Fuel Oils: Handling, Mis-handling, and Contamination (Part 2)

Steven M. Lindholm, P.E., P.M.P.

Part 2:  Contaminants and Damage

Welcome to the second part of a three-part presentation on current trends in fuel oils and fuel oil contamination. EDT hopes you will come away from this series with better knowledge of oil as a fuel, what can contaminate fuel oil, the damages caused by contaminants, and the available means to prevent or identify fuel contamination. This part of the series will cover the typical contaminants found in fuel oil and the damage these contaminants can cause.

Contamination

In a perfect world, the consumer should get exactly what the bold print says. But just like the small print on the chocolate bar, “This product has been made in a facility where nuts are processed, and may contain small amounts of nut product,” contamination cannot be avoided in practice without adequate prevention methods that may become cost prohibitive. In most cases, the amount of contamination is very small relative to the volume of the product, but sometimes even a small amount of contaminant is a big problem. Like pecan fragments to someone who is allergic to tree nuts, certain contaminants in fuel oil can cause big problems. Some common contaminants found in fuel oils are:

Aluminum.  The element aluminum is a wonderful metal when alloyed properly. Introduced into fuel oil, it becomes a nightmare. Aluminum in fuel oil often becomes aluminum oxide –aluminum is highly reactive and will bond with free oxides or hydroxides. Aluminum oxide is a very hard particle – it forms the basis of the gems sapphire and ruby, which are just below diamond on Mohr’s hardness scale.

Iron.  The element iron, or ferrous, is another metal which finds its way into fuel oil. Not as damaging to engines as elemental aluminum, it becomes a pollution problem when it combines with carbon to create persistent soot – a lung irritant. It can also combine with aluminum oxide during combustion, creating thermite, a very hot burning compound.

Silicon.  The element silicon is abundant and, like aluminum, prefers to be bonded as an oxide. Silicon oxide is a staple in most people’s lives as glass or sand – you drink from it, see through it, and walk on it at the beach. But just like that irritating crunch in your sandy sandwich, it is a hard and damaging particle.

Sulfur.  Sulfur is another element which is not fuel but is found in oil. Most sulfur is introduced into the fuel oil from the raw oil stock. A common problem in removing petroleum from the ground is hydrogen sulfide, a noxious gas which is hazardous even in relatively low concentrations. The presence of hydrogen sulfide is an indication of sulfur in the base stock. Sulfur is not damaging like aluminum oxide; instead, it can form sulfuric acid during combustion, leading to ‘acid rain’ and other air pollutants.

Vanadium.  Vanadium is commonly used as an alloy in high strength steels. Vanadium can also be found in small quantities in fuel oil – which does not increase the strength of the fuel, as it does in steel! Vanadium in fuel oil is another abrasive, adding to potential engine damage.

Water.  Water as a contaminant is not necessarily damaging – water injection is used in some engines as a means to increase power. To be effective, though, water needs to be at a controlled amount and injected in the proper sequence. Water emulsified in the fuel oil can be an issue as the percentage is unknown leading to either uncontrolled detonation or lowered energy density. With lighter fuel oils such as gas oil or marine diesel oil (MDO), some cases have been reported where the settling out of water in storage tanks has resulted in microbiological growth.

Microbes.  Should the conditions be right – warm temperatures, water, and fuel oil – anaerobic bacteria can grow in stored fuel oil. Anaerobic bacteria do not need oxygen to thrive; instead, with hydrocarbon material to digest they can feed, grow, and reproduce with a lack of air. 

Polymers.  Because many grades of fuel oil are considered by refineries as residuals – oil left over from the refining processes, the feed stock into intermediate fuel oil and bunker, is often residual low flashpoint oils left over or ‘out-of-specification’ from other processes. Lately, this has included polymers – partly reacted long strings of hydrocarbons which form the basis of plastics and complex lubricants. The process of polymerization – creating larger molecules by reacting small molecules – allows for durable hydrocarbons. Fuel, though, needs to be broken down for combustion, so polymers are not a good choice for power.

