First GT8C2 package tested in the field

In late summer 1999 Alstom Power started construction at its gas turbine test centre in Birr, Switzerland of the first of the new GT8C2 units; the unit with serial number one was to be tested for performance, and for its viability as a packaged turbine, before any GT8C2 was delivered to a customer’s site. Actual testing started in late January 2000. By mid April 2000 the first GT8C2 reached full base load output, despite the fact that the test centre had to be shared with its larger sister unit, the GT26.

GT8C2 testing and performance

Hot commissioning of the GT8C2 started in March 2000. Idle running was achieved within a few days of first start up without significant problems, and after less than two weeks of actual testing, base load and peak load capability were demonstrated. Good run up characteristics of the gas turbine and excellent operational behaviour of the machine were confirmed.

Testing is still going on. The test instrumentation covers standard operation and commissioning measurements plus a variety of special measurements including temperature readings of all critical components in the hot gas path as well as telemetric measurements of the rotor. In addition, hot gas temperatures can be measured, starting from keel probes at the leading edge of vane one and going downstream to a detailed exhaust temperature profile, including gas temperatures along a radius at the turbine outlet.

The extensive instrumenting applied to the gas turbine and good monitoring facilities at the test centre allow for fast evaluation of test runs parallel to testing and are essential for the fast progress of any test programme.

The tests confirmed important component efficiencies. The GT8C2 operating characteristics allowed the variation of operating parameters such as variable inlet guide vane (VIGV), turbine inlet temperature (TIT) and part load exhaust temperature limits over a wide range. Subsequently an optimisation of the combustor and gas turbine operating concept was carried out.

The resulting overall operating concept is shown in Figure 1. The graph shows VIGV, compressor exit temperature, TIT, exhaust temperature as well as exhaust mass flow as a function of relative load of the gas turbine.

Three main changes, compared to its predecessor the GT8C, were made to the operating concept of the GT8C2, as follows.

First, the range of VIGV variation during normal operation has been increased by 50 per cent. The increased operational flexibility of the compressor leads to almost constant optimum operating conditions for the combustor over a wide power range at high part load (>50 per cent relative load at ISO conditions). Further, and more important for cogeneration and combined cycle applications, this leads to higher exhaust temperatures at part load. 100 per cent or higher exhaust temperature is kept all the way down to 35 per cent part load.

Second, the part load exhaust temperature limit was increased from the GT8C’s 540°C to 550°C for the GT8C2.

This change allows retention of a high TIT for a wider load range, providing two major advantages:

• The region of optimum operating conditions for the combustor is further extended, thus reducing CO emissions, which otherwise would increase more quickly at lower firing temperature.

• Temperature gradients in the hot gas path during load changes at high load are reduced.

Interestingly, the increase in exhaust temperature for equal load points (i.e. same percentage part load level) does not increase the metal temperature of critical hot parts as shown in Table 1. This relative reduction of metal temperature is due to a reduction in the compressor outlet temperature. If we reach for example 75 per cent load with an exhaust temperature of 550°C the TIT will be higher than for 75 per cent load with an exhaust temperature of 540°C. However, in order to stay at the same power output point, the extra output from higher TIT has to be compensated by lower mass flow. In order to achieve that, the VIGVs are closed a few degrees further. The reduced mass flow reduces the pressure ratio and therefore the compressor outlet temperature. Because of the improved cooling with lower compressor outlet temperature, metal temperatures of hot gas parts will be reduced, despite the higher hot gas temperature.

For the exhaust end of the turbine as well as for the exhaust gas diffuser the increased exhaust temperature is not critical. In fact the maximum local temperatures with exhaust temperature of 550°C in a GT8C2 stay below those observed for the GT8C at exhaust temperature of 540°C as shown in Figures 2a and 2b. Figure 2a shows the exhaust temperature profile measured at part load conditions on a GT8C. The temperature with a cold stream and clear hot stream is typical for a gas turbine with silo combustor at part load (exhaust temperature of 533°C). For comparison, Figure 2b shows the exhaust temperature profile for a load point with exhaust temperature of 550°C for the GT8C2. The GT8C2 exhaust temperature profile shows very small circumferential variation and only a small variation in radial direction. Thus, even though the average exhaust temperature shown for the GT8C2 is 17°C above the GT8C exhaust temperature shown for comparison, the local temperature peaks remain lower for the GT8C2. Since we never experienced any difficulties with stage three or the turbine diffuser of the GT8C we are convinced the GT8C2 will operate safely with the new operating concept.

