For many storing fuel is an integral part of our business, be it in emergency equipment or stored for everyday use. We rely on this same fuel to be immediately available and of the same high quality as new fuel, regardless of how long it has been stored. We look at how realistic these expectations are and what can be done to meet them.
Oil companies are now refining 80% more from a barrel of crude oil than they were in the 1980’s and these barrels originate from fields deemed uneconomical and of poor quality in the 90’s. As a result today’s distillate fuels are more unstable and contain greater undesirable elements including increased sulphur, all of which pose a greater challenge to the refiners as they endeavor to supply a product to meet ever increasing environmental legislation.
The highest volume of fuel is used by the transport sector; On average, diesel is combusted in an engine within 18 - 24 days of leaving the refinery, consequently there is no requirement to produce a diesel suitable for long term storage. However refineries generally add some stability additives which will enable the fuel to perform satisfactorily for a maximum of 4 to 6 months if it is stored under ideal conditions. Given that marine is anything but ideal and considering from leaving the refinery to getting to the end user can take up to a month or more, it is clear to see the fuel is already degrading even before it is received by the end user.
Meanwhile to meet environmental legislation the engine manufacturers are designing engines with ever higher fuel pressures, presently up to 45,000 psi and some at 60,000 psi with tolerances measured in microns and with exceptionally light pintles in the injectors, these engines demand a high quality fuel or early failures can only be expected.Additionally it is common practice in marine to use the fuel itself as a cooling medium for the injectors. Often more than double the engine fuel consumption is returned to the tank, consequently a molecule of fuel could potentially be subject to these high heat and extreme pressures many times before it is actually combusted in the engine. This causes additional degradation very quickly, a process the humble diesel fuel was never designed for, whereas purpose built cooling and heating oils have complicated additive packages to prevent degradation.
There are many contributing factors to fuel degradation, these include:-
Oxidative breakdown from exposure to air and light
Hydrolytic breakdown through exposure to water
Catalytic breakdown from exposure to certain metalsI.
In reality contamination begins as soon as the tanks are filled and continues until the fuel is used. Without preventative measures the longer fuel is stored the higher the probability of damaging fuel degradation..
Oxidative degradation results in the formation of particulates, water and gum. Oxygen reacts with the hydrocarbon molecules in the fuel causing discoloration. This can occur through tank ventilation, refuelling and can even be trapped during the refining process. These particulates and gum will block fuel filters leading to fuel starvation. Additionally the particulates and gum do not burn in the engine very well and can lead to carbon and soot deposits forming on the injectors and fuel pumps.
Hydrolytic degradation occurs when water finds its way into the fuel tank, this can be through oxidative breakdown or condensation forming on the inside of the tank, water can even be present in the fuel itself. Bio-fuel makes up 12% of the European standard (EN590) and is hygroscopic, absorbing 30 times more moisture from the air than traditional fuels. Build-up of water is one of the most serious issues for modern fuel. It is the home of diesel bug, the collective name of the bacteria, moulds and yeasts form this microbial contaminate. Water can also lead to rusting of internal components and has been known to become superheated at the injector tip on common rail engines resulting in the nozzle being blown off.The corrosion of tanks caused by microbial infestation releases metals back in to the stored fuel. This will accelerate the oxidation reaction resulting in a vicious circle of contamination that can become difficult to control.
Normal un-dyed diesel should be clear and bright with an amber-green colour. As fuel degenerates the heavier components of the fuel blend detach from the solution and float freely in the fuel. These components give degraded fuel a darker colour.You may be unaware that you have degraded fuel - especially in emergency equipment that by its very definition is used infrequently – until you try to use it. Failure to start, rough running, black smoke, reduced RPMs and frequent stalling due to fuel starvation from clogged filters, are all symptoms of degraded fuel.
As well as taking a sensible approach to fuel storage such as ensuring only the freshest fuel is initially purchased, pressing up fuel tanks to avoid condensation build up and keeping it clean, dry and cool, a scheduled maintenance programme should also be implemented following these essential steps:-
Every six months take two fuel samples for analysis. One sample should be taken from the very bottom of the tank for visual inspection to identify free water and contaminants. The other should be taken from the middle of the tank with a fuel sampling “bacon” bomb.
Ideally fuel should be replenished every year, the old fuel should be removed and used elsewhere; of course, in most cases this is not possible or practical. Alternatively a relatively low cost static fuel polishing system could be fitted, for best results this would be fitted with a 2 micron filter and timed to come on at regular intervals.
Fuel degrades at an alarming rate so additives should be added at the appropriate time to maintain the quality. As a minimum requirement the additive package should include; a stabiliser, cetane improver, demulsifier, detergent and a biocide to arrest microbial contamination.
n conclusion, without a good house-keeping process to prevent degradation, it is unrealistic to expect fuel to maintain its quality. Studies carried out by the University of Idaho concluded the contamination and degradation process of stored diesel fuel is well underway within 28 days of storage with the fuel showing 26% degradation at this point, although this may not be common across the board it clearly highlights the issue.
Peter Weide is MD of MarShip, a UK based company specializing in optimising the efficiency of marine diesel engines. Advising on maintaining the cleanliness of Air, Fuel and Lubricating oil, we regularly recommend solutions to operators and appear regularly in industry press, with our full range of diesel additives DieselAid we can offer solutions for most operating conditions and wont advise them if you don’t need them.
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