Commissioning a Himalayan Medium-Head Hydropower Project: The Chamera lll Experience

Chamera III, the three-unit Francis turbine project is located in Himachal Pradesh in the Himalayas

Chamera III is a recently commissioned hydroelectric plant located in Himachal Pradesh, India. It was quite challenging to conceptualize, design, manufacture, install and commission within schedule.

By CK Jain, Arnaud Legrand and Prashant Kaul

Chamera III is located in the Chamba district of Himachal Pradesh in India. It is a run-of-river plant on the left bank of Ravi river. Main components include a concrete gravity dam, intake, de-silting chamber, long head race tunnel, silt flushing tunnel, powerhouse and pressure shaft.

The geology of the Himalayan mountains presented major challenges during the design and execution of the project with respect to the remote access, as well as land slides, soil erosion, rock falls, floods and heavy snow.

Before initiating design of the E&M equipment other major constraints were the package size due to transportation limitations (ie 3 m width and 40 tonnes), access issues due to the remote and underground location of the power house, specific assembly and dismantling requirements of the contract – dismantling of the runner, wicket gates, bottom rings etc. from below through an access gallery and the runner must be dismountable without disturbing the wicket gates. Furthermore, stable turbine governing operation against a given water conductor system ie the long head race tunnel with downstream pressure shaft, as well as the mechanical design of parts to protect against river water containing high volumes of silt abrasive particles (5000 ppm during the monsoon season) and a maintenance friendly design of the assemblies with minimal down time was required given the remoteness of power house. An interchangeable runner, wicket gates, bearing assembly shaft etc. and easy replacement of facing plates, labyrinth rings, shaft seal assembly etc. were required.

Pressure pulsation and cavitation are also to be kept within the stipulated values.

During development of the hydraulic model of the turbine, special attention was given to offer a highly efficient turbine with cavitation-free performance throughout the regime of operation specified in the contract with regards to the hydraulic parameters of project (head and discharge variations). The proposed hydraulic design of the turbine was model tested at the OEM’s accredited hydraulic laboratory.

Material selection and mechanical design were carefully performed to ensure satisfactory performance of the turbine components under adverse operating conditions and to ensure a longer life. Feedback and previous experience from design to commissioning of similar machines was amalgamated at every stage while conceiving the Chamera machines.

Accurate manufacturing of components per design standards, assembly and hydraulic testing of critical assemblies at the workshop ensured smooth and trouble-free installation of the unit on site. All performance parameters predicted during the design stage and guaranteed to the client were proven during performance tests conducted on site, thus validating the design concepts and philosophy.

Hydraulic design of the turbine

Transient analysis
Based on hydraulic data, layout of water conductor system, inertia of rotating system and specific closing law of wicket gates, a detailed transient analysis was carried out to compute pressure rise and speed rise for each possible load acceptance and load throw off condition.

Model test
The hydraulic design of the turbine water passage for the project was developed using CFD analysis. Before starting the design of the prototype turbine, a detailed model test was conducted to study and analyse the behaviour of the model with respect to the hydraulic phenomenon for the complete operating zone, Model tests included efficiency and output, cavitation, axial hydraulic thrust and runaway speed, among others.

Mechanical design of the turbine

The major turbine assemblies of the Chamera III project were meticulously designed according to the specific technical requirements of the project while leveraging design experience from similar machines.

For example, the stay ring was designed for transient load conditions and loads during normal operating conditions, using analytical tools. The results were then validated by Finite Element (FE) analysis. The stay vane profile was designed and checked for resonance phenomenon due to Karman vortices condition. It was accurately machined per close tolerances as defined in the design drawings.

The stay ring was subsequently dispatched in halves to suit transportation limitations outlined above. The two halves were welded together on site. The surfaces that come in contact with the head cover and bottom were ground smooth for clamping them together.

The spiral casing was then welded to the stay ring on site and the hydraulic pressure was tested to ensure leak proof welding. The spiral casing was set in concrete in pressurised conditions that allowed optimizing the transfer of the load to the concrete due to volumetric changes during operation.

The distributor assembly was carefully designed considering the fact that the flowing water contains abrasive silt particles. The best material for wicket gates, facing plates, bushes and seals was chosen based on experience and proven track record. Bronze bushes with a high pressure grease lubrication system were provided to ensure flushing of silt paste that may accumulate between the wicket gate trunnion and the bushes in case of a seal defect.

