Kelly Oeffinger reports how to correctly design nozzles and control systems to minimise blade erosion.
Gas turbine inlet fogging has long demonstrated its ability to increase output, while lowering fuel costs and emissions. But as one Latin American power plant learned, not all fogging systems are the same. System and nozzle design and operation are not only critical to obtaining the greatest power boost, but also for continued safe operation of the turbine. A poorly designed system leads to excessive erosion and eventually catastrophic failure.
The early 1980s vintage power plant contains four Siemens Westinghouse W501 D24/D5 Gas Turbines in a 2 x1 combined cycle configuration - one block producing 290MW and the other 309MW. To boost output, in 2005 they installed fogging systems from a European manufacturer.
The fogging system could inject about 16,000 litres of water into the inlet air stream, with a guaranteed power increase of 10 per cent. To achieve this result, the fogging system used overspray, injecting more water into the airstream than could evaporate before entering the compressor. The fogging units did produce the desired power boost, but also produced severe turbine damage.
Within four years of installing the fogging system, an unusual noise was detected in Unit 2. The unit was brought off line and a subsequent inspection found that one of the fog spray nozzles had broken loose and damaged the blades and diaphragms on compressor Stages 1 through 19. Shortly thereafter, that same unit suffered a catastrophic failure, with the combustion pressure dropping from 125psia to 28psia and output from 66.26MW to 3.72MW in less than two seconds. Upon opening the unit, they found shredded blades and diaphragms, and a build-up of blade debris in compressor bleeding areas.
To determine what caused the failure, the utility requested that the Engineering Department of the local university conduct an inspection and root cause analysis. The engineers determined that operating the fogging system with excess water caused a premature failure in the early stages of the compressor. Then, when nozzle detached and damaged the blades, the repairs caused a change in the natural frequency of the blades, increasing vibration and eventually leading to a surge condition which damaged the high pressure section.
The university also inspected Unit 5 and found premature erosion and material removal from the compressor blades caused by entrained water droplets. Judging by the paths traced on the blades, it appeared that droplets were still impacting the blades up to Stage 8 before fully evaporating. Computational fluid dynamic modelling determined that the use of overspray, poor nozzle placement and a 90 degree angle in the inlet duct caused the fogging droplets to agglomerate into larger droplets that did not fully evaporate. The droplets would collect on the spiral baffle at the turbine inlet and then get sucked into the turbine inlet. Combined with the SOx and NOx in the air, this water led to erosion, corrosion and crystallization on the blades.
While this damage could have caused the utility to cease using fogging on all of its turbines, the benefits of fogging are significant enough that the utility decided to give it a second shot. This time, however, it decided to switch to a Meefog system.
Mee Industries has been building high-pressure fogging systems since the 1960s and has installed inlet cooling systems on more than 750 gas turbines ranging from 5 MW aeroderivatives to 250MW frame turbines and has the knowledge and experience to design a system to meet the exact needs of a particular plant.
In this case, the utility kept the earlier water filtration system, but upgraded the nozzle array and pump skid with a Meefog system consisting of 890 impaction pin nozzles with a 127 micron orifice and operating at 207 bar (3000psig). Each nozzle produces a flow of 0.162l/min for an aggregate maximum water flow of 144.2l/min.
The system can produce an 11° C cooling +0.4 per cent overspray. The system provides 13 cooling stages so the operators can precisely control the amount of water going into the air in order to obtain the maximum cooling available without excess water pooling in the inlet entering the turbine.
With the new Meefog system, the utility achieves a 40MW power increase when fogging on all four turbines, with a maximum 50.4kW power usage when operating all 13 fogging stages. And it doesn't have to worry about the fogging system causing another failure.
For more information at www.engineerlive.com/ipe
Kelly Oeffinger is with Mee Industries, Irwindale, California, USA. http://meefog.com/applications/gas-turbine-cooling