NOTICE CONCERNING COPYRIGHT RESTRICTIONS This document may contain copyrighted materials. These materials have been made available for use in research, teaching, and private study, but may not be used for any commercial purpose. Users may not otherwise copy, reproduce, retransmit, distribute, publish, commercially exploit or otherwise transfer any material. The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material. Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specific conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement. This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law.
Transcript
NOTICE CONCERNING COPYRIGHT RESTRICTIONS
This document may contain copyrighted materials. These materials have been made available for use in research, teaching, and private study, but may not be used for any commercial purpose. Users may not otherwise copy, reproduce, retransmit, distribute, publish, commercially exploit or otherwise transfer any material.
The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material.
Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specific conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement.
This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law.
Since the first geothermal turbine was installed at Lar-derello in Italy, so many geothermal power plants have been developed. Due to corrosive gas and impurities and high wet-ness in geothermal steam, geothennal turbines have suffered damages such as high stress part of turbine rotor including moving blades, eroded part of turbine sealing part and last stage long blade and so on. In these a hundred years, so many technologies on material selection, steam cleaning system, low stress designs to prevent the damages have been developed. And now a day, reliability of geothermal turbine has been significantly improved and the availability has reached to the level of 90% which is very close to thermal units. However, the maintenance works are more important for geothermal turbine to maintain the reliable operation for a long time and to prevent the significant trouble in advance. In this paper, the maintenances for higher reliability of geothermal turbine are introduced, focusing on the rehabilitation of MakBan geothermal power plant in Philippine.
Introduction MakBan geothermal power plant in Philippine has continu-
ously operated since commissioned into service in 1978 for Unit 1 and 2, in 1980 for Unit 3 and 4, and in 1984 for Unit 5 and 6, respectively. The capacity of each unit is 55MW at genera-tor terminal. MakBan is one of the largest geothermal power plants in the world. This power station has greatly contributed to the development of economy in Philippine for many years and also stable operation will be expected in future.
As the deterioration of the power station was remarkably ob-served at many places after the long years operation went on, the rehabilitation work for Unit 1 through 4 was started in 2003.
It was planned in the rehabilitation work, not only to renew the deteriorated parts but also to improve the power output from 55MW to 63.2MW. The comparison between before and after specification is shown in Table 1.
The rehabilitation work was carried out in two stages and was completed as planned.
ÿ lst stage: October 15th 2003 to June 16th 2004 ÿ 2nd stage: May 161h 2005 to November 30th 2005
The rehabilitation work for 1st stage and 2nd stage are as follows.
ÿ Steam Supply System ÿ Steam Turbine and Auxiliariesÿ Gas Extraction Systemÿ Cooling Waler Systemÿ Cooling Towerÿ Generator and Excitation System ÿ Transformerÿ Motor Control Centerÿ DC Systemÿ UPS Systemÿ Instrument and Controlÿ Instrument Air System
Maintenance for Reliable Geothermal Turbine
Hisanori Matsuda
Mitsubishi Heavy Industries Nagasaki, Japan
Table 1. Specification of Plant for MakBan.
Before Rehabilitation After RehabilitationPlant Cycle Double Flash, Condensing Single Flash, CondensingOutput 55 MW 63.2 MWSteam Condition• Pressure• Temperature
Geothermal steam contains impurities such as silica or chloride and non-condensable gas in comparison with thermal power generation. Impurities or non-condensable gas cause scaling or corrosion related problem.
In addition, geothermal steam is usually produced at satu-rated temperature conditions. Therefore, wetness in turbine, especially at last stage blade, is higher compared with thermal turbine unit. And then, the leading edge at the last stage blade is eroded. The deteriorated condition of MakBan geothermal turbine after the long operation and the repairing works for the deteriorated parts, which were carried out to rehabilitate the turbine and maintain the reliable operation in future, are introduced in this section. The rehabilitated parts are shown in Figure I.
Blade
First Through Third Stage Moving Blades ISB design (Figure 2) was applied to the moving blades of
first through third stages. Grooved blades were used in original design. As there is no tenon riveting, the durability against SCC (Stress Corrosion Cracking) or corrosion fatigue inherent to geothem1a1 turbines is remarkable increased, thus improving the turbine reliability.
