An encroachment on hydro

By 2020 more than one in five Indian reservoirs will have lost 50% of their storage capacity due to sedimentation. Drawing on his experience in India, B S K Naidu gives an insight into this growing phenomenon and its implications for reservoir and hydro power plant management

A combination of factors leads to siltation. Soil erosion in tropical climates can be caused by high summer temperatures which cause soil to crack and grass to burn, leaving web-shaped spaces in the ground. The flooding monsoon waters then erode and transport soil. From an Indian perspective, additional factors responsible for soil erosion include:

•The immature geology of the Himalayas.

•Glacial silt unleashed from melting snow.

•Climatological anomalies — droughts followed by floods.

•Landslides.

•Uncontrolled tree felling.

•Heavy pressures of grazing.

•Methods of cultivation.

Eroded soil comprises more than 80% silt, around 12% clay and other remaining materials such as decaying vegetation. Sand (0.06-2 mm in diameter) usually gets deposited along the shores of water bodies (eg river beds) and the finer particles which constitute silt are carried by moving waters, getting sedimented wherever water stagnates. About 20B tonnes of earth material is carried to the seas every year worldwide, of which nearly 6B tonnes is from the Indian sub-continent alone. Global sediment production is over 150M billion tonnes per year.

Sediment deposition in reservoirs is a troublesome process. It creates a variety of problems such as raising stream beds; increasing flood heights; choking up navigation and irrigation canals; and encroaching on the live storage capacity of reservoirs, decreasing their useful life. Data on 116 Indian large dams were analysed to determine the severity of sedimentation. By 2020 over 20% of reservoirs will have lost about 50% of their storage capacity.

In hydro power plants siltation in the generating stream can become a great nuisance. You have to operate a plant with silt laden flows: feeder tunnels become throttled; trench weirs get choked up; hydromechanical gate seals become ineffective; and generating units face increasing erosion damage, leading to the perpetual problem of increased restoration costs and longer downtime.

The Baira Siul project (3x66MW) in Himachal Pradesh, for instance, handles nearly 10,000 tonnes of silt per day, per machine during critical monsoon days. Material loss in guide vanes alone is 10% by weight. The trailing edge thickness of runner blades thins down to 2mm from 12mm. Approximately 50,000 welding electrodes weighing 1.6 tonnes are consumed per unit, per repair season. More than 90% of the silt passing through these machines is quartz.

Siltation is a costly problem. The normal life expectancy of dams in India is assumed to be 100 years but reservoirs in India are estimated to be losing storage capacity at an average annual rate of about 1% due to insidious encroachment by sediment. The effects of silt on power plant maintenance can be significant. Annual maintenance can take the shape of renovation and there are power stations in India where the length of time between major renovative efforts can vary from one to three years. The annual O&M cost of silt-affected power stations can be as high as 5% of the capital cost, against 1.5% in normal cases.

Assessing and monitoring

Siltation can be categorised into two areas:

•Reservoir sedimentation.

•Siltation in the generating stream (the carriage of unconsolidated sediment with particle sizes inbetween those of sand and clay).

Reservoir sedimentation can be assessed and monitored by stream flow analysis and hydrographic surveys using global positioning systems (GPS), echo-sounders and transducers supplemented by computer analysis. Encroachment on live storage can also be monitored by satellite remote sensing which is a quicker and cheaper technique. It should be used to supplement and complement hydro-

graphic surveys which can then be carried out at longer intervals. The removal of sediments from a reservoir may be accomplished by excavation, dredging, drainage and flushing, and sluicing.

Siltation in the generating stream

Silt escaping the desilting system and entering the generating stream affects hydraulic turbines and valves etc. Its assessment and monitoring assumes great importance when considering the operation and maintenance of hydraulic equipment.

