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Fuel Filtration: Protecting The Diesel Engine

Fuel Filtration: Protecting The Diesel Engine

Nov 12, 2019

 

How does the water divider work

Separation of emulsified water from diesel oil is a long-term requirement for diesel engine operation. The water is removed by a fuel-water separation filter installed in the engine fuel system. The most typical filter media found in such separators are hydrophobic barrier media, such as cellulose treated with siloxane. The medium is separated from water on its hydrophobic surface. Water in the fuel oil is removed and deposited upstream of the medium. As more water is expelled, the beads gather into large droplets and are then emptied into a collection cup. Another successful medium is hydrophilic deep coalescence media, such as glass microfibers. The medium has a high affinity for water. When the water in the fuel meets the medium, it binds to the fiberglass, and over time, the fiberglass binds to more water and grows into larger droplets. Water and fuel together flow through the filter. Downstream, water due to its higher density falls from the fuel stream into the water cup, while dry fuel rises from the filter outlet. The effective removal of water from fuel is considered fundamental to maintaining the fuel injection system and therefore meeting emission targets.


Introduction of ultra low sulfur diesel and biodiesel - effects on water separation

When a step change is made in a raw material specification, it usually produces unexpected results. The diesel industry is riddled with unforeseen problems that cascade into areas that seem unrelated to measures taken to meet specific emissions requirements. The transition to ULSD is no exception. To meet the required sulfur content, ULSD is hydrodesulfurized, a refining step that removes sulfur not only from the diesel distillate, but also from the waxy material. The result is ultra-clean fuel. Unfortunately, it is an ultra-clean fuel that has lost its inherent lubricity. Fuel lubricity is critical to emissions compliance because it is the fuel lubricity that protects the injector system from catastrophic wear and thus ensures accurate combustion control. Oil lubricity is also fundamental to engine operation. The fuel system must maintain pressure in order to eject fuel into the cylinder. Leaks due to wear and tear can cause engine failure due to insufficient fuel. Because ULSD has long failed to meet diesel wear requirements, the industry has added fuel additives to ULSD, such as grease enhancers, rust inhibitors and antiwear agents, to restore the required lubrication.

Due to lubrication defects in early ULSD prototypes, biodiesel began to gain a foothold in the north American diesel market. Biodiesel is a compound of fatty acid methyl ester (FAME) derived from a caustically catalytic reaction between methanol and plant/animal fat. Biodiesel improves the lubricity of the ULSD so that its use as a diesel fuel mix generates some independent power. However, the social and political nature of biodiesel has been the main driving force for its entry into the diesel market. Rising oil prices, perceived demand for domestic or "green" fuel supplies and pressure to minimise fossilised carbon

Emissions prompt state and federal governments to encourage or simply require the inclusion of biodiesel in diesel mixtures. The state of Washington, for example, requires 2 percent biodiesel in all diesel mixtures and will increase that to 5 percent before the state increases manufacturing. Unfortunately, these expectations were developed without a full assessment of the impact of biodiesel on the machinery required to burn the prescribed mixture.

Just as hydrodesulfurization has an unexpected side effect on the lubricity of diesel, additives and biodiesel have a less obvious but equally dangerous unintended consequence: the failure of existing oil-water separators. In short, ULSD blends containing sufficient lubricating additives meet wear requirements, while ULSD blends containing biodiesel create conditions where commercial oil-water separators are unable to remove 40-100% of the fuel entrainment water. The danger with this side effect is that the operator has no way of knowing it is happening. Unlike the case where the particle filter creates an excessive pressure difference before the bypass to warn the operator that the filter's life is over, there is nothing to communicate to the operator that the fuel-water separator is not removing water. Oil and water separators rely on operators or automatic valves to empty the water collection tank when it is full. If the collection chamber is not full, it does not mean that the fuel water separator has failed. It indicates dry fuel. The result is a continuous injection of water into the injection system without the operator's knowledge, which is detrimental to water sensitive surfaces and orifices.


Fuel surface activity

The root cause of the failure of fuel-water separators in ULSD and ulsd-biodiesel mixtures is the increase in fuel surface activity. Despite the separate titles, rustproofing agents, lubricating enhancers, antiwear additives, and biodiesel can all be grouped into one molecular family: surfactants. Fuel and water are substances that do not normally dissolve in each other. If forced to coexist, they are most stable as separate layers, with the fuel layer at the top of the water layer. The degree of layer repulsion can be measured as interfacial tension (IFT). If mixed, it forms an emulsion in which water is briefly deposited in the fuel as a suspended droplet. Surfactants are unique molecules because they strongly associate with both fuel and water. When surfactants are present in the fuel, they associate with water and increase the fuel's compatibility with water. The lower IFT between the two fluids reflects improved compatibility. This unique surfactant property allows more water to dissolve into the fuel.

In addition to dissolving more water into the fuel, the surfactant's role in the inactivation of the fuel-water separator boils down to three S's: size, stability, and surface. When a surfactant containing fuel is mixed with water, the resulting emulsion has a smaller droplet size distribution than an emulsion containing no surfactant. This is due to the surfactant lowering IFT. All fuel - water separation media depend on the physical interaction between water droplets and the media. The surfactant produces droplets small enough that many droplets can pass through the medium without encountering it. Surfactants also stabilize the emulsion so that it does not separate, so droplets that do affect the medium are less likely to be distributed from the fuel to the medium. Also, droplets that affect other droplets have difficulty coalescing into the larger droplets needed for successful separation. Finally, the surfactant binds to the medium and the surface of the water droplet and interferes with the unique surface interaction between the medium and water, making the water in the fuel unstable and separating. In general, the addition of additives and biodiesel to ULSD results in the inactivation of the fuel-water separator and the escape of water into the jet manifold.