MATSHELA KOKO
DURBAN - Boiler-tube failures are a significant contributor to the total plant breakdowns across the Eskom coal fleet. On Thursday, October 7, 2021, the total plant breakdowns for the generation fleet were 14 982 MW. Eskom announced that “the fleet suffered a cluster of boiler leaks within a short period of time”. It is this “cluster” of boiler-tube failures that pushed the electricity grid over the edge. It forced Eskom to implement load shedding on Thursday, October 7, 2021.
Boiler tube failures remain the leading cause of availability losses in fossil steam power plants worldwide. In almost all cases of serious availability losses, the problems are repetitive in nature and result in multiple forced outages. Tube failures occur in new and old units; in units that cycle and those that operate under base-load conditions.
A proven way to prevent costly repeat failures is to implement a formalized boiler tube failure reduction program that is fully supported by senior management, and that focuses attention and resources on the relevant operating, maintenance, and engineering controllable parameters.
In 1994 Eskom began working with Electric Power Research Institute (EPRI) in America as part of its boiler tube failure reduction program. A target of 0.5% was assigned for the unplanned capability loss factor resulting from boiler tube failures which represented a major reduction from the level of 1.68% experienced in 1994.
As a young engineer at the time (1996), I had the privilege of being trained and mentored by Dr Barry Dooley of EPRI. In the year 2000, the unplanned capability loss factor resulting from boiler tube failures was reduced to 0.68%. It was because of this achievement that I was appointed a senior consultant responsible for the boiler tube failure prevention program by the then Managing Director of Generation Group, Ehud Matya.
The key to the success of the boiler tube leak prevention program was to understand the mechanism of the boiler tube failure, identify the possible root causes, confirm the actual root cause, and then provide a permanent solution to the boiler tube failure mechanism. This meant that by addressing the root cause, the mechanism was “killed”, rather than “managing the damage” which results in multiple repeat failures.
The “cluster” of boiler-tube failures on Thursday, October 7, 2021, is a clear indication that under Andre de Ruyter the boiler tube leak prevention program has collapsed. The unplanned capability loss factor resulting from boiler tube failures is currently at 2% which is 9% of the total unplanned capability loss factor. It belies the fact that boiler tube failures are preventable.
Experience has shown that repeat rather than random boiler tube failures are the result of a combination of the following factors: Lack of management support and/or lack of resources, the failure or inability to determine the mechanism or root cause of the boiler tube failure, the failure or lack of capacity to provide a permanent solution to dealing with the failure mechanism, and the lack of remedial action plans to eliminate the repeat boiler tube failures.
The age of the plant is not a good excuse for the boiler tube failure problem. The ageing of the boiler and piping components is mainly manifested by the following mechanisms: graphitisation, corrosion, long term overheating (creep), dissimilar metal weld failure, stress corrosion cracking and thermal fatigue. Degradation mechanisms other than creep and fatigue are less accessible to useful life predictions by lifetime consumption calculations. For these mechanisms, qualitative lifetime assessments are based on metallographic investigations.
Unless and until Eskom reintroduces and implements a formalized boiler tube failure reduction program that is fully supported by the board and senior management, and that focuses its attention and resources on the relevant operating, maintenance and engineering controllable parameters, the boiler tube failure problem can only get worse.
The changes in the electricity supply industry, including renewable energy sources (wind and solar) as well as the reduction in electricity demand, affects how conventional power stations are dispatched, operated, and maintained, which, in turn, influences the active boiler tube failure mechanisms and the way boiler tube failures are treated.
Eskom power plants were designed and manufactured to be operated under base-load conditions. Cycling them to meet fluctuating electricity demand causes disproportionate wear and tear on the boiler and turbine plant components, which often leads to damage.
The historical experiences at the Majuba power station provide Eskom with a very good case study on how to deal with the new developments in the electricity supply industry.
Majuba power station was originally intended by Eskom to operate as a base-load, mine-mouth facility. However, problems with the Majuba mine resulted in the need to transport coal to the site by rail and road transport. As a result, power from this station became the most expensive in the Eskom fleet. When the demand for peaking capacity increased in the late 90s two-shifting of Majuba units became an attractive option. The challenge was to define the mode of operation that would permit start-up and shutdown cycles that would not cause significant material damage.
A remnant life monitoring model was developed for the six most critical components which provided a representative status of fatigue and creep damage due to temperature cycling. A set of indicators was developed to ensure good chemistry control for long-term plant health management. The temperature control during start-up required that the oil burners and the milling plant be highly efficient.
The developments at the Majuba power station happened under the watch of Phillip Dukashe who was the maintenance manager at the Majuba power station at the time. He is today the group executive of Generation Group. The impact of cycling power plant operations on the long-term damage to the plant should be a common cause to him.
Load shedding has damaged Eskom’s reputation. The time for good excuses is gone. The Eskom leadership has no choice but to accept that boiler tube failures are preventable, and that their engineers have the skills and the know-how to prevent them. When supported properly the engineers can prevent these boiler tube failures.
The challenge for Eskom to do more with less is real.
Matshela Koko is Eskom’s former interim group chief executive
Daily News