World class outage performance in Malaysia21 June 1999
Some extremely successful overhaul performance has been achieved at YTL's Paka power station, including a benchmark-setting 21 day outage. What are the ingredients of success?
The Paka plant, on Malaysia's east coast, consists of two 404 MWe blocks. Each block has two Siemens V94.2 gas turbines, two single-pressure heat recovery boilers and one steam turbine-generator. The four turbines run in combined cycle, mainly baseload, with natural gas as fuel. The steam turbine condensers are directly cooled with seawater.
Operations and maintenance are contracted to YTL Power Services Sdn. Bhd. (YTL PS), a 51 per cent Siemens/49 per cent YTL company registered in Malaysia, founded in 1993. The O&M agreement is between the plant owner YTL PG and the plant operator YTL PS.
The first hot gas path inspections were due after 33 000 equivalent hours, or about four years service. The main planned activities for the overhauls were to replace the first two rows of turbine rotor blades with new blades and send the third row rotor and the row 1 and 2 stator blades for refurbishment and re-coating. The Inconel mixing chamber portions of the combustion chambers were also to be repaired and refurbished. The original concept for the inspections was to leave the rotor in place during blade changes. Before the overhauls started, it was decided to remove the rotor to be able to inspect and clean the compressor parts and the lower casing.
Planning for the overhauls started back in 1996, closely involving the Siemens Service Department in Kuala Lumpur. Siemens was chosen as the main contractor. The original proposal was to do all the overhauls consecutively, with only short breaks in between, but this would have involved the purchase of two or three sets of new turbine blades. The blade refurbishment time of twelve weeks meant that the refurbished blades from the first machine would only be returned in time for the third overhaul. Economically the best solution was to carry out the overhauls at twelve-week intervals, allowing blades from the first machine to be used in the second, and so on. The additional mobilization costs were much less than the costs of the turbine blades that would be required.
The original schedule for the first inspection was 31 days but in the end, it took 46 days. The inner casing, which is fitted on to the rotor, was found to require more extensive refurbishment and repair work than originally estimated. The unexpected findings on the inner casing and the resulting additional work were the biggest contributors to the extension of time for this overhaul.
A more detailed schedule showing all activities was developed before the overhaul of the second machine. For this overhaul, several major changes were made to the programme, based on the assumption that the scope of refurbishment work would be similar to the first machine. The turbine rotor disks and stator blade carriers were sent to a workshop in Singapore for blade replacement and final machining to size. This avoided additional time required for final grinding of the rotor blade tips in the machine and allowed site staff to concentrate on other aspects of the overhaul. Refurbishment of the inner casing and mixing chambers was carried out more quickly as the amount and type of work to be carried out was already known. This planning and preparation work enabled the second overhaul to be carried out in a time of 28 days, which was already a record time for this type of machine.
For the third overhaul, detailed reviews of the time schedule allowed possibilities for further time reductions to be identified. For instance, the critical path was now seen to be the mixing chamber refurbishment and heat treatment. Carrying out welding repairs to both mixing chambers in parallel allowed the time to be reduced further, although additional personnel and equipment had to be hired. The detailed planning resulted in a reduction of the overhaul time to 23 days on the third gas turbine.
The last overhaul in Paka, which was completed in March 1999, took only 21 days and is the new Siemens benchmark of "19+2" for gas turbine overhauls. That is, one day for preparation, (scaffolding, removal of insulation, etc) nineteen days for the actual overhaul work and one day for final re-commissioning and testing.
Many factors contributed to the reductions in overhaul times, but some of the main ones were:
Shaving off another day
Preparations are now underway for the hot gas path inspections of the two V94.2 gas turbines at YTL's Pasir Gudang power station, on the southern tip of the Malay peninsula. This consists of one 404 MWe combined cycle block. It may be possible to shave another day or so off the overhaul time since the plant is much closer to specialist workshops in Singapore.
|Joint venture approach pays off|
| responsibility-for-operation-and-maintenance-of-the-paka-and-pasir-gudang-power-stations-including-the-acquisition-of-spare-parts-and-consumables-rests-with-ytl-power-services-the-joint-company-of-siemens-51-per-cent-and-ytl-49-per-cent-ytl-ps-is-contracted-to-carry-out-services-for-ytl-pg-with-the-intention-of-li-optimizing-plant-availability-efficiency-and-economics-li-meeting-the-client-s-delivery-obligations-under-the-power-purchase-agreement-ppa-and-the-grid-code-li-minimizing-thTablesAvailability (%) of Paka and Pasir Gudang (combined cycle operation)
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