With environmental emissions standards becoming tighter,
the need for more efficient fuel combustion has increased
dramatically. Burning a leaner flame can keep NOx emissions
low, but increases dynamic instability in the engine. This
instability (combustor dynamics) can result in negative
consequences, such as damage to components in the
combustion chamber (nozzles, baskets, transition pieces) or even downstream
components (blades). This can cause unnecessary downtime, equipment repair
costs, and more importantly loss of revenue.
When dry low NOx (DLN) combustion systems were first introduced, the OEMs recognized that there was an issue
with combustion dynamics. Typically the turbine OEM would offer “seasonal tuning.” Tuning engineers will go to the site at the start of each weather season, typically fall and spring, with their own tuning equipment. They will tune the engine for the best balance between NOx and dynamics based on the current conditions at the site. Once the tuning portion is finished they will take the equipment and data with them. Operators were not left with the capability to make additional tuning changes or the ability to continuously monitor the dynamics and therefore were not able to detect any potential changes that occur within the turbine.
Faced with these challenges, combustion dynamic monitoring (CDM) systems have been developed to continuously monitor dynamics and to avoid high dynamic levels than can cause damage to components. These systems have the additional benefit of indicating changes in dynamics associated with hardware failure, allowing users to pinpoint the combustor
with issues and to take action before additional failures or
damage may occur. Since the introduction of these systems
in the late 1990’s there have been many advances in CDM systems.
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Monitoring Solutions
The GTE Combustion Dynamics Monitoring System has been installed in 100+ s Dry Low NOx power plants. The system provides the ability to tune for emissions compliance as needed and early indication of damaged combustion hardware. This allows operators to run their turbine at its optimum setting, allows preventative maintenance, assists with emissions compliance, extends combustion part life, and can help to increase outage intervals.
Portable Systems
GTE’s portable tuning systems consist of pressure sensors that are connected to sensing lines running to some or all of the combustors. These sensors monitor the pressure fluctuations in the combustor. The signals from these sensors are fed into a PC analyzer that breaks the data down into amplitude versus frequency. The frequency range is typically subdivided into smaller bands, each with their own amplitude limit. The goal when tuning is to minimize these dynamic amplitudes and maintain the best emission levels.
Permanent Monitoring Systems
Permanent CDM systems have been developed for continuously monitoring of dynamics and emissions compliance. These systems provide round the clock monitoring so that tuning changes can be made whenever necessary. They also provide the benefit of detecting changes to the mechanical hardware
within the combustion system. Like the portable systems, permanent systems consist of pressure sensors and PC/analyzer. The original permanent systems provided sensors mounted outside the turbine enclosure. The sensors are then connected through sensing lines (SS tubing) to each combustor. Because of the long sensing lines in this arrangement, the ability to purge any condensate buildup must be provided. This requires additional equipment and maintenance. However there are several advantages of this system. It is the most widely used setup, so there is a larger set of data for comparisons between turbines. Because the sensors are mounted outside the turbine enclosure, the ambient conditions the sensors must endure are relatively mild. This allows less expensive sensors to be used and they have a long life expectancy. Finally, because the sensors are outside the turbine enclosure, they can be serviced while the turbine is online.
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High Temperature Sensors
Sensor manufacturers have recently introduced high (1000°F plus) temperature sensors that can be directly mounted to the combustors. These sensors have been deployed on many turbines and results have been favorable. The primary advantage of this sensor type is that it eliminates the sensing lines and the related purging equipment. This simplifies the install and corresponding outage maintenance, because the sensing lines go away altogether. There is an expectation that these sensors may provide more information because they are providing a more direct 
measurement of the dynamics. There has not yet been enough data/experience to prove this out. The biggest disadvantage currently is the cost of the sensors. As the demand/volume increases the sensor costs should decrease. The life expectancy of these sensors has not been fully determined, although it is expected to be shorter than the low temperature sensors.
Intermediate Temperature Sensors
An intermediate temperature (700°F) sensor has also been successfully deployed. The intermediate sensor provides all the advantages of the high temperature system described above, but uses a lower temperature system mounted in a specially designed standoff tube. The standoff tube moves the sensor into a lower temperature area that the sensor can handle. This sensor has been widely used in similar temperature environments. This system provides a nice compromise between cost and maintenance while still maintaining performance.
Advanced Capabilities and Consulting
Experience developed from this installed base has provided GTE the experience to offer both hardware and operational solutions to Power Plant Operators. Given the 100+ systems in operation, GTE has been able to understand significant operational variation and provide additional service to Operators that resolve issues – some times before the issue is even visible to an Operator. Management and communication of this information by GTE and development of recommendations has been a significant benefit to our client’s profitability and overall availability.
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Combustion
