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Solution saves water at Kriel power station

10th May 2013

By: Yolandi Booyens

  

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The Kriel coal-fired power station, in Mpumalanga, is waiting for an appropriate time to replace a faulty valve, which will require three 500 MW units to be taken off line.

“Taking all three power-station units off the grid would cause a shortage of 1 500 MW of electricity, leading to inadequate supply for that period,” states Kriel power station manager Thomas Conradie.

A crack in the discharge valve caused a significant leak in August, resulting in the loss of 13-million litres of water a day.

The leak sprung when a main cooling water pump backup valve failed, owing to a crack on its butterfly valve shaft housing around the valve stem due to surge and the water hammer phenomenon, he states.

“There is no isolation system downstream from this valve,” notes Conradie, who adds that the cooling water system feeds from six pumps into a common system and produces cooling water for the three turbine units.

The leak on the discharge valve was spraying water about 10 m to 15 m into the air and the pressure inside the system was about 300 kPa.

The water leaking from the north cooling water system was ultimately drained to Vaalpan – a drain water catchment dam outside the power station.

“Concerns that the valve leak could result in catastrophic failure and immediately flood the valve pit were prevalent, as it posed a danger to workers, the environment and significantly stressed power production,” Conradie notes.

“It was a significant environmental and production risk and all attempts to contain the leak were unsuccessful.”

Specialists were invited to the site and advised that three units had to be shut down to enable the team to replace the valve. However, shutting down the units was not an option, as it would have increased the strain on the national power grid.

An effort to contain the leak by recovering the water running down into the effluent dam was unsuccessful, owing to the extent and size of the leak, as the dam started filling up faster. Following this, additional pumps were brought in, which slowed the rising of the water in the dam.

“However, the volume of water kept increasing to such an extent that, after a month, it started to decant into the stream, resulting in a loss of 13-million litres of water a day,” Conradie stresses.

“We then attempted to close the leak with a prop, which was also not successful, owing to the extent of the crack and the leak.”

Kriel power station turbine maintenance and engineering staff proposed the manufacturing of a large steel box that would fit around this 1.5 m valve to contain the leak.

This proposal was successfully implemented on November 13, 2012, and the leak was contained without shutting down any of the power station’s units.

“The cooling water backup valve body normally rests on a concrete plinth,” Kriel power station turbine maintenance manager Sibusiso Ngwenya explains. “This plinth proved to be extremely hard and had to be removed with jack hammers over two days to provide space for the sealing box to be fitted in position.”

“The impact of implementing this innovative repair prevented a forced shutdown with both environmental and power supply implications,” he concludes.

Edited by Tracy Hancock
Creamer Media Contributing Editor

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