless tanker met sales resistance from customers who considered that frame-mounted tanks had greater strength. To dispel such fears simple bending-moment diagrams were produced that proved that the maximum bending stress was well below the stress resulting from the discharge pressure—of say about 30 p.s.i.—applied to the tank.
The articulated frameless tanker gradually became accepted. Perhaps its greatest asset was that the tank portion could be (and was) designed specifically to suit the particular liquid to be carried and the customer's re quirements without being limited by chassis considerations. Furthermore the design permitted a lower centre of gravity, a saving in weight and an increase in payload.
Scammell, being the only British manufacturer of articulated vehicles, thereby amassed a considerable amount of practical experience and became the accepted consultants on hulk-liquid transport problems to the extent that during one period nearly 80 per cent of the company's home market sales were for tanker vehicles.
Mild-steel tanks can be corroded by, or can contaminate, the liquid carried, in which case • other metals or special linings have to be used. These include rubber and lead linings (for acids) and electrically deposited or molten sprayed tin, zinc, cadmium, copper or glass lining for beer, fruit juices or other edible liquids.
At that time beer was occasionally carried in copper tanks and milk in aluminium tanks. Tanks made from these materials, however, having little inherent strength were inserted into a mild-steel frameless cylinder and held down internally on longitudinal timber bearers by securing straps, the annular space being filled with granulated cork for heat insulation. Illustration No. 6 is of such a tank, which was supplied in 1928. This is also of interest as it depicts the tank semi-trailer mounted on an axle having four wheels transversely in line, a design introduced by Scammell.
To fuse the glass lining of a compartmented tank satisfactorily, special construction features are necessary, as is shown in illustration No. 7. The main objection to glass lining is that of excessive weight as the firing tern
perature of 1,600 /1,700deg C requires a tank
shell having a thickness of to prevent distortion.. Also the lining is easily damaged by, for example, a tool dropped through the manhole.
Synthetic resins are an effective and economical means of lining a tank, a lining with the normal thickness of 0.004in. having a weight of approximately 1/20 oz /sq.ft. The resins can be formulated to suit the load, are easily applied and are generally satisfactory, although in the early days the linings tended to flake or craze in the areas of maximum stress.
The world's first articulated vehicle with a frameless tank constructed from aluminium was produced by Scammell during 1931, (illustration No. 8), for carrying kerosene and petrol. It had a capacity of 3,000gal with six compartments and was used in this form when carrying kerosene, but as the Petroleum Conveyance Regulations in force at that time prohibited the use of tankers having a nominal capacity of more than 2,500gal from carrying petrol (even though the load did not exceed this) the rear 500gal compartment was made detachable for this duty. In pioneering such a project special construction techniques were employed and care was taken to prevent electrolytic corrosion by insulating all ferrous parts, such as running gear, turntable brake and other fittings which were attached to the tank.
Stainless steel—ideal
Stainless steel is now rapidly replacing mild steel for tank construction. It is an ideal material from the physical and chemical point of view. Various surface finishes and grades are available, most of which have a mechanical strength superior to that of mild steel, thus allowing plate thickness to be reduced with a corresponding improvement in the payloadto-unladen-weight ratio. Stainless steel is, however, an expensive material, and to reduce the cost of the thick plate required for tanks subject to high internal pressure a bimetal material consisting of a mild-steel base with a stainless-steel skin has been used. This requires special care during fabrication.
Lagging the exterior of a tank is a wellknown method of reducing the loss, or absorption, of heat when transporting a load. Many types of insulating material are available, such as glass silk, slag wool, cellulose acetate film, asbestos fibre, aluminium foil, expanded rubber and cork and more recently a range of plastics, the material selected and its thickness being dependent on the duty. Early experience proved that a bitumastic-skinned material in non-crushable form had an advantage as an insulating material because loaders occasionally stood on the tank despite the availability of walkways. Soft, compressible materials permitted the outer cladding, usually a thin gauge aluminium sheet, to distort, which allowed rain or water from washing hoses to penetrate the insulation. Depending on the insulating material used, the absorbed moisture increased the unladen weight or reduced the insulating efficiency.
For these reasons slab cork was greatly favoured for insulating chilled loads or loads restricted to an upper temperature that did not exceed 110deg C. Above this temperature magnesia or asbestos slabs were used.
In a comparatively new method of insulation, the outer cladding is separated from the tank shell by post spacers, and polyurethane foam is injected into the annular space. When it sets, the foam bonds to the cladding and tank skin and provides an excellent waterproof insulation with a good thermal efficiency suit able for refrigerated loads or hot loads not exceeding 210deg F.
Even the most efficient insulation cannot prevent some passage of heat either into or from the tank. Normally the former is easier to control (except in the case of liquefied gases) because of the reduced temperature gradient compared with the gradient when a hot load is being carried.
Heating methods
Various methods of applying heat to the load or tank have been used and include systems based on steam-heater coils submerged in the liquid or attached to, or in contact with, the outside of the tank shell. Some liquids which are loaded at high temperatures are permitted to lose heat on the journey provided that the temperature can be raised at the terminal before discharging. Submerged steam-heater tubes fitted longitudinally in the lower half of the tank shell having 50 sq.ft. of heating area at a steam pressure of 150 psi. are adequate for most operating conditions.
