– The article is authored by Dr Iyad Al-Attar
Dr Iyad Al-Attar is a Mechanical Engineer, and business innovator. He is also a Visiting Academic Fellow at the School of Aerospace, Transport and Manufacturing at Cranfield University. He received his mechanical engineering degree from the University of Toronto and his Masters from Kuwait University. His PhD, from Loughborough University United Kingdom, is concerned with the performance of pleated filters for HVAC and Gas Turbine applications with special emphasis on the chemical and physical characterization of suspended particles.
Our Stark Reliance on Power
For all the generations that were born to find wall sockets for their phone chargers, televisions, and many electrical appliances, have you ever wondered how long could we survive without power? In fact, we get very irritated if our air conditioning and/or heating systems are off for even short periods.
Today, we feel isolated if the batteries of our smartphones and laptops are not charged. Our patience could dissolve in seconds if we missed our favourite television show or if there was no power to run our home appliances. We even demand that aircraft seats to be equipped with power sockets to charge our devices, in order to stay connected. Nevertheless, how did power make it to our homes and offices? How did our daily lives become so extremely reliant on power and at the mercy of its availability?
Centuries of development
Although the theory of gas turbines and their anticipated functions were established over the past four centuries, the manufacturing of a gas turbine was faced with great challenges. The earlier unavailability of required materials impeded the full implementation of the theory into practice. The gas turbine operation requires components capable of sustaining excessive temperatures for an extended period. Further, additional challenges, such as the growing power demand and fuel price, have highlighted the importance of efficient gas turbine performance.
The definition of gas turbine
The term “gas turbine” describes an engine consisting of at least a compressor, a combustion chamber, and a turbine. It can convert natural gas or other liquid fuels to mechanical energy. In the land-based application, the generated power then drives a generator that produces electricity. Land-based gas turbines are of two types: (1) heavy frame engines and (2) aeroderivative engines.
The working principles of a gas turbine is based on the thermodynamic principles of the Brayton cycle. The objective of operating a gas turbine is to have efficient compression and expansion processes to produce useable power output.
Atmospheric air is drawn through a filter housing installed at an elevated level and then passed through the filtration stages prior to entering the compressor via the bell mouth. Filtered air is compressed to high pressure and temperature before entering the combustion chamber where fuel is injected and combusted at constant pressure. The gases that leave the combustor at high temperatures contain large amounts of energy. Energy is then extracted from the hot pressurized gas, thus reducing pressure and temperature. 50 to 60 % of converted energy is used to drive the compressor and the rest drive a generator or shaft to produce power or the mechanical equipment of interest.
Figure 1: Schematic of a combined cycle power plant.
The challenges ahead
Gas turbines operate in different environments such as tropical, coastal, and desert. They confront a wide array of atmospheric contaminants with various concentrations and particle size distributions. This could lead to performance degradation and components deterioration. Gas turbine engines are challenged by fouling, erosion, corrosion, abrasion, and foreign object damage. The compressor fouling (Figure 2) caused by the deposition of airborne particulate matter, smoke, oil vapour, carbon, sand, and sea salt is responsible for a 70–85% of gas turbine performance degradation . It is described as the deposition of airborne contaminants on the aerofoils and annulus surfaces at the inlet of the gas turbine engine, particularly the compressor section. The presence of pollutants in the atmosphere may cause a compressor performance to degrade due to changes in aerofoil geometry and surface roughness.
Recoverable and Irrecoverable degradation
When compressor fouling can be removed by cleaning, performance losses are regarded as recoverable. The recovery takes place through offline or online compressor cleaning. However, extensive exposure of contaminant deposition can lead to non-recoverable degradation. Non-recoverable degradation in performance due to physical impacts causing permanent defects to gas turbine components during operation. Wear and tear of the profile of the blade profile is a great example of the aerodynamic alternation because of a permanent change in its shape, which affects the integrity of the component.
