The demand for ever more efficient turbines shows no sign of abetting, continually pushing manufacturers to develop more cost-effective and sustainable solutions.
Efficiency and reliability continue to be high priorities for operators of turbines used in distributed generation systems. These performance attributes have become an even higher priority as operators have increasingly incorporated cogeneration technologies and alternative fuel sources into their systems.
The marketplace is responding to the demands for maximum efficiency and reliability in developing new turbines, blades, and even lubricants.
New Units Suit Cogen
Two new gas turbines, one equipped with a single annular combustor (SAC) and a dry low-emissions (DLE) model have been developed by GE Energy to suit cogeneration applications in particular.
The LM6000 PG with a SAC, an addition to the company’s LM6000 aeroderivative gas turbine product line, is designed to provide a 25% simple-cycle power increase and an 18% boost in exhaust energy for cogeneration applications. According to the manufacturer, the unit puts out combined cycle power in the range of 66 MW with efficiencies ranging from 50–52%, depending on selected emissions control methods. The turbine also yields a power density improvement of nearly 20% compared with the company’s existing 50-Hz LM6000 technology.
The improved combined-cycle efficiency of the unit reportedly can reduce fuel consumption by the equivalent of 33,000 barrels of oil per year, compared with aeroderivative models in the same class. According to GE, the unit’s uprate also reduces carbon dioxide emissions by 6,500 tons over the course of a typical
operating year.
The manufacturer incorporated material and technology upgrades from its CF6-80E and GE90 aircraft engine and its LMS100 into the new turbine. According to the company, the LM6000 PG has been designed with attention to commonalities between the 50- and 60-Hz offerings for a wide global experience base.
GE Energy’s LM6000 PH DLE model is said to offer a 25% simple-cycle power increase and an 18% boost in exhaust energy for cogeneration applications. The unit provides combined-cycle power in the range of 65 MW with efficiencies ranging from 52–55%, depending on selected emissions control methods. Power density is improved by nearly 20% within the same footprint as the company’s existing 50-Hz LM6000 technology. As with the LM6000 PG, the LM6000 PH can reduce fuel consumption by the equivalent of 33,000 barrels of oil per year compared with aeroderivative models in the same class, according to GE Energy.
Also like the LM6000 PG, the LM6000 PH reduces carbon dioxide emissions by 6,500 tons over the course of a typical operating year. Additionally, the model reportedly decreases water consumption by approximately 55 gallons per minute at 3,000 hours per year for a typical annual water savings of about 9.9 million gallons.
High-Speed, Low-Pressure Blades Improve Efficiency
Another company division, GE Oil & Gas, has launched a new family of high-efficiency, high-speed, low-pressure (LP) blades for mechanical drive and geared power generation steam turbines. The new LP design has been applied to turbines with a wide range of rotational speeds and sizes to form a family of condensing stages. These blades have been developed to improve the speed, mass flow capability, reliability, and efficiency of industrial steam turbines.
The manufacturer points out that, in the future, the steam turbine will be the natural choice for applications using low-cost alternative fuels, such as biomass, urban refuse, and wood. Although plants that utilize these alternative fuels are designed to reduce emissions, they must also operate efficiently for purposes of economic feasibility.
In response to these demands, the company has developed its new generation of last-stage blades that range from 8-inch blades—the height of the last rotating blades—that operate at 11,250 rpm to 30-inch buckets that run at 3,000 rpm and suit the 50-Hz power generation market. The largest blades are still under development. The manufacturer reports that the blades offer a 12.5% increase in rotating speed capability and a 5% increase in section efficiency, compared with current-generation technology. The higher speed is intended to increase the overall power density.
Michele Taviani, senior service process manager, and Clarice Carmone, steam turbine product line manager with GE Oil & Gas, argue that, in power-generation applications, turbine efficiency is the key Critical to Quality (CTQ) parameter because kilowatt-hour output is king. They add that the new blades were developed to address efficiency. The LP stages have a significant impact on overall turbine efficiency since they reportedly deliver from 20% to 40% of the overall turbine power, depending on the steam balance of the machine, so improvements in the LP stage capability and performance have a significant effect on the entire turbine.
However, optimizing LP stage and turbine performance requires careful work from the start of a project. The company focuses on several areas in developing the LP stages. The flow path inner and outer diameters are limited to fit the hub outer diameter of the last stage blade to the existing set of exhaust casings. The axial length of the stage sections is set to maintain the overall bearing span of the turbine and the rotors are designed with significant stiffness to preserve rotordynamic behavior.
