Proceedings World Geothermal Congress 2010
Bali, Indonesia, 25-29 April 2010
Maintenance and Operational Experience Gained By Operating the Aluto Langano
Geothermal Pilot Power Plant
Merga Tassew
P.O. Box 1233, Addis Ababa, Ethiopia, Tel. 251-11-5512552, Fax.251-11-5525710
mergatassew@yahoo.com
Keywords: Geothermal power operation, geothermal output of 8.5 MW. The two high pressure wells LA-3 and
power plant maintenance, well output decline, Turbine LA-6 are feeding the geo fluid to the conventional steam
derating, solid depositions, turbine of the GCCU. The exhaust steam coming out of the
steam turbine is delivered to a heat exchanger which, in
ABSTRACT turn, boils a binary fluid to drive the organic binary turbine.
Ethiopia is well endowed with geothermal resources, and After its commissioning, the power plant operated for a
current exploration is believed to have studied only a relatively short period of time before a number of problems
fraction of the ultimate potential. The potential prospective arose that severely affected the availability and power
areas are distributed along the Ethiopian Rift valley system output level, ultimately leading to the shutdown of the
which runs in a northeast direction along the entire length power plant. These problems occurred underground, within
of Ethiopia. To begin to realize this potential, in 1998, the the surface (fluid handling) equipment, and in the power
Ethiopian Electric Power Corporation (EEPCO) constructed plant itself. After a number of years, EEPCO issued an
the Aluto Langano geothermal pilot power plant (8.5 MW international tender to rehabilitate the power plant and put it
gross) consisting of two Ormat generating units, one an back into operation.
OEC (Ormat Energy Converter) and the other a GCCU
(Geothermal Combined Cycle Unit consisting of a back
pressure steam turbine exhausting to an OEC bottoming
cycle unit) - each designed to have similar power outputs.
The power plant operated satisfactorily for a short period of
time right after commissioning before a number of
problems arose that severely affected its availability and the
power output level.
Repair and maintenance of the Aluto Langano GCCU has
been completed and the unit was put back into normal
operation in July 2007. During the original operational
period of the pilot plant and after its recent rehabilitation,
experience gained in operation and maintenance will
ultimately be used to develop and harness the huge
geothermal energy reserve exists in the rift valley region of
Ethiopia.
Figure 1: Chart of Energy production before the failure
In the coming five years it is planned to expand the Aluto of the pilot plant
Langano geothermal field in two phases.
Subsequently, EEPCO engaged the services of a geothermal
consultant, Geothermal Development Associates, to do the
1. INTRODUCTION AND BACKGROUND analysis of the problems and carry out the necessary repairs
The Aluto Langano geothermal field is located in the on the units to put it back into operation. Work carried out
Ethiopian main Rift valley, Lakes District, some 200 km as part of the power plant rehabilitation includes:
south of the capital Addis Ababa, which is one of the
principal sectors of the East African Rift system. In this ¾ Reconditioning of the GCCU steam turbine has
field detailed geological, geochemical and geophysical been completed and the unit is put back in
surveys were carried out during the late 1970's and early of operation. This included re-engineering of the
1980s. The result of the studies indicated that a high steam turbine to enable it operates at lower steam
temperature underground fluid exists with an evidence of pressures, upgrading the power plant automation
long time residence occupied by high temperature rocks. system (PLC) and improving the lubrication
Six deep exploratory wells were drilled at Aluto out of system of the steam turbine.
which four wells were successful to supply a geothermal
fluid to the power plant. ¾ Repair and maintenance (servicing and/or
overhaul) of the wellhead valves has been
The ALuto Langano Geothermal pilot plant was built, completed.
commissioned and put into operation in 1998 to
demonstrate the potential of geothermal energy in Ethiopia ¾ Flow tests, go-devil runs, and down-hole pressure
under a turnkey contract with Ormat International. The and temperature measurements to confirm the
power plant is composed of two generating units; the causes of the output declines were undertaken.
geothermal Combined Cycle Unit (GCCU), and the Ormat The result of the tests indicated that the two high
Energy Converter (OEC). Combined, they had a design pressure wells are in good condition and are
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currently supplying the geothermal fluid to the 5. Worn-out steam turbine shaft on the drive-end
GCCU unit. side
¾ Installing data loggers to enable records of 6. Tube leaks in the OEC heat exchangers
wellhead pressure and temperature under static
and dynamic condition. This has helped EEPCO 7. Frequent failure of the cooling fan shafts
personnel to understand the way the wells
respond under dynamic flow condition. 2.1 Solid Depositions in the Wellbore and Surface
Equipment
¾ Analysis and investigation of corroded OEC heat
Liquid dominated geothermal reservoirs are hydrothermal
exchanger tubes to determine the root cause of the
reservoirs that contain circulating liquids which transport
tube failures. The result of the tests indicated
the thermal energy of the hot rock with dissolved minerals
brine carry-over, with traces of chloride, was the
near to the surface by natural fluid circulation. The fluid
root causes of the tube failure. Based on the
comes from such reservoirs may form scale that deposits in
results obtained it was decided to replace all the
the well bore and fluid carrying surface equipment,
heat exchanger tubes (over 2,000 tubes) with
particularly after boiling and brine separation. Therefore the
higher quality duplex stainless steel. Modification
brine scaling potential, corrosiveness, non-condensable gas
work is currently underway to install a duplex
(NCG) and hydrogen sulfide (H2S) content are the four
filtering system to remove any debris and solid
major chemical characteristics that have to be studied and
carryover to the heat exchangers in order to
addressed in the geothermal power plant design.
maintain fluid quality.