Air filtration emerges as a critical (separation and retention) process to capture pollutants prior to introducing the air to the gas turbine engine. Different filtration techniques such as static (Depth) and pulse (Surface) filters shown in Figure 3 are employed to protect the compressor assembly from suspended contaminants. Therefore, accurate filter performance prediction facilitates their appropriate selection in order to avoid premature clogging. This can be facilitated by virtue of capitalizing on multi-stage filtration selected after professional aerosol monitoring has been conducted. This would allow the engineer in charge to recognize the contaminants the gas turbine engine is up against and make appropriate filtration plans and compressor washing techniques to combat them. Furthermore, the detailed account of filter media must be investigated since it is the building block of air filter cartridges.
Compressor washing is another maintenance measure where an engineered injection of demineralized water droplets and cleaning fluids is delivered to the compressor assembly (Figure 4). Compressor washing is widely used to bring compressor blades to their design point and allow the compressor assembly to realize its fullest potential. The three methods of compressor washing are: online, offline, and/or hand-wash. They can be used at different times and are usually implemented alongside various filtration techniques to achieve optimum results.
The balanced approach
Increasing the number of filter stages and/or upgrading their efficiency leads to pressure drop increase of the entire filter section. On the other hand, reducing the number of filter stages and/or their efficiencies provides greater volume of less unfiltered air to reach the gas turbine engine. However, this would subject the compressor assembly to various types and concentration of airborne pollutants deposition to be washed when required.
It is also important to realize that flow rate used in land-based gas turbine is higher than that used in HVAC applications. Therefore, installed air filters are exposed to greater concentrations of contaminants with various size distribution and chemical composition throughout their lifetime. Ultimately, the limitations of air filters and the wide spectrum of suspended pollutant make it difficult to rely on air filtration as a sole technique to maintain gas turbine performance. Therefore, compressor-washing techniques come in handy in rendering the compressor assembly back to its design point. Perhaps, a techno-economic study can reveal the current design limits of both technologies prior to holding any them responsible for engine performance degradation. Particularly, since an efficiency gain as low as one percent is significant to gas turbine operators.
Sustained gas turbine performance translates into maintaining the designed power output, reducing fuel consumption, emission, and the number of outages. Therefore, implementing a balanced approach between professional air filter installation and compressor washing techniques can prove invaluable in designing maintenance measures. Clearly, the objective is to utilize available tools to maintain optimum efficiency of the compressor. However, one must address the inability of air filters to remove all suspended contaminants with various concentrations, size distribution and chemical composition.
Air pollution and sandstorms
Air pollution and sandstorms add to the complexity of filter performance prediction. Research has proven that polluted air and sandstorms expose the installed filters to high particle concentration causing them to bridge with one another leading to dust-cake formation whereby particles begin to agglomerate. Furthermore, high particle concentration alters filter porosity and occupies the interstitial spaces of the filter medium causing a significant rise in the filter’s pressure. Unfortunately, early surface deposition on a depth filter does not warrant the full utilization of filter depth/thickness as shown in Figure 5. It could also reduce permeability of the filter triggering premature clogging and more frequent filter replacements because of the accelerated rise in pressure drop.
A tactic to tweak!
Abiding by international filtration standards alone cannot get the job done. In fact, actual performance of air filters installed in hot and humid climates tends to deviate from the performance predicted by laboratory results. This is particularly true in regions sustaining frequent sandstorms and known to have atmospheric dust (Figure 6) with characteristics deviating from that of commercially available synthetic standard dusts (Figures 7&8). Therefore, it is recommended that any filter enhancement to consider the following three-element plan:
- Conduct chemical and physical characterization of atmospheric dust in the concerned region.
- Investigate the possible impact of those characteristics on the filter selection and performance.
- Revisit existing air filter designs to enhance the gas turbine performance via efficient, economic, and appropriate filtration solutions for the corresponding climate conditions.
There is an opportunity for operators to employ the concepts of the balanced approach detailed in this article to capitalize on the gas turbine performance and environmental impact.
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- Meher-Homji CB. Gas turbine axial compressor fouling – a unified treatment of its effects, detection and control. Int J Turbo Jet Engines 1992;9(4):99–111.
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