3A problems and solutions21 September 2000
A solid body of good operating experience, combining high efficiency with low emissions, is now being recorded by gas turbines of the 3A series. Not surprisingly, in getting to these high performance levels, several difficult issues have had to be resolved along the way.
Developing gas turbine technology to high ratings and efficiencies has posed some stiff challenges.But the 3A gas turbine series, which was developed in response to customer demand for improved efficiencies and lower costs, can now be regarded as having successfully completed its market introduction phase. Consistent and reliable operating performace is now being clocked up under the toughest field conditions.
Siemens launched its new 3A gas turbine series in 1995, the prototype V84.3A (60 Hz model) having achieved a new efficiency record of 38 per cent on the full-load test bed in Berlin at the end of 1994. In 1997, the 70 MW V64.3A geared version (for 50/60 Hz) underwent trials – also on the Berlin test bed.
Although the main reason for the modification outages (but not forced outages) was combustion noise due to flame instabilities, we also made quite a number of modifications to the turbines as part of our ongoing product improvement efforts during the market introduction phase. These served not only to enhance performance but also to prolong service life, to extend inspection intervals and to reduce maintenance effort.
The 3A gas turbine family is based on the 3 series (introduced in 1990) with very similar rotor dimensions (diameter, distance between bearings). The aerodynamics of the compressor were improved in a joint development project with Pratt & Whitney (P&W) through a variety of measures, including use of controlled diffusion airfoils with boundary layer correction. The first four stages were provided with variable-pitch guide vanes to optimise flow across the profile. For this purpose the double-shell casing was initially retained in the 60 Hz version, the V84.3A.
In the course of commercial introduction, the front region of the compressor casing was changed to single-shell design in all machine types. The double-shell casing needed for installation of the variable-pitch guide vanes was dispensed with in the series machines, because the orientation of the guide vanes has now been optimised, and only the inlet guide vanes are adjustable (Figure 1).
In the V94.3A version as originally introduced, which did not have the advanced compressor, resonance occasionally occurred in some compressor blade rows, especially in the case of severe fluctuations in grid frequency (in one turbine this phenomenon even caused forced outages). This resonance was eliminated by fine-tuning the blades for an adequately broad frequency band. This also improved the turbine’s stability at underspeed and overspeed conditions, so that it is also able to tolerate wider grid frequency fluctuations. The final version of the V94.3A, of course, was equipped with the advanced compressor.
We also introduced or further improved the aerodynamics of the turbine, the film cooling and the blading material (monocrystalline alloy PW 1483 and thermal barrier coating (TBC)) in co-operation with P&W (Figures 2 and 3). In the course of the market introduction phase the improvements made to the V84.3A blading were also adopted for the V94.3A turbines. Use of TBC in the first row of turbine blading made it possible to raise the turbine inlet temperature and the baseload rating still further without detriment to the service life.
Regular inspections of the hot gas path during the market introduction phase revealed that the coatings on the first blading rows were being seriously eroded, especially at hot spots with inadequate film cooling air supply. Improvements to the film cooling and the procedure for application of the coating made it possible to significantly extend the service life of the coatings.
One of the advantages of the hollow rotor is that it makes it possible to extract cooling air at the optimum pressure level out of the compressor and direct it via pipes running through the hollow space into the turbine. Wear initially observed at the turbine-side connections was eliminated by modifications performed in the course of scheduled major inspections.
The creep strength of the central hollow shaft was sufficient to enable the turbines to reach the first scheduled major inspection without setbacks or reduced availability. Use of an improved alloy made it possible to significantly enhance the creep strength, and the hollow shaft was replaced during the inspection. The improved 3A series turbines are, of course, fitted with the new hollow shaft.
The annular combustion chamber was introduced in the 3A turbines to achieve low NOx emissions despite the high turbine inlet temperatures needed for high efficiencies. Use of ceramic tiles (Figure 4) made it possible to raise the turbine inlet temperature still further. As when any manufacturer develops a new gas turbine, the problem of combustion noise in the operating turbine had to be brought under control, as this cannot be adequately tackled by theoretical considerations alone. This problem stems from the fact that instabilities in the flame can cause acoustic resonance, particularly in high temperature gas turbines. In the case of the burners in the annular combustion chamber of the 3A series, this resonance stemmed from annular vortices in the burner outlet region (Figure 5). In this region, fuel and hot gas products mix, and the mixture spontaneously ignites after a very short time. The resulting pressure surges triggered oscillations due to acoustic feedback.
