**3. Incorporation of water**

#### **3.1. Sources of water in fuels**

Water is a very common contaminant in all fuels. The most common ways in which water is incorporated into fuel is through contact with air, condensation of water in the air on cold tank surfaces, leaks in floating cover storage tanks and hatches left open. In most cases, liquid water will drip through the fuel with a small amount dissolving in the fuel. Simple contact with air will cause surface layers of the fuel incorporate water based on the solubility of the water in the fuel, the relative humidity of the air and the temperature. Fuel typically arrives from the refinery saturated with water, which makes reducing the dissolved water in fuel impossible to eliminate. Through mixing, water reaches the entire storage tank. Since the temperature and humidity of the air is constantly changing, the amount of water in the fuel never reaches a steady state.

Kerosene based fuels will dissolve between 40 and 80 ppm water at 20°C. The solubility rises with increasing temperature and varies considerably with the composition of the fuel. Fuels with a high aromatic content can dissolve much more water than those with little aromatic content [4]. Single ringed aromatic compounds, for example dissolve 5–10 times more water than similar saturated compounds. Mixing fuels changes the solubility of water, which can lead to water depositing out of the fuel after mixing.

In addition to dissolved water in jet fuel, there is frequently also free water and emulsified water. Free water is water that sinks to the bottom of the storage tank. Since water is significantly more dense than fuel, if through changes in temperature or fuel composition the fuel exceeds its saturation point water droplets will form and sink to the bottom of the storage tank. As the seasons change is repeated, it is possible for a significant amount of water to settle to the bottom of the tank.

An emulsion is formed when one liquid forms tiny droplets of less than 100 microns that are suspended in another liquid. Many emulsions will spontaneously separate if given enough time, but some will remain suspended indefinitely. Emulsions can form with changes in temperature which can lower the solubility below the saturation point. The can also form when new fuel is added rapidly, dispersing free water into the bulk of the fuel as tiny droplets. Naturally occurring surfactants and surfactants formed during the refining process are known to stabilize water emulsions in jet fuel. These surfactants include naphthenic acids and sulfonic acids and they can cause an emulsion to be stable indefinitely [5].

#### **3.2. Effects of water on fuels**

**3. Incorporation of water**

**Table 2.** Some common additives found in jet fuel.

196 Flight Physics - Models, Techniques and Technologies

Corrosion inhibitor/lubricity improver (CILI)

Fuel system icing inhibitor (FSII)

**3.1. Sources of water in fuels**

never reaches a steady state.

to the bottom of the tank.

lead to water depositing out of the fuel after mixing.

Water is a very common contaminant in all fuels. The most common ways in which water is incorporated into fuel is through contact with air, condensation of water in the air on cold tank surfaces, leaks in floating cover storage tanks and hatches left open. In most cases, liquid water will drip through the fuel with a small amount dissolving in the fuel. Simple contact with air will cause surface layers of the fuel incorporate water based on the solubility of the water in the fuel, the relative humidity of the air and the temperature. Fuel typically arrives from the refinery saturated with water, which makes reducing the dissolved water in fuel impossible to eliminate. Through mixing, water reaches the entire storage tank. Since the temperature and humidity of the air is constantly changing, the amount of water in the fuel

Agreement Allowed Required Required Required

Agreement Agreement Required Required Required

**Additive type Jet A Jet A-1 JP-4 JP-5 JP-8** Antioxidant Allowed Required Required Required Required Metal deactivator Allowed Allowed Agreement Agreement Agreement

Kerosene based fuels will dissolve between 40 and 80 ppm water at 20°C. The solubility rises with increasing temperature and varies considerably with the composition of the fuel. Fuels with a high aromatic content can dissolve much more water than those with little aromatic content [4]. Single ringed aromatic compounds, for example dissolve 5–10 times more water than similar saturated compounds. Mixing fuels changes the solubility of water, which can

In addition to dissolved water in jet fuel, there is frequently also free water and emulsified water. Free water is water that sinks to the bottom of the storage tank. Since water is significantly more dense than fuel, if through changes in temperature or fuel composition the fuel exceeds its saturation point water droplets will form and sink to the bottom of the storage tank. As the seasons change is repeated, it is possible for a significant amount of water to settle

An emulsion is formed when one liquid forms tiny droplets of less than 100 microns that are suspended in another liquid. Many emulsions will spontaneously separate if given enough time, but some will remain suspended indefinitely. Emulsions can form with changes in Water can be present in fuel as either dissolved water, emulsified water or free water. Free water is water that is collected at the bottom of the tanks. In a well-designed storage system, the free water collects in well-defined places where it can be periodically drained. The dominant problem with free water that remains in the tank is it forms an interface for microbiological growth, which will be discussed in Section 5. Emulsified water, while it may eventually settle is a more difficult problem since it cannot be conveniently drained and can clog filters and forms a nucleus for ice formation. Typically, emulsified water and free water in the tanks of aircraft are picked up with the fuel and fed to the engine [6].

Dissolved water can be a significant problem since there is no physical way to remove it from the fuel. It is simply pumped into the aircraft. In flight aircraft fuels can experience a wide range of temperatures which certainly change the solubility of water in the fuel. As the temperature of the fuel decreases due to the low temperatures observed at high altitude small droplets of water form. Depending on conditions at low temperature, ice crystals can form and remain suspended in the fuel [7]. In order to reduce the possibility of ice formation fuel system icing inhibitors (FSIIs) have been developed and are required in military jet fuels. Diethylene glycol monomethyl ether (DiEGME) and triethylene glycol monomethyl ether (TriEGME) are two common FSII additives that act by stabilizing water in the fuel, reducing the formation of droplets, which decreases the temperature at which ice can form. A second approach under development is the use of a water reactive compound as an additive to completely remove the water from the fuel system. These compounds are either ketals or ortho esters which spontaneously react with water to form alcohols and either ketones or esters. The products of the reaction with water also act as icing inhibitors [8]. The mechanism of one such additive is shown in **Figure 2** below.

The effect of water on the usability of jet fuel depends greatly on the form of the water. Free water tends to settle to the bottom of the tank in pools. Ribbed tanks are frequently used so that the water will collect in predetermined areas and can be drained from the tank. A problem that can arise from the presence of free water is additive depletion. Some additives such as DiEGME and to a lesser extent TriEGME are much more soluble in water than they are in the fuel and will move from the fuel into the free water resulting in significant or complete additive depletion. Typical fuel systems use a filter or separator to ensure that free water is not pumped into the tank of an aircraft. There is still the possibility of free water depositing from the fuel due to changes in temperature and also changes in fuel source.

**Figure 2.** Reaction of a ketal and an orthoester with water.
