Built in the 1970s using an interesting technological mix of East European equipment based on Western technologies, the Tisza power plant is unique. So there were high expectations when a refurbishment was ordered to prolong its life until 2016.
Vilmos Wolff, Alstom Power Service, Hungary
In 2002 AES Corporation issued a tender for the refurbishment of Tisza II, its 860 MW conventional steam power plant in Hungary. Located in Tiszaújváros, 180 km east of Budapest, the 4 x 215 MW reheat steam cycle plant operates on heavy oil and natural gas.
The goal of the project was to entirely refurbish the plant, increasing its lifetime until 2016, to improve the firing system to meet European emission standards and to improve the comfort of plant operation by installing a new DCS system.
The plant consists of four similar blocks built in the late 1970s. Its main technological mix was very unique for an ‘ex-comecon’ country. Local and Eastern companies originally supplied the main equipment mostly based on Western technologies. The 4 x 215 MW main steam turbines and the 4 x 6 MW feed pump turbines were designed and manufactured by Láng Engineering Works (now Alstom Power Hungária Rt) under a BBC licence (today also owned by Alstom). Slovenske Energeticke Stojarne (SES) Tlmace originally supplied the boilers after licence from Deutsche Babcock and Siemens supplied the control system via Skoda Export. Only the generators and the transformers supplied by Ganz Electric Works represented genuine Eastern (here Hungarian) technologies, but even Ganz has had a licence exchange agreement with Alstom.
Figure 1. Tisza II generating 860 MW at dusk, 180 km east of Budapest
The project was financed by a consortium led by Credit Lyonnais Bank (today Calyon) and secured by a long term power purchase agreement with the Hungarian Power Companies Ltd (MVM).
The tenders for the project were split into seven individual packages. The steam turbine, the boiler, the instrumentation and control, the electrical, the civil works, the balance of plant (BOP) and the oil and gas piping. Alstom Power Service was awarded the package for the steam turbines and BOP, SES Tlmace was awarded the package for the boiler and auxiliaries, Emerson for the new DCS, ABB for the refurbishment of the electrical equipment including the supply of a new transformer, Alpin Saleva for the civil works and Euronorp was awarded the package for the fuel piping.
The 215 MW steam turbines are four-casing reaction type units originally designed for base load operation. Throughout the decades they have been gradually transformed to load following units with a minimum load as low as 50 MW. The Alstom scope of the steam turbine package exceeds the four steam turbines and includes, among other things, the renewal of the four 257 MVA generator rotors and the refurbishment of mechanical BOP within the machine house, including the heater equipment. The scope of the contract concerning the BOP equipment outside the machine house includes the refurbishment of the water intake plant and the fire fighting systems.
The steam turbine refurbishment items are a High Pressure (HP) turbine, an Intermediate Pressure (IP) turbine, combined Intermediate and Low Pressure (IPLP) turbine and a Double-Flow Low Pressure (DFLP) turbine.
The HP and IP turbines were transferred assembled to the factory and disassembled at the workshop. The IPLP and DFLP turbines were disassembled on site, but the rotors, blade carriers and the gland bodies were transferred to the factory.
Figure 2. Extensive rotor diagnostics were carried out and repairs used the latest technology
Alstom replaced the impulse wheels and nozzle rings of the main turbine. The new impulse rings manufactured from forged rings were welded to the HP rotors. The entry notch area of the first blade groove was remachined and replaced with an improved geometry. Rows 1 to 13 were dismounted and all the austenitic spacers replaced. These actions increased the HP rotors’ lifetime.
Extensive rotor diagnostics were carried out including straightness and axial run-out control, dimension measurements, and inspection of welding seams, bend surfaces, bearing areas and coupling surfaces by magnetic particle test.
The sealing trips were completely replaced for all the rotors (together with the moving segments of gland and dummies-piston casings). The bearing areas were remachined improving the necessary surface quality.
All the rotors were balanced and overspeed tested in a balancing chamber. The critical blade rows (HP 1-13, IP 1-9, LP L-0, L-1, L-2) were removed and inspected by visual and magnetic particle testing.
The HP and IP casings blade carriers’ deformations were measured by split plane clearances and deformation of joint surfaces. Alstom eliminated the casing deformations by remachining the split planes, restoring the inner diameters by turning and installing additional rings in some areas in order to have the correct tightening surfaces.
Several diagnostic methods were applied to the casted parts. These were: finding cracks, erosion and deposits by visual checking, measuring the functional diameters and split planes, crack testing on hazardous areas (magnetic particle and dye penetrant method) and a replication test in case of high operation temperatures.
A range of solutions was applied to repair the cracks on the casted parts. Recording of the cracking defect dimensionality (ultra-sonic testing) was undertaken and any cracks that were found were removed by grinding or machining. Also, the relevant welding technology was applied where necessary.
Figure 3. The generators were completely renewed, checked and tested
All the crossover pipes were completely replaced due to a non-appropriate condition of the compensators and insufficient wall thicknesses. All of the radial bearing shells were replaced and machine adjusted to the rotor diameter.
Alstom carried out multiple activities for the turbine auxiliaries. It replaced the existing dual filters in the lubrication and control oil system, refurbished the check valves installed into extraction pipelines (eight pieces), fully replaced LP by-pass pipelines including the compensators, refurbished the turbine turning gear and the jacking oil system and replaced the bleeding pumps.
All of these activities, including the disassembly and assembly of the turbine, took an average of 87 days for each of the three refurbished blocks, which induced a heightened workload pressure at the site.
Generators and auxiliaries
The power plant runs on three ORV220 type Ganz generators and one ORV221 which has a slightly modified stator winding system.
