Gas turbine operators face many operational problems relative to the overall operation of their plants. Many of these problems are mechanical in nature. CT Magnesium Corrosion Inhibitors (CT-30Mg) can minimize many of these problems when caused by chemistry.
High Temperature Corrosion
High temperature corrosion in gas turbines is caused primarily by vanadium. When the vanadium fuel contaminant is burned, it combines with oxygen to form primarily vanadium pentoxide (V2O5). This compound melts about 675 C, well above the temperatures found along the combustion path in the turbine. Vanadium pentoxide is a “solvent” for the passivating metal oxide that forms on the cobalt-chromium-nickel alloy surface. It dissolves the oxide coating causing another layer of the metal to form the oxide, which in turn is dissolved (each time the oxide dissolves, it is removed from the metal surface), which in turn causes another layer of the metal to form the oxide, etc. This process will continue until eventually the metal eventually disappears. When even small amounts of sodium are present the corrosion rate is accelerated. And with a higher sodium to vanadium ratio, the corrosion rate increases. The corrosion rate is also accelerated the higher the temperature of the gas path.
Filters
Filters are very important on gas turbines. Turbines are multi-million dollar pieces of equipment with several components that have very tight tolerances (clearances). The filters are designed to remove as much “solid” material from the fuel as possible to protect flow dividers, fuel nozzles, fuel pumps, etc. Also, the hot gas stream is “blowing” through the gas turbine at a fairly high velocity so any particulates in the fuel that pass through the combustions system can cause erosion of the rotating blades. This is not acceptable.
Low pressure filters are normally installed inside of large filter housings. Paper filters have provided very good service through the years. Paper filters have pore sizes of 5, 10, or 25 microns, depending on the fuel and the turbine manufacturer’s requirements. Less viscous fuels tend to have the smaller sizes.
Metal screens are more robust than paper filters. Metal screens are very expensive to purchase initially, but they can be cleaned (usually in an ultrasonic bath) to remove trapped debris. As their useful life continues, the holes in their surface will become more and more blocked with particles that cannot be removed until the filter screens need to be removed from service permanently.
High pressure filters are located after the main fuel pump. High-pressure filters normally comprise a single sheet of metal screen supported on a stiff “spring” encircling the inside of the filter element. The size of these filter elements are about the same as the paper elements described above. High-pressure filters do very little filtering. Their primary purpose is to act as the last place to stop any particles large enough to damage the flow dividers or fuel nozzles. Still, these filters often end up coated with a waxy material. Increases in fuel temperature are normally sufficient to control this problem. High-pressure filters typically have 75 or 100 micron pore sizes.
Storage Tanks
The major problem with fuel storage tanks is water accumulation in their bottoms. This is quite normal. As fuel (in some cases well over a million liters) is stored in tanks, the water that represents only a tiny fraction of the total fuel settles to the bottom. For example, a relatively dry crude oil may contain only 0.1% water, but in one million liters, this represents 1000 liters of water. Over time, this settles in the tank. A very normal operation at gas turbine sites is draining water from the tank bottoms. There should be a regular schedule set up to do this and it should be adhered to. High levels of the contaminants sodium and potassium are contained in this tank bottom water (as well as any other water found in fuels). If any of this water gets pulled into the suction of the fuel pump and gets all the way to the gas turbine, corrosion damage could result in the hot section. Frequent and regular tank draining is almost as important as using a magnesium additive.
Solutions to Chemical Problems
Problem solutions are divided into two general areas:
(1) fuel washing to remove the water-soluble contaminants sodium and potassium
(2) the subsequent addition of a magnesium additive to inhibit the effects of vanadium, which is not water-soluble.
Fuel Washing
The principle of fuel washing is to mix “clean water” into the fuel and then remove all the water in a subsequent step. Water-soluble sodium and potassium are removed along with the water. Sodium and potassium arise from salts (typically sodium chloride – the salt in salt water – and potassium chloride) that are contained in the crude oil as it is pumped from the ground. Thus the only way sodium and potassium can exist in the fuel is to be present in water droplets. It is important to realize the sodium and potassium are very concentrated in the droplets of water. It is not uncommon to have concentrations above 2000 ppm for both these elements in the water droplets. Thus for a residual fuel oil that contains 1% water and sodium plus potassium of 40 ppm, the water droplets would contain 40/0.01 = 4000 ppm in the water droplets.
To remove the sodium and potassium, it then becomes a “simple” matter or adding a quantity of sufficiently pure water, mix it thoroughly into the fuel to contact the water droplets in the fuel to dilute the concentration of sodium and potassium in the water droplets, and then to remove as much of the water as possible. These steps are accomplished in the following manner.
