Fuel requirements

Fuel requirements

107. In general, a gas turbine fuel should have the following qualities:

(1)  Be 'pumpable' and flow easily under all operating conditions.

(2) Permit engine starting at all ground  conditions and give satisfactory flight relighting characteristics.

(3)  Give efficient combustion at all conditions.

(4)  Have as high a calorific value as possible.

(5)  Produce minimal harmful effects on the combustion system or the turbine blades.

(6)  Produce minimal corrosive effects on the fuel system components.

(7) Provide adequate lubrication for the moving parts of the fuel system.

(8)  Reduce fire hazards to a minimum.

108. The pumping qualities of the fuel depend upon its viscosity or thickness, which is related to fuel temperature, Fuel must be satisfactory down to approximately -50 deg. C. As the fuel temperature falls, ice crystals may form to cause blockage of the fuel filter or the orifices in the fuel system. Fuel heating and anti-icing additives are available to alleviate this problem.

109. For easy starting, the gas turbine engine depends upon the satisfactory ignition of the atomized spray of fuel from the fuel spray nozzles, assuming that the engine is being motored at the required speed. Satisfactory ignition depends up on the quality of fuel in two ways:

(1)  The volatility of the fuel; that is, its ability to  vaporize easily, especially at low temperatures.

(2)  The degree of atomization, which depends upon the viscosity of the fuel, the fuel pressure applied, and the design of the atomizer.

110. The calorific value (fig. 10-21) of a fuel is an expression of the heat or energy content per pound or gallon that is released during combustion. This value, which is usually expressed in British thermal units, influences the range of an aircraft. Where the limiting factor is the capacity of the aircraft tanks, the calorific value per unit volume should be as high as possible, thus enabling more energy, and hence more aircraft range, to be obtained from a given volume of fuel. When the useful payload is the limiting factor, the calorific value per unit of weight should be as high as possible, because more energy can then be obtained from a minimum weight of fuel.

Other factors which affect the choice of heat per unit of volume or weight, must also be taken into consid- eration; these include the type of aircraft, the duration of flight, and the required balance between fuel weight and payload.

111. Turbine fuels tend to corrode the components of the fuel and combustion systems mainly as a result of the sulphur and water content of the fuel. Sulphur, when burnt in air, forms sulphur dioxide; when mixed with water this forms sulphurous acid and is very corrosive, particularly on copper and lead. Because it is impracticable to completely remove the sulphur content, it is essential that the sulphur be kept to a controlled minimum. Although free water is removed prior to use, dissolved water, i.e. water in solution, cannot be effectively removed, as the fuel would re- absorb moisture from the atmosphere when stored in a vented aircraft or storage tank (para. 118).

112. All gas turbine fuels are potentially dangerous and therefore handling and storage precautions should be strictly observed.