Gas turbine design is all about achieving a balance of efficiency and reliability, according to how the turbine is to be used. Lynne Anderson discusses how Siemens’ newest turbine works for cogeneration applications.

The gas turbine market is fully mature – over 50 years of design and development activity having ensured that all products on the market have broadly similar key performance parameters. For original equipment manufacturers (OEMs) this means incremental, small improvements in efficiency via refinements in material technology, cooling technology and aerodynamics leading to reduced losses and increased pressure and temperature ratios.

Gas turbines play a key part in the energy industry and have an astonishingly diversified role. They are used from wellhead to consumer, from the simple cycle machines used to compress gas, pump oil and provide platform power, to the combined heat and power (CHP) gas turbines used to provide electrical power, heating and cooling for industrial plants, institutions and residential dwellings.

Efficiency and reliability are two major parameters that should both be considered at the beginning of a new engine design. What are the target values for a new engine and what is achievable? To get higher efficiency means higher firing temperatures, higher pressure ratios, exotic materials, complicated cooling systems, etc, all factors which jeopardize the cost and the reliability of the product. The aim of the product design team is to reach the optimum balance for these parameters, and for the demands and specifications of the individual customer.

For oil and gas companies, the profit flows when the oil or gas flows; that is, the costs associated with providing the power and pumping capability are small compared to the value of the commodity being pumped, hence reliability far outweighs the advantages of fuel efficiency.

For power producers there is often no use for heat, hence gas turbines are used in combined cycles (CC) where the exhaust heat raises steam to drive a steam turbine. These CC applications are most prevalent for gas turbines above 20 MW and, for these applications, gas turbine simple cycle efficiency plays a more important role in plant economics.

For industrial and domestic users, a gas turbine installation, in most cases, only makes economic sense if the exhaust heat is used; in this case, it is the overall plant thermal efficiency that is most important. Or, to put it another way, the user must gain the maximum benefit from the fuel being burnt as the cost of energy supply for many industries is a major influence on their achieved profit. This is where cogeneration comes in.

Cogeneration plants are an excellent option for meeting the demand for electricity and heat in an economical manner. The combined generation of power and process steam significantly increases overall efficiency. In such plants the residual heat in the exhaust-gas flow is used to raise steam in a heat recovery steam generator (HRSG).

Cogeneration is therefore cost-effective in three respects: cogeneration plants are highly efficient thanks to the high turbine efficiency level, they make optimum use of the fuel and, finally, they save costs for the operators.

THE NEW SGT-750

It is against this backdrop that Siemens recently introduced its 36 MWe SGT-750 gas turbine. Benchmarking was a significant part of the pre-design phase. Siemens went out to their customers and asked what it was that they really wanted when they invested in a new turbine. The answer was reliability – a machine that just keeps on running, as downtime means loss of production hence loss of income.

Consequently Siemens focused on designing a robust industrial gas turbine with a unique uptime capability. Serviceability and robustness were the key features from the beginning. The determining factor here is the 24-hour generator swap. If the customer selects this option, Siemens pledges as little as 17 days’ maintenance time over a 17-year service period.

An advanced 3D visualization programme developed during the design period enabled the engineers to check that the design allowed full serviceability of important components and areas, without major disassembly of the unit.

Virtual design in 3D studios helps to ensure that designs facilitate full serviceability of key components and areas
Source: Siemens

TURBINE DESIGN

For power generation applications, the free, two-stage power turbine runs at a speed of 6100 r/min. For mechanical drive applications the shaft speed is 3050-6405 r/min. The turbine inlet temperature is 1144°C, which is on a par with the smaller SGT-700.

The compressor is a 13-stage axial flow compressor with a 24:1 pressure ratio. It offers a controlled diffusion airfoil for high efficiency and two variable guide vanes for optimized performance even in the most extreme conditions. The compressor section utilizes the conventional horizontal split casing design for unbeatable accessibility, while the hot parts of the unit are surrounded by vertically split single-piece circular components for optimized clearances and stability. This combination of casing design unites the best qualities of two concepts into one.

Two variable guide vanes in the compressor offer optimized performance even in the most extreme conditions. The axial blade attachment grooves allow complete compressor reblading without rotor removal.

The SGT-750 has eight combustor cans which make control and service of the combustor system easier than with an annular combustor system. The combustors and flame heads are based on proven components and global experience from the overall Siemens turbine range. The SGT-750 is designed for single-digit emissions and testing on gaseous fuel so far indicates NOx emissions ≤ 15ppmV (with DLE, corrected to 15% oxygen).

The gas turbine can be run down to 50% load with maintained flame stability, and can operate on dual fuel with on-load switchover between fuels. The materials used throughout the gas turbine, compressor as well as turbines, are advanced but proven. The compressor turbine stages are cooled, while the power turbine is uncooled.

APPLICATIONS

The SGT-750 is intended as a multi-purpose machine for both power generation and cogeneration applications as well as mechanical drive. It was originally conceived as a fit for the future mechanical drive market, which means that it is designed to meet the stringent requirements of the oil and gas industry, including extreme climates.

Initially, however, it is being launched as a power generation machine in order to test the new unit on full power under long periods. The turbine’s high efficiency and rapid start capability should make it a very competitive machine for the power generation market. The turbine is still under manufacturing and the first module will be released at the end of 2011.

SGT-750 sets a new standard of efficiency for industrial gas turbines in this power range. Reaching 38.7% in power generation applications and a full 40% in mechanical drive applications helps push fuel consumption down to even lower levels, still keeping – and improving on – all the benefits of a robust, easy-to-service industrial design.