The schematic diagram showing common locations of solid
2. IDENTIFIED PROBLEMS RELATED TO THE depositions (Teso Merga report, 2001, UNU- GTP) shows
STEAM FIELD AND THE POWER PLANT in Figure 2.
Some of the problems that were identified that relate to the
steam field and the power plant are: The geothermal reservoir that feeds the well bore contains a
variety of dissolved minerals and gases. Tests and
1. Solid deposition in the well bore and surface downhole logs conducted on the Aluto Langano geothermal
equipment field wells indicated that well LA-4 exhibited calcite scale
depositions. This well was drilled in 1983 to a depth of
2. Well output decline 2062 m in the outflow zone of Wonji fault. The logging
result of May 2006 indicated that a go-devil of 3 in
3. Difficulty of operating the two high enthalpy, diameter stopped at a depth of 1085 m and a 4 ½ in go-devil
wells LA-3 and LA-6 in parallel stopped at a depth of 1050 m while logging fragments of
white soft pieces, resembling calcium carbonate, were
4. Journal bearing failure of the steam turbine recovered from both go-devils.
Figure 2: Flow diagram showing common locations of solid depositions
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The blockage observed in well LA-4 has also significantly available locally who could do the de-staging. De-staging
reduced the flow from this well. Subsequently, a well work of the GCCU unit was performed successfully and the unit
over was recommended to remove the obstruction and bring was put back into normal operation in June 2007.
the well back into normal flow. It is expected that, within a
six month time period, the well work over will be
completed and the well will come back into normal
operation.
3. RECOMMISSIONING OF THE GEOTHERMAL
COMBINED CYCLE UNIT (GCCU)
During early operation following commissioning the
GCCU, it was observed that when well LA-6 was opened
and the flow from both LA-6 and LA-3 wells was joined,
the LA-3 wellhead pressure increased while the flow of
LA-3 dropped sharply. This indicates that when the two
wells operate together, LA-3 was unable to flow sufficient
steam at higher system pressure to maintain a stable flow to Figure 3: Plot of GCCU unit Energy Production
the unit. This issue was discussed in detail with the
Consultant and it was believed that the production pressure Note: The unit was out of operation during the period of
of the wells was not sufficient to spin the multi-stage steam April to August due to a reduction gear box failure (RG)
turbine.
During the course of its operation after the rehabilitation,
Destaging of the steam turbine, to enable operation of the routine vibration measurements revealed a gradual increase
GCCU at a lower inlet pressure, was suggested by PB in the vibration levels nearly 9 months after the start of
Power in its final report prepared in 2000. According to PB operation. It was later discovered that the root cause of the
Power, by de-staging the GCCU unit and removing turbine vibration was related to loosening of the inertial masses on
stages, the design steam inlet pressure would be lowered. the pinion gear of the speed reduction gearbox. From the
This would match the stable production pressure of the month of March to August 2008 the unit was out of
wells to the turbine’s capacity. operation in order to rectify the problem with the reduction
gear box (RG). The purpose of the RG is to couple and
The predicted performance of the GCCU unit was as reduce speed of the high speed steam turbine to match that
follows (from the original PB POWER design): of the synchronous generator.
Extensive investigation on the cause of the gearbox
Table 1: problem was carried out. Information was also obtained on
experience with gearboxes at similar geothermal power
Steam Pressure inlet (bar) Steam Flow Power
plants (such as the Puna, Hawaii plant). A repair strategy
to the demister Rate (t/h) output
was developed and the gearbox pinion was sent to the
12.0 29.0 4.0 Ethiopian Airlines workshop for reconditioning. Ethiopian
Airlines reconditioned the RG and carried out dynamic
Design data 10.0 24.05 3.25 balancing to acceptable industry standards. The RG was
before de- subsequently taken to the power plant, installed and the unit
staging 7.5 18.1 2.3 restarted and put back into operation in August 2008.
