Bethlehem Hydro Sol Plaatjie and Merino Power Stations
Developer: Bethlehem Hydro
Consulting Engineers: Aurecon
Main Contractor: Eigenbau (Pty) Ltd.
Sol Plaatjie Project Value: R 26,435,481.43
Merino Project Value: R 32,310,032.86
The developer of the project Mr Anton Louis Olivier from NuPlanet formed a special company Bethlehem Hydro which included local shareholders and financial backers and obtained funding from the Development Bank of South Africa. He took the decision to build two separate hydropower plants near Bethlehem on the As River, utilizing the flow in the River from the Lesotho Highlands Water Scheme. The rivers flow varies between 25 and 35 cubic meters per second.
Eigenbau were appointed as the main contractor and were responsible for:
- All civil and building works including access roads
- Installing the hydro-mechanical equipment
- Electrical in the power in the power station
The two hydropower plants, named Sol Plaatjie and Merino have a combined output of 7 MW, with 3 MW generated at Sol Plaatjie and 4 MW at Merino. Transmission lines connect to the national grid adding approximately 40 GWh annually.
The two turbines are very similar in size and the power stations are similar, both 28 metres high, 22 metres long and 6 meters wide internally, and comprise the power station floor, switchgear room and control room, with the draft tube passing under the floor of the power station. The power stations are each equipped with one emergency gate on the upstream side and two maintenance gates on the downstream side of the power stations. Each inlet works is fitted with three large sluice gates to isolate the flow to the power stations.
SOL PLAATJIE POWER STATION
The intake is located next to the existing Sol Plaatje dam wall with the power house just downstream of the dam wall. The water flowing over the dam wall is diverted to the power plant to generate power and then returned to the river.
MERINO POWER STATION
At Merino a weir in the river diverts the water into an 800-metre long canal. The canal ends at the forebay of the power station. The canal can be isolated by the three manual sluice gates at the head of the canal.
The project was a technically challenging one which required engineering expertise and design flexibility to ensure successful completion.
One of the major challenges faced by the company were the excavations up to 23 metres deep in solid rock. These deep excavations were at the river’s edge and extended 12 meters below water level. The tailrace extended past the rock into very soft material in the river bed. This required constant pumping which naturally increased both the risk, as well as the cost on the contract. Coffer dams at the outlet of each of the power stations ensured that the work could be completed safely.
It was decided early on by Eigenbau to use hydraulic hammers to break the rock in the excavation as blasting would have opened fissures to the river, resulting in a much larger flow of water into the works. Eigenbau had to factor in extra contract time for the excavations because of this decision, but this was more than offset later by not increasing the ingress of water into the excavations.
At the Merino site the layers of sandstone were interspersed with mudstone. This meant that the exposed areas of mudstone in the forebay area had to be protected to avoid weathering once exposed. This was achieved by cladding the mudstone with a reinforced concrete skin sprayed onto the face of the excavation.
The Merino power station had to be moved to find a more suitable foundation as no foundation investigation was done prior to the start of construction due to funding constraints. The subsequent redesign of the power station and realignment of the canal necessitated deeper excavations in solid rock.
The power stations are of conventional design and were constructed in reinforced concrete. The walls were cast directly against the sides of the excavation anchoring the power station and ensuring that floods will not cause flotation of the structure. The transition section from square to round of the draft tube upstream of the turbine was formed with a purpose made shutter. The draft tube downstream of the turbine was fabricated in India by the turbine supplier and placed and cast in position by Eigenbau. The steel was later sandblasted to specification and the corrosion protection coating applied.
During the construction period the scope of Eigenbau’s contract was increased to include the following:
- The supply and installation of the emergency and maintenance gates at each of the power stations plus the sluice gates at the inlet works.
- Gantries, overhead cranes in the turbine hall and the winches to operate the gates.
- Trash screens and hatch covers
- The installation of the mechanical equipment as the supplier was not interested in carrying out this portion of his contract. This was not originally in our contract, but Eigenbau have the necessary experience and skills in their mechanical engineering division and successfully completed the installations. The turbine supplier supplied a team to provide technical assistance and quality checks.
- The installation of the electrical equipment including cables and domestic supply.
The turbines and generators were sourced directly from Boving Fouress of Bangalore in India by the developer.
The contract for the Merino plant has a weir upstream of the power station to divert the water into the canal leading to the power station. The construction of the weir necessitated the construction of coffer dams to divert the river to facilitate construction of the weir. The river was diverted through a completed section of the canal and the inlet works during the construction of the weir across the riverbed. Thereafter the river was diverted over the completed weir and the construction of the canal completed.
Only the first 60 metres of the canal are concrete lined, thereafter it has a combination of vertical walls in the rock and a battered face lined with rock were specified.
The design of the canal was crucial as the available freeboard was very limited. It was thus necessary to limit waves from abrupt changes in flow in the canal. The wicket gates at Merino close in under 5 seconds to shut down the power station when the load is rejected.
An Active Spillway crest gate was installed just upstream of the forebay to accommodate the closures and divert the flow into a bypass channel back to the river. As soon as the power station starts up again the gate closes reinstating the flow back through the turbine.
This gate is manufactured and installed by Eigenbau.
Active Spillway was chosen for this application for the following reasons:
- It is a fully automatic gate.
- Instantaneous response time.
- Fail-safe opening and closing.
- No operator intervention or control equipment.
- Small head required for opening.
- Has operational life exceeding 30 years.
- It eliminated wave surges in the canal.
The Merino and Sol Plaatjie power plants are situated 30 km apart and it was necessary for each site to have its own construction team complete with plant and equipment, concrete batch plant and site camp.
The large number of challenges and obstacles faced on this contract were overcome by the positive attitude and high levels of professionalism exhibited by the construction team, resulting in the satisfactory completion of the project.
HYDRO-MECHANICAL EQUIPMENT AT BETHLEHEM HYDRO
Our contract consisted of the construction of two complete Hydro power stations. This included the construction of the complete civil works including coffer dams, intake works, feeder canal, power stations and the tailrace outlet.
Our contract excluded the supply of the turbines, generators, transformers, draft tubes and control equipment but included the installation and commissioning of all this equipment for both Merino and Sol Plaatjie power stations.
The following equipment was manufactured and installed by Eigenbau.
- Emergency gates 2 no 3,2m wide x 3,2m high, head 24m
- Maintenance gates 4 no 2,5m wide x 3,4m high head 12m
- Gate guides for above
- Sluice gates 6 no 1,5m wide x 2,6m high
- Structural steelwork for the 8 hoists
- Steel stairs and walkways
- Screens 2 no
- Automatic spillway gate 10m wide x 1,5m high (Active Spillway)
The following equipment was supplied and installed by Eigenbau.
- Emergency gate hoists 2 no
- Maintenance gate hoists 4 no
- Power station gantry cranes 1 no 35 ton and 1 no 30 ton
- Jib cranes 2 no
- Sump pumps 2 no
Eigenbau has a mechanical and electrical division which specialises in water and wastewater treatment.