Posted 9th August 2019
Water that gets into a storage tank is the biggest cause of fuel issues with diesel engines, generators, vehicles, plant and boilers. Over 25% of generator breakdowns are caused by bad fuel, not mechanical failure. But how does this water get there?
Diesel and any stored fuel can become contaminated by water from several different causes. It is unlikely to be delivered into the tank from the oil distribution supply chain. This is due to the monitoring at the refinery and the vast volume that goes through the tankers. The fuel is regularly tested during the downstream delivery process, avoiding most issues before reaching the tank.
Faulty lids or caps, poorly fitted vent pipes or general damage to the tank fitting cause water to get into tanks. The other factor is the diesel composition. Since 2011 diesel fuel in the UK has been blended with up to 7% biodiesel, also known as Fatty Acid Methyl Esters (FAME). This blended diesel is hygroscopic meaning it absorbs moisture from the surrounding air.
Temperature changes in the tank between day and night and sunnier days will also allow the formation of condensation in the tank. The condensate forms on the sidewalls of the tank, it then runs down into the fuel, collecting at the bottom of the tank.
Diesel and gasoil fuels are hygroscopic, therefore the fuel will absorb and hold water at a molecular level. The amount of water absorbed will vary with temperature, air humidity and biodiesel content. Water will start to form droplets when the fuel reaches the maximum saturation percentage.
These droplets sink to the bottom of the tank collecting in pools along with condensation or other water. This is when problems occur. As this layer of water increases over time, the level may reach the outlet of the tank, causing severe issues and damage to machinery resulting in breakdowns.
The free water collecting at the bottom of the tank will allow the formation and growth of microbes known as ‘diesel bug’. Diesel bug forms in the layer where diesel and water meet at the bottom of the tank. These microbes feed on the hydrocarbon and create a by-product of mud and sludge along with acids and gums.
This causes rapid oxidisation of the diesel and allows the contaminants to block filters and increase wear to the fuel injection system. Common rail diesel engines run at injector pressures over 2000 bar and require clean fuel and close tolerances.
Wet diesel allows these close tolerance components to wear quickly and become off-spec. Thus, causing soot, incomplete combustion and loss of efficiency and performance. This increases fuel consumption and exhaust emissions.
We measure water content in PPM (Parts Per Million) and Relative Humidity as a percent of saturation (the maximum dissolved water the diesel can hold). 100% saturation is the point where free water forms from the absorbed content in the fuel.
The standard specification for diesel fuel in the UK and Europe is EN590. The EN590 specification includes multiple characteristics but water contamination is the biggest factor for causing problems. EN590 states a maximum of 200mg of water per kilogram of diesel fuel. That’s 200 PPM.
Detecting free water in the bottom of a diesel tank can be achieved with a very simple DIY test with water detection paste and a dip stick. This is an instant ‘yes-no’ visual test. To detect relative humidity or PPM of water, we need to use electronic sensors. We sample fuel quality using sensors installed in the pipework or as part of test equipment.
It is possible to send a sample to a lab for analysis and would be more accurate, however field testing for water content is a reliable test. This is because of changes in the diesel condition between the sample acquisition and the lab test. Diesel saturation point will rise with temperature, so a sample taken at 5°C will have decreased free water and relative humidity than in a lab at 20°C.
How much water content is ok? Simple answer…zero – but impossible to achieve in reality. So 200 PPM inline with the EN590 standard or relative humidity below 70% will not cause too many issues due to the moisture content in the fuel. Especially if this can be maintained at this level.
After a dip test, tank sample or analysis to determine that water is present, what’s the next cause of action? There are several approaches to remove both free and dissolved water. Depending on how big the problem you could:
We are able to advise on the most appropriate course of action; please contact us or call 01722 714514 to speak to an expert.
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