The Yallourn Power Station upgrade
An efficient natural draught cooling tower is an integral part of any power station. Industrial Water Cooling, a company specialising in the refurbishment of cooling towers and evaporative cooling systems, describes a recent successful on-line overhaul at the Yallourn Power Station in Australia.
Founded in 1986, Industrial Water Cooling (IWC) is a specialist water cooling company with a specific focus on evaporative cooling systems and cooling towers. IWC has 25 years’ experience servicing industries including:
- power generation
- mining
- petrochemical
- sugar
- steel and aluminium
- food and beverages
- air-conditioning and refrigeration
For many years IWC has been a member of the Cooling Technology Institute (CTI), based in the US. This association has as its members consultants, designers and constructors, as well as end-users of cooling towers. IWC has extensive knowledge of cooling tower refurbishment and has undertaken in excess of 80% of the natural draught refurbishment work conducted in South Africa. The company recently undertook the on-line refurbishment of a natural draught cooling tower in Australia and has also successfully completed this type of project in South Africa and Zimbabwe. IWC has completed the on-line refurbishment of 12 natural draught cooling towers to date within the power generation and petrochemical industries, and is currently busy with two more.
IWC has extensive knowledge of cooling tower refurbishment and has undertaken in excess of 80% of the natural draught refurbishment work conducted in South Africa.
Yallourn Power Station: project background
Located at the eastern end of the Latrobe Valley, approximately 150km east of Melbourne, Victoria, Australia, TRU Energy’s Yallourn Power Station consists of four brown coal-fired generator units, commissioned from 1973 to 1982. The upgrade project involved replacing the internals of the No 3 cooling tower.
It is a single wet type natural draught cooling tower servicing two generation units. The tower is 131m high, 95m in diameter at the air inlet and has an inside diameter of 61.6m at the top. The cooling tower featured an unusual arrangement of a partial spray tower with a reduced volume of fill, having had the hot water distribution pipes and splash packs installed in the air
inlet below the skirt of the shell.
The cooling tower relied on higher-than-normal pressures for the spray nozzles and featured a large 4m air gap between the distribution pipes and the drift eliminators; the tower performance suffered from the cold air ingress around this gap. Most cooling towers have the fill at the bottom of and inside the shell (i.e. above the skirt of the shell), an arrangement that minimises cold air bypass in a well-maintained tower.
Both the drift eliminators and the distribution system contained asbestos with significant failures evident on both. The existing distribution piping arrangement had been modified in order to isolate some of the distribution pipes; this, in turn, allowed for the partial isolation of the tower for minor on-line repairs.
The thermal performance of the cooling tower was well below that of other towers of similar shell size due to poor design and failing components. Site testing prior to the project indicated a cold water temperature roughly 4ºC higher than the original design level of 26°C, the extent of which varied depending on the external wind. The performance deteriorated over time with the continuing degradation of the tower internals.
Further investigation of the internal heat distribution confirmed a large cold-air bypass through the air gap between the shell and the pack structure. The testing also revealed that the cooling water flow rate was 15-20% below the original design value; this was primarily attributed to flow restrictions at spray nozzles, distribution piping isolation valves and condensers.
Distribution piping, fibreglass supports and walkways being installed in the Yallourn Power Station project.
The deterioration in the tower performance is summarised as follows:
- collapsing drift eliminators
- failure of asbestos cement water distribution pipes
- a large area of CT3 with cold-air bypass where pipes have failed
- fill damage
- cold-air bypass at the perimeter gap between the grids and shell
- reduction of cooling water flow from spray restrictions, isolating valves and also the Unit 3 condenser
Project objectives
The basic project objectives of the cooling tower upgrade are summarised as follows:
- to recover lost thermal performance
- to improve the thermal performance over original design to allow the operation of the two generation units consistently at full load so as to meet the minimum performance guarantees, but allowing for some performance degradation over time
- to remove all asbestos from the cooling tower
- to complete the work without any impact on generation output
Execution
WC split the project work into four separable portions. Design work commenced in May 2009 with site activities on Separable Portion 1 commencing in September the same year. Only a few cooling tower distribution pipes could be isolated at a time in order to ensure that sufficient water flow rate was maintained through the cooling tower. The effect on generating capacity was monitored and this together with generation demand and ambient conditions determined the number of distribution pipelines that could be isolated at any one time. With the distribution pipes isolated it was then possible to remove the fill and the drift eliminators from the cooling tower.
The original splash fill was left in-situ and a trickle pack fill was installed above it, inside the cooling tower shell. In order to achieve, this the distribution piping had to be elevated to create sufficient space. This modification reduced the pressure head available on the spray system, and nozzle sizes and spacings had to be carefully calculated to ensure that the design flow rate was achieved within the flow and pressure constraints of the existing reticulation system. A new distribution piping support structure had to be installed at the higher elevation in order to keep the structural loads to a minimum. IWC designed a fibreglass network of beams to carry the piping as well as an internal walkway system that provides maintenance staff with safe access to each sprayer installed in the cooling tower.
The asbestos cement drift eliminators were replaced with an
extruded PVC equivalent.
Distribution piping, fibreglass supports and walkways being installed in the Yallourn Power Station project.
Thermal performance
Each separable portion took three months to complete and the effect of the upgrade was measured using a simplified CTI test. The tower was evaluated prior to commencing the works and again on completion of each separable portion. The completion of each separable portion consistently yielded an improvement in cooling tower capacity, with the tower exceeding a 100% capacity on completion of the fourth portion.
In conclusion, all of the objectives set for this project were attained. IWC has once again demonstrated its ability to successfully execute the on-line refurbishment of cooling towers both locally and abroad.