The rotors were completely renewed. This operation included the dismantling and testing of the retaining rings, the removal of the winding elements, the complete cleaning and replacement of the slot-wall insulations and the other insulating structural parts as needed. The standard check and test methods were applied in line with the previously submitted and approved Quality Control Plan (QCP) from the first run-out test following arrival at the workshop, up to the balancing-overspeed and running electrical tests at the balancing chamber.
The stators went through a series of checks. Electrical tests were carried out after general cleaning slot wedge tightness tests which included repairs if necessary. The iron core integrity, EL CID and cooler were tested. Antimagnetic steel fixing elements were replaced with ones that were composite-made and the winding-end integrity was checked. Testing and eventual replacement of the RTDs was carried out when needed. The coolwater collector rings were replaced with safer ones on three units. And the mechanical components like bearings and seal rings were checked and repaired when necessary.
The seal oil unit was renewed with new differential pressure regulating valves built in; gas manipulation boards were replaced with new ones using modern handling elements and more reliable material. Last but not least the complete installation of the machine and an active participation on the re-commissioning was also included in Alstom’s scope.
Balance of plant
The BOP package contained the overall reconstruction of the condenser cooling water intake plant and filter plant, gravitation channels, unit-CW pumping stations, internal pipelines, bearing CW pumping station together with transmission pipes, oily waste water treatment plant, raw water intake well-system for boilers, the fire fighting water pumping station and pipe network within the power plant area, as well as the boiler chemical dosing plant.
The customer’s appointed engineer continuously monitored the progress of the work and a pre-agreed reporting form was filled in throughout the activities. A daily log of events and a photographic report of the site and workshop activities were also supplied to the customer.
The refurbishment programme was established in accordance with well defined requirements due to specific restrictions regarding open air work conditions in winter as well as cooling water demand of working units in operation during the refurbishment period. Therefore the detailed time schedule had to be changed several times according to the actual circumstances in the power plant.
The organization of these activities meant a real challenge for field service employees and the customer’s plant operators and engineering company. In spite of these difficulties, each activity was completed with success. Each repaired, refurbished or replaced piece of equipment went through its individual trial run and for the time being they are in commercial operation.
The natural circulation reheat boilers in Tisza II were originally designed for natural gas and high sulphur content, heavy fuel oil firing or a combination of both. The original technical parameters had set maximal continuous rate (MCR) at 670 t/h, superheated steam pressure at 171 bar, superheated and reheated steam temperature at 545/545°C and the feed water temperature at 250°C.
The main targets for the boiler retrofit were: to operate the boilers as a base fuel on natural gas and when natural gas is unavailable, with a light fuel oil having low sulphur content in order to comply with European Union (EU) regulations concerning sulphur dioxide (SOx) and nitrous oxide (NOx) emissions; to extend the boiler service life until 2016; and to increase the boiler operating efficiency and availability.
The scope of the refurbishment works included the following:
- Replacement of the existing old combined gas/oil burners with new low NOx equipment
- Design and installation of the new oil heating system. Replacement of valves, strainers, flow meters, viscosity meters, etc. which may not be suitable for the new fuel oil
- Replacement of the exhausted corroded or eroded pressure parts of membrane walls super heaters and reheaters
- Enlargement of the economiser
- Replacement of drain valves, vent valves and blow down valves
- Replacement of drums and super heater safety valves with silencers
- Refurbishment of the boiler and air heater soot blowers
- Refurbishment of the forced draught fans together with their steam air heaters and the gas recirculation fans
- Refurbishment of Ljungström air heaters including replacement of upper and lower heating surface elements and installation of automatic control of the sealing plate clearance
- Local repairs of the flue gas and air ducting, along with dampers and expansion joints
- Refurbishment of the entire boiler casing plates, refractory lining, insulation and cladding.
Two of the newly refurbished boilers have been equipped with low NOx emission burners from Alstom and the other two with low NOx emission burners from the UK’s RJM Corporation (EC) Ltd. Due to space constraints the boiler’s low NOx conversion was a real technical challenge.
The new DeltaV DCS from Emerson Process Management had to be fully functional from the very first unit and should be able to take over control of the units one by one. The DeltaV system provided an easy-to-use environment for power plant operation and process information in Tisza II.
Figure 4. System components can be added and removed while the system is powered and running
The system integrates the existing automation systems of the power plant to provide benefit from the latest cost-saving technologies. Cabinet footprints are significantly reduced. A system with 600+ I/Os per unit was installed into cable marshalling rooms and the former controller rack rooms were converted to engineering offices nearby the unit control rooms.
Along with the traditional automation system maintenance tools came a set of advanced device diagnostics and calibration tools for easy predictive maintenance. System components including controllers, I/O modules, field devices and workstations can be added and removed while the system is powered and running, with no downtime. Engineering and maintenance efforts are dramatically reduced.
Besides the technical risks, the main challenges lay in the schedule due to various features of the project setup and the executional intervals. The four units had to be shut down consecutively in a row, totalling 16 months. The time schedule was very tight compared to the usual industry standard, leaving very limited float to compensate unexpected technical challenges.
The plant refurbishment project is to be completed in two major phases. The first one was the low-NOx conversion of units two and three. The second phase is the total plant rehabilitation unit by unit. The project is now in its last phase and last stage after the successful re-commissioning of units one, two and four. Unit three is scheduled to reach mechanical completion in December 2004.
The refurbishment will ensure that the plant continues to comply with EU environmental directives and perform in accordance with the terms of its power purchase agreement with the Hungarian electric utility (MVM). Compliance with the EU environmental directives is essential in light of Hungary’s accession to the EU. Multiple fuels are supplied to Tisza II under a long term Fuel Supply Agreement with the Hungarian Oil and Gas Shareholding Company (MOL).
The Tisza retrofit project will allow AES to continue to provide safe, clean, reliable and reasonably priced electricity to the Hungarian national grid.