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GRID STABILITY, POWER AUGMENTATION, SUPERCRITICAL CO2 AND EDDY CURRENT ANALYSIS WERE AMONG THE MOST VALUABLE SESSIONS
The incursion of renewables into the grid generated several lively discussions at the ASME’s International Gas Turbine Institute (IGTI) conference in San Antonio, Texas in June. Speakers addressed topics such as how to: improve grid reliability when wind and solar account for large portions of total power; improve steam turbine design in a renewable-rich grid; and measure and predict wind gearbox failures. The Turbo Expo 2013 event featured over 3,000 participants, over 1,000 presentations and 46 subjectmatter tracks.
Power augmentation was another popular area of investigation. A series of sessions featured inlet chilling, fogging, evaporative cooling and overspray.
Shale gas and supercritical CO2 (S-CO2) also garnered a lot of buzz during the weeklong conference. Claudio Lettieri of the MIT Gas Turbine Laboratory in Cambridge, MA, is working on a new diffuser design to improve efficiency in compressing CO2 for such uses as carbon capture and sequestration (CCS) and enhanced oil recovery (EOR).
“To enable economically viable largescale CCS, it is necessary to devise advanced fluid machinery compatible with supercritical fluids operating at improved efficiency levels and enhanced stability margins,” said Lettieri. “The high pressures and multiphase flow in these compressors and pumps yield major fluid machinery design challenges.”
He noted that pumps are typically equipped with vaned diffusers which provide a high inlet flow angle of around 80°. Industrial compressors, on the other hand, commonly use vaneless diffusers to achieve a wider operating range. Therefore, he has been working on how to introduce a vaned diffuser into SCO2 compression in such a way as to achieve the best of both worlds.
“Multi-stage compressors operating with supercritical CO2 for CCS and EOR applications require stages with a relatively thick shaft, a wide operating range, and need to have good efficiency while operating at very low flow coefficients,” said Lettieri.
Many sessions centered on how to cope with the inevitable increase in renewables on the grid. Thomas Thiemann, head of Siemens Energy Fossil steam turbine R&D, said that steam turbines need to be designed with more flexibility than ever to deal with the power peaks and valleys occasioned by wind and solar.
As an example, Thiemann said renewables accounted for more than 20% of total electrical power generation in Germany in 2012. Government policy requires that the country reach a 35% renewables target by 2020.
Wind and photovoltaic (PV) solar power, in particular, have been experiencing a boom. The PV system price in Germany has dropped by a factor of about three since 2007. As a consequence, it amounted to 5.3% of the country’s annual power generation last year.
This leads to dramatic changes in the operating regime of the installed power plants. One user, he said, had been operating an aging steam unit well beyond recommended transient limits and in a heavy cycling mode due to high prices on the spot market. This caused crack initiation in the main steam valve. Such examples called for another look at basic stream turbine fundamentals.
Thiemann laid out a series of equations to demonstrate that the basic design requirements of a steam turbine running in a steady-state base load mode are quite different for those enduring heavy cycling. To reduce stress and fatigue in a steady-state and maximum-load operation, the answer is typically to maximize casing wall thickness for pipes and minimize rotor diameter. But for cycling, heavy temperature transients call for a change of approach to minimize wall thickness.
“Component lifetime is reduced by a factor of about two if the startup gradient is increased by 15%,” said Thiemann. “If you choose 25% more wall thickness, this results in a reduction in component life by a factor of about 10 when you are cycling. Therefore, steam turbines for highly flexible operation require innovative design solutions and optimization strategies in order to minimize thermal stresses.”
Siemens addressed this by reducing the casing thickness and the rotor loading by implementing a patented high-pressure turbine bypass cooling system for certain applications. This has led to a reduction of the cold startup time by about 50%.
The growing influence of renewables on mainstream power will impact turbine control as well. Will Schulke, a gas turbine systems engineer at Woodward, highlighted controls developed in conjunction with GE for the LM6000 to deal with spinning reserve and grid instability requirements. The goal is to make the turbine more responsive to frequency disturbances. This led to the development of a Primary Frequency Control (PFC) system and a Reserve Margin Control (RMC) system.
“Power grids are continually subject to imbalances in generation and consumption that impact frequency stability,” said Schulke. “Responsive turbine and control systems are required to firm up frequencies in rapidly evolving grid environments.”
The PFC works by running a little below the predicted level of maximum power so that it has a margin available to respond to frequency disturbances. It can be adjusted to varying national and local mandates from grid authorities.
To improve turbine system response and predicted power accuracy, Schulke added that Woodward had also created a new WT6 valve and better metering controls for use in the Sprint system deployed in the GE turbine.
