|Measuring the radial gab of a gas turbine
Gas turbine performance has traditionally been evaluated in terms of thermal efficiency, but in determining overall performance, operators also evaluate availability, reliability, flexibility and, above all, profitability.
The earliest commercial gas turbine engines were used to power aircraft during the 1940s and it was another 20 years before gas turbines became established in the electricity generating fleet.
In the early 1960s, grid disturbances led to electricity blackouts across the southeast of England. With load growth predicted, the Central Electricity Generating Board (CEGB) decided to deploy fast start aero engines as gas turbine generators. A demonstration engine at Hams Hall power station in 1964 was followed by a major installation programme.
Across the Atlantic on a November night in 1965, a cascading voltage collapse blacked out nearly one third of the population of the northeastern US. This led the country’s electricity industry to call on aircraft engine makers to provide small, rapid-start generators that could be deployed across the grid.
There has been continuous development in gas turbine technology since then and their use in power generation has increased rapidly. Gas turbines are now one of the most widely-used power generating technologies. Turbines have become larger, with better thermal efficiency, able to operate at higher temperatures and pressures. New plant can be constructed relatively quickly, but high operating costs and lack of operational flexibility are areas where operators seek improvements.
Turbines used in gas-fuelled generation are sophisticated and complex machines. The compressor draws in air, pressurizes it, and feeds it to the combustion chamber at high speed. The combustion system injects a steady stream of fuel into combustion chambers where it mixes with the air and burns at high temperature to produce a hot, high pressure gas stream. As this passes through the turbine section it expands and spins the rotating blades which turn a generator to produce electricity. The combustion gas is also used to drive the compressor.
Heat from the exhaust gas can be recovered and used in a combined-cycle configuration. The combined cycle is more thermally efficient but operational limitations include longer start-up time, purge requirements to prevent fires or explosions, and the need for a phased ramp-up to full load.
Increased efficiency has been the traditional goal in designing and operating gas turbines. ERA Technology gas turbine consultant Siavash Pahlavanyali reflects: “Over the last 30 to 40 years the trend has been steady improvements in the thermal efficiency of gas turbines. Thirty years ago, E Class turbines operating in single shaft, single cycle had thermal efficiency rates of 30-32 per cent, which has now increased towards 37-38 per cent. In combined-cycle operation, the rate has moved up from 50 per cent to nearer 60 per cent.”
He cautions: “Pushing the efficiency higher may compromise the integrity and design margin.”
Continuing R&D will see the trajectory of improved thermal efficiency continue. Alap Shah, turbine technologies manager at Black &Veatch, expects combined-cycle efficiency to increase towards 65 per cent in the next 10-15 years if economic and environmental drivers continue to push the industry.
He says: “Gas turbines of the future will have higher firing temperatures, better sealing technology and air cooling for hot gas path cooling, rather than steam cooling, increasing operational flexibility. Firing temperature limitations are based on the material technology and metallurgy.
“The trend in the last ten years is for turbine manufacturers to offer higher temperatures on their products. Other contributions to efficiency will come from increased steam temperatures and pressure of the bottoming cycle.”
Manufacturers offer customers upgrade packages to improve output, as well as long-term service and maintenance agreements for their components and systems. A typical upgrade package for an F Class machine might include improved blade aerodynamics, better sealing, advanced materials and improved cooling technologies to allow higher operating temperatures.
Mike Salvatore, Siemens’ technical marketing manager of gas turbine modernizations for the Americas, explains: “Upgrades are driven by customers’ need to maintain competitiveness in the marketplace. We evolve products and services to provide greater capacity and improved efficiency and we offer products to improve operating flexibility.”
|MAN Diesel & Turbo has realized single digit NOx values in the load range between 50 and 100 per cent by optimizing its Advanced Can Combustors (ACC) on a MGT 6100, the single-shaft version of the new MGT gas turbine.
Credit: MAN Diesel & Turbo
Operators of turbines installed in the late 1990s and early 2000s are looking to the OEMs for possible upgrades. Salvatore says: “When we assess what our customers need we pull from our newer, more advanced technology and apply it to the more mature fleet. This means taking advantage of our native knowledge, improved materials, better cooling schemes, more sophisticated gas turbine control system technology and retrofitting them.
“Upgrading is a constant refreshing of the more mature technology that is 10-15 years old. A lot of users of older equipment are trying to operate them like brand new technology. They want large capacity improvements, efficiency, fast starting ability, fast start acceleration and oftentimes this is prohibitive because it could require capital changes to the equipment.”
