Fuel Oil Quality
Effective onboard fuel management is critical in maintaining adequate fuel quality. The primary concerns are particulates, water, and microbiological contamination. Particulate contamination can be caused by dirt, rust flakes, catalyst fines, or other foreign matter introduced into the fuel. It can be introduced from a fuel source, or from the ship's own storage tanks and piping. Particulates generally settle out of solution and accumulate at the bottom of storage tanks. However, biological contamination and storage stability problems also generate particulates that can stay in suspension. Particulate contamination will clog fuel filters, and can potentially damage fuel injection equipment. Timely and periodic tank stripping is the best means to keep particulate contamination manageable.
Incompatibility problems between distillate fuels are rare, but it can happen. Incompatibility will cause rapid formation of particulate matter similar to what can be observed with storage instability. The factors that influence incompatibility are not well understood. Chances for a compatibility problem are highest for cutters that fuel from multiple and remote locations, and from fuel sources that likely originated from different crude oils. Cutters and boats that generally refuel from the same supplier are at minimal risk.
Water can be introduced by condensation, through onloaded fuel, via wet fueling lines, and by ballasting fuel storage tanks. Free water is water that can be removed from the fuel by mechanical means, for example by a centrifugal purifier. Free water, if allowed to remain still for a relatively short time, will settle to the bottom of the tank (or container) as a distinct layer or as droplets. Periodic tank stripping will therefore remove free water.
Entrained water is defined as free water droplets that are suspended in the fuel. Entrained water typically appears as a haze. Dissolved water is water that can not be removed from the fuel by mechanical means and is not visible to the naked eye as haze. The concentration of dissolved water varies with fuel temperature, the relative humidity of the air contacting the fuel, and the chemical composition of the fuel. Water is necessary for microbial growth. Water can also cause damage to fuel injection equipment. Gas turbines are particularly sensitive to water contamination. Only water separators (coalescers) can remove entrained water from fuel. Timely and periodic tank stripping will help prevent the occurrence of problems associated with water.
Marine fuel may have a colored dye mixed into the fuel. Only red dye is used domestically, but blue/green dye may be found in some foreign ports. In the Mediterranean, even black dye may be found. The dye can complicate performance of a clear and bright test. The resulting fuel/dye blend can have a colored tint (red/pink, etc.), but it must not be visually brown or darker in appearance.
Microbial contamination consists of organic debris created by fungi (including yeasts), bacteria, and protozoa. Microbes gain entrance to fuel tanks from contaminated fuel supply sources, from airborne particulates through tank vents, and from seawater if tanks are ballasted. After entrance into a tank, microbes settle on tank surfaces and especially at the fuel/water bottoms interface. Proliferation of those microbes adapted to this environment can occur if free water is present. Growth will be favored by warm environments. Sludges or interfacial mats can form - having a slimy or stringy consistency and appearing as a black, brown or dark olive color. Enmeshed nonbiological particulate matter as well as fungal and bacterial cells may also be included. This kind of contamination can quickly clog fuel filters.
If free water could be eliminated from the fuel, microbiological contamination would no longer be a major concern. Once again, timely and periodic tank stripping will minimize concerns with microbiological contamination. The addition of biocides is another means of preventing microbiological contamination. However, any fuel additive (including biocides) increases the chance of fuel compatibility problems when fuels are commingled (adding a new fuel load to a partially filled tank). The effectiveness of biocides can also be problematic.
Biocides are commercially available products which are toxic to microorganisms. Biocides are not authorized for use in aviation fuels. The most useful biocides for treating fuels are soluble in the fuel but have sufficient water solubility that they can partition into any present free water. Biocides prevent growth and proliferation of microorganisms but will not reduce the amount of microbial particulates taken in from a fuel supply source, or which may have developed in a tank before biocide treatment. To be effective for shipboard use, biocides must not alter the characteristics of the fuel. Biocides must also be compatible with fuel storage, handling, transfer, and delivery systems for equipment that uses fuel. Multiple biocide products were approved for use in the past. However, due to recent changes in the MIL-S-53021A and its associated qualified products list (QPL-53021-9), and concerns with toxicity, compatibility, and other technical issues, only Biobor JF or Nalfleet 9-303 are currently approved for use. Earlier information that stated compatibility problems existed between Biobor and Nalfleet were investigated by the Naval Research Laboratory and found to be in error. There remains a general compatibility concern with fuel that has been treated with different additive products.
F-76 and F-44 (JP-5) are the only marine fuels that have "built in" protection against problems associated with long term storage. NPD MGO, and other commercial fuels, are not delivered with a storage stability "guarantee" and therefore should be consumed as soon as possible - but not to exceed six months. Operational indications of unstable fuel are very similar to that of microbial contamination. It will most likely be manifested by frequently and quickly clogged fuel filter elements. Filter elements clogged by unstable fuel products may appear to be coated with a black gummy substance or a gel. The most recent information provided by Navy fuel experts is that there is no direct link between fuel haze and a fuel's storage stability characteristics.
Clear and bright visual testing upon receipt of fuel is important. Clear and bright testing can identify water and particulate contamination, and to a limited degree, microbiological contamination problems. This test is particularly important when receiving an approved DESC bunker fuel product, or emergency substitute fuel. A microbial contamination problem can show up as fine particulates which may or may not settle out to the bottom of the sample bottle. If fuel passes clear and bright criteria, and onboard fuel stocks are routinely rotated, chances are slight that a fuel quality problem will occur before the fuel has been burned.
When required by operational emergencies to accept fuel that will not pass clear and bright visual inspection criteria, minimize the amount of fuel taken onboard and burn it as soon as possible. However, the most likely cause of cloudiness, or haze, is entrained water. DESC contract requirements for NPD MGO requires that if a sample fails clear and bright visual testing, it must then be subjected to a water & sediment centrifugal test (max allowable water and sediment content is 0.05% vol). If the fuel passes this more rigorous test, it is acceptable for use.
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