Our solution to this problem is to add cylindrical extensions to the burner outlets. This cylindrical burner outlet (CBO) concept and its function are also shown in Figure 5. The CBO ensures that the ring vortices have less fuel to mix with, so that the vortices do not ignite before the main flame and so can no longer cause resonance. What is more, the uniform flow pattern helps to stabilise the aerodynamics and to reduce flow/flame instabilities.
We have now accumulated several thousand hours of positive operating experience with the modified burner (designated HR3, see Figure 6). Otahuhu, Seabank and Peterhead have been fitted with HR3 burners since November 1999, January 2000 and March 2000 respectively. Experience to date suggests that:
l even at outputs above the full-load rating, NOx emissions remain below the warranty limits;
l operation is stable over the entire operating range, with acceleration of combustion chamber wall elements less than 10 per cent of the allowable figure;
l the Active Instability Control (AIC) feature, which counteracts acoustic noise with dampening of outer face fuel valve motion and prevents the build-up of resonance in the burners, is required only in special circircumstances;
l the functional capability of the burners has been demonstrated both under cold and hot ambient temperatures.
Combustion chamber heat shields
We have accumulated more than eight million hours of operating experience with ceramic tiles in our fleet. The design and material of the tiles is similar in the 2, 3 and 3A gas turbine families. The tile holder mechanism used (Figure 7) has shown an excellent service record ever since our early turbines.
With the changeover to the 3A turbines we experienced problems due to high mechanical loads on the tiles, when the acceleration of the combustion chamber wall due to combustion noise exceeded the retaining forces of the tile holders. This resulted in tiles being lifted off the casing structures and fracturing on re-impact.
Optimisation of the burners to prevent combustion noise minimised acceleration of the casing, and a system for monitoring combustion noise and acceleration has been introduced. New tile holders with leaf springs enhance the retaining forces acting on the tile and extend the operating range within which no lift-off occurs.
We now have encouraging operating experience with this solution too. Of course the tiles, like any other combustion chamber lining, remain a high-maintenance item due to the extremely high thermal stressing they sustain. However, this effort is much less in the 3A turbine series than in other types since the good accessibility of the hot gas path through the manhole means that tiles can be replaced without having to disassemble the combustion chambers.
Operation in the field
Fifty-two 3A machines (types V64.3A, V84.3A and V94.3A) have now been delivered. Some 42 have seen their first firing, and twenty-six have been handed over to the customer.
Altogether, the turbines delivered to date have clocked up over 300 000 equivalent operating hours (EOH). The turbines that have been in commercial operation longest have each attained up to 25 000 EOH at availability levels of over 95 per cent.
The turbines with the longest operating experience are the V94.3A machines at the 1390 MWe Didcot B plant, the most powerful combined cycle power plant in the UK. The two 3A turbines in unit 6 there (in commercial operation since August 1998) have amassed over 400 starts and over 30 000 hours in service, altogether about 50 000 EOH.
Didcot B and our advanced gas turbines are closely linked with each other: this is where the first V94.3A turbines (apart from those in Genelba) went on line, this is where the turbines did their service trials and were tuned for on-line operation, and this was where the new technology had to demonstrate its performance under real-life operating conditions. Our thanks go to the management and staff of Didcot, but also to our other customers, for their co-operation and support during what were not the easiest of times. In Argentina, the V94.3A machine in the Lujan de Cuyo combined cycle plant, which was taken over by the customer in June 1998, has already clocked up over 200 starts and almost 15 000 operating hours, equivalent to 22 000 EOH, at an availability of around 95 per cent. While in Portugal, the three V94.3A turbines in the Tapada do Outeiro single shaft combined cycle plant were taken over successively between March and August 1999, and have together achieved over 50 000 EOH at an availability of over 95 per cent.
At the Otahuhu single shaft combined cycle plant in New Zealand, the higher rating version of the V94.3A has already run for over 8 000 EOH. This world-leading plant boasts a capacity of 380 MWe and an efficiency of 58 per cent.
The two V94.3A turbines in the Seabank combined cycle power plant in the UK, taken over by the customer on 14 March, have together already achieved well over 20 000 EOH.
Also in the UK is the Cottam Development Centre, where a V94.3A machine representing the latest stage of development of has been in commercial operation since September 1999 in a single shaft combined cycle plant, having currently clocked up about 5 000 operating hours (10 000 EOH). The Cottam plant is operated within the scope of a long-term agreement between PowerGen and Siemens on demonstrating and testing advanced gas turbine and combined cycle technology in a phased test programme.
Further turbines of the higher-rating type are in various phases of commissioning. Figure 8 shows a 3A gas turbine during manufacture.