•Silt characteristics: silt data should essentially constitute particle size, shape, hardness and concentration. An appropriate proforma for petrographic analysis of sediments should be used for data collection and analysis. It is also important to record the silt erosion damage of critical underwater parts of Francis, Pelton and Kaplan turbines. This must be done scientifically and systematically through well designed formats.

•Silt measurement and erosion monitoring: silt measurements can be done at the intake just before water enters the headrace tunnel and at the tailrace just after it leaves the turbine draft tube, or at least at one of these two points. While physical or manual measurements of silt may continue, to ascertain particle size, shape and hardness, photoelectric silt meters may be installed for continuous on-line measurement of silt concentration in parts per million (PPM). Erosion monitoring of underwater parts can be done through video probing after dewatering the machine. A video probe’s large detailed images of internal turbine wear can save costly disassembly time and hence overall shutdown periods.

Accurate silt measurement and erosion monitoring are crucial in hydro plants handling silt laden flows. Equipment users need to avoid speeding up erosion and must abide by the manufacturer’s recommendations on operational strategy related to silt concentration, and preventive maintenance related to silt characteristics etc. Having given a guarantee for material loss due to cavitation/silt erosion over a certain period based on silt data, equipment manufacturers generally like to be reassured of the conformity of the actual data with the projected values. If the actual data is different, they would be well within their rights to seek suitable amendment in the guaranteed values of erosion damage.

Major developments in tackling siltation and its impact on hydro turbines include catchment area treatment (CAT) of the river valley upstream of the project site. Biological measures involved in CAT include plantation and pasture development. Engineering measures include check dams and walls; contour bunding; gully plugging; wire crating; benching and berming; rock bolting; and networking drainage wells.

To prevent silt from entering the generating stream effective desilting arrangements are important in a hydroelectric project. The removal of accumulated silt and its environmentally responsible disposal must also be considered. In order to prevent silt entering into the generating stream, the following solutions are suggested.

Hydraulic design

Silt erosion should be treated as one of the most important factors during the selection of turbines in the overlapping head ranges. The following should be considered:

•Radial machines are more susceptible to erosion due to centrifugal forces.

•Runner damage may be less in Kaplan than Francis turbines, though higher costs are involved in the dual regulation system in the former.

•The design head should be taken as the apparent head multiplied by the specific gravity of the silty water at its maximum concentration. Peripheral velocity of the turbine runner in silt laden flows should not exceed 40m/sec.

•A deeper setting of the turbine is normally called for, compared to the machine operating with clean water, to avoid ‘cavitation-silt erosion synergy’. However, during the monsoon (when silt concentration is at its peak) the turbine automatically sets deeper with respect to the rising tail water level (TWL). Variation of TWL during the monsoon and lean season should be very clearly explained to the turbine supplier to optimise the machine setting. The tailrace weir may be constructed to optimise the turbine setting.

•The hydrofoil cascades, particularly the runner, can be redesigned and optimised using in-house hydrodynamic design software. Erosion should be taken into account through imposing certain boundary conditions such as larger sized but fewer blades.

•In the case of large projects with high silt concentrations, model testing with sediment (alumina) laden flow may be desirable to observe the critical areas of the flow path prone to erosion. This can either be modified from a hydraulics point of view or may be protected through special coatings.

Mechanical design

•Every turbine in a power station should have a separate main inlet valve (MIV) in silty situations, and even independent penstocks to facilitate repair of MIV without having to stop adjacent turbines.

•Turbine shaft seals should have rubber rings in place of carbon rings and the design should allow dismantling and replacement in the shortest possible time.

• The erosion resistance of martensitic (13:4/Cr:Ni) stainless steel is better than other materials. This should be used for crucial components like runners, guide vanes, labyrinth seals etc, with additional coatings wherever necessary and feasible. Austenitic steel may however be used for top cover and bottom ring liners.

•The thickness of runner blades should be increased in the areas prone to erosion (runner outlet edges near the skirt in Francis turbines, and near the peripheral sections and outlet edges in Kaplan turbines).

