A technical team serving the requirements of the Drakenstein municipality in the Western Cape has designed one of the country’s largest pump and turbine stations to be used for water reticulation to the nearby town of Paarl during dry periods, and to generate electricity for the city during the remainder of the year.
The city’s engineer for water operations, Hein Henning, says they were faced with the dilemma of having to pump water to their neighbour for just one month a year. For the rest of the time the pump station would be dormant and need expensive maintenance due to possible corrosion, perishing, lubrication and bearing damage to the idle infrastructure.
Instead, the progressive municipality chose to innovate and generate an income while simultaneously reducing the requirement for maintenance by effectively allowing the pumps to run backwards against the pressure of water from its elevated reservoir and generate electricity for the region’s electrical grid.
The commissioning of the pump and turbine (PaT) station effectively brings online one of the largest such projects of its kind in South Africa, able to generate 57 kW, which is enough to power the entire region’s water infrastructure of treatment plants, pumphouses, offices and other infrastructure.
“The water comes from the City of Cape Town Wemmershoek Dam and is gravity-fed to our city regions via the main reservoir,” explains Henning. “For one month of the year, the pumps need to be switched on to pump 400 litres per second upstream to Paarl, where after they would usually be switched off.
“However, the higher elevation of the reservoir allows us to rotate the pumps backwards to act as small turbines for the rest of the year which is able to generate electricity at 80% efficiency. Unlike turbines however, the pumps are standard stock items from the supplier, KSB Pumps and Valves, and do not require specialised support and servicing. What’s more, the pumps are durable enough to allow us to design the infrastructure with a 40-year lifespan.”
Before commissioning the project, the municipality had spelled out its requirement to replace its existing 96 litre-per-second pump station with a new one and discussed the ability to generate electricity in the off season with consulting engineers, Aurecon. Having designed and planned a solution, the main contract was awarded to Hidro-Tech Systems for mechanical and electrical work.
Brian Cooper of Hidro-Tech says that despite the relative simplicity of the mechanical design, it required complex integration of controls to deal with the hydraulic force and convert the electricity generated into a useable form. This required pump speeds to be controlled via a variable speed drive to prevent over speed, as well as convert the wave produced to a pure 50 Hz sinusoid compatible with the city’s power grid.
“The system is designed with full PLC control that can be managed on site HMI, or accessed remotely to give users control and monitoring of the site wherever and whenever required,” he elaborates. “The industrial network automates the entire system including control and monitoring of the pumps, valves, level indicators, flowmeters and other equipment critical to the systems efficient operation.
“In addition to providing seamless control of the system, it also ensures redundancy in all operating conditions and provides emergency measures in the event of failures, or parameter changes as required to rectify any operating problems incurred.
Following the commissioning of the pump station and turbines the system has begun operating at full capacity and the Drakenstein municipality has started reaping the benefits of its forward-thinking water management strategy.
“We are pleased with the outcome and commend everybody involved in the project for their innovation and dedication to building a system that can be used as a blueprint by other municipalities around the country,” concludes Henning. “It has been a brilliant project with many challenges and plenty of innovation. We look forward to tackling similar projects in the future.”
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