The collapse of the upper section of Silo 20 at Majuba Power Station in Mpumalanga, following a catastrophic wall fracture in November 2014, posed a significant design and engineering challenge for consultant Hatch.
Eskom’s second-largest power station, Majuba has an installed capacity of 4 110 MW. Located between Volksrust and Amersfoort in Mpumalanga, Units 1 to 3 have a capacity of 657 MW each, while Units 4 to 6 have a capacity of 712 MW each.

The power station has several firsts to its name: it is the only coal-fired power station in South Africa that is two-shifted, while it is also the only power station to have axial FD and ID fans. In addition, Majuba uses both indirect dry cooling and wet cooling. An astonishing 700 trucks a day deliver 0.74 MT of coal by road, with 0.42 MT a month delivered by rail.

The upper section of Silo 20 collapsed in November 2014, following a catastrophic wall fracture. The conveyor transfer house and coal conveyor gantry sections supported by Silo 20 also collapsed. This damaged the head end of the overland conveyors, take-up structures, and the tail ends of the over-silo link conveyors.

The coal plant at Majuba is configured in such a way that it is dependent on the distribution of coal to the desired silo from the transfer house on top of Silo 20. The structural failure of Silo 20, together with the associated conveying infrastructure, meant that no coal supply to any of the silos was possible.

Eskom’s recovery plan for the coal plant adopted a phased approach. Phase A, the ‘Immediate Solution’, was implemented directly after the incident. Phase B, the ‘Gap Solution’, involved the temporary re-location and repair of the tail end of the boiler incline conveyors fed by Silo 20.

Phase C, the ‘Interim Solution’, involved construction of a temporary conveyor system that bypasses the coal silos, and provides a reliable coal supply to the generating units over the medium term. This system is currently in place, with the generating units able to produce electricity at normal capacity.

Hatch is providing engineering design services for Phase D, the ‘Permanent Solution’. The materials handling solution here has been developed in accordance with the results of the Silo 20 structural investigation process, as well as the determination of the current conditions of Silo 10 and Silo 30.

“Hatch won a competitive tender to provide engineering services for the Permanent Solution, consisting of a detailed civil and structural design for the plant, as well as a basic design, for all associated interfacing plant, including Bulk Material Handling (BMH), Electrical, Control and Instrumentation (C&I) and Low Pressure Services (LPS),” Project Manager Rodney Goosen explains.

Carrying out such a major construction project at a fully-functioning power station poses a major challenge. “It is an interesting project from a construction point of view. The project scope is large and complex, with the main focus being a very tight deadline. To date, the project has been a success,” Goosen comments. Hatch’s full scope of work on the project is Silo 20 reinstatement, strengthening of Silos 10 and 30, and all supporting systems. This entails the provision of a fully-integrated plant, inclusive of all associated civil and structural, mechanical, electrical, C&I, LPS infrastructure, and interfacing with the existing plant.

It comprises the following:
• Civil and Structural – Construction of silos (reinstatement and strengthening), lift shaft, transfer houses, conveyor gantries, supports, accesses and supporting structures for all disciplines, including new structures, as well as repairs to damaged structures where required.

• BMH – Detailed design and construction of all associated coal conveyor systems and ceramic lining of silos.

• Electrical – Detailed design and reinstatement of electrical supplies for the project, including lighting, earthing and protection systems.

• C&I – Detailed design and reinstatement of the supporting C&I systems for the project.

• LPS – Detailed design and reinstatement of the fire protection systems for the project.

The Hatch team has been on-site since February 2016, with the project expected to be completed early next year.  The 65-m-high Silo 20 has been refurbished using sliding platform technology. Instead of demolishing Silo 20 in its entirety, the engineering design team suggested that the base and bottom half, from the hopper up, be incorporated into the new structure.

The sliding methodology used for the silo refurbishment requires the assembly of the system at ground level, with the system then being lifted incrementally as concrete is poured and set over the steel reinforcing fixed inside the forms.

Once the required height has been achieved, the sliding system is dismantled. Concrete was supplied from an offsite batch plant about 10 km away, where the challenge was ensuring a precise concrete mix at the correct temperature.

Upcoming milestones include conveyor construction and lifting of gantries and the transfer house sections to the top of Silo 20, which include lifts in excess of 40 t. A critical consideration here will be no disruption of the coal feed while commissioning the different conveyors at the live power station.