SESSIONS AT IGTI SHOW SHOWCASE INLET AIR FILTRATION,
LNG TURBOMACHINERY, AERODERIVATIVES AND DUCT FIRING
BY DREW ROBB
The annual Turbomachinery Exposition, organized by ASME’s International Gas Turbine Institute (IGTI), covers a lot of ground. More than 3,000 participants came to hear over 1,000 technical papers presented in over 400 technical sessions.
Tracks include heat transfer, electric power, aircraft engines, ceramics, fuel types, combustion, emissions, controls, cycle innovation, fans & blowers, cogeneration, manufacturing, marine, oil & gas, steam turbines, and lots more.
Within that packed program, some of the standouts sessions were about duct firing, inlet air filtration and using aeroderivatives in LNG applications. They were delivered by speakers from Watson Cogeneration, Bechtel and EPRI. They provided tangible advice to plant operators and maintenance personnel about how to improve operations, boost power output, save money and select equipment.
Inlet air filtration
Inlet air filtration and cooling was the subject of several sessions. Steve Ingistov, Principal Engineer at the Watson Cogeneration Plant in Carson, CA, covered site and laboratory tests of inlet air filters from four different manufacturers.
A total of 720 12¾ inch diameter, 40inch long filtration elements are placed inside each inlet air intake of four GE Frame 7EAs at the plant. The right choice of filter can reduce costs on many fronts. With so many filters per gas turbine (GT), the need for regular replacement mounts up. But more importantly, loss of power can hurt profitability. The wrong filters will let particulate through and that can lead to the deposition of tiny particles inside the compressor.
“Fouling of the compressor due to poor filtration can lose a lot of power,” said Ingistov. “If you try to save money on filters, it can cost you a lot more in the long run.”
Offline water washing may solve most fouling issues. However, a machine might be down for up to 16 hours while it is being done. As plants, such as Watson Cogen, provide steam and power to a refinery, downtime is expensive.
Another factor to consider in filter selection is location. Watson Cogeneration is situated by the coast in an area subject to fog. The presence of fog and rain swells the pleats in some filters. This causes a sharp rise in intake air pressure as there is less area for the air to pass through.
Both lab testing and field testing must be done, said Ingistov, to see which filters are best. Filter selection based on one parameter, such as filter efficiency or pressure could easily lead to an incorrect choice. It requires a look at all parameters to fully understand which filters to choose.
For example, statically precharged filters may initially operate well. But tests showed their efficiency drops rapidly after a few months of operation. Buying precharged filters without testing their effectiveness once the charge diminishes could lead to an expensive mistake, he said.
Ingistov went to great lengths to test four types of filters to see which would be best for his facility. He conducted a series of tests, including dry and wet tests as well as lab and field tests.
A test on one parameter might show Filter A to be best, and another test on a different test might show Filter B as tops. However, deluge testing for onehour and threehour increments demonstrated that filters doing well based on one parameter often performed poorly under wet or foggy conditions.
“Within 30 minutes, pressure soared on two of the filters due to the presence of water,” said Ingistov. “Only one filter performed well on the 3hour deluge test.”
As a result of these tests, Watson Cogen changed filters. The results are tangible. From having to shut down to clean a compressor every month, the plant now only needs to conduce offline water washing once per year.
The filtration theme continued with a look at economic optimization of inlet air filtration for GTs. Dale Grace, Principal Technical leaders at the Electric Power Research Institute (EPRI), said that selecting the appropriate level of filtration for a GT helps to minimize overall costs and maximize revenue.
“Even a minimum level of filtration protects against erosion and corrosion of the compressor blading,” said Grace. “Increasing filtration efficiency levels to High Efficiency Particulate Air (HEPA) improves compressor performance and power output.”
Air filtration efficiency is a nonlinear function of particle size. Minimum Efficiency Reporting Value (MERV) ratings are used to rate the ability of an air cleaner filter to remove dust from the air as it passes through the filter. The higher the MERV number, the higher the rating. F8 is a HEPA filter.
The bottom axis of the chart shows particulate matter (PM) size measured in microns. PM10 equates to cumulative concentration of small particles under 10 microns in size, and PM2.5 is under 2.5 microns. Particle sizes under 2 microns are most likely to cause fouling of the compressor.
