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New gas turbine designs push the envelope.

A trio of new gas turbines point the way to more efficient and cleaner power generation for utilities, mechanical drives for industry, and compressors for pipelines and platforms.

Industrial gas-turbine manufacturers continue to refine their machines to perform beyond predecessors' limits. This was borne out by the three newly designed gas turbines showcased at the 42nd annual Turbo Expo, sponsored by ASME's International Gas Turbine Institute and held in Orlando, Fla., in June. The Cyclone from European Gas Turbines (EGT), ABB's GTX100, and AlliedSignal's ASE120 all draw on previously proven designs that have been refined to make them more fuel-efficient, less polluting, easier to operate and maintain, and longer-lived.

Three major factors are driving the development of new machines like those unveiled at Turbo Expo, according to Holger Lukas, technical director of Encotech Inc., a consulting engineering firm based in Schenectady, N.Y. "First, the installation costs of new gas turbines are low. Second, the time needed to install gas turbines is also low, averaging under a year for simple-cycle operation [fuel burned to turn an electrical turbine] and under two years for combined-cycle operation [where exhaust heat is captured to produce additional power]. Lastly, the sharp competition between turbine manufacturers has driven the dollar-per-kilowatt costs down to about $300 per kilowatt-hour, about the same as it was in the late 1970s."

Gas-turbine designers are creating the next generation of machines by adapting existing designs. "Scaling proven parts up or down enables engineers to draw on proven aerodynamics, performance characteristics, and materials when designing parts for new turbines, a more cost-effective alternative than designing from scratch," Lukas said.

Although the EGT, ABB, and AlliedSignal turbines are distinct machines, they share certain design features that illustrate trends in turbine manufacturing: dry-low-emissions (DLE) systems, split casings, variable stator blade stages, dual-fuel systems, single-crystal cast blades, and advanced cooling systems. DLE systems replace water or steam injection systems used for emissions controls, which can add to the turbine's overall cost, particularly in arid regions. Variable stator blade stages enable operators to optimize the turbines for different ambient conditions, i.e., temperature and humidity, or different operating regimes (basically part load and full load). The ability to burn natural gas or distillated oil ensures turbine operation if either fuel becomes too expensive or less available.

"Because blades break across the interface of two crystals, single crystal casting makes parts inherently stronger and corrosion-resistant," Lukas added. "And because manufacturers cannot go much further with metallurgy, advanced cooling of the turbine combustion system and blades extends their performance life."

BOOSTING CYCLE EFFICIENCY

EGT in Lincoln, England, a subsidiary of GEC Alsthom, developed the 13.4-megawatt Cyclone gas turbine to serve power-generation and mechanical-drive applications, including offshore oil and gas platforms, industrial power generation, and pipeline transmission.

Engineers from EGT increased the compressor and high-pressure stages, and added a free power turbine to their proven Tempest gas turbine to create a train shaft engine with a simple-cycle efficiency exceeding 35 percent. This high simple-cycle efficiency, coupled with the turbine's capability of raising more than 27,000 kilograms of steam per hour at 10-bar pressure, can provide overall plant efficiencies in excess of 80 percent.

The Cyclone compressor is a zero-stage version of the Tempest compressor, creating an 11-stage design with transonic flow conditions. This results in a pressure ratio of 16.7:1, compared with 14:1 for the Tempest, at a compressor rotor speed of 14,100 rpm. The inlet guide vanes and first four rows of stator blades on the Cyclone use variable geometry to provide surge control during start-up.

Advanced cooling was a key design consideration for the Cyclone compressor turbine. The two-stage, overhung design with rotor and stator blades is cooled by compressor delivery air. Air bled from the compressor also cools the rotor disks. The unshrouded turbine blades are inherently more temperature-resistant and have lower stresses, thus allowing increased power per stage.

The turbine is scaled from the Typhoon design. It has interlocking, shrouded blading to reduce tip leakage and allow operation over a wide speed range. The power-turbine operating temperature is sufficiently low to avoid cooling the rotor and stator blades. The power turbine produces full power between 8,000 and 10,000 rpm, enabling direct-drive speed matching of typical compressors and thereby eliminating the need for a gearbox. Both stages of stator vanes are cast as a complete ring to minimize losses and simplify assembly.

The Cyclone is equipped with six reverse-flow tubular combustion chambers positioned around the high-pressure casing. These can burn either natural gas or distillate fuel, with automatic changeover between fuels accomplished at any load.

The burners incorporate the DLE combustion design that is standard on EGT equipment. Fuel and air are pre-mixed at the swirler before being ignited, reducing nitrogen oxide emissions to 25 parts per million when burning natural gas and 50 parts per million when burning distillate. The low and even flame temperature reduces the generation of N[O.sub.x] and minimizes temperature deviation.

The DLE design also addresses emission control at part load by reducing the airflow through an air-bypass system, and adjusting the pilot burner keeps both carbon monoxide and unburned-hydrocarbon emissions in check.

