A series of upgrade packages has been has been developed by Alstom for its mature fleet of gas turbine models to enhance their output and efficiency and extend maintenance intervals. The packages will ensure that the fleet remains competitive in today’s deregulated power market.

Eric Jeffs

There are still gas turbines in operation today that were installed in the 1950s, long before the combined cycle came on the scene as the system of choice for new generating plant in a deregulated market. But now there are many more modern units that were installed in the last 25 years and that are capable of being refurbished and upgraded to remain competitive in the deregulated environment.

Alstom, as the inheritor of the original Brown Boveri and ABB gas turbine designs, has built up a substantial business of upgrading and refurbishing these gas turbines so that they can continue in operation at higher power and efficiency, often by taking advantage of new technologies that have been applied to the more advanced gas turbine models that have recently come to market.

Alstom Power Service, headquartered in Baden, Switzerland, is the responsible division for a gas turbine service business which has grown substantially in the last five years as more customers see that there is value in increasing power and efficiency to enhance revenues at times of high fuel prices.

Figure 1. Sectioned drawing of GT11N with new turbine and compressor upgrades, the EV burner package, and at right the variable inlet guide vane conversion
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Gas turbines fall into three categories. The historical units, about 150 in the Alstom fleet, are those that were installed before 1970: BBC Types 10 and 12 and some of the early sequential combustion designs which are still in operation. The owners are comfortable with them and they are easy to maintain with a simple technology. Eventually these will be replaced with new equipment.

The mature fleet includes those gas turbines that were in production up to about 1990. These include the GT11N, GT13E and GT8B/C, and the series GT9, 11D and 13D. Later engines are now beginning to receive upgrades including the GT13E.

Why upgrade?

Gas turbine components wear out in time and have to be replaced. When this time arrives depends on how the machine has been operated and on the fuel used, which can range from crude oil through diesel, naptha, and kerosene to natural gas, industrial waste gases and blast furnace gas. Frequent stopping and starting will add equivalent operating hours to shorten the maintenance intervals, as also will extended operation at peak power rating.

Gas turbines designed 20 years ago were of a simpler technology than those coming into service today. So part of the upgrade process is to apply modern technology where appropriate to improve the efficiency of components and of cooling arrangements in the power turbine stages. The object of the upgrade is to recover lost output due to operational degradation, and add to it, to match as near as possible what the new technology design would have produced had it been available at the time of original manufacture.

Computer-aided design has revolutionised the design of compressor and turbine blades and vanes. Suppose there are 100 blades in each stage of the compressor. The incoming air flow can be divided into 100 equal parts and the computer can then calculate the work that can be done by each blade of that stage and also optimise the blade profile to minimise aerodynamic losses. The output of that first stage now defines the inlet conditions for the second stage and so on down to the final stage. The sum of the changes results in a higher mass flow and improvement in compressor efficiency, and no two rows of blades need have the same blade profile.

A particular example of this in the 60 Hz market is the complete redesign of the GT11N compressor using Controlled Diffusion Airfoil blade profiles for all rows, as used on the GT24, GT26 and GT11N2 compressors, to produce a more efficient design for the smaller gas turbine. The first example of the new GT11NMC compressor as part of an upgrade package was installed at Aguaytia, Peru in 2003 and further installations here and elsewhere are planned or already implemented.

