Gas Turbines - Fuels — Page 367, Lesson 464

All right, let's get into the fuels chapter. We're going to start with a very practical, hands-on skill: looking at a fuel sample and figuring out what's wrong with it. Imagine you've drained a small sample of fuel into a clear container. If that sample looks cloudy or hazy, there are a few possible culprits. The key is to watch how that cloudiness behaves. If the cloudiness rises quite rapidly towards the top of the sample, then you've got air present in the fuel. But if the cloud falls quite slowly towards the bottom of the sample, then that tells you water is present in the fuel. Now, as a general rule of thumb, a cloudy appearance usually indicates the presence of water. So that's your first diagnostic check: air rises fast, water sinks slowly. Now, let's talk about why water is such a big deal, because a certain amount of water is present in all fuel. It's normally dissolved within the fuel, but that dissolved water gives rise to some serious fuel system problems. The first is icing. As an aircraft climbs to altitude, the fuel is cooled, and the amount of dissolved water it can hold is reduced. So water droplets form, and as the temperature is further reduced, those droplets turn into ice crystals which can block fuel system components. That's a critical failure mode. The second problem is fungal growth and corrosion. There's a microbiological fungus called Cladosporium Resinae that is present in all turbine fuels. This fungus grows rapidly in the presence of water to form long green filaments which can block fuel system components. And here's the nasty part: the waste products of the fungus are corrosive, especially to fuel tank sealing substances. To combat these problems, we add something called FSII, which stands for Fuel System Icing Inhibitor. The inclusion of FSII in the fuel will help to overcome both the icing problem and the fungal growth and corrosion problems. Now, there are other additives we need to cover. First, there's HITEC, which is a lubricity agent. A lubricity agent is added to the fuel to reduce wear in the fuel system components. Then we have static dissipater additives, which partially eliminate the hazards of static electricity generated by the movement of fuel through modern high flow rate fuel transfer systems. Next, corrosion inhibitors protect ferrous metals in fuel handling systems, such as pipelines and storage tanks, from corrosion. Interestingly, certain of these corrosion inhibitors appear to improve the lubricating qualities—the lubricity—of some gas turbine fuels. Finally, we have metal deactivators, which suppress the catalytic effect which some metals, particularly copper, have on fuel oxidation. Let me pause there on that last one, because it's a subtle point. Copper can act as a catalyst, speeding up the oxidation of the fuel. The metal deactivator suppresses that catalytic effect, keeping the fuel stable. Now, back to water. Water is always present in fuel, and the amount will vary according to the efficiency of the manufacturer's quality control and the preventive measures taken during storage and transfer. But there are further measures we can take to minimize water accretion once the fuel has been transferred to the aircraft tanks. That brings us to water drains. If the fuel can be allowed to settle after replenishment, then the water droplets, being heavier than the fuel, will fall to the bottom of the tank and can then be drained off through the water drain valve. That's why we sump the tanks—to let that water settle out and get rid of it before flight. So to tie it all together: you check your fuel visually for cloudiness, you understand that water is the enemy causing icing and fungal growth, you rely on FSII to inhibit icing and fungus, HITEC for lubricity, static dissipaters for static electricity, corrosion inhibitors for ferrous metals, and metal deactivators for catalytic oxidation. And finally, you use the water drain valve to physically remove settled water from the tanks. That's the complete picture of fuel handling and additives.