Research on optimum sediment exclusion19 August 2008
Sediment exclusion is a major concern when designing and operating high head run-of-river hydro plants on sediment-loaded rivers. However, Meg B Bishwakarma suggests that guidelines for optimum sediment exclusion (OSE) have not been well developed to date, and suggests that more research is needed to investigate the causes and mitigation of sediment induced impacts
When it comes to developing a run-of-river (RoR) hydro plant on sediment-loaded rivers, the design approach based only on the availability of water for power production is not always realistic during operation. The sediment concentration in the available water can often significantly limit production during monsoons, and it is important to incorporate such losses in the economic analysis during the planning and design stages. However, the relationship between characteristics of sediments and their effects on different turbines is not well known yet.
The author conducted a study on sediment exclusion for Jhimruk hydro power plant in Nepal based on the available data from monsoon seasons between 1994-1997. The study revealed that when the power plant was operated for a period of about 10 minutes with sediment concentration of around 24,500 PPP, the estimated cost of repair was found to be about 10 times higher than the generated revenue during this operation. This example reveals that it is not always economical to operate a power plant without understanding the economic value of the water utilised for power generation. There is a level at each high head RoR plant where the costs are higher than the benefits of continued generation due to excessive sediment induced wear and resulting maintenance costs (Støle and Karki 1999). To establish a limit for concentration cut-off, measurement and analysis of sediments and corresponding effects on power plant operation and maintenance needs to be carried out.
Figure 1 shows a sediment concentration duration plot for Jhimruk river at the intake. The horizontal axis represents percentage of monsoon (mid June to mid September) time and the vertical axis shows the sediment concentration measured at the intake. If 3000 PPM is taken as the maximum level of concentration at the intake when JHP can be operational, then the power plant needs to be shutdown for about 20% of the monsoon time – meaning 20% of the expected generation is lost. It is therefore important to determine a level of concentration cut-off based on the cost benefit analysis as it has impact on generation losses and also losses caused by the sediment induced wear.
Optimum sediment exclusion (OSE)
Optimum Sediment Exclusion (OSE) requires documentation and analysis of time series of sediment data, corresponding effects on turbines and the associated costs due to the sediment induced problems. Due to the lack of a required database and information regarding the sediment characteristics and their relationship with the power plant features, economic optimisation of the size of settling basins has not been practically possible. The general design criteria for the settling basin has been to adopt about 98% trapping efficiency for particle size of 0.2mm. The author studied two power plants in Nepal and one in India, which revealed that the settling basins built on these projects fulfil the stated design criterion, however sediment-induced turbine wear problems still exist. This reveals that the adopted general design criterion is not adequate to address the issue of sediment-induced problems.
Generally, an attempt has been made to address sediment induced problems through some operation and maintenance strategy based on past experiences. For instance, the Jhimruk power plant (3x4MW) in Nepal has adopted an operation strategy to shutdown the power plant when sediment concentration exceeds 3000 PPM to avoid excessive erosion. Similarly, Marsyangdi hydropower plant (3x23MW) has adopted a routine of maintaining one unit each year on a rotational basis. However, the most cost-effective option for these power plants has not been judged fully due to the lack of a required database for optimisation study. Therefore, the adopted strategies may not be optimum, as the decisions are not based on economical analysis.
OSE studies need to be done in a holistic manner. To address the sediment-induced problems in a cost effective way, it is necessary to combine the mechanical and civil aspects of a power plant. To the author’s knowledge, a comprehensive research programme to assess the causes and effects of the sediment-induced problems aiming for a cost effective solution to a hydro power plant has not yet been conducted. It has therefore been difficult to develop guidelines for the design and operation of a sediment exclusion system as well as the turbine and its accessories.
Framework for OSE
Optimum sediment exclusion with respect to sediment induced turbine wear problems is a multi disciplinary approach. From a technical as well as an economical point of view, it is not feasible to manufacture a turbine having 100% resistant capacity against wear or to construct settling basins capable of removing 100% of the suspended sediments. Therefore, it is important to achieve an optimum combination of the design and operation of a sediment exclusion system together with the turbine and accessories in high head RoR hydro power plants so that an acceptable solution to sediment induced problems can be obtained. The framework for an OSE is to monitor parameters associated with this and analyse them in terms of cost and benefit. The main aim is to operate the power plant in an optimal way for an adopted operation regime. Støle (1997) and Lysne et al. (2003) outline the following key issues involved with OSE:
• Design, construction, operation and maintenance of the sediment exclusion system.