5.0 12.15 1.35 4. TUBE LEAK OF THE HEAT EXCHANGER OF
THE OEC UNIT
7.0 26.8 2.9 The Ormat Enegy Converter (OEC) is an organic turbine
that uses N-Pentane as a working fluid. The two medium-
With first stage 6.0 23.0 2.5
enthalpy wells, LA-4 and LA-8, are the primary energy
removed
source to boil the organic fluid in the tube and shell heat
5.0 19.2 2.1
exchangers. The OEC unit is comprised of a one pass
Removal of the 5.0 29.0 2.6 vaporizer and four stages of pre-heaters; the heat exchanger
first two stages tubes were made of 316L and duplex stainless steel except
for the first and second passes of the preheater which were
Actual 5.14 ~29.0 3.05 made of carbon steel.
performance of
the steam Approximately one year following the original
turbine after commissioning of the OEC unit, in December 1999, a
removal of the pentane leak was observed through the non condensable gas
first two stages (NCG) vent and caused the stoppage of the OEC unit. The
results of investigations carried out subsequently indicated
that pentane had leaked into the geothermal fluid stream
due to vaporizer tube failure.
Based on the above and the actual flow tests conducted by
the rehabilitation consultant (Geothermal Development During the recent rehabilitation of the Aluto plant, in order
Associates - GDA), it was recommended to remove the first to investigate the root causes of the tube failures, samples
two stages of the rotor. The modification of the turbine of the failed tubes and other relevant materials were sent to
rotor was implemented by shipping it to Conhagen a metallurgical testing company in the USA to carry out a
workshop in Texas, USA, as there were no workshops failure analysis. The company, Jonas Inc., conducted
metallurgical analysis of the tube material and compatibility
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of the chemistry of the geothermal fluid with that of the logic controller (PLC), Supervisory Control
tube materials. Acquire Data Acquisition (SCADA) system and
Human Machine Interfacing (HMI) of the power
plant.
Table 2: Results of tube inspection
6. CONCLUSION
Tube Number of Pit Depths Pit Diameter
pits per foot (mm) (mm) As described above, Ethiopia is endowed with considerable
on ID geothermal resource potential. But, up to now, only a tiny
fraction of this enormous resource has been used. The
1 12 0.2 to 0.9 0.2 to 1 success with the re-commissioning of the Aluto plant will
now pave the way for future expansion of power generation
2 23 0.2 to 0.9 0.2 to 1 at Aluto and at other geothermal prospects.
1 8 0.2 to 0.9 0.2 to 1 Bringing the Aluto Langano geothermal power plant back
into operation has been a high priority for EEPCO in order
to maximize the return on the capital invested in the project,
provide power to the national grid system, and
The result of Jonas’ investigation indicated that localized demonstrates that geothermal power from Aluto site as well
pitting inside the tubes was due to high concentrations of as from other fields can reliably meet the future electricity
corrosive impurities carried into the heat exchanger by demand in Ethiopia.
brine carry-over during the initial operation of the plant.
Jonas concluded that the heat exchanger material was not Expansion of the Aluto Langano geothermal site to larger-
compatible with the chemistry of the Aluto wells. It further scale power generation is in the pipe line and the experience
indicated that if the heat exchangers were operated under gained in operation and maintenance of the existing pilot
the same conditions, further damage would occur to the plant will ultimately be used for larger scale development.
heat exchanger tubes. The recommendation was to replace
the existing heat exchanger tubes with a more resistant Close monitoring of the resource (reservoir performance,
grade 2205 Duplex Stainless Steel. fluid chemistry and steam quality) and performance of the
power plant will ensure the reliable operation of the pilot
5. EXPERIENCE GAINED plant as well as provide useful operational data that can be
used for future power plants in the country.
1. Experience gained in refurbishment of wellhead
master valves and bleed lines is very important.
This included experience with the assembly and ACKNOWLEDGEMENTS
disassembly of master valves, hydrostatic testing, My deep thanks to my company for granting me permission
acquisition of spare parts, use of specialized tools to present this paper on World Geothermal Congress
and equipment, and local manufacture of some (WGC) 2010 conference. I also thank the Aluto Power
valve parts. Plant Manager for providing me energy data to include into
this paper.
2. The knowledge acquired in steam field resource
management and wellhead equipment operation REFERENCES
would ultimately be used for Aluto expansion and
Consultant Report (Geothermal Development Associates,
other geothermal prospects.
GDA) submitted to EEPCO, as per the Contract
3. Experience was gained with the operation of a Agreement (RHGP 1/97, June 2006) - Consultancy
new, digital, Kuster K-10 tool for down hole services and Repair of the Alutoto Langano GPP.
pressure and temperature logging. Jonas Inc. Report: Analysis of Heat Exchanger Tube pitting
Aluto OEC -Unit, unpublished.
4. Knowledge was acquired in the use of wellhead
data loggers and interpretation of the logged data. PB POWER, Unpublished Report submitted to EEPCO:
Aluto Langano Geothermal Pilot plant Project, June
5. Experiences was gained with the refurbishment of 200.
cooling tower fans, specifically, in vertical shaft
alignment, and correct tensioning of the torque Geothermal Development Associates: Final Project TOR
transmission belt, which will be used to resolve Reports, November 16, 2007 and April 1, 2008.
problems that may arise with the cooling system. Teso Merga, 2001: Effect of Solid Depositions and
Methods of control. UNU - GTP, Iceland, Report 9.
6. Knowledge was acquired in updating the power
plant automation system. This familiarized plant
personnel with the operation of the programmable
