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Early warning system: an installation at Connah''s Quay power station, UK, demonstrates how monitoring and analysing combustion driven oscillations in dry low N[O.sub.x] gas turbines can give advance warning of failures in combustion components.

Article Excerpt
Competitive electricity markets everywhere now require plant operators to achieve greater reliability and availability from their generating units, increasing the importance of monitoring systems that provide in-depth information about key aspects of plant health.

The application of on-line combustion monitoring (OLCM) aims to help generators meet and respond to changing energy markets by raising their confidence in the health of the combustion system, giving them early warning of the risk of component failure or the onset of deterioration that could result in major damage downstream in the machine.

The work is based on monitoring the combustion-driven oscillations--dynamics--that are characteristic of modern, dry low-N[O.sub.x] (DLN) gas turbines, in which a lean premixed approach has been adopted by most manufacturers. These combustion-driven oscillations can result in severe component damage leading to reduced reliability and availability. Improvements can be achieved by monitoring these oscillations to provide:

* A component of an overall sophisticated gas turbine health monitoring system

* Optimised combustion performance and reduced risk of failures; and

* Input for analysis of any combustion failures.

Connah's Quay

Power Technology's work concentrated on GE 9FA turbines, of the type installed at Powergen's Connah's Quay power station in North Wales (Figure 1). The aim was to develop the capability to continuously monitor combustion dynamics--as opposed to the post-overhaul tuning activities undertaken by the original equipment manufacturer--that would establish whether there was any correlation between the combustion dynamics levels and combustion component damage.

[FIGURE 1 OMITTED]

The gas turbines (Figure 2) at Connah's Quay have 18 cylindrical combustors around their circumference, each containing five burners fuelled with natural gas. Fuel distribution between four of the burners and the fifth is varied to achieve optimum combustion performance in terms of both emissions and combustion dynamics across the operational load range of the engine.

[FIGURE 2 OMITTED]

In common with several other combined cycle gas turbine sites, the power station had experienced problems with the premature failure of combustion components, increasing the risk of debris damage occurring in the turbine.

The main focus was on the combustion liners of which the inner surface has a ceramic thermal barrier coating (TBC) to protect the complex metal alloy from the high temperature combustion gases.

With a design life of 8000 hours at baseload operation, they should only require replacement at each annual overhaul. However, at Connah's Quay this life expectancy has sometimes not been achieved.

The combustion liners are designed for the DLN system in which the natural gas is premixed with large quantities of air, resulting in lower temperatures than those from diffusion flames. However, CFD studies (Figure 3) carried out...

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