• Design, manufacture, installation, operation and maintenance of the turbine and replacement of its accessories.
• Generation losses due to reduced turbine efficiency, down time for maintenance and shutdown during high sediment concentration.
• Reduced sediment exposure of the turbine through real-time sediment monitoring guided operation regime.
Research programme on OSE
Conducting research on OSE is a cyclic process as shown in Figure 2, where a series of activities are involved. The aim of OSE research is to enable the power plant designers and operators to optimise the power generation with respect to given sediment and flow characteristics. Working on OSE research is to improve the performance of existing as well as planned hydro power projects. Increasing performance means maximising the benefit from a hydro power operation, which in turn will help minimise construction costs and overall environmental effects caused by the construction of new hydro plants to achieve the equivalent energy.
Objective of OSE
The overall objective of an OSE research programme is to generate a database for the optimisation of RoR hydro power plants with respect to sediment exclusion. The specific aim is to determine the cost per tonne of sediment on a particular hydro power plant having its own sediment characteristics and power plant features.
The interests of the power plant owners and the equipment manufactures are crucial to realise such a research programme. In a global context, it is recommended that hydro developers and operators try to bring in funding agencies, equipment suppliers, designers, academics and research institutions for joint research on OSE, integrating experiences and knowledge from the operation of existing plants throughout the world. A suitable institutional set-up is important to conduct such a research programme. Figure 3 shows a link between the major parties involved in the development of hydro power as key research partners in this endeavour.
The power plant owner/developer could initiate, coordinate and finance an OSE research programme cooperating with other relevant organisations as shown in Figure 3. Academic and research institutions with knowledge and expertise on instrumentation, data acquisition and analysis could help implement such a programme. The output of OSE research will provide a good basis for the optimisation of a power plant with respect to sediment exclusion.
Quality assurance and training of personnel
Consistency on sediment sampling and the laboratory analysis are the important parameters to achieve accuracy in sediment load estimation. Similarly, consistency in the measurement of generator output, inlet and outlet temperatures, inlet pressure, and tailwater level etc, is important to achieve accuracy in turbine efficiency measurements. Inconsistency in measurement instrument, methods and procedures used to measure sediment and efficiency at different times and locations leads to inconsistency in the results. It is important to develop standards for measurements. The standard should include – but not be limited to – instruments, data acquisition techniques, recording formats, methodology and procedures for measurements, etc. Such standards should enable engineers and technicians to record necessary data and information in a proper and consistent manner. Training and motivation of the involved personnel is key to maintaining efficient and systematic operation of a measurement programme. The involved staff should have both the skills and knowledge to understand the principles of measurements, laboratory analyses and data interpretation. Therefore, training of personnel involved in the measurement programme is equally important.
Expected outcome from OSE research
The OSE research programme should aim to establish relationships between the parameters such as sediment characteristics, flow characteristics and their corresponding effects on turbine operation. The relationship shown in Figure 4 is expected to be developed from OSE research.
Ultimately, the outcome from the OSE research programme will facilitate for the optimisation of the design and operation of RoR hydro power plants with respect to sediment induced turbine wear. A theoretical optimisation curve for a hydropower plant is shown in Figure 5.
The initiated OSE programme
Realising the need for optimum sediment exclusion and economic operation of hydro power plants built on sediment-loaded rivers, a broader research programme was initiated through a collaborative effort involving the following institutions: Hydro Lab (co-ordinators), NTNU (Department of Hydraulic & Environmental Engineering and Water Power Laboratory), GE Hydro, Sediment System, Kathmandu University and Butwal Power Company Ltd. Himal Power Limited and Statkraft SF.