Low Flashpoint Oils.  Low flashpoint oils make good fuel. But, when the intent of the fuel is to create a more stable fuel oil with a higher flashpoint, adding low flashpoint fuel into the blend is not improving the fuel oil. The reason for adding low flashpoint oils is to decrease viscosity of the fuel oil, improving the ‘pour point.’  This, though, is at the risk of increasing the volatility of the fuel, including potential ‘boil off,’ or gasification, within fuel lines.

Paraffins.  Paraffin is petroleum wax. Paraffins are long chain polymers which form from petroleum oil. Like the wax of a candle, paraffin can burn, just not at the rate expected for combustion.

Asphalt.  Asphalt is the ultimate residual in oil. Asphalt remains after all other lighter oils have been distilled off, and often includes sand fines (silicon dioxide).  Occasionally, this end product is left in the heavy fuel oil, causing problems during separation and combustion.

Some of these contaminants are present in the base petroleum oil as it is pumped from the oil field, while some are added during the refining process. Sulfur, asphalt, water, and silicon are present in the raw oil as it comes out-of-the-ground. Aluminum, iron, vanadium, and silicon can be part of the catalysts used to separate the lighter oil distillates from the residual oils which form the heavy oils, or bunkers. Polymers, paraffins and low flashpoint oils are sometimes part of the blending feed stock used to modify the residual oil for viscosity and flashpoint.

Damages

Contaminants in fuel oil can often be the source of damage to combustion power machinery. When investigating the cause of damage to combustion machinery such as boilers, turbines, and engines, common causes are wear-and-tear, improper or inadequate maintenance, or improper use. Employing an expert with experience in power machinery can help determine which of these is the likely culprit in machinery breakdown cases. Once these common causes are ruled out, the investigator should consider fuel contamination as a potential cause – especially when confronted with damage to some of the following:

Fuel Treatment Equipment.  Separators and filtration equipment can be damaged by asphalts, paraffins, and polymers in contaminated fuel oil. These contaminants damage treatment equipment by clogging or unbalancing the centrifuges, resulting in increased filter change periods, separator cleaning, or damaged bearings.  Aluminum, vanadium, and silicon as oxides are abrasives which increase wear on separators and increase clogging in filters. Look for swirl wear from abrasives or bearing wear from polymers and asphalt. Microbes can also be a source of fuel treatment issues; look for organic slime.

Burners and injectors.  The burners of boilers and injectors in diesel cycle or turbine engines are similar in function – they inject fuel into the combustion chamber.  Abrasives such as aluminum oxide, silicon oxide, and vanadium can cause premature wear of the spray valve components, leading to an inability to hold pressure in the fuel system or problems with combustion control. In the worst cases, this can lead to explosions as fuel is leaked into an unpurged boiler prior to ignition or the injectors emit excess fuel into an engine cylinder. Polymers and paraffins can also damage these injectors and burners, causing the spray valve to stick open by fouling plungers.

Valves and Pistons. Internal components of engines and boilers can be damaged by metallic oxides and metal contamination. Aluminum and iron in fuel oil form higher temperature burning compounds. The damage can be discerned on valves by erosion on the stem and valve seat, and by roughened surfaces, which look like fire erosion, on piston heads.

Turbine blades.  Like the exhaust valves of diesel engines, the blades of turbine engines or turbochargers can be damaged by combustion of metal compounds. Compounds of aluminum and iron can continue to combust outside of the combustion chamber and be carried onto the turbine blades. The damage may appear as erosion and pitting on the leading edges of the turbine blades.

Employing an expert with experience diagnosing combustion related damages will save the operator time and the expense of haphazard replacement. Where the cause of damage is not straight-forward – and with combustion machinery like engines, boilers, and turbines, damage to components may have multiple facets – it is best to have an expert look with an unbiased eye at all possibilities.

This concludes the second of three blog posts regarding fuel oil, fuel oil contamination, and its consequences. In the third and final part of this series, we will look into the trends in fuel oil supply that influence contamination and some measures which can be taken to mitigate the risk of contaminant damage.