Third, the VIGV will be opened slightly for maximum performance at moderate ambient conditions and will be closed to design position at high ambient temperatures. A variation of the VIGV pitch angles was carried out for VIGV open positions beyond the “design-open-position” as shown in Figure 3. This was done to demonstrate flexibility of combustor operation and confirm the solidity of the compressor. Similar tests had already been carried out on the GT8C where the improved compressor blading is available as retrofit and has been operating without any problems since 1997 (more than 24 000 operating hours on one unit alone).

Because of material limitations, for example on the large casting parts, to ensure a normal lifetime for these components, even in hot regions, the compressor exit temperature is limited. When opening the VIGV, the inlet mass flow increases, leading to a higher-pressure ratio and consequently to a higher compressor exit temperature.

GT8C2 packaging

Packaging of small gas turbines and even large aero-derivative engines has been available for decades. The difference in the GT8C2 case was the challenge of packaging a large heavy duty gas turbine that integrates as many functions and as much equipment as possible to substantially reduce site work, but nonetheless ends up as a package which is still transportable, in respect of both dimensions and weight.

Packaging concept

In recent years fast track projects have been realised in 4 to 5 months from contract signature to power on-line. Out of these four to five months, typically 2 months, if not less, are available to assemble and commission a gas turbine package at the site. Consequently, the goal for GT8C2 was to reduce the 4 months at the site typically required for GT8C to 7 weeks. Figure 4 shows a possible fast track project schedule. It assumes that infrastructure at the site is excellent and also that subcontracts are negotiated in parallel with the EPC contract, so that site work can begin very shortly after contract signature and notice to proceed. The main step-up (station) transformer is not part of the OEM supply.

In order to complete site work within two months, the GT8C2 gen-set was packaged into as few modules as physically possible without exceeding block sizes that were still transportable under normal circumstances.

The following major modules were created:

1. GT block package, containing the gas turbine and gear with air intake manifold integrated and all pipe connections and wiring completed in the factory. The GT block package was shipped with its enclosure in place and functional testing completed in the factory (see Figure 5). The package weighed 163 tonnes, and was 11m long, 5.6 m wide and 5.2 m high.

2. Single lift auxiliary skid (SLAS), containing all mechanical systems required to operate the GT genset, namely the lubricating oil tank integrated into the base, supporting lubricant and power oil system equipment, liquid fuel system, natural gas system, heat exchanger serving to cool the lube oil, water pumps (2x 100 per cent) and accumulator for the water cooling system (common for TEWAC generator and lube oil). Fire fighting system and compressor for combustion supporting air complete the list of mechanical items integrated into the SLAS. The MCC panels and the I/O module for SLAS related equipment were also added to this 2nd generation single lift auxiliary skid. This integration of electrical equipment makes the SLAS completely autonomous and allows functional tests before shipment to the site (Figure 6 shows an inside view of the single lift auxiliary skid).

3. Electrical module, containing the control system Egatrol 8 (based on Advant hardware as used in the entire Alstom Power GT family), the MCC for all equipment other than SLAS, generator protection and automatic voltage regulation, DC & UPS system as well as battery room. The latter has its own access door and is separated from the control room by a sealing wall.

Other modules include the high voltage electrical module (Figure 7) and other modules containing the stack and main transformer.

The generator was shipped to the site with piping pre-installed but without the enclosure, which was installed within a few days.

Table 2 shows a comparison of total shipping modules GT8C versus GT8C2. The drastic reduction of the number of items to be handled and installed at the site, combined with maximised pre-testing at the factory ensured achieving the set target for the schedule of site related activities.

Test run

The permit to construct the first GT8C2 at the Alstom Power gas turbine test centre was obtained in early September 1999 and construction began soon after, the major blocks arriving at site during early November. The test centre for the GT8C2 genset exactly followed the standard outdoor lay-out. Only the main step-up (station) transformer has a position different from the standard, as we were able to use the generator of the GT26 being tested at the same site.

Installation of the GT8C2 genset package was essentially complete on January 25, 2000, only 9 weeks after the main blocks had arrived at the site (including a ten day Christmas break). Since erection took place during the worst weeks of the Swiss winter season, it was a rather tough test for the concept, but fully demonstrated the feasibility of the originally planned short schedule.

For project specific reasons, ie in order to reduce the overall block size (Swiss road system limitations and related potential transportation cost) the thermal block package was not completed in the factory. Therefore some additional work had to be performed at the site. Although completion of the thermal block package was carried out separately from the remainder of the genset, it was still possible to verify the erection time goal with the standard configuration. Figure 8 shows an overview of the package at Birr while Figure 9 shows the standard layout for the GT8C2.