Provisions were made to facilitate easy assembly and dismantling of critical parts like facing plates, wearing rings, wicket gates and bushes. The bottom ring and discharge ring were designed to facilitate removal of the runner without disturbing the wicket gates.

Size of balance pipes carefully designed to control axial thrust even with worn out labyrinth rings.

Head cover and bottom ring were designed to offer optimized rotation and to eliminate probability of wicket gate jamming during normal and transient operating condition.

Rotating parts (runner, turbine shaft)
The most critical part of the turbine runner was hydraulically and mechanically designed to give optimum performance at the guaranteed operating zone. FE analysis was carried out to validate stress and displacement levels at normal and transient operating conditions. Natural frequencies were out of excitation frequencies generated during operation.

Bearing and shaft seal
A shell type guide bearing was used to ensure optimum bearing performance with respect to temperature rise and vibration of the machine. The Alstom self-cooled bearing does not require any heat exchanger for oil cooling. The rotating oil bearing tank is always in water. This design feature also eliminates maintenance related to the heat exchanger.

An Axial type shaft seal is located above the bearing which facilitates easy dismantling of the shaft seal assembly. The shaft seal is lubricated using clean water (filtered water taken from the cooling water system) with higher pressure than the water on the other side of the seal. The seal material is proven and gives maximum wearing resistance to unfiltered water containing abrasive material.

Inlet valve
A butterfly-type inlet valve was provided as security device for the turbine (in case the wicket gate fails to close) It is designed to close against maximum flow with counter weights only (in case of failure of the oil pressure system).

The valve plug and body were carefully designed to allow minimum leakage across service and maintenance seals of plug (minimum deformation of body and rotor at operating pressure).

Bronze valve trunnion bushes were provided with an HP grease lubrication system to prevent entry of silt particles in to bush area.

Manufacturing

All incoming material (plates, castings, forgings) was purchased according to Alstom’s technical specifications and quality plan (prepared per guideline of code and the client’s requirement).

Fabrication, stress relieving, machining and assembly of critical turbine components were carefully planned and executed to avoid site installation problems.

Major highlights:
Complete draft tube elbow and liner were proof assembled and match marked at workshop before dispatch.

Machined stay vanes were accurately positioned and welded on rings. Complete assembly was stress relieved and machined accurately. Stay ring halves were assembled and matched marked. Stay ring and head cover / bottom ring coupling holes were drilled through segmented template to avoid mismatch at site.

Spiral casing was proof assembled in segments to avoid mismatch issues at site.

Distributor components were manufactured with high accuracy. Distributor with operating mechanism was erected at shop and operated to ensure smooth movement.

Individual runner blades were machined by CNC machine before welding to band and crown ensuring a high degree of accuracy in water passage of runner.

Coupling bolt holes for runner and shaft were made by CNC operation using machining templates.

Complete runner assembly was statically balanced before dispatch

Site installation and testing

Erection and installation of three turbine units was carried out on site and on schedule. Site protocols were prepared for major dimensions and functional requirements of assembly.

Successful dry commissioning of the units proved the readiness for wet commissioning and commercial operation.

Wet commissioning of all three units went quite smoothly and took place within a span of one month. The performance of all the machines was satisfactory and met client expectations. During the commissioning process the guaranteed technical parameters were checked one by one on site to validate the design and ensure the installation met client requirements. After acceptance of all guaranteed parameters, the units were handed over to the client for commercial operation. Some of the important commissioning tests carried out on site at Chamera III are detailed below:

Head and discharge measurement
HWL: 1382 m (1380 MDDL)
TWL: 1171.2 m (when 3 units running at full load of 77 MW)
1168.4 m (as per contract)
Static head: 211.3 mWC
Spiral pressure: 21.5 Bar (with 1 unit)
19.5 Bar (with 2 units)
19.4 Bar (with 3 units)
GV opening: 94% for 78.5 MW (turbine output) 3 units in operation
Discharge was measured ultrasonically
Measured value: 41.05 m3/ sec
Theoretical value: 41.76 m3/ sec (rated flow)
Pressure taps and flow measurement instrument mounted in inlet valve gallery

Load throw off test
The load throw off test was carried out when all three turbines were running at full load. The pressure rise and speed rise were found to be well within an acceptable limit:

Guaranteed pressure rise: 257 mWC
Allowable speed rise: 483 rpm

Bearing run tests
The temperature of the turbine bearing and shaft seal bearing were stable and within an acceptable range. At full load:

Turbine bearing temperature: 41 degrees
Shaft seal temperature: 10 degrees

Vibration tests
There are no specific vibration values defined in the contract, however, usual acceptable values per ISO 10816 – 5 are 2.5 mm/sec.

