Gas Turbines - Fuel Systems — Page 376, Lesson 472

Right, we’re now into the fuel system components themselves. We’ve already covered the fuel tank and the low-pressure pump, so let’s pick up with the next component in the line — the fuel heater. The fuel heater completes the warming of the fuel and the elimination of ice crystals that may occur. So its job is twofold: it finishes the job of heating the fuel, and it gets rid of any ice crystals that might have formed. It uses compressor delivery air to warm the fuel — that’s the hot air bled from the compressor section of the engine. The heater may be automatic, working in conjunction with the FCOC — that’s the fuel-cooled oil cooler — to maintain a predetermined fuel temperature. Or it may be manual, selected by the flight engineer. So you have two modes: automatic, where it works with the FCOC to hold a set fuel temperature, or manual, where the flight engineer selects it. Next is the fuel filter. The fuel filter is in the low pressure side of the system. Its job is to protect the delicate control components within the HP fuel pump and the fuel control unit — the FCU — from any dirt or contamination. So it’s a protective component, sitting on the low-pressure side, guarding the sensitive parts downstream. Then we have the flowmeter. The flowmeter measures the instantaneous fuel flow in gallons per hour or kilograms per hour. It may also include an integrator to sum the total amount of fuel used since the engine was started — that’s the totalizer. So the flowmeter gives you the instantaneous rate, and if it has an integrator, it also gives you the total fuel used since start. Fuel pressure and temperature may be sensed at this point in the system and indicated to the pilot to allow the system to be monitored. So at this stage, we can pick up fuel pressure and temperature readings and show them to the pilot for monitoring. Now we come to the high pressure fuel pump — the HP pump. It’s driven by the engine high pressure shaft through the HP gearbox, and it raises the pressure and flow required for the demanded engine thrust setting. So the HP pump is driven off the engine’s high-pressure shaft via the HP gearbox, and it boosts the fuel pressure and flow to match the thrust the pilot is demanding. The HP pump illustrated is representative of the type used in some engines — it’s an axial piston type pump. Other engines may use a spur gear type HP pump, which is simpler but will still supply the pressure and flow required. Any excess is recycled back to the inlet side of the pump. So you have two types: the axial piston pump, which is the more complex one shown in the figure, and the spur gear pump, which is simpler — and in that simpler design, any excess fuel is just recycled back to the inlet. Let me show you the axial piston pump figure so you can see the layout. So to recap the sequence: fuel heater warms the fuel and removes ice crystals, the filter protects the HP pump and FCU from contamination, the flowmeter measures instantaneous flow and possibly total fuel used, pressure and temperature can be sensed for monitoring, and then the HP pump raises pressure and flow to match the demanded thrust. That’s the fuel system components in order.