As a start of the research on OSE, sediment and efficiency measurement were carried out on one of the turbine units at Jhimruk and Khimti hydro power plants in Nepal during the monsoons of 2003 and 2004 respectively. This first phase of OSE research mainly focused on the development of required instruments and procedures for measurement. The author was involved as a Research Engineer in this programme with responsibility for developing equipment and procedures to monitor sediments passing through the turbines.
Required data and observation methods
A number of data need to be measured to study OSE. Collection of different data requires knowledge, instruments and observation methods. The data observation methods and analysis standard are important in order to make the observed data comparable with data measured at different power plants. The most important data required to be documented for OSE in each of the hydro power plants are: suspended sediment (concentration, particle sizes and mineral content); flow and power output of the turbine; turbine efficiency; turbine erosion; repair, maintenance and replacement costs; and consequences on power supply regularity due to sediments
To the author’s knowledge, some of the instrument and data recording methods are available – but most are not. For example, the instrumentation and methodology for turbine efficiency measurement and manual sampling for suspended sediment concentration are available. However, standard instruments for monitoring high frequency suspended sediment concentration, turbine erosion and proper methodology for documenting costs related to sediment-induced problems are not well developed.
Suspended sediment measurement
Measurement of suspended sediment involves analyzing a sample of water for concentration, particle size distributions and content of minerals. All these parameters are useful for the planning, design and operation of RoR hydro power plants. Sediment data provide a good basis for the selection of sediment exclusion systems and to help make decisions on the plant operation regime for a particular power plant. Online sediment measurement is needed to capture the variations in sediment concentrations and to adopt a sediment-guided operation of a power plant. Standard methods are available for the analysis of particle size distribution and mineral content. The SMOOTH online sediment monitoring system developed at NTNU can be employed for the sediment concentration measurement.
Turbine flow measurement
Measurement of flow through a turbine is required to compute the sediment load passing through each of the turbine units. Accurate measurement of flow through a turbine is a challenging task. There are various methods developed for measuring total flow through the turbines. However, not all of them are suitable for measuring the flow through an individual unit. Flow through an individual turbine unit can be measured using the Ultrasonic, Winter Kennedy, Pitot tube, and Acoustic methods, etc.
The easiest method for turbine flow measurement is to use Ultrasonic Flowmeters. However, finding a suitable location for this type of flowmeter may be a challenge in the power plants, as it requires a straight pipe section of about 10 times the internal diameter. Winter Kennedy method may be used for measuring flow in Francis turbines. The Pitot tube and Acoustic methods may be employed in both Francis and Pelton turbines. However, the excessive sediment concentration may alter the results in both of these cases. In the case of Winter Kennedy and Pitot tube methods, clogging of pipes due to sediments is a common problem. Similarly, flow measurement by acoustic method is also affected by sediment concentration in turbine flow. Therefore, it is important to consider these factors during flow measurements.
Turbine efficiency measurement
Efficiency of a turbine is defined as the ratio of the power produced and the hydraulic power available just upstream of the turbine. Efficiency measurement is done for two main purposes. In new turbines, the efficiency measurement is done to check the contractual compliance, whereas in the old turbines it is done to check the status in order to quantify the improvement potential. Efficiency tells the operating status of a turbine, which is linked to the electricity generation capacity. Periodic measurement of the turbine efficiency helps to discover the improvement potential of an existing turbine.
The loss in production means loss in revenue. The reduction in turbine efficiency is largely dependent on sediment-induced erosion. The same turbine can last for several years if operated in sediment free water, however it may not survive even a single monsoon if exposed to heavily sediment-loaded flow like the power plants built on Himalayan rivers. Therefore, measurement of turbine efficiency together with sediment concentration and sediment-induced turbine erosion is vital for such power plants. The efficiency testing can help save the turbine from potential irreparable damages as well as unacceptably high generation losses. The relationship between reduction in efficiency and sediment load can be established by measuring absolute efficiency before and after the monsoon season together with sediment load. The Thermodynamic method can be employed to measure the absolute efficiency of turbines.