Shaft displacement measurement
Maximum admissible values for shaft displacement per ISO 7915 – 5 for 333.33 RPM are 0.135 mm (radial displacement – S3 Smax) and 0.240 mm (diametrical displacement – S3 Sppmax) at normal operating zone of 75% to 110% of load

Noise tests
Per the contract, noise levels should not be higher than 85 dB outside of the pit on the turbine floor.

Silt concentration
Silt concentration measurements are underway and Alstom is analysing the results to establish final values. Silt concentrations must be monitored closely and should issues arise from elevated silt concentration then available solutions that exist will need to be evaluated.

Today, per present practice, the client is stopping the turbine at silt levels of 2000 PPM.

Shaft seal pressure measurement
The inlet cooling water pressure supplied to the shaft seal shall be sufficiently higher than the shaft seal back pressure to avoid ingress of unfiltered water in to seal area

Inlet valve performance
The Inlet valve opening and closing operation was quite smooth during dry stroking as well as during wet commissioning. There was no appreciable leakage across the service seal of the valve. The differential pressure across the valve was well balanced (within 1 Bar).

Field efficiency tests
Performance tests were carried out on unit three as per “Thermodynamic method” and described in IEC 60041 after approximately 2800 hours of operation. Constraints during efficiency tests:

— Net head of 200 m was never achieved during testing
— Grid frequency was not stable (varying from 49.96 Hz to50.35 Hz)
— Turbine critical parts slightly eroded (increase in wicket gate clearances and labyrinth ring clearances.
— Weighted efficiency measured on site met the guaranteed values according to IEC codes.

Power output tests
As the net head on site did not match the guaranteed net head for the values, the transposition rules were applied per IEC to calculate the capacity of the unit:

— Output measurement at 90% opening of wicket gates when transposed to rated head of 200 mWC corresponds to 83.75 MW which exceeds guaranteed value of 78.6 MW.
— Output measurement at full opening of wicket gates when transposed to rated head of 200 mWC corresponds to 91.71 MW which exceeds guaranteed value of 86.4 MW.
— Output measurement at full opening of wicket gates when transposed to minimum net head of 188.6 mWC corresponds to 83.98 MW which exceeds the guaranteed value of 78.6 MW.

Conclusion

The journey of the Chamera III project from concept to commissioning was challenging but achieved very satisfactory results at the end. All stipulated goals in form of guaranteed parameters were fully met on site. All three machines are continuously running per the load requirement of the grid.

The head water level of 1397.0 m in the reservoir is yet to be achieved due to civil work constraints; however performance of the machine will remain excellent under maximum net head conditions.

CK Jain and Arnaud Legrand are at Alstom, from their French and Indian offices, respectively. Prashant Kaul is the general manager of the Chamera III project at NHPC.

Project parameters
Client:
NHPC
Type:
Vertical Francis
Output:
3 x 77 MW (with 10% overload)
HWL:
1398 m (max) 1397 m (Full Reservoir lvl) 1380 m (Medium Draw lvl)
TWL:
1168.8 m (flood level)
1168.4 m (max) 1167.58 m (min)
Net head:
227 m (max)
200 m (rated) 188.6 m (min)
Discharge:
46.31 m3/sec (max)
41.76 m3/sec (rated)
RPM:
333.33
Speed rise:
20% (max)
Pressure rise:
45% (max)
Elevation of c/l of turbine:
1158.0 m

Turbine parameters
Runner discharge diameter:
2200 mm
Distributor diameter:
3083 mm
No of wicket gates:
20
Type of wicket gate bushes:
Grease lubricated
Spiral inlet diameter:
2318 mm
Bearing diameter:
650 mm
Type of bearing:
Shell type
Runner dismantling:
Bottom
Runner shaft coupling:

By torque transmission pins
Inlet valve diameter:
2500 mm
Type of inlet valve:
Butterfly valve
Type of trunnion bushes:
Grease lubricated
Nominal governing oil pressure:
61 Bar

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