He expands: “We can help by making improvements to the existing configuration; we can implement the latest technology in terms of modernizations and upgrades of the components, thereby resulting in improved performance levels and reliability.
“Our customers are pushing for these upgrades, as replacing their older equipment may not be feasible in the short term. As the OEM with the most specific expertise on our products, we work closely with owner-operators to enhance the ‘asset value’ of their existing equipment.”
Third party specialists can advise on how to fine-tune and get the best from systems. Upgrading the gas turbine may necessitate work on the balance of plant, and involving a third party in the upgrade can be advantageous.
Shah explains: “We have been involved in several combined cycle upgrades where OEMs offered several gas turbine upgrade packages to the owner and the owner hired us to evaluate these options in conjunction with the HRSG, steam turbine and balance of plant equipment to find a sweet spot in terms of overall plant performance upgrade. In that role we take the upgraded performance from the turbine supplier and we integrate that with the overall plant thermodynamic model.
“With the help of this integrated model, we study and evaluate the equipment such as HRSG, steam turbine and boiler feed pumps, and systems such as high pressure and reheat steam, boiler feed system etc. We perform a debottlenecking study to find the constraint and upgrade that equipment or system if it makes economic sense.”
Upgrades are expensive and an understanding of the current and future operating requirements of the plant is necessary in deciding what improvements are appropriate. Shah says that bigger CTG upgrades are not always better for the combined cycle plant.
Pahlavanyali points out: “I see clients who pay to get very advanced coatings on their blades. But if the machine runs at 80 per cent of base load for most of the time, it means it has not been operating efficiently. In which case that upgrade does not make sense.”
Extending maintenance intervals
All OEMs recommend specific inspection and service intervals, but as condition-based monitoring becomes more sophisticated, operators may be able to extend these intervals, giving greater availability and profitability.
Black & Veatch’s Shah explains: “As manufacturers get more information and experience from their operating fleet, they can reduce margins and be more aggressive in allowing turbines to operate for longer times between maintenance intervals.”
Maintenance intervals for hot gas path inspection have typically increased from 24,000 hours to 33,000 hours, while the interval for major inspection is up from 48,000 hours to 66,000 hours.
Shah says: “The time to be considering an upgrade is when a major overhaul is needed. For example, a rotor inspection will come at 100,000 hours. Replacing an existing rotor might cost up to $10 million. This is the logical time for thinking about an upgrade.”
Flexibility, the ability to start and stop frequently and rapidly, is important where gas turbines are used on networks that give priority dispatch to renewable sources such as wind and solar. This can mean a change in role for gas turbine generation, from base load to intermediate duty.
Gas turbines in combined-cycle power plants do no not give their best results when operating under variable loads. Getting the best out of combined-cycle plant requires operation at constant base loads or peak operating conditions. However, single-cycle gas turbines are now able to operate more efficiently at lower loads, and maintain emissions compliance over wider load ranges.
Pahlavanyali told PEi: “A UK client recently told me he can’t make any profit on combined cycle with 50 per cent efficiency, but they make a good margin and profit when they run the same machine as on-call, fast response simple cycle.”
OEMs are designing upgrades that give better flexibility as well as improved output and efficiency. Shah says: “One significant change we are seeing in the market is that operators have greater flexibility to operate their turbine either with higher efficiency and lower maintenance intervals, or lower efficiency and higher maintenance intervals. Operators can make a choice and balance between those two parameters, usually dependent on the cost of gas.” This flexibility is a recent addition to the options manufacturers offer when operators upgrade their turbines.
Advances in software
Sophisticated control systems are used to improve output – for example, GE’s OpFlex software upgrades in the control system. Eric Kauffman, product strategy director of software and analytics for GE’s Power Generation Services business, describes some of the advantages.
“This allows customers to extend output on cold days by controlling the combustion system more precisely,” he says. “This enables the gas turbine to maintain a higher level of output when the temperature drops below 59à‚°F. We also have products which allow customers to peak fire their machine while remaining emissions compliant.”
Another OpFlex technology fine tunes operators’ control of the water spray within the heat recovery steam generator, which controls the steam temperature (attemperation). Kauffman explains: “We found customers who use more attemperation spray than necessary to keep the steam temperature from rising too high. The result is that the system runs less efficiently.
“Our new software uses a model-based control approach that anticipates the attemperation requirement based on the gas turbine activity. The result is that our customers do not have to suppress the temperature, which, in turn, stays more stable and leads to an increase in efficiency of the steam turbine.”
