Gas turbine performance influencers
The performance of all gas turbines is affected during operating because the air being consumed contains airborne particles that will affect the performance in a variety of ways. One of the most readily available indicators of losses in gas turbine performance is a drop in output, either mechanical or electrical, depending upon the installation, although this can sometimes be overlooked, especially if the gas turbine is not operating at part-load conditions rather than at base load. This is because when at part-load, a gas turbine has a degree of flexibility within the operating parameters and limitations to meet the demand placed upon it by increasing the fuel flow or permitting a higher exhaust gas temperature to suit the situation. Whilst this allows the gas turbine to continue to operate, the full impact of the degraded performance may not become apparent until weeks or even months later at an outage.
An alternative method, which requires some calculations, is that of compressor efficiency. The losses experienced can be split into two groups, recoverable and non-recoverable, and affect various sections within the gas turbine in different ways. Recoverable losses can be regained during normal operations. The main type, due to compressor fouling, is usually rectified by carrying out a compressor wash; this can be either online, whilst the gas turbine is operating, or offline, when it is shut down. Non-recoverable losses are due to both the normal wear of internal components, such as blade tips that increase the tip clearance, and also to the ingested material that causes erosion or corrosion.
The damage that the impact of airborne particles in the airflow causes to the compressor blades is called erosion and alters the aero foil profile of the blades thereby changing it from the design point. These changes can be a reduction of the blade chord, which shortens the blade length. This, in turn, increases tip losses, blunting the leading edges and sharpening the trailing edges.
Corrosion, on the other hand, is induced by contaminants that adhere to, and react with, the material of the blades, which will lead to pitting of the surface. This causes roughness and disruption of the airflow, which could ultimately lead to failure of the equipment due to localized stress points.
The challenges ahead
Now that we understand a little of the history of gas turbines, their benefits of operation, and the consequences of not looking after them, let us look to the future.
As we have already stated, gas turbines operate in different environments, such as tropical, coastal, industrial, and desert climates. They also need to deal with a wide array of atmospheric contaminants with various concentrations and particle size distributions.
In this new century, should we ask ourselves if there is a place for gas turbines when we are constantly asked to focus on renewable energy? We understand a great deal about the operation of gas turbines, and, with some consideration and adoption of new technologies, perhaps they can help us buy some time until we can establish a true zero carbon footprint and hydrogen economy.
A gas turbine is at its most efficient when it is operating at base-load conditions, which is full power. Although it takes away some of the engine’s flexibility, it means the engine is producing less emissions than at a part-load setting. If we then ensure we can provide an in-let filtration system that is designed for the specific environment in which the engine is operating, that system will not impede the flow of air into the compressor. Add to this the concept of keeping the compressor clean and free of potentially damaging fouling, and we have an engine that is operating close to its peak performance.
If we then address the way that we handle/treat the emissions from the exhaust, all of a sudden, we have sustainable operation capable of producing large amounts of power.
Of course, it could be argued that not all that power is required all the time. If so, why not use the excess power to produce hydrogen, charge batteries, and fill caverns with compressed air?
We simply need to reevaluate the way we use energy in a responsible way. Let’s look in more detail at the influences and technologies available.