Looking at efficiency against particle size highlights the tremendous variation that exists, especially in lower efficiency filters. Grace urged attendees to conduct a thorough analysis that looks at multiple factors. As well as raw numbers, he said to consider initial costs, operational costs and ongoing maintenance costs for the filter.
EPRI recommended a complex methodology to account for all aspects of the life cycle. EPRI’s Air Filter LifeCycle Optimizer (AFLCO) software is a tool that attempts to account for the influence of the specific GT, operating conditions, ambient conditions, load profile, filtration choices, and wash type and frequency. It also attempts to quantify revenue and cost considerations over many years.
LNG and aeroderivatives
Aeroderivative engines in mechanical drive applications for LNG liquefaction was the topic addressed by Matt Taher, Principal Rotating Equipment Engineer at Bechtel Corp.
In early LNG liquefaction plants, steam turbines were utilized for compressor drives. The first GTs in LNG liquefaction were deployed in 1969. In an LNG liquefaction plant, the GT drivers and refrigeration compressors strongly influence overall plant performance and efficiency.
After the first aeroderivative was applied at Darwin LNG in 2006, there has been a continuing growth in the use of these engines for LNG mechanical drive. Natural gas accounts for a quarter of global energy demand of which 9.8% is supplied as LNG. LNG volume in 2016 was 258 MTPA (million tonnes per annum), a 13.1 MTPA increase over 2015. LNG production is predicted to jump at least 50% by 2035.
The energy intensive LNG liquefaction process consumes 6% to 8% of the gas being converted. Bringing that number down raises plant efficiency. High efficiency GTs help in reducing fuel draw.
Some of the desirable GT characteristics for mechanical drive LNG duty are high fuel efficiency, multiple shafts for easier startup and turndown, a large power output to produce more LNG and shaft speeds to match refrigeration compressor requirements. Aeroderivatives satisfy all four characteristics and are also easily swapped out for maintenance purposes, which aids plant availability.
The thermal efficiency of aeroderivatives is higher than traditional used heavy duty GTs. That’s why aeroderivatives such as the LM6000, LM2500, Trent 60 and the LMS100 are being used or considered for mechanical drive LNG duty. According to Taher there are 94 aeroderivatives used in LNG mechanical drive around the world. Most of these are in Australia and the US.
The weight of aeroderivatives is another factor that leads to their selection. Taher compared two models with similar output. The Frame 5D has an output of 32.5 MW, an efficiency of 29.5% and weighs 65 tonnes (flange to flange). The LM2500+G4, on the other hand, has an output of 34 MW, efficiency of 41% and weighs 7 tonnes.
LNG mechanical drive turbines normally operate at high power and many are sited in hot areas or places with warm summers. Therefore, inlet air cooling is often utilized.
Taher pointed out that aeroderivatives are more sensitive to hot ambient conditions than heavy duty GTs. Aeroderivatives can lose as much of 1% power per 1°C rise in temperature compared to 0.7% for a traditional heavy duty GT.
Taher mentioned that the importance and growth of aeroderivatives in industry has resulted in the API 616 standard now being updated to cover the use of aeroderivative engines.
John Gulen, Principal Engineer at Bechtel Infrastructure & Power, provided a session on Heat Recovery Steam Generator (HRSG) duct firing, which is typically used to increase output on hot summer days when GT airflow and power output lapse.
The aim is to generate the maximum amount of power when it is most needed. However, this is achieved at the expense of heat rate.
Duct firing entails burning additional fuel in the transition duct between the GT exhaust and the HRSG inlet. In many cases, though, the burners are located behind the first superheater and between the reheater tube banks. That is why duct firing could be more accurately referred to as supplementary firing.
“Under certain boundary conditions, duct firing in the HRSG can be a facilitator of efficiency improvement,” said Gulen. “When combined with aeroderivative GTs with highcycle pressure ratios and low exhaust temperatures, duct firing can be used for small but efficient combined cycle power plant designs as well as more efficient hotday power augmentation.”
This is a possible way for natural gas plants to better support variable renewable generation. In addition, repowering an obsolete coalfired power plant slated for retirement with a highly fired, singlepressure HRSG and an advanced GT can be a costeffective method to add capacity. The GT generates additional power while boosting steam cycle output.
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