Hydrodynamic tilting-pad journal and thrust bearings support the Cyclone compressor rotor and the free-power-turbine rotor, enhancing stability and extending performance life. A standard lubricating system supplies filtered mineral oil to the turbine and driven equipment by an auxiliary gearbox-driven main oil pump. The oil reservoir forms an integral part of the underbase to reduce the turbine's footprint. An ac-motor pump provides lubrication before start-up and after shutdown. A dc-motor-driven pump provides for safe shutdown or power outage.

During start-up, an ac-motor-driven hydraulic pump uses lube oil to provide high-pressure oil to the hydraulic start motor. This rotates the turbine via a self-synchronizing clutch mounted in the auxiliary gearbox located at the air intake end of the turbine.

The Cyclone package is designed to accommodate many different applications. In power generation, the standard Cyclone generator with an integral speed-reduction gearbox is mounted on a separate underbase for ease of installation. A stainless-steel enclosure can be used to protect the turbine in offshore or coastal environments such as oil platforms. A painted carbon-steel acoustic enclosure reduces noise levels to 85 decibels.

On-site maintenance is facilitated by the vertically and horizontally split intake casing, which allows access to the inlet bearings without dismantling the ducting. The horizontally split compressor casing enables the compressor rotor and blades to be inspected or cleaned on-site. The combustion chambers, fuel injectors, and igniters can also be inspected without removing the main engine casing. In addition, multiple borescope ports permit visual inspection without the need for disassembly. Inspection intervals are necessary every 8,000 hours.

The Cyclone controls include integrated, local control systems mounted on the turbine underbase, which minimizes installation time and cost. The first shipments of the Cyclone are scheduled for early 1999.

SIMPLICITY EQUALS RELIABILITY

ABB has aimed its 43-megawatt GTX100 industrial gas turbine at the 40- to 50-megawatt power range, a growing sector of the power market accounting for 4 to 5 gigawatts annually. Within this market is a sharpening demand for combined cycle or cogeneration by independent power producers and small utilities seeking to provide more-efficient energy to customers in a more competitive market created by deregulation in the Western countries. In addition, the economies of South Asia, East Asia, and Latin America are turning to independent power producers and cogeneration to provide electricity for industrial growth that the local grid cannot supply.

Another important cogeneration application for the GTX100 is industrial processing, such as petrochemicals and cement, that requires large amounts of both steam and electricity. To meet that need, ABB engineers designed the GTX100 to provide 37-percent efficiency in simple-cycle operation and 54-percent efficiency in combined-cycle mode, with an output of 62 megawatts.

The GTX100 has a simple, single-shaft design to strengthen its reliability. The shaft is supported by two hydrodynamic tilt-pad bearings. The cold-end drive of the unit provides for an axial exhaust from the turbine end. When the turbine is used in combined-cycle or cogeneration configuration, this feature permits an efficient exhaust diffuser to be installed. It also allows easy access to the boiler. The diffuser connects to the exhaust stack or directly to the waste heat recovery unit.

The new turbine's 15-stage compressor is an aerodynamically scaled-down version of ABB's GT24 and GT26 compressors. The first three stages on the GTX100 have variable geometries to maintain high, partial-load efficiency and low emissions over a wide operating range. The stator is vertically split to provide maintenance access, and the rotor is electron-beam-welded. The chromium-steel rotor blades were fashion as controlled diffusion airfoils. A portion of the compressor had abradable seals, another method of ensuring performance.

The annular GTX100 combustor is equipped with 30 of ABB's first advanced environmental (AEV) burners, an upgrade of ABB's low N[O.sub.x] EV burner. These burners keep emissions of N[O.sub.x] and CO below 15 parts per million with gas fuel, 25 parts per million in liquid fuels.

The AEV cone has four slots for the compression air to enter, with the gas fuel entering along the edges. This is twice the number of slots as the EV, providing a better mix of fuel and air and lowering emissions further. When liquid fuel is used, it enters the AEV through a central nozzle in the tip of the cone. Unlike some dual-fuel turbines that require the fuel system to be switched over from liquid to gas, the GTX100 can burn both fuels at the same time.

The GTX100 turbine has a three-stage barrel design with disks bolted to the stub shaft. The first two stages of vanes and blades are cooled using air bled from the compressor to extend their life. For the same reason, the first stage blades are cast from a single crystal material. The external cooling of the turbine stator decreases tip clearance and increases efficiency. The GTX100 compressor casing is vertically split to ease service access. In fact, the entire gas turbine can be replaced in one day, minimizing maintenance downtime.

The first GTX100 unit will be ready for testing by mid-1998. After six months of testing, the first units will be commercially available.

AIR-STAGED COMBUSTION

Engineers at AlliedSignal Engines in Phoenix redesigned their own TFE 1042 afterburning turbofan engine to develop the 10-megawatt ASE120 industrial gas turbine for power-generation and compressor-drive applications. The former purpose is particularly good for military aircraft, including more than 100 indigenous defense fighters flown by China. The ASE120 is designed to provide thermal efficiency exceeding 35 percent, with greater than 96-percent reliability at 7,910 rpm.

