Posted On : Wednesday, February 15th, 2023 in News
The Steenbras Hydroelectric Power station was the first of its kind in Southern Africa, and to date has served the city of Cape Town for over 40 years. The hydro-electricity generated is distributed by the local (municipal) grid. Two dams are located at different elevations (300m), connected by a low-pressure tunnel through the mountain, to the high-pressure tunnel / steel penstock 3.8m in diameter, which is buried below ground to reduce environmental impact.
The penstock feeds water gravitationally through four turbines, generating 180MW of electricity for the City of Cape Town. The internals were originally coated with a thin solvent-based epoxy, which blistered. The internal lining was refurbished after 18 years in service with another epoxy lining and blistered again.
Our partners in South Africa, Cape Town, were awarded the tender for the reline of the steel section of the penstock. It took 12 months of planning, design, fabrication and testing to prepare for this challenging and time sensitive project.
Above ground view of the site seen from the first access point. The second access point seen circled in red
SCOPE OF WORK:
- Corrosion protection over of 7600m² of steel penstock pipe.
- Corrosion protection of four draft tubes on the downstream sections / discharge side of the turbines.
- Protection and skimming of sealing surfaces of the sluice gates leading to the four draft tubes.
- Protection of all concrete to steel interfaces.
- Design and installation of work surface access throughout the penstock – this included the elevated horizontal and the angles walkways, cantilevered access towers in vertical sections and bridges over vertical bifurcations.
- Design, building and installation of ventilation and air control devices within the penstock.
- Installation and running of all plant equipment used for the project.
COATING SYSTEM USED:
- Corrocoat Zip E – a high solids glassflake epoxy designed for service in immersed conditions.
- Plasmet Underwater Putty and Underwater Laminate – an epoxy glass flake coating system intended for application and cure underwater.
SITE CHALLENGES:
- Access – only three manholes for access to the main penstock were available. One of which had a down vertical shaft. Some 120 tons of access equipment had to enter and be removed through these access points. Dedicated rope access teams were assigned to deliver and install anchored walkways on steeply inclined sections, and cantilevered access to towers in vertical sections. Man-riding hoists were on standby for grit removal and emergency evacuation.
Cantilevered access tower installed in a vertical section
- Rope Access for Large Work surface access – governed by the severe time constraint, a need to have larger teams working on a section drove planners to customize and use scaffolding at angled sections.
Rope Access professionals were essential to installing the customized scaffold and stair system that allowed a larger work force to work. However, some areas could only be reached via rope access alone.
Bespoke scaffold staircase Rope access for final inspections
- Air and dust management – the need to work parallel in multiple areas called for the design and installation of air blocking devices and fans, allowing control over air movement direction and dust. The system proved effective enough to control humidity in the pipe, protecting blasted steel from flash rusting.
- Grit removal – Corrocoat designed Hoggajet pneumatic vacuum systems were used to remove in excess of 800 tons of blast media from inside the tunnel, at distances of up to 200m from manholes.
- Water leaks – seepage water from the upstream concrete sections of pipe – mechanical blocks were built and water pipes installed at the cracks, to divert water away from the steel section. In certain cases, the coating specification had to be adjusted due to the constant flow of water. Corrocoat’s Underwater Epoxy system was successfully applied to these areas.
Draft Tubes
- Safety – The major consideration during planning was to develop work methods that would ensure safety of up to 120 staff working with flammable material, deep inside confined spaces, in dusty conditions, on very steep slopes, above vertical drops, with extremely limited escape routes. A rope-access evacuation team was on standby 24-hours per day.
Despite all of the challenges, work was completed to the full satisfaction of Corrocoat South Africa’s client.