Normally, production log sheets are maintained in every power plant to record the generated energy. The energy meter shows the produced energy over time which is used for billing. This provides a record of accumulated production. However, energy generated by an individual unit is more important from an OSE research point of view. Recorded production together with the correspondingly measured flow provides additional information on relative efficiency of the turbine. The increase in flow for the same power production after a certain period is an indication of reduced turbine efficiency. A systematic recording of power output from each unit in every power plant is a useful data source for further analysis. Manual records on an hourly basis can be kept in a standard format. However, automatic logging of production with short (about 10 mins) time interval is preferable to obtain accurate results and cover variations.
Sediment induced turbine wear is a common phenomenon in hydro power plants built on sediment-loaded rivers, thus it is important to monitor the erosion together with other parameters mentioned in previous sections. The benefit of erosion monitoring can be two fold; firstly generating a database on turbine erosion and secondly adopting a preventive maintenance programme.
To the author’s knowledge, no standard instrument has been developed for online monitoring of the turbine erosion. Commonly adopted indirect methods include weighing the runner or guide vanes before and after the erosion, estimating erosion based on the weight of the consumed material in the welding, and evaluation of the photographs or images taken by video probing. With the methods presently available, it is not only difficult to compare the available data but also to figure out the relationship between the erosion and the efficiency loss. Therefore, proper instrumentation and methodology are necessary to measure the sediment-induced wear of the turbines. Such instrumentation is expected to enable the power plant operator to monitor the erosion and plan for necessary repair before the turbine is irreparably damaged. By measuring turbine erosion together with the sediment and corresponding losses in turbine efficiency, the relationship between these parameters can be established.
Recording of cost
Recording of costs associated with sediment-induced problems are needed to derive the cost per tonne (or per kg) of sediment for a particular power plant. Separate recording of costs particularly associated with sediment-induced effects are often missed. The author carried out analysis of the sediment-induced effects in terms of cost for Jhimruk hydro power plant in Nepal. However, the analysis could not yield satisfactory results due to the lack of reasonable cost data associated with the sediment-induced effects (Bishwakarma 1999). Sediment induced turbine wear includes the costs of; construction of sediment exclusion system; operation and maintenance of sediment exclusion system; maintenance of turbine and replacement of parts; protective coating (if it is applied); loss of generation due to downtime; loss of generation due to loss in efficiency; loss of generation due to high sediment concentration; and reduction in power delivery regularity
It is important to study the percentage contribution of each of the items on the total cost and identify the major contributing items. Some of the items may have very little or negligible contribution in the overall costs, and as such may be neglected. Before developing formats for recording the cost data, one should think of the complexity involved in this venture. The recorded costs at one power plant may not be comparable to the costs recorded in another. This is because the following factors change depending on the location of a power plant: manpower costs; energy price; administration of maintenance works; type and quality of equipment and parts; type and quality of protective coatings; and source of supply of manpower, equipment and parts
In the case of an individual power plant, these factors will have little or no influence if similar procedures and equipment is applied every year for operation and maintenance. In such a case, the direct costs of workers, loss of energy and necessary equipment can be used. The factors mentioned above are key to make the recorded costs comparable among different power plants. In order to make cost data comparable, the best way could be to record the required data in raw form (in terms of quantity). Such data may include loss of generation in KWh, numbers and types of parts maintained or replaced, man-hours consumed by different staff, etc.
Based on these raw data together with the information obtained during interview, a shadow costing can be done later. This may prove useful for discovering the cost per tonne of sediment passing through a turbine at a particular hydro power plant.
In developing hydro power projects on rivers carrying high sediment loads, appropriate attention should be given to the sediment exclusion aspects. Design of RoR hydro power plants based only on the availability of water is not realistic as the content of sediment in water limits the production – most often in the monsoon season.
The available knowledge to deal with sediments is not sufficient to address the issue of OSE with respect to sediment-induced turbine wear problems in RoR plants. Therefore, more research and development is needed to investigate the causes and mitigation of sediment-induced impacts. Instrument and methods are required to observe data to study the relationship between sediment characteristics, power plant features and sediment induced problems. The initiated OSE research has not been accelerating due to the lack of funding, however some knowledge has been gained over the years.
Conducting OSE research is a collaborative effort. The power plant owners need to play a more active role in this endeavour. Academic and research institution could contribute significantly. The collaborative efforts of the key players involved in developing hydro power could help convert the existing challenges into opportunities.