Preventing Corrosion in Gas Turbine Fuel Systems

Most failures of fuel system components, such as flow dividers and fuel pumps, are caused by the effects of corrosion. Rusting of iron and steel can jam rotating components, while surface pitting of many materials from traces of acids in the fuel can weaken parts. It can be shown that preventing fuel system corrosion can substantially increase the reliability and availability of the gas turbine. Part of the reason that corrosion is such a problem in gas turbine liquid fuel systems is that most land based turbines operate most of the time on natural gas. This means that the liquid fuel system sits idle for most of the time. The liquid fuel almost always contains some water from sources such as condensation in the tank. Water also enters the fuel system from moisture rich combustion gases that can leak past check valves while the turbine is operating on natural gas. When the liquid fuel system is idle, this water separates from the fuel, collecting into a layer inside components. This forms an area of rust in the bottom of any unprotected component. This rust can prevent components from operating or can plug orifices and fuel nozzles downstream when the liquid fuel system is operated. Corrosion in the fuel system can also come from traces of acid in the fuel. This usually comes about when operating on crude or residual oils with a high sulfur content. Water washing and heating of the fuel as part of the fuel treatment process may cause a chemical reaction that produces small amounts of sulfuric acid in the fuel. Over time, this acid may cause surface pitting in fuel system components. These pits can encourage and speed the formation of fatigue cracks in highly stressed parts such as shafts or bearings. These fatigue cracks will eventually cause the failure of these parts. Two methods have been used to protect fuel system components against these corrosion problems. The first is to substitute inherently corrosion resistant materials, such as stainless steels, in place of existing materials. This is effective, but it is relatively expensive and creates lubrication and wear problems. These problems come from the poor wear characteristics of stainless steels and many other corrosion resistant materials when rubbing against other metals. Cathodic protection is the other method of protecting components against corrosion. This consists of introducing sacrificial anodes of a material, such as magnesium or zinc, that is more electrochemically active than frequently used materials such as cast iron or steel. The presence of these anodes reverses the corrosion process and makes the iron and steel parts the cathodic or protected end of the corrosion reaction. These anodes must be changed periodically since they are gradually consumed during use.