In the low-pressure compressor of the ASE120, engineers eliminated the TFE1042 bypass flow to increase efficiency, and provided a split case to ease maintenance. They equipped the low-pressure compressor with stainless-steel blades, vanes, and disks to make them more durable, and used low-aspect-ratio blading to improve surge margin. A patented air-stage combustor uses a DLE system developed by AlliedSignal in conjunction with Optima Radial Turbine BV in Hengelo, the Netherlands, called OPRA ("Small-Scale Turbines," August 1996). This dual-fuel system is designed to achieve less than 10 parts per million of N[O.sub.x] and 10 parts per million of CO on natural gas. Using liquid fuel, the DLE system will keep N[O.sub.x] under 25 parts per million and CO under 50 parts per million.

The main components of the air-staged DLE system are a convectively cooled combustor liner, an electronically actuated air-modulating valve placed outside the combustion chamber, a dual-fuel air blast nozzle, and a premixing/prevaporized venturi. The air-modulating valve divides the engine airflow between the combustion air and the bypass air. These components provide full control of the fuel-to-air ratio, which determines flame temperature. Thoroughly mixing vaporized fuel with air before ignition reduces emissions.

A pair of three-way valves control the flow of air into the combustor. Two premixers send combustion air into the burner. Remaining air flows through either dilution orifices or the variable bypass circuit located at the burner exit and upstream from the high-pressure turbine stator vanes. The dilution orifices are sized for maximum power, and bypass orifices are sized for idle power.

Heat transfer through the combustor liner wall is enhanced by small cast ribs. A shroud on the outside diameter directs air and maintains flow velocity needed to cool the liner. A thermal barrier coating applied to the liner reduces wall temperature.

Computational-fluid-dynamics analysis of the ASE120 combustion chamber was performed using CFD-ACE software from CFD Research Corp. in Huntsville, Ala. Rig tests are currently under way at AlliedSignal's Phoenix facility to validate the design and CFD predictions.

Designers cast the ASE120's high-pressure turbine blades of single crystal material, and improved nozzle cooling by increasing the number and location of holes on the nozzle. The ASE120 is equipped with a heavy-duty single-pad starter gearbox that reduces costs and facilitates maintenance by replacing the lightweight magnesium gearbox currently on the TFE 1042. Cost reduction and maintainability were also the reasons the engine will use Allen-Bradley's industrial grade programmable logic controller.

While AlliedSignal has overall program responsibility for developing the ASE 120 plus assembling and testing its gas generator, the new turbine represents the culmination of an international partnership. Mitsubishi Heavy Industries Ltd. in Tokyo designed and fabricated the unit's power turbine, and will incorporate the ASE120 into its AS9 mechanical-drive package. Shipments of the AS9s are expected to begin in the fourth quarter of 1998 for a variety of power-generation, compressor-drive, and marine-propulsion applications.

Two firms based in China, Aerospace Industrial Development Corp. (AIDC) and Super Precision Heat Treatment Co. Ltd., are partners on the ASE120. AIDC performed the mechanical design of the low-pressure compressor, and Super Precision is manufacturing the low-pressure compressor case and high-pressure turbine seal disk. Hyundai Space & Aircraft Co. in South Korea, which is also a partner, fabricated selected components such as the burner.

AlliedSignal is adapting other engines it originally developed for commercial and military end users to create new industrial turbines. For example, engineers are using the TF40 and TF50 engines, from U.S. Navy air-cushioned landing craft known as LCACs, to develop the ASE50, a 3.8-megawatt industrial gas turbine that will incorporate the air-staged DLE system, a new air inlet, variable inlet guide vanes, a higher-efficiency impeller, an improved diffuser, an accessory gearbox, and Allen-Bradley state-of-the-art turbine controls. The ASE50 will be available in 1998.

Another offspring of the TF40 and TF50 turbines will be the TF50A. This 5,114-horsepower turbine will incorporate a new low-pressure power turbine, now under development, and a new compressor to meet the demands of fast ferries and other high-speed marine craft.

Encotech's Lukas predicted that future turbine designs will rely on making parts more thermally resistant to raise the operating temperatures of gas turbines and thus their performance. "We have reached nearly the limit of what we can do with high nickel and cobalt alloys parts, so either thermal coatings or ceramic parts will probably be the next step," he said.

Another design strategy Lukas described is using steam rather than air as a coolant. This will raise the steam and cycle temperature, and thus turbine efficiency. "Westinghouse and General Electric are already using steam to cool the stationary and some rotating elements of the turbine on their advanced designs," he said.
COPYRIGHT 1997 American Society of Mechanical Engineers
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Copyright 1997 Gale, Cengage Learning. All rights reserved.

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Author:Valenti, Michael
Publication:Mechanical Engineering-CIME
Date:Aug 1, 1997
Words:2565
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