Cyrus Meher-Homji, technology manager for turbomachinery at Bechtel Corporation, talked about the various power augmentation approaches for mechanical drive turbines. “The reduction in output of mechanical drive gas turbines curtails plant power output with an increase in air compressor inlet temperature of the order of 0.7%/°C for heavy duty gas turbines and approximately 1%/°C for aeroderivative turbines,” he said.
While adding inlet cooling can minimize production swings, designing refrigeration compressors driven by gas turbines for high ambient temperature swings poses a design challenge due to power limitations at high ambient temperatures and high refrigerant condensing pressures. This is being addressed by EPC companies in the current batch of LNG plants being built.
There are currently ten large-scale LNG trains with power augmentation either operating or under construction. The first plant using aeroderivatives was the Darwin LNG facility which has been in operation for over six years. The Darwin LNG facility has six LM2500+G4 gas turbine compressor drives all using evaporative cooling. Four plants under construction in Australia and the U.S. use evaporative cooling. Six LNG trains under construction use inlet chilling with compressor inlet temperatures in the vicinity of 10°C.
“The need and type of gas turbine power augmentation deployed depends on specific plant and project drivers, operating conditions, project economics, and other factors,” said Meher-Homji. “Gas turbine power augmentation has numerous benefits for mechanical drive gas turbine applications and specifically for multi-shaft aeroderivative engines with low net work ratios that exhibit a high sensitivity to ambient temperature.”
As the ambient temperature increases from 20°C to 35°C, LNG production drops by about 8%. This temperature swing can occur monthly or in some cases even daily.
Media evaporative cooling has been used at the Darwin LNG plant in Australia for six years on mechanical drive turbines and is being deployed on four LNG trains under construction, while inlet fogging has been used extensively for gas turbines in power generation service, added Meher-Homji.
He compared the two: water quality requirements for media evaporative coolers are less stringent than those required for direct fog cooling systems. Media type cooling, however, requires more water for blow down, and water quality has to be monitored to avoid more frequent replacement of media pads.
Inlet chilling has not been extensively used for mechanical drive turbines. Currently, six LNG trains (36 gas turbines) are under construction that will use inlet chilling. In these systems, a circulating refrigerant in a mechanical chiller package produces cold water which is then pumped to cooling coils within the inlet duct. The advantage of mechanical chilling is that the gas turbine compressor inlet air temperature is not as dependent on humidity as it is in evaporative cooling. But the cost is much higher.
“A judicious selection of the climatic design point is important for any power augmentation system,” said Meher-Homji. The final selection of the design point depends on the specific application, the revenue generated by mechanical drive application, and installation cost. Each cooling technology has specific characteristics related to design criteria to maximize power output, economics, and investment return.
The next paper analyzed the effect of water temperature on the performance of gas turbine inlet air-fogging systems. “A slight decrease in droplet size may be observed at higher water temperatures, and the use of warmer water increases evaporation efficiency,” said Mustapha Chaker, senior mechanical engineer at Bechtel. “However using colder water marginally improves cooling efficiency, which is our ultimate goal, as cooler air means higher density air, and consequently higher power boost of the gas turbine.” There is no significant impact of water temperature on inlet fogging, he concluded.
The number of wind sessions at IGTI continues to grow. Jonathan Sullivan of the Department of Engineering Science at the University of Oxford, for example, addressed wind turbine gearing and its future applicability to gas turbine operation.
At high speed, scuffing and surface roughness begin to take effect, he said. At high load, on the other hand, bending fatigue and fracture are the more common issues; at low speed and low-to-medium load one sees adhesive and abrasive wear.
Sullivan has been analyzing how eddy current measurement can be used to predict component health across the entire gearbox and how to isolate these degradation factors in their early stages (Figure 2). “Eddy currents are very good at detecting the interface between two surfaces,” said Sullivan. “You don’t want the signal to vary between gears, for example.”
His team has developed a plastic idling gear that can be placed over an actual gear. It contains sensors for measuring eddy currents. But these sensors can also be placed in other ways without the need of a plastic overlay.
Oxford University has assembled a test rig to check radial and axial movement of gears as well as pitch and yaw alignment. Measurements can be made on two identical teeth on connecting gears, for example, to determine if they are rotating at exactly the same speed. The test can also detect gaps between meshing gears and cracks in the root of the gear.
“These irregularities can’t be seen with the naked eye and don’t affect power output, yet eddy currents can detect them, and this can potentially be used to prevent catastrophic failure in the future,” said Sullivan.
He gave the example of small cracks isolated on an eddy current graph. At light load the crack developed and finally led to a broken gear tooth 50,000 cycles later. “I don’t believe there are any other technologies that will provide this type of failure prediction,” said Sullivan.