POWER GENERATION/COGENERATION

It is anticipated that the main applications will be in simple cycle and cogeneration. The designers selected a high pressure ratio in order to achieve a high simple-cycle efficiency, and a fairly moderate firing temperature to ensure a very reliable engine. Although the relatively low exhaust gas temperature of 462°C (864°F) may limit the potential heat for high-pressure steam, it gives a wide potential for steam flexibility with supplementary firing in the HRSG.

The SGT-750 is the perfect option for base load, standby power and peak lopping. The fast start-up (ten minutes from start to full load) and cycling capability both support intermediate to continuous operation with improved turndown capability, high efficiency and low emission levels. Through the use of a free power turbine, the 36 MWe SGT-750 is also well suited where grid requirements call for maintained power output in the event of frequency drop.

With its high exhaust energy, the SGT-750 is a nice fit for efficient power generation and cogeneration. The sheer robustness and stability of the 37 MW SGT-750 makes it a perfect option for mechanical drive applications within the oil and gas industry. The dual-fuel online switchover capabilities provide a unique built-in flexibility when it comes to ambient climate, and perfect adaptability to fixed or floating installations, onshore and offshore, upstream, midstream or downstream.

The SGT-750 is delivered as a complete package. The footprint of the SGT-750 gas turbine package is similar in dimensions and layout to that of the smaller SGT-700 (driver package: 12.8 m x 4.3 m / power generation package: 20.3 m x 4.75 m). The SGT-750 gas turbine train comes mounted on a base frame with single-lift capability and a base-frame split between driver and driven equipment.

The lubrication oil system is installed inside the package and all auxiliary systems are mounted on the base frame. Liquid fuel, water injection and fire-fighting systems are kept separate.

SGT-750 power generation plant configuration
Source: Siemens

CONTINUOUS GAS TURBINE DEVELOPMENTS

Siemens’ industrial gas turbines, ranging from 5 to 50 MW, have clocked up an impressive record in all applications. Across the range, they have amassed a total of more than 57 million operating hours, providing dependable power to some 500 operators in more than 80 countries around the world. The units are under continual development, matching market requirements in power output, reliability, emissions, efficiency, fuels capability and price.

In the interests of economic efficiency, the R&D teams have addressed not only further development of advanced materials and designs for hot gas path components, but also serviceability and maintainability, including advanced repair, non-destructive inspection tech-nology, remote monitoring and diagnostics, and new maintenance concepts with extended maintenance intervals. The increased reliability of the 47 MW SGT-800, for example, has led to a maintenance interval extension from 20,000 to 30,000 EOH, which increases availability by about 1%.

MARKET DEVELOPMENTS FOR POWER GENERATION TURBINES

As Siemens has a full range of both gas turbines and steam turbines, combined cycle is a natural configuration to offer its customers. A cogeneration plant can be configured as a combined-cycle with both a gas turbine and a steam turbine, especially if only low pressure steam is needed. Supplementary firing can be used when the steam demand is higher than can be met by the unfired system, by burning additional fuel at the entry to the boiler. In this scenario the boiler can be modified to enable steam to be raised even if the gas turbine is shut down.

A specific variant of the combined-cycle configuration is the simultaneous generation of electricity and district heating, the most common and economic form of cogeneration widespread in northern Europe and Scandinavia, where economic heating is a prerequisite during the long winters. District heating requires large quantities of low-pressure steam or hot water for space-heating of industrial or domestic premises.

A greater percentage of exhaust heat can be recovered using this system since feedwater temperature is low, allowing a corresponding reduction in boiler exhaust stack temperature. The process is governed by the total heat load and electrical energy is supplied to premises as an additional benefit.

A showcase example is the Rya CHP plant in Gothenburg, Sweden. This plant provides 30% of the city’s electrical power (261 MW) with 3 x SGT-800 gas turbines and an SST-900 steam turbine. The plant also provides 35% of district heating for the city (294 MW) at an overall plant efficiency of 92.5%. Annual carbon dioxide emissions have been lowered by over 500,000 tonnes.

Municipalities, hospitals, universities, offices and commercial buildings and government buildings are sectors currently benefiting from Siemens’ CHP district heating installations. For total climate control, the turbine exhaust gas can be directed into a fired absorption chiller or its steam can be used to provide mechanical energy for other forms of refrigeration. Trigeneration, where power, heat and cooling are produced simultaneously from a single fuel source, is the most economical and comprehensive form of cogeneration. The amount of heat and cool generated can be varied according to facility needs.

A showcase example of this application is the Riverbay Co-Op Development (New York, USA), a trigeneration scheme providing electricity, heating and cooling for 60,000 residents in the Bronx area of New York. Some 40 MW of electrical power is produced by two SGT-400 gas turbines and an SST-300 steam turbine. Up to 16 MW of electricity not used locally enters the New York power grid. The steam generated by the exhaust heat is also used to provide heat in the winter and cooling via absorption chillers in the summer.

CONCLUSION

The design parameters of the gas turbine core engine need to be chosen carefully to balance their influence on the reliability, maintainability, cost, efficiency and emissions of a gas turbine based power plant. Balancing efficiency and reliability has been shown to be a key design decision.

Gas turbines are used in power plants and mechanical drive applications and, as these plants can be configured in a number of ways, the gas turbine manufacturer needs to balance the requirements of each user to optimize the design. This is a constant and major focus of the Siemens design engineers and is a feature of the full range of industrial gas turbines from 5MW to 50MW, including the recently launched SGT-750.

Lynne Anderson is with Siemens Industrial Turbomachinery, Finspong, Sweden. Email: lynne.anderson@siemens.com

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