The Performance Degradation Of Gas Turbine
Before we start the degradation, we just got to understand the mechanism of degradation
Mechanism of degradation
Depend upon the sort of fuel gas and therefore the geographical location, the mechanism of degradation is assessed as follow,
- Fouling
- Corrosion
- Erosion and Abrasion
- Particle Fusing
Fouling
Fouling is caused by the adherence of particles on the airfoils and annulus surfaces. The adherence will increase by oil or water mists.
The result’s a build-up of fabric that causes increasing the surface roughness and, to a point, changes the form of the airfoil. Particles that cause fouling are typically smaller than 2 to 10 μm. Smoke, oil mists, carbon, and sea salt are common things with creating fouling within the turbine.
Around 50 to 60% of the fouling can control by a correct selection of air filters. Fouling also can be reduced by the detergent washing of components.
Corrosion
Corrosion is that the wearing of surface metals thanks to a reaction of the metal with the corrosive medium(unlike erosion or fretting which are mechanisms of the erosion of surface metal thanks to mechanical surface action). Usually, the metal reacts with oxygen within the air but there are many other chemical reactions that will participate within the different corrosion mechanisms.
Certain sorts of corrosion (such as oxidation, sulfidation, and hot corrosion) are mainly attacking the recent section of the turbine.
Other types, like crevice corrosion and pitting, are rather found within the compressor section of the turbine. For turbine applications, the foremost important corrosion mechanisms are hot corrosion and cold corrosion.
These mechanisms are relevant for gas turbines and may impact their life and performance. Corrosion within the turbine is especially thanks to inlet air contaminants and turbine fuel.
Fuel side corrosion is usually more noted and severe with heavy fuel oils and its distillates when compared with the gas.
Due to impurities and additives within the liquid fuels that leave aggressive deposits of corrosive products after combustion. Salt-laden air also can cause corrosion on the unprotected parts of the turbine.
Erosion and Abrasion
Erosion is that the abrasive removal of fabric from the flow path by hard or incompressible particles impinging on the flow surfaces.
These particles typically need to be larger than 10 μm in diameter to cause erosion by the impact. Erosion can become a drag for engines using water droplets for inlet cooling or water washing.
Abrasive solid particles attack rotating parts. Collisions between high-speed rotating blades and airborne particles end in metal fragments remove from the blade surfaces.
Particle composition and shape can significantly affect erosion rates. Blade profiles are so carefully designed that even minor abrasions can alter the profiles, it’s going to affect the performance and cause vibration thanks to unbalancing.
Erosion is an upscale problem, since it causes permanent damage, eventually requiring parts refurbishment or replacement. Erosion is proportional to particle concentration and, in severe service with poor filtration, can significantly reduce turbine life.
Particle Fusing:
The fusion of particles on hot surfaces of the turbine can cause affect the performance of the turbine. If the particle within the fuel undergoes the filter and its fusion temperature is less than the turbine operating temperature, the particles will melt and stick with hot metal surfaces.
This will cause severe problems since the resultant molten mass can block cooling passages, alter the surface shape, and severely interfere with heat transfer, often resulting in thermal fatigue. Affected surfaces are usually permanently damaged and can eventually need replacement.
Gas Turbine Performance Degradation
Gas turbine performance degradation are often classified as
- Recoverable Loss
- Non-recoverable loss.
Recoverable Loss
The recoverable loss is typically accompanied by compressor fouling. It is often partially rectified by water washing or, more thoroughly, by mechanically cleaning the compressor blades and vanes after opening the unit.
Eg: Fouling
Non-recoverable Loss
The non-recoverable loss is primarily thanks to an increase in clearance of the turbine and compressor and changes in surface finish and airfoil contour. thanks to this, there’s a discount in component efficiencies.
This reduction cannot recover by operational procedures, external maintenance or compressor cleaning. But only through replacement of affected parts at recommended inspection intervals.
Quantifying performance degradation is difficult because consistent valid field data is tough to get. Also, various site conditions and parameters cause the non-recoverable loss within the turbine.
They’re the mode of operation of the turbine, contaminants within the air, humidity, fuel, and diluents injection levels for NOx.
Improving turbine Performance
Recoverable Loss
By periodic water washing and mechanical cleaning of the compressor & turbine blades of the turbine will improve its performance.
Recent field experience indicates that frequent off-line water washing isn’t only effective in reducing recoverable loss but also reduces the speed of non-recoverable loss.
Non-recoverable Loss
After the primary 25,000 hours of the turbine operation (in this case, gas is that the fuel of gas turbine), its performance degrades to twenty to six from the guaranteed performance conditions (refer below figure).
Inspection of the turbine after 25,000 hours of operation is named “Hot gas path inspection”
Generally, High-Pressure Turbine (HPT) components are renewal at a 25,000-hour interval to enhance the blade life and performance restoration.
The results of the replacement of the HPT components is 60% or more restoration of the non-recoverable performance loss, counting on the extent of labor accomplished. The vertical axis represents the facility variation in percentage. The horizontal axis represents the turbine running hours.
After the recent section repair, the share of power variation reduces. Now the facility generation is almost 0.5%.
The overhauls at about 50,000 to 60,000-hour intervals (normally depends on manufacturer recommendation) entail more comprehensive component restorations throughout the engine. As an end in nearly 100% restoration of the non-recoverable performance. within the figure, the turbine overhaul took after 57,000-hours.
One generalization which will be made up of the info is that machines located in dry, hot climates typically degrade but those in humid climates.
When using liquid fuel, which is more corrosive than gas, an identical but more rapid pattern of degradation occurs, leading to approximately an equivalent 3% to five-level at the standard 12,500-hour liquid-fuel HPT repair interval.
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