First signs of intermodal transport are generally traced back to experiments by Malcolm McLean (at that time working for the McLean trucking company and later SeaLand) in 1956. The first experiments involved loading some 35ft road trailers onto a vessel rather than unloading the contents of the trailer and loading these on the vessel by traditional cargo sling methods.

McLean’s experiment quickly showed that there was no need to take the wheels along and that more efficient stowage and handling methods were available if only the box part of the trailer was hauled aboard ship. This led to the separate container and chassis units, and to the standards for container size and handling equipment that remain in use to this day.

An intermodal portable tank consists of a single, cylindrical vessel (the tank body) within a rectangular steel framework. The frame is built according to International Standards Organisation (ISO) specifications. The most common frame sizes range from 20ft to 30ft. The typical container height is 8 ft and 6 inches, and standard width is 8 ft. The frame design must meet strict standards, including stacking, longitudinal and lateral inertia. The tank design, which varies according to the commodity and customer’s specification, must also meet specified requirements. These standards are determined by a number of national and international government agencies. The weight of an empty tank container generally varies between 3000 and 5000 kg.

The tank body is usually built from stainless steel, carbon steel or aluminium and has one or more loading openings at the top (manholes), and usually one central discharge opening at the bottom. Safety valves avoid excessive pressure build up inside the tank shell, whilst triple closures (a combination of a foot valve, a product valve and a drip cap) avoid undesirable leakages of the loaded product during transit. Tank containers are usually discharged using ordinary air pressure (average pressure of 2 bar) to push out the loaded product. Products with high flashpoints are discharged using a pump, whilst vapour return lines are used to avoid any air contamination during the loading and discharge process. Many tanks are insulated by a variety of materials to maintain the temperature of the carried product.

Tests

International government agencies like Lloyds, Bureau Veritas, ABS London and Apragaz cover the manufacture, inspection and maintenance of tank containers, by specifying strict regulations.  Tank containers are subjected to a number of tests relating to the structure and other elements. Some examples of these tests are:

-Stacking test
-Lifting by the top and bottom corner fittings
-Impact test
-Pressure test
-Air test
-Water test
-Inspection of all welding

Each tank is initially tested immediately after construction. The tank is then supplied with an initial construction certificate and a printed calibration chart for the full contents of the tank. Official tests are repeated on a regular basis, depending on the type of tank and its length of service, to ensure maximum safety of the tank container. The air test for example, to see whether the tank is still completely airtight, is repeated every two and a half years. However, quality tank container operators, including Huktra, conduct these tests more regularly, to increase safety even further.  After each test the construction certificate is renewed in the form of an approval certificate.

Generally, a tank container consists of two main sections: the frame and the cylindrical barrel made of stainless steel. However, each tank also has several additional features. The features vary from one tank container to another, below we give you some examples of features.

On top of the tank container, you can find a walkway and collapsible safety handrails. To get on top of the tank, there is also a ladder at the back of the container. Many tank containers also have a dipstick in stainless steel and a properly engraved calibration chart. The chart enables you to calculate the volume of the product to be loaded, expressed in liters. The dipstick gives a visual check on the actual volume that has been loaded into the tank container.

Two other standard features are the document-tube and the labelholder. The document-tube should be watertight and of sufficient size to contain all the necessary load documentation. It should be located in a protected and secured area to avoid document theft during transit, a recognised problem within the industry. The labelholder is secured to the tankshell, and displays the safety labels required to identify the tank contents. Well-equipped tank containers have several labelholders.  More information on dangerous goods safety marks can be found here.

The tank container also has an identification plate or certificate plate, with general information on technicalities and origin of the tank.

Intermodal tank containers always carry a registration number that identifies the owner and the specific unit. This registration number consist of four letters, the last of which is always ‘U’, denoting a container. Six numeric digits follow the four letters, specifying the tank number. A seventh digit is normally added as a check digit.

For example: The tank container on the left has registration number HUKU 352271-7, stating it is a container (fourth letter is a ‘U’) owned by Huktra NV (HUK is the registration code), with registration number 352271. The check digit is 7.

Naturally, the livery of a tank container also helps to identify the owner. Usually the frame is painted with high chemical resistant paint, in one of the colours of the owner, and logos are placed on the different sides of the tank.