Productivity and Efficiency measurement in Hydraulic Turbine

Raju Shrestha1, Dr. Bhola Thapa2, Prakash Gautam3, Piyush Upadhyay3, Productivity and Efficiency measurement in Hydraulic Turbine, "Building new Nepal: Challenges and Opportunities”, 10th National Convention,, Nepal Engineer's Association, Kathmandu, Nepal 11-13 April 2007.

Introduction

Nepal’s indigenous energy resource is hydropower and its potential is estimated to be 83,000 MW, Out of which nearly 560 MW is harnessed so far, which accounts only about 1.32% of technically/ economically potential. Historically, the system loss in Nepal’s electricity 70’s is as high as 36 %. After the establishment of Nepal Electricity Authority and formation of loss reduction division in 1980, the power loss was reduced to 29.1 % (Shilpakar, 2006?). The trend of loss in NEA system can be seen in Figure 1. The loss in the electricity system includes both technical and commercial (non-technical) losses. Some of the factors affecting technical losses are overload, long lines, unbalanced load, poor power factor, transformer location, loose connections, leakage through tree branches and non-standard line materials. Similarly non technical losses are caused by manipulating metering system of electricity consumption or bypassing electricity from electric meter. By introducing various losses reduction programs NEA was able to bring down the losses to a minimum value of 22.5 % in the FY 1997/98. However, the loss has once again increased to 24.2 percent in FY 2002/03 mainly due to pilferage. NEA's has strategy to bring down the loss below 22 % in 2006/07 and further reduction of 1 % thereafter (NEA 2006).

Figure 1 Distribution loss of NEA over the years (Shilpakar, 2006)

The priority of NEA is to reduce non-technical losses because it is cheaper compared to reduction of technical losses. The non-technical loss can be reduced by the support of civic society and government in enforcing the law. However reduction of non-technical loss alone can not help in reducing loss, but technical losses (around 15%) should also be reduced through improvement and reinforcement of distribution systems. If we increase the production in the same system or decrease rate of efficiency drop within the turbine, the productivity of hydropower system improves.

Productivity and turbine

Less developed nations, in particular, face tremendous pressures to improve productivity for meeting expectations of the people for improving their quality of life and strengthening their national economies to survive and succeed in the competitive world (Pant et al 1999). NEA at present is in great financial pressure and also not able to provide quality service to customers and hence considering productivity activity should be importance in NEA. According to Prof. Ichiro Nakayama (1963) the basic principle of the productivity can be said to be the realization of maximum output with minimum input. Viewed in this way, a certain relationship of efficiency emerging from input and output should be from the core concept of productivity (Monga 1999). Productivity approaches quite often focus on reducing wastes of all forms to improve productivity. The water flowing from the power house without turning turbine runner can be considered as waste of raw material. The main reason for wastage of water passing through hydropower plant is technical problem.

 Even though productivity movement in Nepal initiated together with First Five Year Plan (1956-61), that was generally centered around manufacturing industries. Ajit N.S. Thapa (1999), a management expert observe the main challenge for the planner is the utilization of water resources in the country rather that improving productivity of energy sector. In his opinion, the priority for productivity movement as a strategy to make productivity movement effective in hydropower sector is “Setting up of manufacturing and service industries around the generation, transmission and distribution of hydro-electricity (providing both forward and backward linkages), needs to be taken up more vigorously. Some efforts have been made to manufacture small capacity hydro turbines and electricity generators and HMG should encourage the private sector to do more in this direction” (Thapa 1999). With the operational experience of small and large hydropower project in the country, only creating the facility to manufacture turbines and generators is not sufficient, rather improvement of efficiency of such equipment is more improvement for productivity movement.

The synergic effect of emanating from efficient utilization of three factors of production: human ware, software and hardware, result in the improvement of productivity in an organization (Chapagain 1999). Prof. Dinesh Chapagain presented two distinct approaches to enhance productivity as: (i) Innovative and system oriented (ISO) approach and (ii) Practice and human oriented (PHO) approach. Based on the major differences in two approaches highlighted by Suzuki (1993) (in Chapagain 1999), ISO approach is considered suitable for improving productivity of hydraulic turbine due to effect of sediment in river water.

The capacity of hydropower plant depends on available head and discharge of water. What portion of available hydraulic energy is converted in to mechanical energy is normally ignored in Nepal. There are several factors which plays a role in hydraulic efficiency of turbines. Such efficiency may not have much importance in wet season due to abundantly available water to produce required amount of power, but it has great significance to increase production in dry season.

