Gas turbine losses: Recoverable and non-recoverable

Below are excerpts from the paper “Assessment of Recoverable vs Unrecoverable Degradations of Gas Turbines Employed in Five Natural Gas 

Compressor Stations” 

presented at the ASME Turbo Expo 2015 held in Canada. 

The authors of the paper were K K Botros, C Hartloper, NOVA Chemicals, Centre for Applied Research Calgary, Alberta, Canada; H Golshan and D Rogers of TransCanada Pipelines Limited Calgary, Alberta, Canada.

"Gas turbines, like other prime movers, experience wear and tear over time, resulting in decreases in available power and efficiency. Further decreases in power and efficiency can result from erosion and fouling caused by the airborne impurities the engine breathes in.

To counteract these decreases in power and efficiency, it is standard procedure to ‘wash’ the engine from time to time.  In compressor stations on gas transmission systems, engine washes are performed off-line and are scheduled in such intervals to optimize the maintenance procedure. This optimization requires accurate prediction of the performance degradation of the engine over time.

A previous paper demonstrated a methodology for evaluating various components of the GT gas path, in particular the air compressor side of the engine since it is most prone to fouling and degradation. The methodology quantifies the engine-performance degradation over time, and indicates the effectiveness of each engine wash.

In the present paper, the methodology was extended to assess both recoverable and un-recoverable degradations of five gas turbine engines employed on TransCanada’s pipeline system in Canada. These engines are: three GE LM2500+, one RR RB211-24G, and one GE LM1600 gas turbines.

Hourly data were collected over the past four years, and engine health parameters were extracted to delineate the respective engine degradations. The impacts of engine loading, site air quality conditions and site elevation on engine-air-compressor isentropic efficiency were compared between the five engines.

For a given gas turbine engine, recoverable degradation (recoverable by means of soak wash) correlates well with cumulative engine air intake between soak washes for that engine at the specific site condition.

The rate of un-recoverable degradation in terms of permanent loss in compressor isentropic efficiency per 1000 OH correlates well with engine loading (defined as hourly averaged shaft power / site rated power). 

The RB211-24G engine installed at Station 1 is base-loaded and operated on a tight range along the maximum compressor efficiency line, while the LM2500+ and LM1600 engines installed at other stations are operated over relatively wider ranges at part loads. As such, RB211-24G engine seemed to be subjected to un-recoverable degradation more than the rest and needed to be overhauled after three years.

Engines employed in compressor stations in a forested area that have less dust in the air degraded (from recoverable sense) at a much lower rate than engines located in the prairies where there is likely to be more dust in ambient air. Hence, frequency of engine washes is not critical for the former as compared to the latter.

An extension of the current work (i.e. future work) could include assessment of the air filtration performance on each engine, and their impact on respective engine degradation (recoverable vs. un-recoverable), particularly since the five engines examined in the present work are located at different sites of different air quality and environment."