Silt erosion...trouble for turbines16 April 2001
Silt is a big problem for hydro plants. Not only can it erode and damage plant components, it can reduce efficiency of machinery and ultimately affect the generation capacity of a plant. In high head, silt prone schemes Pelton turbines may prove to be the most economical choice for long term power production
Many hydro project sites in the Himalayan range in northern India, and in the northeastern region of the country, face severe silt erosion problems. An inspection of several existing hydroelectric power stations located in these regions has confirmed the severity of silt on the critical underwater parts of turbines and other components of the power station.
Silt has not only damaged/eroded the components of these plants but has resulted in a steep fall in efficiency of the affected machinery, which can only be rectified if timely and thorough repairs are carried out. Siltation has resulted in increased operation and maintenance costs and reduced availability of these stations for power generation.
Planning for power stations where silt is a potential threat, purely on the basis of the techno-economics of initial investment, is short-sighted. For high head, silt prone hydro power schemes, the concept of working out the theoretical annual energy potential without taking into account the probable annual energy loss due to forced shutdowns imposed by repairs on underwater components, needs to be reviewed.
To choose the best turbine for the job, a dual cost (initial investment cost plus operation and maintenance cost) long term planning concept should be applied which is more realistic and beneficial for these power stations.
Soil erosion is primarily caused by rainfall. The quantum of soil erosion depends on the quantum of rainfall and its intensity, soil erodibility, available vegetation or crop cover, slope length and gradient, and the management practices adopted by the concerned authorities. Deforestation of land in the upper reaches and grazing of animals on waste land accelerate the rate of soil erosion. Soil is ultimately washed into rivers and flows into intervening dams/reservoirs and water conductor systems of power stations.
High concentrations of silt are associated with floods. In the monsoon period the issue of silt becomes increasingly critical.
When judging the danger of silt erosion it is important to note the quality of silt (the percentage and shape of the quartz contents and other hard materials) rather than the total amount of silt. In most of India’s northern and northeastern rivers, 90% of silt is made up of hard materials such as quartz, with a hardness in the range of 5-7Mhos.
Despite taking account of available design measures to counter silt at silt prone hydro power plants, siltation will continue to create problems until resistant materials or long lasting cost-effective surface coatings are developed for the underwater components of turbines. Unless research and development efforts are undertaken in this area, lasting solutions will continue to be a distant dream.
Francis turbines have a higher geodetic head and are 3% more efficient than Pelton turbines, so Francis turbines are generally preferred by planners and designers of hydroelectric power schemes. Francis turbines also run at a higher speed which makes the generator and overall power house costs cheaper. However, the civil costs (deep excavations/concreting for the runner, and a unit dewatering system which is necessary for a Francis installation) can offset the cost advantage of a higher speed generator. But overall, the initial investment costs for a Francis installation are usually less than for a Pelton installation.
Recent technology has brought about the development of a high capacity Francis turbine which can achieve efficiencies as high as 95-96%. Comparable Pelton turbines can achieve efficiencies in the range of 92-92.5%.
Cavitation and silt erosion of water borne parts of the machine, especially the runner, guide vanes, labyrinth seals, nozzle and needle assemblies, and main inlet valve seals are regarded as the most serious problems for large capacity high head turbines.
The amount of erosion and consequential loss of effici-ency is difficult to compare for Francis and Pelton turbines and may be assumed to be equal for both. However, where the water contains large amounts of sand for heads above 400m, erosion damage is generally more severe in Francis turbines.
Besides ease of replacement and repair, the downtime for inspection, removal and repair, and reinstallation of underwater parts of Pelton turbines is much less than for Francis turbines. This is a significant and distinct advantage for silt affected power stations.
In-situ inspections and removal of the runner, needles and nozzles are quicker in Pelton installations as no dewatering is needed. The repair time of a Pelton runner is less because of the simpler profile of its buckets. However, due to the complicated profile of a Francis runner, the accessibility of blades for repair is quite difficult and time consuming. Besides this the replacement of labyrinth rings, guide vanes and guard rings – which are liable to erosion damage in Francis turbines – is also time consuming.
In silt affected Francis turbine power stations, the frequency of repairs and inspection of underwater parts is generally much higher and the overall forced downtime per year for repair is usually much longer. Consequently, the forced annual loss of energy generation in hydro power stations could be much higher with Francis installations.
When planning the lifetime costs of a hydro plant, it should be taken into consideration that more Pelton runners are required as the theoretical designed life for a Pelton runner is usually 20 years, compared to 35 years for a Francis runner. However, the cost of these extra runners are offset by the increased availability of the power station for generation.
Making the right choice
In silt prone hydro power schemes with average high heads, and where quartz and other hard materials are the dominant constituents of silt, installing a Pelton type turbine may be a better choice with regards to long term operation and maintenance costs. However, every hydroelectric project has several other specific considerations such as large head variations, type of power house (underground or surface), proven design and manufacturing capability, transport limitations and availability of expertise for repairs, which may finally dictate the choice of turbine.