When applied, maintained, and properly operated, gas turbine technology offers high reliability and low life-cycle maintenance costs. Successful operation requires hands-on understanding of model characteristics, monitoring systems, important operating limits and ranges, safety hazards, tolerance for starting transients, lubrication requirements, fuel and air quality requirements, capacity, cooling requirements, and control interactions. The following points are significant:

• Rapid thermal transients associated with start-up and rapid load increases and decreases are life-reducing events for gas turbines and should be avoided to the extent possible. Good operations and planning can minimize the need for thermal transients associated with unnecessary starts and stops.

• Fuel quality must be maintained to assure maximum life and performance from any gas turbine. Equipment damage and degraded performance may result from a number of causes, including poor quality fuel.

• Performance indicators that require shutdown include off-line water washing, fire detection, fuel leaks, lubricating oil leaks, control system malfunction, low operating fluid levels, and driven load hazards.

• Monitoring the quality of lubricating oil and management of injection water quality are needed, along with appropriate corrective action when problems arise.

• Provisioning and inventory maintenance of spare parts needed to support the operation are a significant part of good operations management.

Turbine suppliers offer training and instruction manuals with correct operating procedures. Third-party firms also offer services that include system operation, usually for a fee, a share of the savings they generate, or both.

Viable model rating and service life depend heavily on site conditions, including compliance with air and fuel quality specifications. Fuel- and air-born alkali metals in trace quantities with small amounts of fuel-born sulfur are particularly destructive to the high-temperature alloy materials used for combustion and hot turbine parts. This so-called hot corrosion can render a 30,000-hour life turbine unserviceable in less than 2,000 operating hours. Selection of pipeline quality, natural gas fuel limits the potential for fuel-bound sulfur and reduces or eliminates the impact of alkali metals.

Air-born sodium (in common salt) and other alkali metals can be controlled by application of high-performance primary air filters with moisture barriers as required. Further protection is offered by properly scheduled off-line water washing of the compressor section to periodically remove contamination not prevented by the primary air filters.

Axial compressors tend to separate contaminants from primary air and store previously dissolved solids as a coating in the compressor air path. This separation results when evaporation liberates dissolved solids from water droplets in the primary inlet air as temperature rises in the compression process. Evaporated water vapor continues with primary air, while solids settle and coat compressor surfaces. As the process continues, the coating thickness increases until it can no longer adhere to air path surfaces and chunks are reintrained with primary airflow. Concentrations of sodium and calcium are introduced with primary air into the combustor, slugging the hot gas path with vaporous alkali sulfates. Vaporous contaminants condense in the turbine as work extraction lowers gas temperatures, and the resulting globs impact and stick on stator and blade surfaces, starting the irreversible hot corrosion process.

Almost all distillate fuels contain trace quantities of sulfur. Further, improper handling after refining can contaminate distillates with alkali metals as well. Barge transport in salt water and fuel contact with transport tankers, storage tanks, and piping with residual or crude oil residues are examples of fuel quality contamination sources. Appropriate remediation steps can eliminate these sources of contamination. When heavy fuel oil is used, Vanadium is also a concern and usually requires treatment with additives.

Figure 10-62 is an estimate of the effect of fuel type on gas turbine maintenance. Natural gas is considered the optimum fuel and establishes a baseline maintenance factor of 1. Heavier hydrocarbon fuels show a maintenance factor ranging from 3 to 4, due largely to a frequently contained corrosive element and a higher amount of radiant thermal energy, which results in a subsequent reduction in combustion hardware life. Distillate fuels are shown to have maintenance factors ranging from the baseline up to as high as 3, depending largely on contaminants picked up during transportation.

Three basic steps can prevent hot corrosion:

1. Select a turbine model with the best available alloys and coating, as demonstrated by successful operation in field applications at the capacity and efficiency required to meet economic performance expectations.

2. Analyze the installation site to determine which hazards exist.

3. Select the design features required to produce economical operation at site conditions by eliminating or to c a F



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