Equipment World Digital Magazine
Issue link: http://read.dmtmag.com/i/565622
September 2015 | EquipmentWorld.com 38 corrosion in diesel fuel storage tanks, fuel dispensing pumps and related piping configurations. One of the concerns with ULSD fuel was lack of lubricity. The cheapest way to remove sulfur dur- ing refining involves hydrotreating, a process that removes sulfur and cetane by treating it with hydrogen. Unfortunately, hydrogen is highly reactive and also reduces the lubric- ity, or lubrication properties, of the end-product. Sulfur serves as a lubricating medium and the reduction of that sulfur (from 500 ppm to 15 ppm) causes a reduction in lubricity. After numerous complaints, refiners started adding lubricity additives to the process to compensate. Many people who service equipment still complain of evidence of poor lu- bricity, especially in older engines. Diesel is a perishable commodity Even if everything else is managed correctly, when you store diesel over time, chemical reactions can compromise the fuel's quality and cleanliness. There are two main types of chemical reactions. One is oxidation, which occurs when the fuel is exposed to oxygen or oxygen-bearing matter. The second reaction is hydrolysis, which occurs when the fuel is exposed to water. Both reactions produce chain re- actions within the fuel, resulting in a fuel that appears darker in color and more translucent. Contami- nates produced under these condi- tions include varnishes, gums and sludge that separate out of the fuel and settle. Most all diesel fuel, including ULSD, has a shelf life from three to six months. This can be extended by adding stabilizers, restricting wa- ter intake through proper storage, filtration and restricting heat. But diesel is far more susceptible to wa- ter solubility issues than gasoline. Ethanol creeping into diesel supplies Another problem that's cropping up is corrosive damage to diesel stor- age tanks. This once took years but now happens in mere months. According to a study conducted by the Clean Diesel Fuel Alliance, "Diesel tanks suffering from ex- treme corrosion contained trace amounts of ethanol and had low pH values in the 2.5 range. A low pH value means the fuel is high in acid. A pH level below 7 is consid- ered acidic; those below 5.5 to 5.6 are considered problematic. The low pH from the study was a result of acid from the bacterial interac- tion, in particular from a bacteria called acetobacter, which is attract- ed to ethanol. So how did the ethanol get into the diesel? One possible source is the issue of "switch loading." Fuel-hauling tanker trucks are not used exclu- sively for one particular fuel. One day they could be transporting ethanol-blended gasoline and the next day transporting diesel. The ethanol gasoline remaining in the bottom of the tanker from yesterday would end up mixing with diesel fuel for today's run. Water: an ever present concern Because diesel is heavier than gasoline, water is more soluble and more readily held in suspen- sion in gasoline as compared to diesel. This is especially true of ethanol-supplemented gasoline. Also, volatility (how easily a fuel vaporizes) is lower in diesel than gasoline, which allows air and dampness to penetrate the diesel fuel in both vehicle and bulk stor- age tanks more quickly than with gasoline. Additionally, diesel fuel tanks are subject to water condensation because, unlike gasoline, diesel fuel has no vapor pressure to displace or push out air. When a fuel tank warms up on hot days, the air expands and is forced out. As the tank cools at night, thermal contraction causes humid air to be sucked back into the tank and wa- ter condenses on the cooler inner tank wall surfaces. maintenance | continued Since some water is inevitable, one solution is a routine treatment of fuel storage tanks with a biocide treatment program to kill tank bacteria microbes. " "