Material technology

•Welding repairs of underwater turbine parts are commonly attempted with austenitic stainless steels which show lower wear resistance compared to the parent material. Special alloy systems need to be developed for the purpose.

•Since most of the turbine runners and guide vanes use (13:4/Cr:Ni) stainless steel, suitable matching electrodes of martensitic steels should be used for major repair welding, since only such electrodes can provide a hard and tough weld. For high quality repair welding, pre-weld and post-weld heat treatments are necessary for martensitic steel components.

•Extensive studies should be carried out on welding repairs of underwater parts. Studies on weld deposits using alternate layers of austenitic and martensitic steels are also necessary to explore the possibility of eliminating the need for pre-weld and post-weld heat treatments.

•All efforts should continue to develop processes suitable for in situ weld repairs and coatings. Ultrasonic stress relieving may also be tried at sites.

•Hard metallic coatings, like plasma coatings utilising chromium boride, may be used on runners and guide vanes made of (13:4/Cr:Ni) stainless steel. This is the best option at present for base material as a second line of defence — with these coatings the service life of critical underwater components can be increased substantially. Development of the cold hard coating technique should be continued to save on costly heat treatment processes.

•While limited success has been achieved in coating guide vanes, efforts should continue in finding viable solutions for turbine runners and their envelopes which are badly affected.

•Amongst the non-metallic options, DuraTough offers a promising solution to the ‘cold and quick’ repairs of eroded components like runner and guide vanes. Non-metallic ceramic coatings have also been reported successful in some cases, although they offer little protection against cavitation.

•Epoxy and polyurethane based plastics are useful for coating the surfaces of spiral casing and draft tubes in reaction turbines, and nozzle pipes in Pelton turbines. DuraQuartz is recommended for repairs of concrete components.

A common knowledge or general awareness of silt problems is not enough to tackle this problem. Hydro power engineers must have a ‘silt consciousness’ during investigations, design, operation and maintenance, and even uprating and refurbishment. More research and development is needed into the causes and mitigation of silt erosion impacts. Experiments and field observations are needed to understand the true relationship between the silt erosion rate and silt concentration, particle size, shape and hardness; wear resistance of base material; relative velocity of water; angle of attack of silt particles; and chemical properties of water.

Test rigs capable of handling silt laden water need to be developed to establish:

•Performance of specially hydraulic-profiled turbine components under silty flow conditions.

•Correlation of turbine efficiency and turbine settings with silt characteristics.

•Prediction of precise locations of damage for hydraulic improvement or at least protection zoning for coatings.

•Extent of damage on profiles under turbine flow conditions in different operating regimes.

•Better understanding of cavitation-silt erosion synergy.

International conference

In recognition of the need for an increasing awareness of siltation and its impact on hydro power plants, the first international conference on Silting Problems in Hydro Power Plants was organised by Central Bureau of Irrigation & Power at New Delhi in October 1999. The main conclusions from discussion there were:

•The cost of catchment area treatment (CAT) should be apportioned amongst various beneficiary sectors (agriculture, soil conservation, horticulture, pasture development, animal husbandry, social welfare, environment, transport, forest, irrigation, flood control and hydro projects) in proportion to the benefits accrued to them.

•Establishments in the Himalayas responsible for hydro project implementation should abide by ISO14000 on environment management.

This covers catchment area development and watershed management.

•The use of satellite remote sensing techniques for estimating live storage capacity loss due to sedimentation was recommended. Hydrographic surveys conducted at longer intervals and remote sensing at shorter intervals will complement one another.

•Periodic flushing of existing reservoirs should be carried out according to a set schedule to reduce sedimentation and to maintain live storage.

•Two or three types of desilting systems need to evolve as standard cost-effective designs for small hydro (up to 25MW).

•The operating schedule of power stations and their individual machines should be established to minimise exposure to silty water during high concentrations.

•Computer software could be developed for making erosion calculations or simulations etc for designing various silt exposed components.