Early in the development of gas turbines, during the 1950’s, the corrosive effects of vanadium became obvious. Many gas turbine manufacturers embarked on research programs to discover a solution to the corrosiveness of vanadium. As a result of all this work, one metal stood out as the most economical and effective of those tested: magnesium.
The minimum treatment ratio of 3 parts of magnesium to 1 part of vanadium was determined to be correct in the late 1960’s to early 1970’s. Initially, the treat rate was set at 3.5 to 1 to insure adequate magnesium would be added. The more appropriate 3:1 was agreed upon as an industry standard since the early 1980’s. The actual stoichiometric amount of magnesium required to just react with vanadium to make safe compounds is only about 0.7:1. However, additional magnesium is added because not only is the desired magnesium orthovanadate formed, but other less desirable magnesium vanadium compounds are also made. To force the reaction to the desired product, more magnesium is required. Other magnesium products are also formed (magnesium oxide and magnesium sulfate). More magnesium needs to be added to offset these less desirable compounds. And finally, since the time allowed for the reaction is very short (high gas velocity in the region of the flame), the greater the amount of magnesium added, the greater are the opportunity for a vanadium atom to find a magnesium atom.
Most if not all gas turbine locations that use additives have selected either sulfonates or carboxylates. CT-30Mg is a truly oil-soluble magnesium additive, that may appear to be more expensive to use than water solutions, but is also much more convenient. Another advantage of CT-30Mg is that it is delivered to the user ready to be used. With water solutions, it is necessary to batch dilute the crystals and to either take a chance on the concentration, do an analysis, or treat with higher levels than needed to be on the safe side. Using more additive than required reduces any cost advantage the water-soluble products might have had.
Regardless of the source of magnesium, the mechanism to solve vanadium corrosion is the same: the melting point of the vanadium contaminants must be raised above the gas turbine temperatures that are encountered. By adding magnesium, vanadium orthovanadate is formed instead of vanadium pentoxide. This reaction is reproduced below:
V205 + 3Mg0 ——————> 3Mg0.V205 (often rewritten as Mg3V208)
Magnesium orthovanadate melts above 1200 C. This temperature is well above typical gas turbine temperatures, especially blade temperatures due to blade cooling. When the system temperature is lower than the melting point of a compound, the compound (magnesium orthovanadate in this case) is not melted, it is a solid. Thus magnesium orthovanadate is solid in the gas turbine. Vanadium pentoxide is only corrosive while it is molten. When converted to the orthovanadate (in the flame) it will pass harmlessly through the system. Thus by adding the appropriate quantity of magnesium (3:1) the system will be protected from corrosion. This has been the case for well over 40 years of magnesium use in gas turbine applications using heavy fuels.
The only disadvantage to using magnesium additives in gas turbines is that the increased amount of metal that needs to pass through the gas turbine will lead to more rapid deposits on turbine blades. This becomes a problem as the deposits add to the weight of the rotating section of the gas turbine. This extra weight causes a loss of power output from the gas turbine. Eventually the gas turbine will need to be stopped to clean the deposits from the blades. With heavy residual fuel oils, the need to stop operation may be every 200 hours of operation when the metal content is high. This operation is completely normal and expected. In fact, these cleaning cycles were considered when the fuel for the gas turbine was being selected.
To clean the turbine, they are often stopped completely. All heavy oil fired gas turbines have a washing system as part of their equipment. The turbine inside, after cooling, is sprayed with water. In some installations it may be standard procedure to completely fill the turbine section. The deposits are allowed to soak for a set period of time and then the water is drained out. This normally will return the turbine to full power. The washing operation is made easier by another compound resulting from magnesium: magnesium sulfate. Magnesium sulfate results from the combustion of sulfur found in all fuels.
In the oxygen rich gas turbine environment, much of the sulfur will be converted to sulfur trioxide. This combines with magnesium oxide and forms magnesium sulfate.
Magnesium sulfate is very water-soluble. So during washing operations the magnesium sulfate in the deposits dissolves, thereby loosening all other deposit materials. This makes the washing operation fairly easy to perform.
Two cautions need to be made concerning washing.
- Each turbine manufacturer has its own recommended procedures. This can also vary among different types of gas turbines from the same manufacturer.
- Another compound formed from magnesium can interfere with good washing. This compound is magnesium oxide. When turbine temperatures are too high, magnesium sulfate can be converted back to magnesium oxide. Magnesium oxide is not water-soluble. Thus if there is too little magnesium sulfate, incomplete washing may result. This can be compensated for by lengthening the soak period and by repeating the washing cycle as required. This decision will be up to the turbine operator and turbine manufacturer.