Degradation of performance depends upon environmental conditions and how often the compressor is cleaned. A clean compressor environment can in fact reduce the degradation effects. About 2 to 3 % degradation is expected after about 24000 hrs of operation which can be recovered up to 90%
Technical audits are a useful tool addressing issues like reliability and availability. It is also an ideal tool for plants that are in operation for more than ten years and would like to upgrade their systems / equipment to current state of art.
Audits are conducted on operation and maintenance practices being followed, operational and maintenance issues, forced outages, operating history, maintenance / inspection history etc.
Maintenance and operating costs are also influenced by the quality of the air that the turbine consumes. In addition to the deleterious effects of airborne contaminants on hot-gas-path components, contaminants such as dust, salt and oil can also cause compressor blade erosion, corrosion and fouling. Twenty-micron particles entering the compressor can cause significant blade erosion. Fouling can be caused by sub-micron dirt particles entering the compressor as well as from ingestion of oil vapor, smoke, sea salt and industrial vapors. Corrosion of compressor blading causes pitting of the blade surface, which, in addition to increasing the surface roughness, also serves as potential sites for fatigue crack initiation. These surface roughness and blade contour changes will decrease compressor airflow and efficiency, which in turn reduces the gas turbine output and overall thermal efficiency.
Generally, axial flow compressor deterioration is the major cause of loss in gas turbine output and efficiency. Recoverable losses, attributable to compressor blade fouling, typically account for 70 to 85 percent of the performance losses seen. compressor fouling to the extent that airflow is reduced by 5%, will reduce output by 13% and increase heat rate by 5.5%. Fortunately, much can be done through proper operation and maintenance procedures to minimize fouling type losses. On-line compressor wash systems are available that are used to maintain compressor efficiency by washing the compressor while at load before significant fouling has occurred. Off-line systems are used to clean heavily fouled compressors. Other procedures include maintaining the inlet filtration system and inlet evaporative coolers as well as periodic inspection and prompt repair of compressor blading.
There are also non-recoverable losses. In the compressor, these are typically caused by non-deposit-related blade surface roughness, erosion, and blade tip rubs. In the turbine, nozzle throat area changes, bucket tip clearance increases, and leakages are potential causes. Some degree of unrecoverable performance degradation should be expected, even on a well-maintained gas turbine. The owner, by regularly monitoring and recording unit performance parameters, has a valuable tool for diagnosing possible compressor deterioration.
It is not recommended to remove the filters when the turbine is running. However, pre-filter can be removed while the turbine is running and can be cleaned.
The estimated life of compressor rotor blades is approximately 200,000 hours. However, this life depends upon factors like environment, seacoast etc.
Erosion and corrosion are caused by high humidity, salts in the atmosphere, acidic atmosphere etc. The degradation can be faster in the turbines where systems like inlet fogging is used (which create high foggy environment in the compressor inlet). Erosion on the blades (like thinning) reduces the compressor efficiency. Hence, more than the mechanical integrity of the blades, it is the loss of efficiency which should be addressed by replacing the blades. It may be noted that the compressor rotor blades can be replaced only at a rotor repair shop. It is also recommended to replace compressor stator blades along with rotor blades. It is not uncommon to see replacement of compressor blades for units located nearer to seacoast at 110,000 to 120,000 hours.
It is recommended that thorough examination is made of compressor blades at every major inspection. Critical examination is needed at the second major, i.e at 96,000 hours to evaluate useful life of the compressor blades.
The intervals are based on the fouling of the compressor, in turn the environment, rather than a fixed interval. It should be done at least once in a year even though the environment is clean.
Both are complimentary. The online wash system if used continuously can keep the compressor blades relatively clean by reducing the accumulation of fouling. The loss in output due to fouling can be reduced to approximately 1 to 2 % per year if online washing is used compared to usual loss of about 4 to 5 % if only offline wash is used. During online wash only water is used whereas offline wash uses detergent wash. As detergent wash can remove greasy material, even though online washing is used, it should be followed with offline wash at least once in a year.
We can recommend a comprehensive list of recommended spares for each inspection of a gas turbine after studying details of the unit (like fuel used components installed at present etc.). We have optimized such lists based on our experience so that you will utilize all the parts in the kit and do not store extra in the store (unnecessary inventory). This gives added benefit of less inventory and reduced maintenance cost.
Please contact us with details so that we can develop customized lists for you.
The frequency of preventive checks to be performed on auxiliary equipment is given in O&M manual under a separate section on maintenance of Auxiliary Equipment.
In general, it is recommended that calibration of all switches & transmitters be done along with HGP / Major Inspections. Cleaning and resetting of LVDTs and calibration of control valves be done once in a year depending upon dust seen around LVDTs.
The most common failure reason for Moog Valves is the chocking of filters and internal passages due to contamination of lube oil. The need for clean lube oil (free from suspended particles and water) need not be over emphasized.
It may be noted that the ‘Last Chance’ filter located in the hydraulic oil line before the Moog Valves has a facility to automatically bypass the filter in case the DP across the filter is high. Hence the contaminants can reach the Moog Valves.
It is recommended that these ‘Last Chance’ filter elements be replaced once in a year (they are use and throw type). Any time, a dirty filter element is replaced it is advisable to change the associated Moog valve also (the valve might have been contaminated as the filter bypass mechanism might be working). The removed Moog valve can be cleaned and may be reused in the next opportunity.
Whenever, a Moog valve is replaced, it is advisable to check polarity of each coil wires by stroking the valve with each coil independently. Any polarity reversal in a single coil can be detrimental to control stability.
When ‘Fired Shutdown’ is selected, the control system reduces the fuel to the turbine in a predetermined rate till about 20% of the speed where the fuel is totally cut off. In case, the required fuel flow is less than the calculated fuel flow at any point of time, flame out occurs. You need to check the calibration of the fuel control to solve the problem. In case this repeats, please contact us.
Servo valves do not have a specific design life. It depends upon external factors like contamination of lube oil, sharp edges cutting the leads etc. Depending upon individual plant’s experience on contamination in lube oil, the replacement strategy can be worked out. To reduce problems with servo valves, It is suggested to replace the servo valve annually and sending the removed valve for ultrasonic cleaning. Mere replacement of the pencil filter in the servo valve does not give desired results always.