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Diesel fuel will always contain a certain percentage of water. The goal is to keep water levels within acceptable limits, well below the saturation point. Removing excessive water from fuel can be a challenge; therefore, the most effective approach is to take every reasonable measure to prevent water from entering your tank and monitor it regularly. This way the need for water removal can be kept to a minimum. In order to develop a good water management strategy, it is important to understand how to measure water content and evaluate the results.
Water has always caused rust and corrosion of fuel system components and infrastructure. Modern fuel systems are much less tolerant than lower pressure systems, that manufacturers now specify zero free water must reach the engine.
Water causes damage to both fuel tanks and engine parts. Rust and corrosion in the tank create hard particulate that is passed along in the fuel, causing engine wear. Component life is also shortened by water etching, erosion, cavitation and spalling, such as:
Rust: In contact with iron and steel surfaces water produces iron oxide (rust). Rust particles that get into the fuel, like other hard particulates, will cause abrasive wear to parts. Premature wear can cause part failures.
Corrosion: Corrosion is one of the most common causes of injector problems. Water combines with acids in the fuel to corrode both ferrous and non-ferrous metals. This is made worse when abrasion exposes fresh metal surfaces that readily corrode. The injector shown on the right was installed new but failed in under 300 hours due to rapid corrosion.
Abrasion: Water has lower viscosity than diesel, therefore providing less of a lubricating "cushion" between the opposing surfaces of moving parts. This leads to increased abrasive wear.
Etching: Etching is caused by water-induced fuel degradation which produces hydrogen sulfide and sulfuric acid that "eat" metal surfaces.
Pitting and Cavitation: Pitting is caused by free water flashing on hot metal surfaces. Cavitation is caused by vapor bubbles rapidly contracting (imploding) when exposed to sudden high pressure, which causes them to condense back into a liquid. These water droplets impact a small area with great force, causing surface fatigue and erosion.
Spalling: Occurs due to hydrogen embrittlement and pressure. Water is forced into microscopic cracks in metal surfaces. Then, under extreme pressure, it decomposes and releases hydrogen in a “mini-explosion” which enlarges the cracks and creates wear particles.
Ice: Free water in fuel can freeze, creating ice crystals that behave just like any other hard particulate. They can create wear in fuel systems and (in large volumes) clog fuel filters. A fuel filter's job is to protect the engine by stopping hard particulate. Engines and filters do not differentiate between dirt and ice. Damage caused by ice can be hard to correctly diagnose since the ice will melt and disappear long before a lab examination can occur.
Water also contributes to or aggravates a number of additional issues, like the following:
Soft Solids: Water is polar. Certain chemicals in additives are polar. Hydrocarbons are non-polar. This means that water and polar chemicals are attracted to each other. In the presence of free water, the chemical molecules will sometimes disassociate themselves from the hydrocarbon chain of the additive and combine with water molecules to form a new substance. The new material is a soft solid that precipitates out of the fuel and can rapidly clog filters or create engine deposits. See additive stability for more information.
Microbial Growth: Like most living organisms, bacteria and fungi (molds) need both food and water to survive. If free water is present microbial growth can proliferate, creating slimes that foul your fuel and acids that corrode your tank and fuel system.
Fuel Oxidation: Free water accelerates the oxidation process and encourages the formation of acids, gums and sediments known generally as fuel degradation products.
All diesel contains some percentage of dissolved water. The water molecules remain part of the fuel until there are too many of them. The point at which the fuel can hold no more water is called the saturation point. The quantity of water in fuel is measured in ppm (parts per million). As long as the water stays below the saturation point as dissolved water it is typically not too much of an issue. Significant problems start when water separates from diesel and becomes free or emulsified water. Emulsified water is another form of free water; the droplets are simply so small as so well mixed into the fuel that they remain suspended rather than dropping to the bottom. There are no "droplets" when water is fully dissolved in fuel.
Water can come from a wide variety of sources, some of which can be extremely difficult to control.
There are several methods for measuring water content in fuel. Some are done in a laboratory, some can be done onsite. It is important to understand the type of information these different tests can provide. Perhaps the most common method for testing for water in fuel tanks is to "dip" the tank using a special indicator paste on a long dip stick. This method is fast, easy and can be done on site, it will tell you if there is free water in the tank bottom.
Water monitors (sensors) can be installed inline and give reliable real time results. They measure the dissolved water content in fuel and return the relative humidity of the diesel as a percentage. The maximum result is 100%, meaning that the fuel has reached its saturation point and can hold no more water in solution. This test method will not tell you how much free water there is in the tank.