Alternatively, electric immersion heaters or electric skin heaters, controlled by an adjustable thermostat, can be used, which are wired to accept both a.c. or d.c. current of varying voltages when the system is plugged into an outside supply. Should it be necessary to carry the liquid at a constant temperature, or if the temperature has to be increased during the journey, electric power from a 15kW electric generator driven by the veh ice's propulsion engine is fed to the heaters while it is in motion on the road.
A Propane-gas-fired furnace can be built into a tank for heating the load when an intense heat with a rapid rise in temperature is required. Illustration No. 9 shows such a furnace installed in a heavy bitumen tank.
Filling and discharge
Tanks can be filled by gravity, counter-pressure or by vacuum, and discharged by gravity, pump or internal air pressure. The majority are filled through an open manhole or by a pipe coupled to a top-filling connection, these together with the discharge fittings having to conform to Home Office regulations if the load has a flash point of 73deg F or less, and to the Customs and Excise requirements if conveying dutiable liquids.
Pressure filling with the use of air or an inert gas in a counter-pressure system can be applied to liquids such as carbonated beer or highly inflammable liquids such as carbon hisulphide. The system is coupled to the discharge valve of the tank or via an internal syphon pipe, the displaced air or gas passing through a sight glass and pipe connection on the top of the tank. A tank designed for filling under vacuum is constructed with circumferential stiffening ribs, which enables it to be exhausted down to 15in. of mercury.
When a tank is to be used solely to transport a particular load at a reasonably constant temperature and viscosity, a pump of an appropriate size and running at an appropriate constant speed to avoid cavitation is probably the most suitable means of discharging the load. But when the liquid is corrosive, has a high temperature or has a viscosity that is liable to vary considerably, air-pressure discharge has many advantages. It is usually quicker and it reduces the risk of contamination. Being compressible, air is selfadjusting to discharge restrictions of head and viscosity, and the air compressed in the tank follows the end of the load and scours clean the delivery hose and rising main. The disadvantage of air-pressure discharge is that it is practicable only when applied to cylindrical-section tanks, and as there is a growing demand for vehicles to be more streamlined and of reduced height (which necessitates the use of stepped tanks of elliptical or non-circular section) discharge by air pressure is diminishing.
It was this demand for a streamlined low height aspect as applied to a 3,600gal petrol tanker (the maximum capacity permitted at that time) that prompted Scammell to investigate the design of a frameless tank of stepped elliptical construction. Illustration No. 10 shows the first of this type produced in 1943, since which date the basic design has been developed and is now, with variation, the most commonly used for bulk-liquid transport vehicles.
Plastics
Operators are constantly calling for a better payload-to-unladen-weight ratio. While aluminium is currently the lightest material in general use for tank construction it is not suitable for all liquids or duties and reinforced plastics are now being considered as an alternative material. The word plastics (used in this sense) is a genetic term covering a vast range of resin formulations which can be supplied with or without various chemical fillers or additives to suit specified duties and it is this "formulation latitude" that opens up an interesting prospect for the future. Tanks have been made from sheets of glass-fibre reinforced material with bonded joints following conventional fabrication practice. In 1963/1964, Scammell arranged for the production of a glass-reinforced plastics tank using an entirely new and revolutionary construction process. Basically this consisted of winding resin-coated glass fibres round a collapsible mandrel or former and of building up the layers diagonally until the tank shell was of the required thickness. This tank had to carry an edible liquid, so the resins for the internal skin of glass fibres were formulated not only to be inert to attack by this liquid but also to provide a high gloss finish for hygienic purposes. The liquid had to be transported hot and insulation was required to reduce loss of heat during the journey. But instead of this being applied to the outside of the tank, as is the usual practice, the glass fibres forming the middle of the tank skin were laid on at an increased pitch or angle and so formed a thickness of many layers of air -trapped honeycomb cells sandwiched between the inner and outer skins. Furthermore, as the load had a deleterious effect on paint, the resins for the final layer were impregnated with a pigment of the operator's fleet colours. This overcame the need for continual repainting.
At present practically any liquid can be carried in road vehicles. It can be refrigerated or heated while it is being transported and it can be loaded and discharged by equipment carried on the vehicle, which is operated mechanically, hydraulically or electrically. In the last-named case the system can be plugged into an outside supply if quietness of operation, for example in night deliveries, renders the running of the vehicle's propulsion engine undesirable. Loads can be carried hot —some oxidized bitumens being loaded at 410deg F—or cold: the 34,000 cu.ft. of oxygen, carried in liquid form in the tank, depicted in illustration No. 11, is loaded at -297deg F.
High pressures are no great problem. Petroleum gases are carried at a comparatively low pressure, typically at about 180 p.s.i. compared with 3,000 psi. when hydrogen is carried in the vehicle (illustration No. 12). In this case multiple small diameter tanks are used which enable the stress in the skin of the tank to be reduced to manageable proportions.
Possibly on the ground of public safety increased capacity may not be permitted for tankers carrying certain chemicals or inflammable liquids; also some of the provisions contained in the recently published -European Agreement Concerning the International Carriage of Dangerous Goods by Road" (ADR) may be incorporated into British controls such as the Petroleum Conveyance Regulations and so affect design; but whatever the pattern, bulk transportation will assuredly increase.