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Photo: NVision
CFD simulations are used to design energy-efficient blades. |
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Photo: NVision
CFD simulations also show engineers how airflow can affect turbine performance. |
All blades feature an integral cover that operates in contact with the adjacent covers over the entire continuous speed operating range, a design that ensures that the blade rows behave as a continuously coupled structure that provides high stiffness and damping characteristics and causes low vibration stresses. The company has transferred its knowledge acquired in the full milling of closed impellers for centrifugal compressors to the production of steam turbine stationary parts. Part counts have been reduced significantly, as the LP nozzles are composed of four diaphragms. The reduction can cut LP stage replacement time significantly.
Another emphasis is improved aerodynamic and aeromechanical quality. With traditional blade and spacer solutions, the company argues, the fidelity of the nozzle throat distribution to the aero-design specification depends on the capability to control the tolerances of single parts and making adjustments during assembly. With full milling, the precision and repeatability of the throat distribution along the nozzle row is directly connected to the milling machine accuracy for greater uniformity. Improved airfoil geometric quality is said to translate directly into higher stage efficiency and lower excitation stimulus on the rotating blade. The mechanical design was verified with 3D finite element (Ansys) models.
“We basically redesigned the airfoil of these blades in order to improve efficiency compared with blades that were designed 20 or 25 years ago,” says Taviani. “The airfoil is totally different. One important point is that the vane of the blades is totally free—there is an integral roof above the blades. The flow to the blades is quite free; it’s not disturbed by any kind of damping system.”
The first application of the new stages is an 8-MW power generation turbine located in a waste-to-energy plant in Italy. The train consists of a turbine running at 9,690 rpm, a gear that reduces the speed to 1,500 rpm, and a four-pole generator. About 5% of the main steam flow is also extracted from the turbine upstream of the second drum. Broad flexibility is required in the plant, which is equipped with an incinerator that has three lines of burners.
The steam produced can vary from 20–100% of the nominal flow, and the air-cooled condensing system is capable of maintaining a vacuum pressure of 0.13 bara at the nominal flow, whereas a reduced mass flow can cause the condensing pressure to be decreased to 0.07 bara.
The stages are able to sustain high efficiency over this wide operating range. Another characteristic of this type of plant that presents an engineering challenge is the poor, i.e., low-temperature, steam condition at the last stage blade exit. This condition pushes the vacuum level to very low values to maximize efficiency. The combination of these two factors leads to very high moisture content at the last stage exit, which, under normal conditions, would cause significant wetness-related efficiency losses. The high moisture can also cause erosion of the last stage blades, as liquid droplet erosion is strongly related to the blade tip speed and moisture content.
To address this situation, the manufacturer has designed the stages with erosion protection technology for high-speed applications by using a martensitic stainless steel that reportedly has erosion resistance almost 10 times higher than standard 13% Cr steel, for the last stage blade base material. Additionally, the company applies a special WCr coating using a high-velocity oxygen flame process.
Blade Design Optimization Improves Efficiency
Just outside the realm of actual manufacturing is third-party turbine blade design optimization. NVision Inc., which specializes in three-dimensional digitizing, reverse engineering, and prototyping services, utilizes its Maxos scanner—manufactured by Steintek GmbH—for the 3D inspection of turbine blades. The scanner is designed to evaluate up to seven axes of motion.
The instrument is being used to improve the performance of blades used in a major turbine manufacturing partnership. Toshiba GE Turbine Components (TGTC), a joint venture formed between Toshiba and General Electric to produce large blades from 26–52 inches for steam turbines, manufactures turbine blades at its Yokohama, Japan facility. The blades feature complex mid-span geometry that provides support for the midsection of the airfoil and requires the examination of numerous cross-sections. According to TGTC, the scanner has reduced the time required to inspect and measure steam turbine blades from 280 minutes to 45 minutes.
The scanner used by TGTC has a proprietary non-contact probe consisting of a concentrated light that collects 100 individual points per second. The scanner uses five axes to reach every point on the blades and also generates specific measurements of critical areas. A servo-controlled swivel head positions the sensor in two rotating axes and the instrument is said to measure 180 coordinates per minute. It reportedly is the only non-contact system capable of accurately measuring uncoated highly polished metal and is said to have an overall accuracy of 10 microns, even on highly polished and shiny materials. The machine reportedly provides accuracy within 0.0004 inch and a resolution between measured points down to 0.0002 inch. The scanner’s software is configured with an overall best fit of the measured geometry to allow a part with an error to fit within the overall tolerance envelope of the reference data.