The Karl Fischer titration method is a laboratory test used since 1935 for determining water content in a fluid sample. The test is highly precise and requires only a small sample size. It detects even small amounts of dissolved water, down to about 50 ppm in diesel fuel. It can measure water content both below and above saturation level (dissolved and free water). In laboratory practice it can be used to determine water saturation level of fuel under different conditions. While laboratory tests are typically more precise than field tests, they can be much less accurate. This may seem confusing. The reason that the laboratory test may be less accurate is that the sample itself may have changed between the time it was taken from the tank and the time it is tested in the lab.
One of the characteristics of diesel is that it holds more water in saturation when it is warm compared to when it is cold. If the diesel in your tank is cold it may be over the saturation point. In this case there will be free water entering your equipment, which can cause huge problems. If you send this same sample to a lab, it will likely be warmer in the lab than in your tank. The fuel will warm up, the water will go back into solution, and it may look like you have no problem at all. The same sort of diagnosis difficulties can happen with ice crystal problems. The "evidence" goes away at room temperature.
The easiest answer would be none. But this is neither practical nor realistic. All diesel contains some percentage of water. The most important thing is to keep the water below its saturation point so that it stays dissolved rather than entering your equipment as free water. Equipment manufacturers specify that zero free water must reach the engine. Saturation points vary from roughly 50 ppm to 1800 ppm based on temperature and on the petro diesel/biodiesel ratio. As you can see on the chart, biodiesel can hold significantly more water in saturation than than its petro equivalent. Blending bio and petro diesel together, however, does not result in a mathematically proportional moisture content. The blend will hold less in solution that the sum of the parts, meaning that free water precipitation may occur when the two are mixed.
In order to understand how to keep water out, one must first understand how it gets in. Water can come from a wide variety of sources, some of which can be extremely difficult to control.
On delivery from supplier: Diesel is relatively clean and dry when it leaves the refinery, yet diesel deliveries will include variable amounts of water. The quantity of water you receive from your supplier depends largely on circumstance and handling practices. What can you control? Beyond potentially switching suppliers or negotiating a contract which puts the burden on the distributor, you can try the following:
Ingress from atmosphere: Just like the air, diesel has a relative humidity, and the two tend to equalize. Meaning, if the air is more humid than the fuel, then the fuel will absorb moisture from the air. If, however, the air is dryer than the fuel, then moisture will evaporate back up into the air until the relative humidity of both is equal.
Free water fall-out: Diesel holds a certain amount of water in solution (i.e. dissolved water). When water content goes over the saturation point, the excess water will fall out as free water. This occurs when the total water content increases or when the diesel cools. Your diesel may hold 90 ppm dissolved water when warm, but only 60 ppm when it cools down due to colder weather. The 30 ppm difference falls out as free water and settles to the bottom of the tank.
Condensation in tank: When it is warmer outside the tank than it is inside, condensation will form and this "sweat" will enter the fuel. This can occur over and over, creating more free water each time.
Leakage into tank: Rain, pressure washing or ground water can all be sources of water getting into a damaged or improperly sealed tank. Inground tanks (at filling stations, for example) can sometimes have inlets below grade. The area around the cap can easily fill with rain water. If the water level is above the cap when removed, gravity will cause the water to flow right down into the tank.
Water removal is easier in regions with over 500 ppm sulfur diesel than it is with ULSD (under 15ppm sulfur). The filter skid to the left, for example, was designed for use in South Africa, where it is quite effective. Coalescers and water separators function much more effectively in higher sulfur fuels. This is because higher sulfur diesel needs much less additive and, subsequently, contains far less surfactant.
Surfactant is a soapy substance that coats over coalescing/water separating medias, severely compromising their performance.
The increased amount of surfactant in ULSD disables coalescing media, rendering its effectiveness questionable at best.
Manufacturers publish filter efficiency based on the current industry test standards. The current standards were developed a number of years ago and are designed for lab comparison testing using a consistently treated fuel. This works well for comparison testing, but does not necessarily reflect filter efficiency in real world conditions. To treat ULSD fuel for lab testing, the standards require the removal of all surfactants. In the real world, ULSD with surfactant removed (AKA additives) would destroy engines. All ULSD that is fit for use in equipment contains additives and surfactant, therefore the fuel itself effectively disables coalescing filters.
So while you will probably not see a decrease in published coalescer efficiency levels, what you will notice in literature is the increased mention of water absorbers. Companies that still market coalescers to ULSD areas very frequently now mention the necessity of adding water absorbers after the coalescer. There is no other way to ensure that free water has been removed.
Unfortunately, the best way to remove large volumes of settled water is to drain the tank. Very straightforward, but not cheap or convenient. Ambient moisture and condensation can be prevented from entering the diesel through the use of good desiccant breathers in combination with a blanket of dry air (or nitrogen) fed into the tank's headspace and out through the breather. As explained earlier, the relative humidity of the diesel will tend toward the relative humidity (or "dryness") of the air. Moisture in the diesel will, with time, be released back into the dry air until the diesel is just as dry as the air.
The key to good fuel water management is to minimize dissolved water content and eliminate all free water.