Article by Capt. Steve Katz
Clean Diesel Fuel
Diesel fuel is the largest operating expense of a diesel engine. A constant clean supply of fuel is crucial to proper operation and efficiency. Diesel fuel can degrade if left to sit unattended, collecting moisture and then growing microbiological “bugs”.
Bugs
Today’s ultra-low-sulfur diesel (ULSD) does not contain the sulfur that helps kill microbiological “bugs” (bacteria and fungi) that can live in diesel fuel. Microbiological “bugs” can grow in a diesel fuel tank when there is a presence of water. Microbial colonies proliferate at the interface between fuel and water in the tank. This growth is often mistaken as algae which needs light to live and grow; there is no sunlight in a closed fuel tank so algae cannot survive.
Microbes are everywhere; they need food and water to live. In this case, the diesel fuel is the food and if there is water in the fuel, the microbes can easily make a home in your fuel tank. Once they start to grow and multiply they can eventually damage your fuel supply.
These “bugs” can be viewed with a microscope, though the presence of microbes is usually more often discovered by looking at your used fuel filters. If the fuel filter is coated with stinky, slimy black goo, then you probably have a microbe problem. There are laboratory tests available that can analyze your diesel fuel and report back the presence of contamination and microbes in your fuel.
Removing bugs
If you have a microbe problem in your fuel, the best way to eliminate them is to remove the water, then kill and remove the remaining microbes. If not done properly, the microbes can easily grow back.
While you cannot sterilize your fuel and tanks, you can reduce and discourage biological activity to a level where microbes are not a problem.
There are fuel additives called biocides that can kill the living microbes and are often added to the fuel tank as a maintenance dose or kill dose. If you add a kill dose, expect the microbes in the tank to die resulting in a bigger problem until the fuel returns to its normal composition. The dead microbes will clog up your fuel filters until all of the debris are eliminated from the tank. This do it yourself approach can take days or weeks to rid the tank of live and dead microbes, with numerous fuel filter replacements while operating your boat.
If you have a bad microbe infestation, it is best to call in a professional fuel cleaning company. They will have the proper tools and equipment to pump out your contaminated fuel, filtering it multiple times and returning your cleaned fuel to the tank. During this process, the interior of the tank needs to be cleaned too, since the microbes can attach to the sides of the tanks and baffles. Generally the cleaning companies use high pressure fuel to wash down the tank sides and bottom. A process like this can take a day or more on a large boat.
Water
Eliminating the water in your diesel fuel can reduce or eliminate the ability of microbes to grow and live in your tank. There are three types of water that can exist in diesel fuel - dissolved, emulsified and free.
Dissolved water is the result of diesel fuel being hygroscopic. Diesel fuel can attract and hold water from the environment. This can be from humidity in the air or water forming by condensation on the inside of the fuel tank.
Emulsified water is when water is thoroughly mixed into the diesel fuel. This situation is similar to a bottle of vinegar and oil salad dressing. Shake it up and it is mixed nicely, let it rest a while and the dressing settles back into two components. On a boat, emulsified water can occur after refueling or a rough day at sea and usually settles out once the agitation action has stopped.
The most common form of water in a fuel tank is free water. This is water that gathers at the bottom of the fuel tank. Water is heavier than fuel and therefore will settle to the bottom of a fuel tank. A fuel tank’s pick-up lines that supply fuel to the engine are often an inch or more off the bottom of the tank, allowing water and contaminants to remain at the bottom of the tank without being ingested into the engine. Once the level of water increases above the pick-up, water will be introduced into your filters and possibly the engine. If you get water in your fuel filters, there is probably more water in the bottom of tank. The easiest way to check the fuel tank for water is to use a water finding paste, coated on a dipstick that can reach to the bottom of the fuel tank. This will allow you to determine if you have water and the amount.
Removing water
If you do find water in your tank, it needs to be removed or else microbial growth will occur. The water at the bottom of the tank serves as a perfect medium for these microorganisms to live in. The microbes feed and live at the fuel/water interface and can even float in the free water below that level. Some fuel tanks have a way to drain water from the bottom of the tank, while other tanks may need to be pumped out using a tube or hose that can reach to the bottom of the tank.
Sampling your fuel and testing for water and microbes should be part of your preventative maintenance schedule. Some boats are equipped with a fuel polishing system, a way to filter your fuel when the engines are not in operation. Changing fuel filters at regular intervals or even more often is a good practice. There is no such thing as "too clean" when it comes to your fuel.
Captain Steve Katz is the owner of Steve’s Marine Service and holds NMEA, AMEI and NMEA2000 certificates along with ABYC Master Technician certification and factory training from many manufacturers.