Exploring maintenance trends and needs for LM series industrial gas turbines when used in peak and base load operations. By Gregor Stöcker

Aeroderivative gas turbines are a popular choice for energy generation thanks to their fast response times, reliability, efficiency and, most of all, their flexibility.

Because they are based on advanced aircraft engine technologies and materials, they are significantly lighter and have a smaller footprint than their heavy industrial gas turbine counterparts.

With up to 45 per cent efficiency compared to up to 35 per cent for heavier industrial gas turbines, these turbines are often seen as ideal choices in smaller scale energy generation of up to 100 MW.

The turbines are also popular due to their fuel flexibility allowing a combination of natural gas and liquid fuel operations. And they look set to remain in demand in the coming years.

The LM6000 turbine is well suited to peak usage.
The LM6000 turbine is well suited to peak usage.
Credit: MTU Maintenance

According to a 2016 study by research firm Technavio, the global aeroderivative gas turbine market is expected to grow at a compound annual growth rate of nearly five per cent between 2016 and 2020.

And speaking at the Western Turbine Users Inc. Conference last year, Axford Consulting indicated that Asia, Africa and the Middle East would continue to be growth regions, with orders from Mexico and Russia bolstering figures for the North American and European regions respectively.

Usage of aero-derivative gas turbines covers a broad range of peak and base load applications.

Base load maintenance

Base load operations are typically characterized by ’24/7′ continuous operations with only two shutdowns a year for half-yearly inspections.

Primary concerns for base load operations are maintaining and enhancing turbine performance, ensuring operational reliability and, of course, minimizing the associated costs of these efforts.

Turbines vary greatly according to their usage and operating environment and all have individual needs.

When it comes to the LM2500 and LM6000 engines, this is as wide-ranging as the variants themselves: As ‘oldies but goodies’, the turbine portfolios go from newly-introduced to sunset models and cover many different standards and modifications.

In comparison, LM5000 turbines are in the ‘sunset’ phase. The turbines are based on the CF6-50 aero engine and were first introduced in 1978.

Parts of turbines used for peak operation are likely to see more thermal distress
Parts of turbines used for peak operation are likely to see more thermal distress
Credit: MTU Maintenance

As they are no longer in production, GE has been phasing out aftermarket support and transferring it to suppliers such as MTU Maintenance in recent years.

However, despite their age, LM5000s are still going strong – which means that the maintenance focus is on continuing operations until planned end-of-life in a cost-effective way.

In such cases, MTU Maintenance focuses on improving repairs so that new parts aren’t needed as often.

Furthermore, we implement strategies such as incorporating technical improvements from other LM engines.

For instance, a better stationary oil seal can replace a Teflon seal on the LM5000 to prevent oil leakage and increase durability. We are also working on a dovetail coating refurbishment of HPC blade stages 3 to 5 instead of having to conduct a blade replacement – creating significant savings for this high cost item.

Additionally, MTU Maintenance advises regular maintenance to help avoid more costly issues building up – such as regular water washing in the field.

Another tip is to protect inlets during downtime to avoid corrosion that can lead to removal problems and, ultimately, higher scrap rates of parts such as compressor vanes.

Generally speaking, regular inspection is always recommended, in particular for parts such as the LM5000 compressor front frame case which has a reputation for forming cracks. Early recognition and monitoring can prevent more serious, unserviceable damage and is always a good idea.

Peak load generation

Peak load generators are often used to support the grid at times of peak demand, to compensate for any fluctuations in the grid caused by renewables, or as an emergency supply option during extreme conditions, such as typhoons or tornadoes.

Although aero-derivatives are well-suited to peak load usage due to their fast start-up, stop and response times – the LM6000 family for instance, is five-minute start capable and able to reach maximum power in five minutes – though this type of operation can require more in the way of maintenance than base load operations.

This is due to multiple starts and shutdowns in a short timeframe. MTU Maintenance for instance has observed that parts of turbines used for peak operation are likely to see more thermal distress and material fatigue (thermal cycling).

In terms of the LM6000 series, fluctuating loads can lead to disproportional pressure ratios between seal air and sump air, which can result in small oil leakages, and in turn deposit buildups or even damage to bearings.

Furthermore, MTU Maintenance also tends to see more wear on the rotating seals, honeycombs and stationary seals. While these findings can occur in base load operations, this generally happens much later and after a much higher number of operating hours. To ensure peak load turbines run smoothly, therefore, we recommend scheduling half-yearly inspections and recalling the turbine once it reaches 4000 hours of operations or has completed 450 fired starts.

Communication

The abovementioned maintenance concerns and trends are just that – something to think about and be aware of when it comes to maintenance planning.

Early recognition, regular checks and an experienced partner can go a long way to continued operational performance and reduced costs in the long term.

Open communication regarding expectations, pain thresholds – for instance cost versus turnaround time – and turbine maintenance schedules/future plans are also key to a great maintenance partnership.

Gregor Stöcker is Director Sales Industrial Gas Turbines, MTU Maintenance, a leading maintenance provider for GE’s LM series of aero-derivative industrial gas turbines.