Addition of a demulsifier is normally done first. Water and oil/fuel can form an emulsion when mixed vigorously. Petroleum emulsions are stabilized by naturally occurring materials found in nearly all petroleum products (asphaltenes, waxes, sediment, dirt, metal soaps, and other materials). And after an emulsion of oil and water forms, the water will resist being removed from the fuel. If this occurs during fuel washing, all that is accomplished by “washing” is to add additional water (normally containing more sodium and potassium) to the fuel. So in order to eliminate existing emulsions and prevent the formation of new emulsions a demulsifier is added first. Emulsifiers mimic the chemical structure of many of the materials that stabilize emulsions with an oil-loving tail and a water-loving head. This allows them to penetrate the emulsion stabilizing film that forms on water droplets. However, specialized molecular branching of the demulsifier disrupts the film, which allows for coalescence of the droplets. As the droplets grow, they become large enough that gravity will cause them to “drop” from suspension in the fuel. Addition of water follows the addition of the demulsifier. Normally between 5 and 10 percent of the fuel volume of water is added. The quality of the water should keep the contained sodium and potassium as low as practical. It is not necessary to have water with zero sodium and potassium for it to be useable; their concentration should just be low (typically in the range of 50 ppm together). The water is then intimately contacted with the fuel to insure that the fuel droplets of water are diluted with the fresh water. For electrostatic precipitators, this is normally accomplished with a mix valve. In the case of a centrifuge, this is done with a mix tank. A mix valve is a globe type valve that is closed sufficiently to obtain a pressure on the fuel/water/demulsifier stream as it passes the valve. The net result is the passage of the materials through the valve causes the water to break into very fine droplets. These droplets will mix with the droplets that are already present in the fuel. This achieves the desired dilution of the “brine”. In a mix tank, slowly rotating paddles are used to mix the fuel and water mixture. Since centrifuges have more difficulty breaking emulsions, the mix tank causes fewer emulsions for the centrifuge to resolve. Electrostatic units are more efficient in breaking emulsions, so their mixing method can cause emulsions, but this allows much more intimate mixing.
The fuel/water mixture is fed into the electrostatic unit or the centrifuge for resolution of the mixture into clean fuel and water. Both methods follow Stoke’s Law. Stoke’s Law is a relationship of how materials settle. The application of Stoke’s Law can be viewed watching raindrops falling down a windowpane.
Both electrostatic and centrifuge methods minimize viscosity by heating the oil (because an electrostatic unit is closed, higher temperatures can be achieved). Heating also greatly reduces the specific gravity of the oil and of the water, although only to a lesser degree. Beyond this the two methods differ. Centrifuges increase the gravity term due to centrifugal force, while electrostatic units rely on increasing the diameter of the water droplet. Since the diameter is a squared term, many times the electrostatic method can effect water separation more quickly. However, normally the selection of a fuel washing method is dependent on the size of the installation: electrostatic treaters being more efficient for larger installations. Forward clean fuel to storage. No matter how the fuel is washed, after it has been washed it will be sent to clean fuel storage. Before the fuel is sent to storage there is one critical step that must be performed: the quality of the fuel must be determined. Remember, the whole purpose of fuel washing is to remove sodium and potassium to a level that meets the turbine manufacturer’s requirements for these elements (0.5 to 1.0 ppm depending on the manufacturer and the fuel). Samples of the “clean” fuel will be checked for these elements using an emission spectrometer. Only if the fuel meets the specification will it be sent to storage. If it does not, it will be returned to raw fuel storage, where it will then be returned to the fuel washing process along with other fuel from raw storage. With both electrostatic and centrifuge systems, it is normal to have to return fuel at the start of the operation until the entire system has reached equilibrium. Also, the level of demulsifier being used may not be proper to sufficiently lower the amount of water in the fuel to meet the trace metal requirements. Once everything is correct, all subsequent fuel should meet the required specifications. Only spot samples will be taken once the system is at equilibrium. Treated fuel storage has several advantages. The fuel will continue to shed water, which will actually improve the quality of the fuel as the gradual drop out of water will also carry more sodium and potassium with it. And more importantly, the treated fuel will be readily available when the plant requires it. Most fuel washing plants are sized smaller than the hourly requirements for fuel if all the gas turbines would be operating. This is done as a cost savings measure and also recognizes the fact that most plants do not run around the clock, nor are all gas turbines always operating. The fuel washing plant can easily keep up with fuel needs by running during gas turbine down times.