First tests with number 2 diesel fuel were successfully completed in September 2000. At the end of September, measures required to obtain a permanent operating permit from the Swiss authorities required an interruption of testing. Testing of the unit on liquid fuel (diesel number 2) resumed at the Birr facility in late February 2001 and are ongoing.

Tests in summer 2001 will also include air intake fogging system tests that were started last year. Tests of a power augmentation system using steam and/or water injection into the GT will conclude the test programme some time in late 2001.

Baku cogeneration project

Quick installation was successfully demonstrated at Birr, a fact that in the meantime has been confirmed with the installation of two more GT8C2s, namely serial nos. 2 and 3 at Baku in Azerbaijan.

The Baku 1 GT8C2 cogeneration plant is the first commercial application of the unit, and the first power station in Azerbaijan built with Western gas turbine technology.

  The turnkey plant was ordered in February 1999 with a twenty months project schedule and was handed over to the client exactly on time, in October 2000. All the tests including the reliability run were successfully completed to the full satisfaction of the client, Azerenerij, within the contracted time frame. The actually measured performance was better than guaranteed. The preliminary results show:

• power output was measured at 1.6 per cent above the guaranteed 53.3 MW (site conditions);

• net fuel utilisation as measured was more than 1 per cent better than guaranteed (88 per cent as opposed to 87 per cent guaranteed);

• net steam production including supplementary firing fully achieved (200 MWt at 16 bar / 285°C, at battery limits);

• NOx emissions were far below the 50 vppm for 100 per cent plant load, also when operating with 100per cent supplementary firing.

The key factors were:

• The feed back from the test centre in Birr

• Maximum possible local contribution (architect engineer, civil construction, electrical and mechanical erection, etc);

• Prompt delivery of GT, gas turbine packages and HRSG in order to allow sufficient time for the local erection companies;

• Thorough supervision of all local contributions in order to stay within the schedule and achieve acceptable quality.

In general it can be said that the modular delivery of the GT equipment, ie the packaging concept with just a few large “almost ready to run” packages has proven fully effective in achieving a short and trouble free erection and commissioning process.

The only additional work to be performed during the project execution was in connection with the fuel gas compressor, the HRSG and the high voltage switch yard. These items are plant specific (equipment & connections) and thus outside the GT8C2 standard scope.

Commercial operation

Baku 1 (Figure 10) including the cogeneration part of the plant was put in commercial operation on October 17, 2000 with 249 operating hours. Unit 1 is operated in base load mode and achieved 2000 operating hours in January.

Unit 1 had to be shut down in January to make the plant interconnections with unit 2, at which time the first Inspection of unit 1 was carried out. The inspection was detailed and the results were very satisfactory – no adverse findings regarding the compressor, the hot gas path or on any of the auxiliaries were recorded.

Some deposits were found on the first vane row of the turbine. The material found is under analysis. However, as far as currently known, the dirt can only come from intake air or the natural gas received for combustion. Since neither the source of the dirt nor the path along which the contamination is finding its way into the machine is clear at this point, further investigations are ongoing and/or are scheduled. No negative effects on performance or on the state of the parts concerned have become visible up to this point.

Unit 1 has been back on-line since January 20 and continues to operate in base load mode.

Unit 2 has been commissioned and simple cycle operation started on 24 February 2001. The cogeneration section will be finished in the autumn of 2001.

Erection and commissioning of Unit 2 was completed within 3 months from the arrival of the major blocks at site. Given the local circumstances at the site, this is reasonably close to the standard site-completion time goal set for GT8C2.

Performance

The expected performance data were fully confirmed. Continuing with the main features and lifetime critical parts of the reliable GT8C minimised design risk and proved to be very advantageous. The packaging concept, unique for this size gas turbine, fulfilled the expectations regarding erection and commissioning times. Valuable information was gained with the test unit in Birr regarding package optimisation. The resulting improvements were already implemented and confirmed successful during the erection and commissioning of Baku unit 1 and further with Baku unit 2. These facts are proof that the GT8C2 programme goal of marrying a state of the art heavy-duty gas turbine with an advanced packaging concept has successfully been reached.

Baku gas turbine operating data and experience to this point, reinforced with results of the first Baku inspection, confirmed the test results first gained at the Birr test bed. Taking over main features and lifetime critical parts of the reliable GT8C minimised design risk and proved to be very advantageous. Introduction of the latest Alstom family combustor was very successful in terms of lowering emissions and achieving excellent operating characteristics.
Tables

Table 1. Turbine temperature measurements for exhaust temperatures of 540 °C and 550 °C
Table 2. Comparison of shipping modules GT8C and GT8C2