More intelligent control of the steam turbine means less movement of valves and other parts, which reduces thermal and mechanical wear. GE has been remotely monitoring turbines for 15 years. Sophisticated software enables the OEM to look at how customers operate at a plant level and identify improvements.
Kauffman says: “We are always challenging ourselves to get more info and deliver more value with the remote monitoring and diagnostics data that we are collecting.”
OEMs’ recommendations for inspection intervals are based on operational hours. The schedule might be every 48,000 hours major inspection, every 24,000 hours hot gas path inspection and every 12,000 hours combustion inspection.
Pahlavanyali says: “That is not always sufficient: the manual is the same wherever you are operating your machine, but operating in the desert or by the coast means there may be differences.”
He believes that by paying more attention to condition monitoring and condition assessment of components, operators may be able to safely extend operational inspection intervals by condition. How can operators determine this?
A gas turbine includes a lot of monitoring tools. Measurements of temperature, pressure, vibration, oil quality and other factors are recorded and can be used to improve understanding of the machine’s condition.
Sharing the operational data with the OEM or with third party experts can help analysis. More information can be gained from visual inspection during maintenance. An inspection engineer carrying out visual inspection and looking at records would be able to determine the likely condition of components such as blades and then advise whether the blades need repair or can remain in service.
Blades can be removed and tested to establish if the blade can remain in service, if it needs recoating or if it should be scrapped. Pahlavanyali explains: “We have looked at hundreds of turbine blades in the last 20 years. In many cases the nominal design life was finished and the operator replaced the component, but often they could extend the life.
“We carry out a lot of tests and, if the result is that they can put the blade back into operation for another 24,000 hours without any problem, this almost doubles the design life. With blades costing from à‚£200,000 to à‚£2 million ($336,000 to $3.4 million) this represents a lot of money.”
The same approach can be applied to other components and processes. While the OEM may recommend offline compressor washing every two months, Pahlavanyal says: “In my experience compressor washing intervals should be based on the condition of the machine. The recommendation is to base it on the amount of drop in pressure, but I think the best tool is experience. Any time there is a sign of degradation – material degradation or performance degradation – then wash the compressor (online or offline) to take it back to normal.”
One of the challenges of flexible use of gas turbines to back up renewable sources of power generation is that part-load operation could significantly increase emissions. Operating gas turbines at low loads may lead to significantly higher levels of CO2 and NOx gases.
Emissions increase during the low-load phases of combined-cycle plants to allow the rest of the plant to operate safely for the desired heat level.
|The new MGT 6100 single-shaft gas turbine before being put through its paces on the test bench in Oberhausen, Germany
Credit: MAN Diesel & Turbo
MAN Diesel & Turbo, which makes a range of small gas turbines designed for use in industry as mechanical drives for, e.g., compressors or for decentralized electricity generators, has substantially reduced the NOx emissions of its MGT gas turbines for a wide operating range down to very low part load operation by refining the combustion chamber design and using premix technology.
Dr Sven-Hendrik Wiers, vice-president gas turbines, explains: “We achieved single-digit NOx by designing a new state-of-the-art combustion chamber for our new turbine.”
The reduction in NOx emissions to single digits (in parts per million) is aided by using an advanced can combuster to homogeneously premix the fuel with the combustion air before it enters the combustion chamber. The pre-mixing eliminates fuel-rich hot streaks, which significantly reduces NOX gases.
Wiers says: “The ambition was to have a very efficient gas turbine. Efficiency is about improving compression of air, improving sealing technology, improving hot gas part lifetime, improving cooling technology and optimizing turbine inlet temperature levels.
“We applied modern, available design tools to improve the efficiency of compressor and turbine, and then to achieve perfect matching of turbine and compressor. We adopted jet engine secondary flow technology as the front runner in gas turbine design methodologies in turbines.
“We applied the philosophy of jet engines to improve our sealing technology where possible.”
Modern gas turbines are proving to be the fossil-fuel technology of choice offering lower emissions than other hydrocarbons, operational flexibility, and the potential for high cycling and peaking, fast startups and load ramps.
However, they are expensive to run and to maintain and operators seek the best upgrade and maintenance solutions from OEMs and third parties.
Saà…¡a Ovcar of gas turbine specialist Inspiro calculates that “for a typical CCGT plant with plant efficiencies of over 50 per cent, maintenance costs may represent up to half of the total cost of electricity production.”
As he says, “It is therefore of highest importance for a power plant’s long-term competitiveness to consider all available options to reducing these costs.” That is the challenge facing gas turbine operators and manufacturers alike.
Penny Hitchin is a journalist focusing on energy matters.
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