Protection of gas turbines against degradation

Below are excerpts from the paper, 'Gas Turbine Degradation' presented by Rainer Kurz, Cyrus Meher-Homji, and Klaus Brun at the 43rd Turbomachinery & 30th Pump Users Symposia (Pump & Turbo 2014) September 23-25, 2014, Houston, Texas.

While engine degradation cannot entirely be avoided, certain precautions can clearly slow the effects down. These precautions include the careful selection and maintenance of the air filtration equipment and the careful treatment of fuel, steam, or water that are injected into the combustion process.

The site location and environmental conditions which dictate airborne contaminants, their size, concentration, and composition need to be considered in the selection of air filtration. Atmospheric conditions such as humidity, smog, precipitation, mist, fog, dust, oil fumes, and industrial exhausts will primarily affect the engine compressor. Fuel quality will impact the hot section. The cleanliness of the process gas, entrained particles, or liquids will affect the driven equipment performance.

While the rate of deterioration is slowed by frequent online washing, thorough on-crank washing can yield a more significant recovery. However, if the wrong detergents are used, some of the solvent may stick to the blades, or the washing process simply transports contaminant from the front stages of the compressor to rear stages or the turbine section. No matter how good the washing, the rear stages of the compressor will not get cleaned effectively. If the compressor blades can be accessed with moderate effort, hand cleaning of the blades can be very effective.

The rate of degradation of components depends to a large degree on the amount and size of degrading substances entering the engine. The quality of air, the type and quality of fuel, the quality of water or steam determine the rate at which the engine losses power and efficiency.

While engine degradation cannot entirely be avoided, certain precautions can clearly slow the effects down. These precautions include the careful selection and maintenance of the air filtration equipment, and the careful treatment of fuel, steam, or water that are injected into the combustion process. It also includes obeying manufacturers’ recommendations regarding shut-down and restarting procedures. For the driven equipment surge avoidance, process gas free of solids and liquids, and operation within the design limits need to be mentioned. With regards to steam injection, it must be noted that the requirements for contaminant limits for a gas turbine are, due to the higher process temperatures, usually more stringent than for a steam turbine.

The site location and environment conditions, which dictate airborne contaminants, their size, concentration, and composition, need to be considered in the selection of air filtration. Atmospheric conditions, such as humidity, smog, precipitation, mist, fog, dust, oil fumes, or industrial exhausts will primarily affect the engine compressor. Fuel quality will impact the hot section. The cleanliness of the process gas, entrained particles, or liquids will affect the driven equipment performance. Given all these variables, the rate of degradation is impossible to predict with reasonable accuracy.

Thorough on-crank washing can remove deposits from the engine compressor blades, and is an effective means for recovering degradation of the engine compressor. The engine has to be shut down and allowed to cool down prior to applying detergent to the engine compressor while it rotates at slow speed. Online cleaning, where detergent is sprayed into the engine running at load, can extend the periods between on-crank washing, but it cannot replace it. If the compressor blades can be accessed with moderate effort, for example, when the compressor casing is horizontally split, hand cleaning of the blades can be very effective.

Air Filtration System

Fouling of inlet filters occurs progressively over time. This leads to increased pressure drop in the inlet system, and as a result, reduced engine power and efficiency. Figure 27 shows the relative impact of the pressure loss in the inlet system on power and efficiency. Self-cleaning filters, where appropriate, or changing of filter pads or cartridges can reverse this pressure loss. It must be noted that air filtration systems are always a compromise between filtration effectiveness, pressure loss, and size or cost of the system.

The filtration system has to be appropriate for the type of contamination that is expected. Some types of filters are very effective for small particle sizes, some are specifically designed for high dust loads; others are effective in keeping droplets (with potentially dissolved contaminants) out of the engine. A complete discussion of air filtration options was given by Wilcox et al., (2011). Figure 27: Impact of inlet pressure loss on engine power and heat rate Schroth and Cagna (2008) reported, that the use of three stage high efficiency filters (H11 end stage) allowed keeping the performance deterioration below 2% for a year of operation without having to water wash the engine. The results were gathered for machines in the size range of 4.5 to 232 MW, operating in facilities in Southeast Asia and Europe. The useful filter life time for all stages was reported to exceed one year, and reached up to three years, although the ultimate useful lifetime had not been determined, since the final pressure drop of the filters had not been reached at the time the report was written. In this study, the authors also evaluated the trade-off between a higher inlet pressure drop versus lower fouling rates of a three-stage high efficiency filter (F6-F9-H11) system and a two-stage system (F6-F8). They found that the higher pressure loss of the three-stage system is more than compensated by the reduced fouling and the lower frequency of shutdowns for water washing.

Similar results are also reported by Hepperle et al., (2011), where upgrades in filtration efficiency significantly reduced the power loss from fouling. The biggest effect, a reduction in power losses due to fouling of 3.5% per year, was reported when an H10 rear stage was added to an existing G4-F8 air filtration system.

Water washing

Several different methods of gas turbine compressor cleaning have been applied over the years, but “wet cleaning” has been found to be by far the most effective and economic technique. However, today's sophisticated large industrial engines and blade coatings require appropriately designed cleaning systems to ensure safety, reliability and optimum efficiency. Two different wet cleaning techniques, known as offline (crank wash) and online cleaning, are generally applied. Under extreme fouling conditions, hand washing of the IGVs may have to be conducted if time permits. During overhauls, hand cleaning of the full axial compressor is most effective.

(Dr. Rainer Kurz is Manager, Systems Analysis, at Solar Turbines Incorporated in San Diego, California. He is an ASME fellow, and a member of the Turbomachinery Symposium Advisory Committee.

Cyrus Meher-Homji is an Engineering Fellow and Technology Manager at Bechtel Corporation working with the LNG Technology Group as a turbomachinery advisor on the aeromechanical design, selection, and testing of large compressors and gas turbines.

Dr. Klaus Brun is the Director of the Machinery Program at Southwest Research Institute. He is the chair of the ASME-IGTI Board of Directors, an editor of Global Gas Turbine News, Executive Correspondent of Turbomachinery International Magazine, and an Associate Editor of the ASME Journal of Gas Turbines for Power.)