Productivity of Hydraulic turbine can be increased after setting up the strategy on the basis of information obtained by measuring efficiency. The efficiency of the turbine decreases as a result of erosion of the turbine component due to sediment load passing through a turbine over the time. This can be useful for maintenance strategy to increase productivity.

The method and result of efficiency measurement of two types of hydraulic turbines, namely (1) Francis turbine - Jhimruk Hydropower Plant (HPP) and (2) Pelton turbine – Andhikhola HPP are presented in this paper. Sand erosion of turbine component is one of the main reasons of decrease of turbine efficiency once it is brought in to operation. This is one of the main challenges for operation and maintenance of turbines in Nepal due to excessive amount of sediment in Himalayan Rivers. The results of efficiency measurement can be used to develop maintenance and operational strategy of hydropower plants to increase productivity of hydropower plants.

Losses and Efficiency of Turbine

Normally hydraulic turbines are designed for the life even up to 50-100 years without deteriorating it performance. But the life of turbine is reduced and performances of the turbines are also decreases sharply due to effect of sand particles present in the river water. The concentration of sediment in the Himalayan Rivers is quite high and these sediments are composed of quite hard abrasive particles. Despite of the sediment removal facilities at the hydropower projects, complete removal of sediment particles from water is not possible. Sediment settling basins are designed to remove particles larger than 200 μm in Nepal and particles smaller than 200 μm are allowed to pass through the power house. Even these fine particles erode turbine and other under water components making the surface rough. Such sand erosion is a main cause of loss of turbine efficiency in Nepal during operation. 

The drop in hydraulic efficiency in turbine is due to several reasons such as leakage of the water without doing useful work, secondary flow within the flow field or friction loss due to roughness of the surface. Sand erosion of turbine may cause these effects. The nature of drop in efficiency in Francis and Pelton turbines due to sand erosion are different. In Francis turbine, highest drop in efficiency is at part load whereas in Pelton the highest loss is at BEP which is schematically demonstrated in figure 2.

Figure 2: Efficiency curve for normal and eroded turbines (Brekke, 1978)

The loss of efficiency in eroded Pelton turbine is due to combined effect of following reasons:

1.        Loss of water through eroded entrance lips.

2.        Change of flow direction due to erosion at outlet edge of bucket and braking effect by back hitting

Similarly the loss of efficiency in high head Francis turbines are due to three major reasons: These losses are illustrated in figure 3.

1.        Increase in clearance between guide vane and face plates

2.        Friction loss due to roughening of surface of guide vane, top cover, blade surface

3.        Leakage through seal rings

Figure 3 Energy losses in Francis turbine (Brekke, 2000)

Hydraulic efficiency of turbine is measured in absolute and relative terms. Thermodynamic efficiency measurement is one of method measuring absolute efficiency measurement. Hydraulic efficiency measurement involves measurement of head and discharge and this has high uncertainty due to high uncertainty in discharge measurement. The thermodynamic method results from the application of the principle of conservation of energy to a transfer of energy between water and the runner through which it is flowing. Relative efficiency is measured based on the pre-measured efficiency. The pressure difference measured in two points just upstream the runner is the function of efficiency change of the turbine.

Thermodynamic Efficiency measurement

The thermodynamic efficiency measurement method uses the principle of conservation of energy during transfer of energy between water and the runner through which it is flowing. The specific mechanical energy available at the runner is determined by measurement of performance variable (pressure, temperature, velocity and water level) and from the thermodynamic properties of water. To establish the efficiency, the need to measure the discharge is eliminated by using the specific mechanical energy together with the specific hydraulic energy, which will have otherwise large uncertainty. The uncertainty of the thermodynamic efficiency measurement is normally within ±1% and limitation of this type of efficiency measurement is the net head of the plant should be more than 100 m.

Measurements setup

The thermodynamic efficiency measurement is performed according to IEC Publication 41, "Field acceptance tests to determine the hydraulic performance of hydraulic turbines, storage pumps and pump-turbines". The thermodynamic efficiency measurement includes temperature-, pressure-, kWh- and flow rate measurements. The measurement setup for Francis unit is shown in the figure 4. The temperature-, tail water level- and kWh-measurements are logged online in a computer. The pressure-, flow rate- and temperature measurements for the leakage water are logged manually.                                                                  

Figure 4 Measurement setup for the thermodynamic efficiency measurements for Francis unit.

