Uprating the GT24s and GT26s: challenges and benefits20 December 2000
The new "B rated" version of the advanced GT24/GT26 gas turbine is designed for improved performance and maintainability. A programme of modifications currently underway around the world aims to make sure that the full potential is achieved.
Introducing new technologies is never easy, particularly when dealing with such large, complex and finely honed machinery as high efficiency, low emissions, industrial gas turbines – with perhaps 2000 individual components apiece. Further challenges come from introducing new technologies to a “hot market”, with a booming order book but with the potential for amassing large costs should teething problems arise, as they inevitably do.
Each of the major makers has faced its share of difficulties in the commercial introduction of new generation gas turbine technology. Alstom, as several recent public announcements have indicated, was not immune to challenges similar to those already faced by GE and Siemens.
In 1996, Alstom (then ABB) took the unique direction of sequential combustion. Since then the company has introduced 24 first generation, or A version, units into commercial operation. The fleet leader, Taranaki in New Zealand, has been in operation since 1998 and has accumulated more than 19 000 fired hours.
In the late 1990s, to meet increased demands from the market for even higher efficiencies, Alstom, with feedback from units in the field, made the decision to uprate the machines in terms of both performance and maintainability. This B version is beginning to show its capabilities, but only after the typical introduction issues have been addressed.
The A rating
The first GT24 (the 60 Hz version) ever to go into operation was at the Gilbert Station in New Jersey, United States, a simple cycle plant, where first ignition took place in September 1995. Extensive prototype testing was done on the Gilbert unit, which is now used for peaking support services on the PJM (Pennsylvania–Jersey–Maryland) system.
The first GT26 (the 50 Hz version) to enter operation was at Alstom’s Birr test facility in Switzerland. The Birr unit started up in November 1996.
The GT24/GT26 gas turbines are advanced machines, optimised for combined cycle use, capable of high efficiency, low emissions and great operational flexibility. The sequential combustion system employs two annular combustors, the EV combustor and the SEV (Sequential EV) combustor. There is a single stage high pressure (HP) turbine between the EV and SEV combustor. Downstream of the SEV combustor are four low pressure (LP) turbine stages.
Air discharged from the 22-stage compressor, which achieves a pressure ratio of 30:1, is guided along the combustor inner and outer liners to the EV burners. Here fuel is mixed with the high pressure air and ignited, producing the hot gas to drive the HP turbine. The gas is reheated in the SEV combustor, into which fuel is injected and ignites spontaneously, and undergoes its final expansion through the four stages of the LP turbine. The result is high power density and low NOx,with high exhaust temperatures, up to 640°C, over a wide part load range.
In the light of commissioning and operating experience with the early GT24s and GT26s a number of improvements and modifications were made. These included, for example, changes to some of the high pressure compressor components to eliminate unnecessary rubbing.
The B rating
After about 50 000 hours of operating experience with the earlier, A rating, GT24/26 turbines, the uprated version of the technology, the B rating, was introduced. The B rating is essentially a package of design enhancements which aim to improve overall performance and maintainability and take onboard operating experience feedback from the earlier machines over the 1995-97 period, while, of course, retaining the basic features of the GT24/GT26 technology.
The main improvements included in the B rating include:
l Improved cooling and seals in the LP turbine section, including repositioning of holes through which cooling air is passed and provision of additional cooling holes. Also has included addition of thermal barrier coating to some components.
l Adoption of single crystal material for certain blade rows, which eliminates crystal interfaces and therefore improves integrity under tension, leading to better creep behaviour under high temperatures and better tolerance of cycling conditions.
l EV burner modifications. In the A rating technology the 1 to 1.2 power scale up needed to go from the 60Hz model to the 50Hz model was achieved by keeping the number of EV burners the same (30 in both cases) but using a bigger size in the GT26 (50 Hz) machine. In the B rating, the burner design has been standardised, with identical EV burners used in the 50 Hz and 60 Hz models, but different numbers of them, 24 for the GT26 and 20 for the GT24. In addition the B rating burners are designed to be retractable, which greatly reduces maintenance times – allowing removal and replacement in days rather than weeks. The new burners are also cast rather than welded. Finally the overall reduction in their number, with consequently fewer holes needed in the turbine casing, also has benefits in terms of mechanical integrity.
