Within each of the plant focus areas, the following approach is being followed:

• Plant condition assessment: expert teams have been established and are undertaking plant inspections to identify critical risk areas. This is only a first-level review to identify critical maintenance requirements. These reviews also focus on possible shortcomings in work practices and identify mentoring, training and skills development requirements. The German utility RWE is providing support in this regard.
  
  Contracts have been placed with original equipment suppliers (OEMs) to undertake technical reviews of plant components and to assess their condition and performance against similar units elsewhere in the world.  This is to pre-empt possible plant failure patterns and make modifications to our maintenance approach.
  
  
• Undertaking critical maintenance requirements: the effect of very abrasive coal currently being used at various power stations, coupled with the high loads at which stations are running, has led to increased wear and tear on plant components. With the limited and ever-decreasing availability of maintenance time and increasing wear rate, the performance of priority maintenance is critical. As far as possible such maintenance is being fitted into the normal maintenance schedule, but there is already not enough opportunity to undertake all the required maintenance. Plant maintenance philosophies are therefore being reviewed in conjunction with OEMs to ensure that the available maintenance time is used optimally.
  
  
• Outage effectiveness: we are acquiring modular spares for major, critical components to reduce planned outage durations, since the replaced component can be refurbished once the unit returns to service and not during the outage, which allows for better use of scarce, skilled resources. These spares also serve as immediately available spare components in the case of plant failures.
  
  
• Development of mentoring and training programmes: Eskom has a substantial store of technical training programmes and material. Guidelines are being developed to ensure a better understanding of critical operating and maintenance practices. We are also revising training material to reflect the changing operating conditions of power stations.
  
  
• Plant performance monitoring: electronic monitoring of critical power station components and total plant performance is being improved greatly. To this end a control room is being established at head office, from where all critical elements of plant performance will eventually be monitored. At this stage the focus is on plant availability and speedy recovery from plant breakdowns.
  
  
• Plant performance stabilisation: our ultimate goal is to achieve stable and predictable plant performance. The above initiatives will determine the appropriate level of sustainable performance that can reasonably be expected from power stations in general and plant components in particular. However, this level of performance can only be achieved if we and our contracted suppliers have the correct level of skills in engineering, operating and maintenance. Furthermore, performance can also only be sustained if the appropriate level of maintenance can be undertaken at scheduled times. Operating plant beyond its design capability results in premature failure and unpredictable performance.

Boiler tube failure reduction research

Eskom is driving a ‘boiler tube failure reduction programme’ to reduce the contribution of boiler tube failures to the unplanned capability loss factor to less than 0,5% by identifying the causes of the boiler tube failures. The focus is on the mechanism identification at the time of the failure with a full metallurgical failure investigation following soon after the failure occurred.

A database has been populated with information obtained on a research project initiated to investigate the corrosion of water-touched tubing and specific deposit mass data relating to each unit at each power station. The anticipated goal of the exercise is to give system engineers the necessary information to determine a trend in terms of failure mechanisms and assist them in determining the time to the next chemical clean as well as tracking the success of mitigation measures taken to reduce corrosion-related failures.

Since fly ash erosion is the major contributor to boiler tube failures, a strategy has also been developed to reduce the occurrence of boiler tube failures resulting from fly ash erosion. These studies and the subsequent modifications of the aerodynamic flow within the boilers have now minimised fly ash erosion.

The use of cold air velocity distribution surveys is playing a major role at the stations in locating areas within the boilers that are prone to excessive erosion rates due to mal-distribution of the aerodynamic flow.

Cold air velocity distribution survey in progress.

Recent surveys at Arnot, Matla, Kriel and Komati have highlighted that substantial modifications are required to the boilers to minimise fly ash erosion. This is especially the case at Arnot and Matla where the aerodynamic flow within the boilers has not been quantified. Recommendations to minimise the erosion rates have been submitted to the stations and have been or are to be implemented and subsequent surveys have been scheduled.

In one particular boiler a specific area was located where erosion rates were approximately 7 000% above the average, which would have resulted in tube failures within weeks of operation.

As shown above, the flow at entry to the finned economiser was successfully modified at this section and the erosion rates were reduced to 17% above the average by a relatively inexpensive modification, which demonstrates the effectiveness and value of the technique that was developed in-house.