The measurement setup for Pelton unit is shown in the Figure 5. The temperature-, tail water level- and kWh-measurements are logged by a computer. The pressure-and tail water level- are logged manually.

Figure 5 Measurement setup for the thermodynamic efficiency measurements for Pelton unit.

Relative Efficiency measurement

The relative efficiency is used in order to determine change of efficiency over the period of time, which gives the shape of the efficiency curve. The absolute value of the turbine efficiency can not be determined by this method. This is performed by recording the pressure difference between two measurement points.

In this case pressures upstream to runner are measured form two points in radial distance from the centre of the spiral casing of Francis turbine and bifurcation of Pelton turbine

The relative efficiency measurements were carried out in order to investigate the change of the turbine efficiency at different sediment concentrations. This information will be valuable for the turbine operation.

Measurements setup

The relative efficiency measurement includes pressure- and kWh-measurements. The measurement setup is shown in Figure 6. The measurements were logged by a computer with time intervals of 10 minutes. The relative flow measurements were carried out by the Winter-Kennedy method. This flow measurement was calibrated by the thermodynamic measurements.

Figure 6 Measurement setup for the relative efficiency measurements.

Result and Discussion

The thermodynamic efficiency measurements were carried out at Francis turbine unit 3 in Jhimruk Hydro Power Plant in 1st September and 11th November 2003 and the results are shown in Figure 7.

Figure 7 Results from the thermodynamic efficiency measurements

The efficiency loss in this period was 4 % at best efficiency point and 8 % at 25 % load. The losses are mainly coming from the guide vanes, runner and sealing rings as shown in figure 3.

The thermodynamic efficiency measurements were carried out at unit 1 and 3 in Andhi Khola Hydro Power Plant. These were done on 1st and 2nd of June 2004 and the results are shown in Figure 8.

Figure 8 Results from the thermodynamic efficiency measurements

The hydraulic efficiencies were 72.7% and 80.4% at the best efficiency point. Unit 1 has the worn-out runner and unit 3 has newly maintained runner. 

The results shows that the efficiency was low for the worn our turbine due to sand erosion. Hence the strategy can be made on the basis of information gathered by efficiency measurement of hydraulic turbine.

Strategy to Improve Productivity

Based on the information available from efficiency measurement, maintenance strategy can be developed considering the productivity and optimal operating cost. Modification in maintenance schedule, change in turbine design, arrangement of inventory for maintenance and preventive measures by surface coating could be some of the strategy for improving productivity of hydraulic machine in hydropower plant. The multi jets Pelton turbine can be run with one or less jets in dry season to improve productivity.

Major inventory for maintenance for sand erosion is spare runner, guide vane and nozzles. Similarly turbine surface can be coated with hard erosion resistant material to minimize the effect of sediment. If we know the efficiency of the turbine and hence improvement in productivity and the cost of the spare parts inventory and maintenance, it will be possible to perform cost benefit analysis and optimization of power plant operation.

Two other possible strategies are discussed here:

Maintenance Schedule

Maintenance schedule involves the timing of the maintenance and interval between the maintenance. A efficiency measurement done on a turbine installed in Jhimruk hydro power plant shows that the efficiency of the turbine was very low after the monsoon months and the sediment induced erosion and these eroded turbines then run throughout the dry season as well as shown in Figure 9.

Figure 9 Maintenance in May

The loss of generation will be significant because of the low turbine efficiency. Also, already eroded turbines are more vulnerable to further erosion. It is thus interesting to look upon measures to improve the turbine efficiency and overall system condition periodically. A maintenance stop is proposed in September, just after the worst monsoon months of June, July and August. The advantage of having the annual maintenance stop in September is that new refurbished turbines will run throughout the dry season when the water is scarce. Thus, the dry season energy generation will be increased because of the higher turbine efficiency.

Figure 10. Proposed maintenance in September [Jonas 2004]

After the dry season the turbine units will be relatively little erode, because there is only 20 % of the annual sediment load present in this season. Therefore the turbines will start the monsoon generation with relatively high efficiency, and then get erode completely in the monsoon before the annual maintenance stop in September again reestablishes peak conditions. Figure 10 visualizes this thought, where the green areas are excess energy generation.

Additional design

Additional design is to add a system in the turbine to increase efficiency or to decrease the rate of falling efficiency of the turbine due to sand erosion.