Steve Kersen, vice president of sales and marketing for NVision, argues that no contact means greater efficiency than touch probe-type measuring devices. “It can scan much, much faster because it’s not touching the part—we’re using light,” he says. “Secondly, it can take far more data than a touch probe. It also measures more accurately, because the other machines have what we call ‘cosine error,’ where they lose their vector on leading and trailing edges. The light doesn’t do that, because we assign it a vector.”
Also according to NVision, its Contract Service Division recently reverse-engineered the entire core of a steam turbine for a major original equipment manufacturer (OEM) in six weeks, compared with six months that the OEM had budgeted for the project. “You’re talking about the central rotor housing with all the blades on it, plus all of the castings of the housings that surround that—two big castings that go together, plus all the auxiliary boxes and the diaphragms,” says Colin Ellis, NVision’s engineering manager.
The turbine rotor measured 11 feet in length and 6 feet in diameter and was not available as a computer-aided design (CAD) model. Over the course of three weeks, technicians scanned all of the turbine components using an NVision HandHeld Non-contact Scanner and touch probe at the OEM’s site and the Maxos scanner in NVision’s Wixom, MI, facility. Kersen says that, at minimum, the scanner saved the manufacturer hundreds of thousands of dollars.
Next, technicians used NVision software to convert the point clouds to STL file format. Over the course of another three weeks, the STL model was converted to a fully parametric CAD model by hand, to correct machining inaccuracies in the as-built parts. The CAD models provided by NVision were used by the turbine manufacturer as the basis for computational fluid dynamics (CFD) simulations that were used to design new energy-efficient blades and diaphragms that improved the efficiency of hundreds of existing turbines. The CAD models were used to accurately simulate the as-built turbine geometry, and the CFD simulations were used to show engineers how airflow affected the performance of the blades and quickly evaluate alternative geometries.
Mission-Critical Lubrication
Turbines operate in a high-temperature, mission critical environment, and keeping them operating dependably is a natural concern of distributed generation facility managers. According to Shell Lubricants, its application-specific oils are formulated to prevent the formation of deposits and sludge. Additionally, the products reportedly meet most OEM recommendations, provide high water separation, and resist aeration and foaming. The company recently added two products to its line of turbine oils.
“Both types of turbine oils are formulated for use in both steam turbines and in gas turbines in standalone mode, as well as combined-cycle systems that have maybe a single oil reservoir supplying both a steam and gas turbine,” says Felix Guerzoni, product application specialist for Shell Lubricants.
Guerzoni notes that both oils are formulated to prevent the buildup of varnish and sludge on journal and thrust bearings that support the turbine rotor, as well as in hydraulic control systems when used for this function, in gas, steam, or combined-cycle turbines. He points out that a problem with these turbines is varnish or sludge buildup in control valves or inlet guide vanes, or in filters used in servo valves.
Turbo Oils CC are designed to have high oxidative stability and sludge control, and the surface properties suit combined-cycle turbine technology, as well as existing gas and steam turbine plants. Their deposit resistance is intended to lubricate gas turbine bearings with minimal deposit buildup or sludge formation to reduce the risk of servo valve problems that can cause turbine trips. The additive system of the product is intended to provide reserves of wear protection for highly loaded gears, while maintaining deposit resistance and oil life. The oils meet or exceed ISO 8068 L-TSA, L-TSE, L-TGA, L-TGB, L-TGE, L-TGF, L-TGSB and L-TGSE; and ASTM D 4304 Type I and Type 2 standards.
“The Turbo CC are well-suited for use in heavy-duty gas turbines, such as the GE Frame 7 and 9 in particular, in the US market,” says Guerzoni. “The biggest issues for these turbines are servo valve sticking and varnish issues.” The Turbo Oil CC product is formulated with high-quality base oils and antioxidants, is a high-load-carrying oil, and includes rust- and corrosion-prevention additives that improve the stability of the oil.
Formulated to meet Mitsubishi Heavy Industry (MHI) combined-cycle gas and steam turbine standards, Turbo Oil J is said to provide reduced deposit-forming tendencies according to the newly emerging industry Dry Turbine Oil Oxidation Test (TOST). The product is designed with water-separation characteristics to prevent corrosion and the other damaging effects of water circulating in the system. It is said to tolerate the traces of contamination that can often lead to blocked filters and extra maintenance. The product is also formulated to resist foaming and rapid air release, which helps prevent pump cavitations, oil pressure drops, and lubricant starvation. It meets or exceeds ASTM D 4304 Type I and ISO 8068 L-TSA, L-TGA standards.
Guerzoni explains that the slight difference between Shell’s CC and J oils lies in their additive chemistries. Both oils use API Group II base oils and Turbo Oil J meets the requirements of the Dry TOST Test, which is being considered for use by ASTM as an international standard.