Unit 6 of the recently completed six-unit Medupi coal-fired power station was the first unit to go into commercial operation. Synchronisation of the generator to the transmission grid had occurred five months before commercial operation. Prior to the admission of first steam to the turbines, the boiler underwent a three stage cleaning process, which was performed by the boiler contractor, to ensure that debris left over in the boiler from construction was removed and to avoid damage to the turbine when steam was admitted. Steam blowing of the boiler was the penultimate stage of boiler cleaning and contractually the steam would have been deemed clean when the steam cleanliness acceptance criteria were met.
The steam cleanliness acceptance criteria, which were set by the turbine contractor, relate to the number and size of indentations caused by particles striking a given area of each target plate situated in the temporary piping downstream of the inlet valves of the high pressure and intermediate pressure turbines. For each target plate, values were prescribed for these variables and for the flow conditions that should prevail in the pipe upstream. The boiler contractor had to meet these requirements. Unfortunately, there was a mismatch between the steam cleanliness requirements set by the turbine contractor and those included in the boiler contract. The less stringent steam cleanliness requirements set for the boiler contractor in the boiler contract meant that the boiler would not be adequately cleaned, from the point of view of the turbine contractor.
The boiler contractor designed a temporary pipework system for the steam blow-through process that permitted steam to bypass the turbines and exhaust to the atmosphere through a silencer. During steam blowing, the prescribed pipe flow conditions for accepting the steam were not being met, even after a large number of blows had been conducted. Mathematical modelling of the process revealed that the required pipe flow conditions could not be attained at the intermediate pressure turbine inlet and as such, the steam blow-through pipework was inadequately sized. The solution was to redesign the temporary pipework, and manufacture and install a new system of pipework, all of which would have taken a couple of months. Business needs required an alternative solution, and so a decision had to be taken on the way forward. Engineering judgement, based on operating and maintenance experience with the current fleet, suggested that the steam was sufficiently clean to be admitted to the turbine, with little risk.
Of the two feasible options available to the project team, admission of steam after a defined number of blows was accepted. Care had to be exercised to manage the risk that the potential turbine contractor non-compliance to any of the performance guarantee conditions could be blamed on poor steam quality. An analysis of the risks associated with this option was conducted and controls were adopted to mitigate the risks. Eventually, steam was admitted to the turbines. Subsequent inspections and tests conducted on the turbines indicated minimal damage and no loss of performance.
This paper describes the Medupi Unit 6 steam blow-through problem and the analytical process that revealed the inadequacy of the blow-through pipework. It describes also the process of analysing the risks associated with admission of first steam to the turboset, the decision processes that were followed to admit the steam, and the process of managing the identified risks through the controls that were put in place.