Inspecta gives Sweden’s largest oil tank a health check-up

It is hard to check if something is damaged if you can’t see that damage. A good example of this is the largest tank of crude oil at Preem Lysekil, one of Europe 's most modern refineries, with an annual capacity of 11.4 million tonnes. The tank has a capacity of 60 000 cubic meters of steel, where the bottom and part of the external skin is covered with epoxy paint, making it difficult to quickly investigate or notice any internal and external corrosion damage.

Preem tank Inspecta is the only company in Sweden license to use SLOFEC ™, (a German scanning system) based on eddy currents in combination with induced magnetism. The method is fast and reliable - when Preem hired Inspecta to check the tank floor in Lysekil, we checked 3 000 m2 within forty hours, the equivalent to a mile-long walk.

- Speed ​​is one of the benefits of this system. We can reach up to 20-30 m per minute depending on parameters such as heating coils, welded joints and sectional barriers. The scanner, with its eight sensors, can handle material thicknesses of up to 35 mm and any paint or rubber coating of at least 2 mm and, in favourable cases, considerably more than 5 mm, explains Johan Forster, one of Inspecta’s seven highly-experienced SLOFEC operators.

In Lysekil, the tank’s base consists of 8 mm carbon steel, increasing to 14 mm near the outer skin. The storage tank’s diameter is 62 meters.

- We prepared for the job by using computer software to draw up a plan of the area to be examined. Another important part of our work is to calibrate the scanner and set the signal from the sensors using a reference sheet with premade errors, such as leakage damage of thicknesses of 25%, 50 % etc. A crack across the plate activates all sensors.

Information in real time

An important advantage of SLOFEC is that the sensor signal is analysed in real time. One can immediately see on the computer screen where any damage is and how extreme it is in the form of markers using six different colours.

Johan Forster, Inspecta
Johan Forster, Inspecta
- The signals travel in a certain phase up and down the material and the scanner has a built-in decoder that measures the length of each scan. As a result, we can see the location of any damage very quickly in relation to the scan. If a problem is indicated, we verify it with a thickness gauge, says Johan.

Reports are printed on paper and each sheet can be presented in a pdf file.

Easier to find corrosion damage 

The overview image shows trends, such as the location of where water run-off has occurred and where any damage is located. It makes it easier to find the causes of any corrosion.

- The only thing that is a bit tricky is to get all the way over the skin due to the radius of curvature. That ‘dead zone’ is reduced by the fact that the scanner's handle can be rotated, but the smaller the tank, the more difficult it is to access a zone of 10-40 mm at the wall. In addition, the thermal impact of the joints makes the signal difficult to interpret, so we always carry out a manual measurement of any critical zone.

Preem’s tank was found to be in good condition. But Johan has previously encountered severe corrosion.

- Inspecta checks storage tanks, boiler tubes and pipes (even when hot) for localized damage in many locations e.g. tank depots, paper and chemical industrial sites, usually for repeat customers. New regulations are aimed at a higher damage identification sensitivity, about 25 % of the thickness as opposed to the previous 50%, and this method is more sensitive, with our accurate reporting giving early damage indications that facilitate timely customer maintenance.

For more information, please contact
Mats Bergman, Inspecta, +46 8 5011 3056

*) SLOFEC ™ (Saturation LOw Frequency Eddy Current) is a rapid technique for non-destructive testing. It is based on the eddy currents induced in the material being examined and then amplified using a unidirectional magnetic field. When damage occurs there is an increase in the magnetic field density in the remaining metal, which is recorded by the sensors as a change. Both sides of the material can be examined due to the separation of the signal phases. The results are analysed in real time and reported on a colour scale. SLOFEC can be used on hot surfaces and both ferromagnetic and non-magnetic materials, from carbon steel to duplex stainless steel.


Author: Tage Eriksson

Page created: 12 Nov 2013