The fleet leaders for the B rating were, in the case of the GT26, the Enfield combined cycle plant in the UK, which first entered operation in 1999 and, for the GT24, the Agawam combined cycle plant in the United States, which was first fired in the latter part of 1999.
One early phenomenon was encountered at Enfield, in October 1999, when boroscope inspection revealed loosening of the locking piece of row 16 in the compressor. It turned out that an unusual combination of circumstances during commissioning had allowed the locking piece to detach. Locking pieces with a simple modification are now being fitted across the fleet.
Boroscope inspection at Enfield also brought to light, again in October 1999, a second issue: cracking, after a few hundred operating hours, in certain parts of the EV outer liner. The geometry of the liner had been changed relative to the A rating machines to accommodate the new burner configuration. The solution here was to stiffen the components and so alter their natural frequency. Modified parts have now been installed across the fleet and have been operational since February 2000.
The third, and most recent, issue to be encountered was overheating in stator heat shields of blade row number 2 in the LP turbine. This first came to light at the Agawam plant on 24 June 2000, at around 500 hours of operation. Once again the problem was revealed by boroscope inspection, which showed deformation of the shroud. The turbine was opened and the root cause analysed. A modification was developed and as this was being validated the decision was taken to take the risk of starting to manufacture the modified components, two sets initially, assuming the validation would prove successful.
The root cause of the overheating problem turned out to be incorrect positioning of the hole designed to take cooling air through the heat shield and the solution consists of adding cooling by provision of a new hole through the heat shield.
By 7 August the row 2 heat shield modification had been installed at Agawam and the plant resumed commercial operation. Shortly afterwards the row 2 heat shield modification was also done at Enfield, which was back on dispatch by mid-August. The row 2 heat shield modification has also now been implemented in the majority of B version units, including Tocopilla, Monterrey and Midlothian.
A programme of rolling outages is now underway around the world to get the B rating machines operating as originally projected. Some of the delivered engines are being run with an interim operating concept to provide high availability while modifications are being validated. In addition several A machines will get some of the upgradeable features of the B technology.
Including units in operation, under commissioning and under construction there is now a population of 80 GT24/GT26 machines around the world – excluding projects in the pipeline. Of the 52 GT24s, 15 are A rating machines, the rest Bs, while nine of the 28 GT26s are B rating machines and the rest are of the A type.
Contrary to rumours, Alstom has confirmed it is continuing to market and sell GT24/GT26 units, and indeed is increasing production capacity to meet the high demand.
Recent experience with gas turbines around the world, has served to emphasise the vital importance of extended testing at realistic operating conditions. At its Birr test facility, Alstom has the opportunity to be able to perform tests on a full working power plant which can dispatch more than 260 MWe to the Swiss grid. The GT26 there has 2000 measuring points over and above the normal complement of instrumentation that would be found on a typical such turbine. New components can be fitted to the machine for thorough testing prior to commercial introduction. However there are limitations. It is a test and development facility with its own agenda of investigations and there are therefore limits on the number of hours of extended operation that can be achieved (eg it is currently being used to look at dual fuel cycling operation so is required to do numerous starts and stops). Thus there will always be an essential role for extended tests on prototypes operating under real conditions in the field.
At the time the LP turbine row 2 issue came to light, the Birr machine had not been converted to a full B rating, consequently the three issues highlighted above were found in the field, during routine boroscope inspections.
However, the Birr machine now has the full B features and in the last couple of months a full load thermal paint test has been done there, which promises to be provide some very important insights over the next few months. In the test every variety of gas turbine component was treated with thermal paint which changes its colour in accordance with the maximum temperature seen over a particular time. The Birr machine was run up to full load and then dismantled. The results are currently being analysed, but are expected to reveal areas where further optimisation could be achieved, providing an “additional channel in the feedback loop”, according to Jonathan Lloyd, Alstom’s GT24/GT26 product manager. “Paint tests have been done before,” he says, “but not at full load operating conditions. This is the only way of knowing how the various components react together.”
Overall, it is likely that testing at Birr will take on a much bigger role in the introduction of future gas turbine innovations.
A few years ago gas turbine buyers were demanding ever higher efficiency from the turbine vendors. What they now want is reliability and availability.
TablesThe GT24 and fleet, showing both "A rating" and "B rating" machines Status of GT24/26 machines, as of 1 December 2000 Summary of GT24/GT26 sales to date (including both A and B rating)