Advanced technology gas turbines have reached >60% net efficiency – but how do you keep them at this level? By Tim Nicholas
Gas turbines have reached new heights in efficiency, with the most efficient, such as the H-class, giving power plants net efficiencies of more than 60% when run in combined cycle mode.
This is a marked increase from older versions such as the E-class, which offer efficiency levels in the region of 50%, and F-class at 55%.
Indeed, large-scale H-class gas turbines are more efficient and flexible in operation and, as they generate more power at the highest efficiency, can move power plants up in dispatch order.
They offer fast start-up and ramp rate capabilities; greater turn down, and so more cost effective ‘spinning reserve’; better fuel efficiency, and lower operating costs – but these supreme levels of efficiency need careful protection if they are to be maintained over the lifetime of the turbine.
The lower efficiency of E-class turbines means changes in aerodynamics have limited impact on performance. To this end, more rudimentary filtration solutions that only protect these rugged units from the damage caused by large particles in the inlet air flow will likely give enough protection for most operators.
It is accepted that the higher efficiency and output capacities of F-class turbines require a somewhat more sophisticated filtration solution to protect performance, life, and power output. The increased efficiency and generating capacity of the more recent H-class or equivalent GTs, however, require a new level and approach to protection to maintain their outstanding levels of performance.
The latest, advanced GT technology achieves high levels of efficiency through more refined modelling of heat transfer, higher firing temperatures and precise ‘super finish’ aerodynamic design.
3D printing techniques allow more elaborate and optimized blades with advanced aerodynamics, new placements of cooling passages, and true 3D profiles.
Other design features include enhanced air cooling flow, improved design of hot gas path components to reduce temperature and stress gradients, and upgraded thermal barrier coatings.
The fine tuning of these machines and the ‘super alloys’ they employ require more rigorous protection from the fouling and damage that finer particulates and contaminates in the inlet air flow can cause.
As advanced GTs also generate significantly more power than previous turbines, even a small compromise in filtration effectiveness comes at a high cost. Focusing in on pressure drop alone, this could equate to 0.4% loss in output over the lifetime of a filter within the system.
For a gas turbine rated at 300MW, this is around $150,000 dollars of lost revenue per year and $255,000 for a 510MW turbine.
On top of this, a sub-optimum filtration solution leaves the compressor more likely to be fouled, thereby reducing power output as contaminants in the air flow adhere to turbine blades and altering the aerodynamics of the turbine.
Having the right filtration solution means performance can be better maintained, the frequency of offline washes reduced, maintenance costs lowered, availability improved, and profitability optimized, all of which give a clear return on investment.
Considerations for advanced GTs
For high efficiency gas turbine, inlet health starts at the inlet house. No two plant installations are identical and filtration design should take into consideration all local environmental factors.
Is the installation near to the coast? Are there extreme temperatures? Does the area experience high humidity or regular fog events? What are the levels of dust or sand in the area? Are there other industrial settings nearby or building works that may increase dust levels and introduce a particular type of contaminant?
Large gas turbines consume huge volumes of air; and contaminants such as dust, sand and salt can cause fouling, pitting and corrosion to the blades, stators and buckets, decreasing turbine efficiency. Weather conditions including rain, snow, mist, and fog also need to be considered in the filter house design and filter selection.
Larger dust or sand particles can erode the special super alloys, finishes and coatings inside the compressor and turbine – and may eventually lead to severe machine damage. They can also plug cooling holes, leading to melting or distortion of hot section components.
Finer dust particles and other types of contaminant stick to blades, affect operating aerodynamics, reduce efficiency and therefore increase operational costs. The hygroscopic nature of salt makes it particularly challenging because it can quickly move between dry, sticky and liquid forms.
As well as adhering to blades and affecting efficiency, chloride in salt can start pitting corrosion in compressor blades. The sodium in salt can combine with sulphur in fuel in the hot section of the turbine to form sodium sulphate, which causes accelerated corrosion and ultimately catastrophic failure of very costly hot section components.
As particles build up on the compressor blades, the reduction in output power and increase in heat rate mean an offline wash is ultimately required. The more frequently this maintenance procedure is carried out, the greater the cost impact through lost power output, reduced availability, and increased rates of fuel usage.
Moisture is a threat to gas turbine performance as it can comprise of high concentrations of small droplets, such as in the form of fog, that become trapped in fine, high efficiency filtration media, creating sudden rises in pressure drop across the filtration system.
In this and other forms such as rain and mist, it can also combine with dust to form mud and also quickly change the physical state of hygroscopic salt particles from solid to sticky liquid form. All these factors need to be considered in the design of the air intake system.
The different threats to GT performance often require multiple stages of filtration. It is vital, that none of these stages are compromised. Each filter needs to fit perfectly, and work in tandem with the other filter stages to maximise performance.
The filtration system is only as good as its weakest part. A simple change in prefiltration can have measurable implications on a final filter and result in decreased turbine performance.
The example in Figure 1 shows the realworld impact of filter selection and the effect it has on pressure drop.
Fitting the wrong filter results in much faster rises in pressure drop, which will reduce GT runtime, and leads to much greater losses in power output. In an operational time of 8,000 hours (typical change-out period) this example highlights hundreds of thousands of dollars lost in treating filters like commodity items.
A sudden change to pressure drop is far from desirable and can result in GT runback, unplanned turbine outage or even damage to the filter house.
To assist with the planning of operations and maintenance activities, increases to the pressure drop should be slow and predictable.
Individual filters react in varying ways to the effects of weather and differing contaminants and the performance of every filtration stage is crucial to the optimisation of maintenance cycles.
Filters will also require cleaning or changing over time and the frequency of this operation will depend on the filter house design and the filter elements installed and, again, every filtration stage will impact the overheads involved.
Measuring filter performance
Although there are standards to gauge the dry particulate efficiency level of a filter, the only real test is how the turbine performs in the real world, monitoring the level of output and heat rate of the turbine and how this varies over time.
The return on investment of a filtration solution will come from protecting gas turbine efficiency levels, prolonging turbine life, increasing availability and reducing maintenance overheads to minimise the total turbine lifecycle costs.
As the efficiency of gas turbiness reach new levels, the importance of an optimised filtration system is increasingly critical to maintain high turbine performance.
Filters specifically designed to protect these advanced performance machines take more than just lab-tested filter media efficiency into consideration. They are designed to fully optimise lifecycle costs, withstand harsh installation conditions, effectively remove all types of contaminant, and work reliably and predictably to ensure high levels of GT efficiency and availability are maintained.
ABOUT THE AUTHOR
Tim Nicholas is PowerGen Market Manager, Gas Turbine Filtration Division, at Parker Hannifin.