Press Coverage

40 Spring 2019 Corrosion forms from the inside out, so the extent of the problem is not generally known until external problems manifest, which can often be too late to rectify. As a rule, diesel fuel should be stored for no longer than six months without being checked by fuel testing to ensure that it meets specifi cation before it is used. In a 12-month period, Crown Oil Environmental was called out to 482 instances of corrosion. The vast majority of these cases could have been avoided by regular checks, simple tank maintenance and keeping fuel levels to a maximum to reduce the risk of moisture. Corrosion culprit There are several types of corrosion, including: general, local, pitting and weld metal corrosion. General corrosion occurs throughout a tank whereas local corrosion appears in specifi c areas where water collects or fl ows. Pitting is found along horizontal surfaces, the bottom of tanks and where water accumulates. Once pitting corrosion becomes thicker than one-third of the wall depth, it manifests as a problem. Weld metal corrosion occurs when welding metal interacts with metal in the fuel tank, triggering an electrolytic action. Troublesome tanks and pipework Good practice when storing fuels onsite dictates that tanks and pipework should be regularly tested to ensure integrity. While many different types of corrosion can occur, most are not visible by exterior examination only and further internal testing is required. Internal testing should be done by confi ned space entry using a trained specialist team to ensure health and safety. The contents of the tank should fi rst be removed, followed by a full internal clean down by pressure jetting to eradicate any loose metal exposing any pitted or rusted areas. This should then be checked by non-destructive testing (NDT) to test the metal thickness at various points in the tank. This process looks specifi cally at the base plate as this is not clearly visible unless the tank is cleaned. Tank thickness can range between 6 – 12 mm depending on the overall size of the tank or the site specifi cation. If the tank thickness has reduced by over 20% once tested, then action should be taken. Pipework should be checked as frequently as tanks by pressure testing and where possible by NDT if the pipework is above ground and clearly visible. Suffering from sludge? When a tank becomes corroded, the oxidation of hydrocarbon ingredients is increased, which reduces the quality of fuels contained and leads to sludge, bacteria and fungi build-up. Corrosion on the bottom of a tank reduces fuel lubrication and clogs fi lters, whereas in the upper fi lter spaces it can obstruct the operation of release detection and ultimately lead to equipment failure. Corrosion of tanks and build-up of damaging bacteria such as fuel algae and sludge are responsible for 62% of equipment failure. Modern matter Fuel tanks also face a number of challenges due to the nature of products stored. The introduction of ultra low sulfur diesel (ULSD), coupled with the increase in bio content and modern refi ning techniques, means that the risk of contamination and general corrosion or pitting is now higher than ever. While some corrosion is inevitable, a wide range of techniques and materials have been developed to prevent it. A proactive over a reactive approach must be adopted to maintain fuel quality. Figure 1. Aboveground tank storage. Figure 2. Industrial tank clean. Figure 3. Man entry tank clean.

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