Figure 2. GT11D turbine as originally installed at Beluga power station with the side-mounted combustion chamber which was changed to a top-mounted unit when GT11N was introduced in 1990
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The Aguaytia power plant is on a jungle site close to the Brazilian border approximately 2000 m above sea level. There are two simple cycle GT11N gas turbines operating at the site on natural gas from local fields. One unit has the new compressor, which has resulted in a power increase from 75.4 to 85.9 MW and efficiency from 32.4 to 33.2 per cent. This is entirely without any change to the firing temperatures, nor does it require any additional power augmentation measures. The values relate to the site ambient temperature of 30.6°C

The main gain in gas turbine efficiency from an upgrade comes from the power turbine where – besides improved aerodynamics – there are also issues of metal temperature and cooling efficiency. Twenty years ago turbine inlet temperatures were much lower than today. On the GT13D3 it was 990°C. Modern designs operate at about 300°C hotter and have better base materials and coatings and much more efficient blade cooling arrangements. Much of this is applied to the turbine blade design for Alstom’s mature fleet, for example, by improving the cooling concept to permit higher operating temperatures. For the GT13D3 upgraded to GT13DM, this is a temperature increase of 30°C to 1020°C which is combined with more efficient blading to give up to 12 per cent increase in power output.

Environmental issues

Alstom’s first low NOx EV burner conversion on the mature fleet was in 1990 at MK11 in the Netherlands. The principle of Low NOx combustion is to first premix a defined quantity of fuel with air in a concentrated vortex. Once the vortex breaks down after the burner, a lean combustion flame is created.

As of March 2005, 34 EV burner packages had been installed on Alstom’s mature engines. Of these, nine are on three plants in the United States and 25 in Europe, the Middle East and Asia, including the GT13Es at Killingholme (3) in the UK, and at Hemweg (1), Netherlands.

Figure 3. The converted gas turbine at Beluga with the new top-mounted combustion chamber and DM turbine upgrade package
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Another innovation on the F-class gas turbines that can be used on older gas turbine types is the variable inlet guide vane used for part load operation in order to keep the part-load efficiency of combined cycle power plants at an optimal level. A variable inlet guide vane can be fitted by drilling the casing and linking the blades to an external actuator ring. This can be supplied with a compressor upgrade package if a customer wants it.

Extended inspection intervals

The major maintenance event for an Alstom gas turbine is the C inspection, which on the mature fleet is normally at 16 000 to 24 000 equivalent operating hours (EOH). Operators of the upgraded GT13DM have the option of running at the higher temperature to increase power output, or running at the original temperature and thereby extending the C inspection interval to 32 000 EOH. They can also run at an intermediate output and accept a less extended maintenance interval. For operational flexibility a switching option exists so the customer can change the operational mode of the engine.

In addition to upgrades, other performance improvement possibilities come from the general changes in operating practice, which were not being used at the time the gas turbines were built. On-line compressor washing did not come into widespread use until about 1993 so that for the mature fleet some operators have taken an on-line washing system. Depending on where a power plant is located different aerosols and fine dust can get through the best intake filters and settle on the first rows of the compressor blades causing a loss of output over time.

A 100 MW power plant might lose 3 MW per week through compressor fouling and might have to shut down every weekend to perform an off-line wash. On-line washing daily might recover 150 kW/day and reduce the rate of loss of power and thereby extend the time between off-line washes.

In Europe in the mid 1970s as the 100 MW-class gas turbines were launched by the two major manufacturers, the static frequency converter (SFC) was introduced as a means of starting these large machines. An SFC acts on the generator excitation so that it functions as a large motor to accelerate the gas turbine up to speed. The SFC is a solid-state system with no moving parts and has contributed to the high starting reliability of gas turbines. Alstom launched this method on the GT11N fleet in the early 1990s, but it was in use on GT8B and GT11Ds before this time.

The SFC has been used on all GT11N engines since the implementation of this rating in 1990. Since then, upgrades to the SFC have been carried out to reduce starting time.

Upgrade results

Niederoesterreichische Elektrizitaetswerke AG was one of the first companies to install a GT13D combined cycle at its Korneuburg site near Vienna in 1980. The plant continued in operation until a breakdown from turbine blade damage in 2003. Parts of the thermal block had almost reached the end of their life, but as luck would have it Alstom had a GT13DM thermal block in store at its Berlin factory which it was able to send to replace the damaged unit at Korneuburg. The DM turbine upgrade was incorporated and an EV burner package fitted. The plant returned to service in late 2004.

