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More power, longer life

Yallourn Power Station cooling towers

Credit: IWC

A project at a coal-fired plant in Australia shows how cooling tower refurbishment can significantly improve power generation efficiency and reduce a plant’s environmental footprint, writes Viv Quann

EnergyAustralia’s Yallourn Power Station in Victoria recently underwent a major refurbishment project.

The work, carried out by South African company Industrial Water Cooling (IWC), successfully restored the 30-year-old cooling tower to its original design capability and extended the lifespan of the tower by a further 20 years.

The original Yallourn Power Station was built between 1973 and 1982 and consists of four brown coal-fired generator units with a design capacity of 1480 MW. The station supplies approximately 22 per cent of Victoria’s electricity and approximately 8 per cent of Australia’s national electricity grid.

IWC – originally founded in 1986 as Industrial Water Cooling – was commissioned to refurbish the station’s cooling tower Number 3 (CT3). CT3 is a wet-cooled natural draft cooling tower that services two of Yallourn’s four generator units, which produce just over 50 per cent of the station’s power generating capacity.

Before the refurbishment, CT3 was performing well below cooling towers of similar shell size due to its poor design and aging internal components. Deterioration included collapsing drift eliminators, broken water distribution pipes, fill media damage and maldistribution of water due to missing and damaged sprayers and distribution pipe-end caps.

Pre-refurbishment testing showed that the tower’s re-cooled water temperature was 4à‚°C higher than original design of 26à‚°C, while the cooling water flow was 15-20 per cent below design due to flow restrictions at spray nozzles, distribution piping isolation valves and condensers.

In addition, performance was further undermined by a large cold air bypass running through the air gap between CT3’s shell and pack structure.

IWC chief executive Roger Rusch says: “The test results gave us a clear indication of where refurbishment was needed, enabling us to set project objectives. These were primarily to recover lost thermal performance; improve the thermal performance to allow the operation of the two generation units to meet the minimum performance guarantees; and to remove all asbestos from the cooling tower. The most important factor was to complete these without any impact on power generation output.”

It was unfeasible for the cooling tower to be taken out of service as it was responsible for servicing generator units that produced over 50 per cent of the station’s overall power generation. This meant that only a small area of CT3 could be isolated at any given time during the refurbishment process to ensure there was no significant thermal performance deterioration.

Online refurbishment technology

To keep CT3 fully operational during the repairs and upgrades, IWC applied its online refurbishment technology.

Rusch explains: “Only a few of the cooling tower’s distribution pipes could be isolated at any one time to ensure that sufficient water flow rate was maintained through the cooling tower. In order to determine how many distribution pipes we could work on at one time, we constantly monitored the effect on generating capacity together with generation demand and ambient conditions.

“Once the distribution pipes were isolated in the portion we were refurbishing, it was then possible to upgrade the water distribution system inside the isolated section, as well as remove and replace the fill media and the drift eliminators. In the isolated portion, work was completed in practically dry conditions.”

As part of the Yallourn upgrade, IWC redesigned CT3’s water distribution system. This included the installation of a new distribution piping support structure at a higher elevation than the old structure in order to improve water flow.

Performance has improved as anticipated

Credit: IWC

To keep the structural loads to a minimum, IWC also designed and installed a fibreglass network of beams to carry the piping, as well as support an internal walkway system, providing staff with safe and easy access to each sprayer. Previously, operational access had been limited, making it difficult to maintain the distribution system.

CT3’s original splash fill was left in-situ and a trickle pack fill constructed out of non-corrosive and non-clogging polypropylene was installed above this original fill. To reduce the degradation of cooling tower performance caused by the effects of high wind speeds, trickle packing was installed at varying thicknesses across the tower. Packing ranged from 2.5 layers thick in the middle, reducing to 2 layers towards the shell.

Rusch says: “Inside a cooling tower, the fill is the medium used to put as much water surface area in contact with as much air as possible for the longest amount of time possible. Water is cooled as it trickles down the fill medium, thus aiding the heat transfer process. Inside Yallourn’s CT3, we installed polypropylene trickle packs, which are recommended for use in cooling water with a high solid content where film packs would be easily clogged.”

To close the large cold air bypass running through the air gap between the cooling tower’s shell and pack structure, IWC installed a dense, stainless steel mesh around the perimeter of the cooling tower shell.

Asbestos removal

CT3’s original drift eliminators and distribution pipework, which were made of asbestos cement, had collapsed, and needed to be removed and replaced during the refurbishment.

Inside an evaporative cooling tower, drift is the unnecessary loss of liquid water to the environment via droplets that become entrained in the leaving air stream. These water droplets often contain harmful chemicals, thus negatively impacting the surrounding environment.

Rusch explains that “drift eliminators are designed to contain large water droplets that are circulating inside the cooling tower’s air stream. They do this by causing the droplets to change direction and lose velocity on impact with the drift eliminators’ blade walls, from where they then fall back into the tower.

“After removing all traces of asbestos inside CT3, IWC then installed our proprietary extruded PVC drift eliminators, which keep drift losses to less than 0.02 per cent of the re-circulating water flow rate. PVC eliminators provide low-cost mist elimination with maximum corrosion resistance.”

Lifespan has been extended by 20 years

Credit: IWC

Refurbishment of each of the four cooling tower portions took six months to complete, with the overall project completed in approximately two and a half years.

After each portion was finished, IWC conducted a simplified performance test in accordance with the Cooling Tower Institute (CTI) Acceptance Test Code 105. Work on the next section only began after test results showed performance had improved as anticipated.

The completion of each separable portion consistently yielded an improvement in cooling tower performance, with the tower exceeding a 100 per cent original design capacity on completion of the fourth portion.

The refurbishment not only restored the tower’s performance but significantly improved it, thereby improving the power utility’s generating efficiency. The lifespan of Yallourn Power Station’s CT3 has also been extended by a further 20 years for its owner, EnergyAustralia.

Viv Quann is Public Relations Officer at Industrial Water Cooling (IWC)