Eskom said the first unit of the Medupi power station (namely, unit 6) is close to first synchronisation. However, the power utility warned that only step necessary before finally admitting steam to the turbine is to clean the boiler and associated pipe work from all scale, welding residue and contaminants, arising from the construction process.
“Initially, cleaning is done by means of chemical flushing, which removes all major contaminants as well as any foreign objects, but this is followed by a process known as steam blow-through. Specific steam flow conditions are prescribed to achieve a minimum velocity and disturbance factors inside the various elements of the boiler,” Eskom said.
It added that in principle, steam is generated by the boiler itself, operating with lower than normal temperatures and pressures, and with fans and some coal burners fully functional. The steam is delivered in short blasts, typically for 10 to 20 minutes at a time. So far, a number of steam blows have been performed.
“Once the steam flow and disturbance factors reach optimum levels, cleanliness of the steam is determined by measuring the number of microscopic impacts on steel plates, placed into the path of the steam exhaust.”
Eskom has previously indicated that whilst the commissioning process towards the synchronisation is at advanced stage, each of the remaining steps requires critical assessment of the inherent risks to both safety of people and the equipment integrity before proceeding to stage.
A team of Eskom experts has evaluated the risks in proceeding to restore pipework to the turbine. Whilst specified cleanliness levels have not been completely reached, a full technical risk analysis has been performed, which shows clearly that the consequences of admitting steam which is not quite clean enough are only slightly accelerated erosion of the high pressure or intermediate pressure turbine blades and possibly valve seats, during the first few days of operation. Thereafter, stable, reliable and predictable operating conditions will be achieved.
To further reduce the already low risk, Eskom has developed a set of robust mitigating actions which involve regular internal inspection, bringing maintenance intervals forward, typically to within a year or so of operation.
“On the basis of the above, a management decision was taken to proceed with the reinstatement of the pipework and that should enable to send steam to the turbine whilst implementing the mitigating strategies.”