Figure 4. The GT13DM at Korneuburg with the new thermal block with DM turbine arrangement and combustion chamber with EV burner array
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In 1993, Aluminium Bahrain installed six GT13D3A gas turbines in two combined cycle blocks each with three gas turbines which were site rated as 72 MW at 35°C. All six units will receive the DM turbine upgrade and the EV burner package as each comes up to its next maintenance outage. Three DMs have already been implemented, two with EV combustors. A DM upgrade involves the replacement of the turbine blades and vanes with new computer designed profiles, and fitting a new vane carrier. These changes result in improved aerodynamics, sealing and reduced cooling air requirements. On the second upgraded unit, completed in June 2004, power output increased by six per cent to 76.3 MW at site ambient temperature. There was a five per cent increase in efficiency and emissions are down by 90 per cent with NOx at 36 vppm. The other benefit of the upgrade is that the maintenance interval has extended from three to four years.

In Alaska, Chugach Electricity Authority has a 30-year old combined cycle at Beluga, some 50 km southwest of Anchorage. As originally completed in 1977 it had two GT11D5 gas turbines and a steam turbine. The original GT11D had the combustor standing beside the machine with a large U-duct connecting to a flange on the bottom of the rotor casing. At Beluga in addition to the GT11DM turbine upgrade the U-duct was removed and a new EV combustor mounted on top of the modified casing. The removal of the U-duct is an optional part of a DM upgrade and is required to achieve the full maintenance interval extension.

Reconfiguring the hot gas path optimises gas flow and contributes to improved power and efficiency. The original GT11D5 gas turbines were ISO rated at 72.5 MW, and the turbine upgrade and combustor relocation together result in around a ten per cent power increase to about 80 MW and approximately six per cent improvement in efficiency. But the big gain for the operator is the extended maintenance interval: the C inspection interval for these early machines was 16 000 EOH but reconfiguration, with U-duct removal, has extended it to 24 000 EOH, and with additional modifications this can be further extended to 32 000 EOH.

Figure 5. The gas turbine at Korneuburg during erection in 2004 with completely new thermal block and the EV burner assembly
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The Tung Hsaio site in Taiwan is approximately 150 km south of Taipei on the west coast of the island. Alstom supplied four of the six combined cycle blocks on the site, the first in 1985. Six GT11N units have since been converted to GT11NM. Three units received turbine upgrades between March and May 2004, which entailed replacement of all turbine blades with units of advanced aerodynamic design and fitting a new vane carrier to improve sealing and blade cooling efficiency. EV combustor packages were also fitted. Power gain for these three engines was between 13.3 and 13.7 per cent with approximately 14 per cent reduction in heat rate. Upgrading of the three turbines of block 5 was completed in March this year, with the units running to full load with excellent results. One of these machines also had the new GT11NMC compressor installed as part of the upgrade.

Changing operations

The upgraded engines return to service in a vastly different market to that when they first started in commercial operation. Deregulation has created a competitive environment that has put the emphasis on reliability and availability. But higher output and efficiency are also important at today’s fuel prices. The converted plants being more flexible will start up and load more quickly and will be important in the competitive market in two ways.

First in the deregulated market a growing number of merchant plants compete in the spot market to sell to electricity distributors and large industrial consumers. If a plant, for whatever reason, cannot supply its customers it must buy power to sell on and replace the lost output. Here is one situation where an older upgraded power plant can start up quickly to supply some of the lost output. This is also particularly significant in a country such as Germany with a large number of wind generators where a back-up power supply must be available for when the wind doesn’t blow.

For an industrial CHP scheme deregulation provides a market for surplus power, which may occur as a result of changes in process demands. Increase in power demand through process changes with higher efficiency will marginally reduce fuel costs. In all cases where a low emissions combustion package is part of the upgrade then there is a beneficial improvement to the environment.