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Load shedding: How series of failures, design flaws brought Medupi to its knees

Oct 22 2019 11:59
Chris Yelland, EE Publishers

Following major problems that brought Eskom's Medupi power station to its knees during the height of the load shedding last week, the power station's manager, Rudi van der Wal, is leaving Eskom to take up a position overseas.

Van der Wal resigned before the mess started, and was due to leave in about two weeks. But he was made to step aside and replaced before his pending departure.

Other staff at Medupi have been dismissed for negligence by Eskom's head of generation, Bheki Nxumalo, in what Eskom COO Jan Oberholzer calls "consequence management" following major problems at the power station that contributed to load shedding last week. Additional problems at Medupi have now been identified that were not previously disclosed to the public by Eskom management.

Medupi and Kusile are emerging as major threats to the adequacy of the power system to meet demand in South Africa.

For some time, the stackers that load coal from Exxaro's Grootegeluk mine onto the stockpiles at the Medupi stockyard, and the reclaimers that take coal from the stockpiles and feed it by conveyor to the power station boilers have not been working.

To get fuel to the Medupi boilers the coal stockyard was bypassed, and the single main coal conveyor feed from Grootegeluk mine fed the power station directly with no redundancy. Medupi was effectively running with minimal coal stock at its disposal.

Thus, when the 4.5 km single main coal conveyor feed from the mine failed on the Eskom side because of a broken belt, Medupi was left with no coal feeding its boilers apart from about one or two shifts worth of fuel in the bunkers at each boiler.

To add to the disaster, it is now clear that blocked chute and belt rip detectors on the conveyor were not working. Thus, the conveyor did not trip when it should have done so to protect the system. This caused damage to some 700 m of belt, as well as damage to the conveyor structure.

With the main coal conveyor from Grootegeluk mine down, and the stacker/reclaimers at the Medupi stockyard not working, coal then had to be transported by truck from the stockyard for offloading at the emergency Buffalo conveyors at the boiler bunkers in order to get fuel to the boilers.  

This was a logistical nightmare due to the huge volumes of coal involved. Congestion by coal trucks forced the power output of Medupi to be reduced to a third of normal, contributing to load shedding. Then problems were experienced with the ash-handling plant, which conveys ash from coal that is burned in the boilers to ash dumps. Staff working on the bunker feed problems had to be shifted to deal with the huge build-up of ash. 

The good news is that the main coal conveyor from Grootegeluk mine is expected to be operating again shortly, which will alleviate the logistical problem of transporting coal by truck and manually offloading it at emergency conveyors to feed to the boiler bunkers. The bad news is that the stacker/reclaimers are still not working. So, with the coal stockyard bypassed, Medupi remains vulnerable and dependent on a single, long, conveyor feed from Grootegeluk mine without any redundancy if it should fail or trip for whatever reason.

In summary, the known problems at Medupi power station include:

1.      The main conveyor from Grootegeluk mine feeding coal to Medupi failed, causing belt and structural damage;

2.      Protection devices on the conveyor failed to detect the fault timeously;

3.      Because the stacker/reclaimers at the Medupi coal stockyard are not working, stockyard is bypassed;

4.      Coal was being transported by truck from the stockyard to the boiler bunkers, causing massive congestion and forcing reduced power output;

5.      The coal mills are of wrong design, causing excessive wear and tear, with increased maintenance and downtime.

6.      The design height of the boilers is inadequate (too low), and cause excessive temperatures within the boiler and flue gas.

7.      The excessive flue gas temperature is causing premature failures of pulse-jet fabric filters, which are essential for fly ash collection and pollution control.

8.      The ash handling system is unable to cope with the volume of bottom ash and fly ash from the boilers.

* Chris Yelland is investigative editor atEE Publishers.

ee publishers  |  eskom  |  chris yelland  |  load shedding  |  medupi
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