 The flow rate from the leakage flow in the upper sealing ring was measured separately. These measurements are given in Figure 11 and shows that the flow rate increased significantly in the period between the two efficiency measurements. This change in flow rate is because of the erosive wearing the upper sealing ring. It is believed that the same wear occurred in the lower sealing ring and therefore the same increase of leakage flow rate.

Figure 11 Leakage flow rate in upper sealing ring

The efficiency loss due to the leakage flow in the upper sealing ring is given in Figure 11. This loss can be divided into two types of losses:

·      Loss due to the flow rate that is entering the sealing ring. This loss can be calculated based on the leakage flow rate.

·      Loss due to the friction between the runner and the upper cover. This loss can be calculated based on the temperature of the leakage flow.

In Figure 12, the two loss components are shown. In the last measurement, the friction and the flow loss have about the same value for all generator outputs. The flow loss from the first and the last efficiency measurements are shown in Figure 13. The difference between these two is about 0.5 % efficiency for all generator outputs. The same trend is expected to happen in the lower sealing ring. Thus, 1% efficiency loss is because of the erosive wear of the sealing rings.

The temperature measurements of the leakage water carried out on 1st September are not good enough for the comparison given here, and they are therefore not presented. However, it is believed that the change of the friction loss is about the same as for the flow loss. If so, 2% efficiency loss is because of the erosive wear of both sealing rings in the period between the two efficiency measurements.

Figure 12 the efficiency loss from the upper sealing ring leakage flow

The loss from the upper sealing ring is shown in Figure 11. Here it is shown that the total loss in the leakage water from the upper sealing ring is 4.1% at 1 MW generator output and 2.1 % at 4 MW output. It is likely to believe that the same loss occur in lower sealing ring. Thus, the total losses in the leakage water from both sealing rings are 8.2% at 1 MW generator output and 4.2 % at 4 MW output.

The erosion of sealing ring and hence loss from leakage from sealing ring can be prevented or reduced by pressurized clean water injection in Labyrinth seal. This is in conceptual stage. Further research in this technology can enhance the productivity of the turbine. The areas where these leakage flows occurs and the direction of flow of pressurized clean water are shown with red color in Figure 13.

Figure 13 Clean water injection into labyrinth seals, concept drawing [Dahlhaug]

Conclusion

The activities to monitor and improve productivity of hydropower plant are not very common in Nepal. The efficiency and productivity of hydropower plant decrease due to erosion of turbine components due to sand in the water. The efficiency of the eroded turbine can be monitored by Thermodynamic and Relative efficiency measurement procedures. The measurement at Jhimruk Hydro Power Plant, Pyuthan at unit 3 (Francis turbine) shows 4 % efficiency drop in best efficiency point in two months period of monsoon season (1st September and 11th November 2003). Similarly efficiency measurements in Andhi Khola Hydro Power Plant, Syangja at unit 1 and 3 (Pelton turbine) show that 8 % efficiency difference in best efficiency point (1st and 2nd of June 2004). Unit 1 had the worn-out runner and unit 3 had newly maintained runner.

The findings of the efficiency measurement can be used to make maintenance strategy of eroded turbine. The rescheduling of maintenance activity can increase productivity substantially. The replacement of repaired turbine is proposed after the monsoon season rather than present practice to repair and replace turbine in dry season. The modification in the turbine design may reduce leakage of water and hence improve productivity. The concept proposed in this work is to introduce clean water in the labyrinth seal.  

Reference:

IEC 41 (1991) Field acceptance tests to determine the hydraulic performance of hydraulic turbines, storage pumps and pump-turbines. Third Edition 1991-11 Published by the International Electro technical Commission.

Bhola Thapa (2004) Doctoral theses at NTNU, Sand Erosion in Hydraulic Machinery.

Pradhan et al. (2004) Pradhan, P.M.S, Dahlhaug, O.G, Joshi, P.N, St?le, H, Sediment and     Efficiency Measurements at Jhimruk Hydropower Plant – Monsoon, Technical report from Hydro Lab

Jonas Jessen Ruud (2004) Master theses at NTNU, Sediment handling problems, Jhimruk Hydroelectric Center Neapl.

Raju Shrestha (2004) Report, Thermodynamic Efficiency measurement in Nepal.

Dinesh Pant et al. (1999) Dinesh Pant, Pushkar Bajracharya, Madhav Pradhan, Current Issues on Productivity