In addition, 17-4 PH (17Cr-4Nickel Precipitation Harden-ing stainless steel) has been adopted for the first through third stage moving blades, which is in the most corrosive zone and is susceptible to scale deposits, although 12% Cr stainless steel is usually used for the blades. The 17-4 PH steel has higher
corrosion fatigue strength than 12% Cr stainless steel as shown in Figure 3.
Fourth and Fifth Stage Moving Blades
Steam condition at turbine inlet is almost saturated and wetness in turbine, especially at last stage blade, is higher than
thermal turbine case. Therefore, the leading edge at fourth and fifth stage last blade is fitted with stellite shield strip by silver soldering to prevent the erosion. However, after the long operation and in the wet condition, the stellite shield strip was eroded as shown in Figure 4. In MakBan case, the stellite strip at all fifth stage blade was replaced. If the erosion has propagated to blade base material beneath the strip, the blade must be replaced totally with new one. Therefore, it is important that the strip should be checked during periodic
Table 2. Erosion Judgment Criteria.
Rank Erosion Status Countermeasure
AEarlier stage of erosion-initiated with(1) Pear-skin like surface on stellite shield(2) Initial attack on blade tip silver soldering
Not required.
B
Intermediate stage of erosion - with(1) Stellite erosion reaching 30-50 % deep
into stellite shield.(2) Blade tip erosion reaching the interface
between silver solder/blade base material
Not required.
C
Alert-1 stage of erosion - with(1) Stellite erosion reaching 50-80% deep
into stellite shield.(2) Blade tip erosion at leading edge almost
through stellite thickness.
Usable as it is, but recommended to replace stellite shield in 1-2 years.
D
Alert-2 stage of erosion - with(1) Stellite erosion reaching the blade base
material.(2) Erosion affected zone spreaded more
than in C.(3) Stellite replacement required before next
O/H.
Recommended to replace stellite shield immediately.
E
Terminal stage of erosion - withserious erosion at blade tip, back (convex side), and stellite shield bottom blade base material (Refer to “Figure 5”.)
Recommended to replace blade im-mediately.
Figure 1. Rehabilitation Parts.
Figure 2. ISB for 1st – 3rd Stages.
Ti-6A l -4V
17-4P H
12Cr
Corrosion F atigue S trength(in Ge othe rmal S team)
10 5 __ 10 6 10 7
10 8
Cyc le
Figure 3. Comparison of Each Blade Material.
Matsuda
757
inspection in accordance with the judgment criteria as shown in Table 2 and Figure 5.
Blade GrooveAll blades at first through third stage and one grouped
blade at fourth and fifth stage were removed to inspect the blade groove.
Stress concentration occurs at the area where section area changes. In view of stress concentration, the special care is taken for the blade groove. For blade groove, large corner “R (Radius)” and “Taper shape” are applied to minimize stress concentration. However, the static (centrifugal) stress acted on the corner “R” parts at the blade groove is relatively high. Therefore, after the long operation, stress corrosion cracking (SCC) may be occurred at the blade groove by a combination of high tensile stress, the presence of chlorine Ion or other harmful impurities in the steam and the material.
As the result of magnetic particle testing (MT) and Replica after removing the moving blades, the blade groove of each stage were founded the crack indications as shown in Figure 6. The indications on the second and third stage were removed within the allowable thickness. However, in the first stage blade groove case, the crack indications were not removed within the allowable thickness. Consequently, the blade groove was entirely removed and re-formed by welding as shown in Figure 7.
Rotor is the most important part and special attention to be made to prevent SCC. CrMo V material is applied to HP rotor in thermal turbine unit. Low alloy steel such as CrMo V is not sus-ceptible to SCC, however we have
minimized the concentration of Sulfur and Phosphorous to minimize susceptibility to SCC and improve toughness. The material code of low sulfur CrMo V rotor is 10325MGB and
Figure 4. Eroded blade at last stage blade.
Trailing edge of strip
Ç Mean 1/2 of blade
thickness
Section A-A
Stellite shield strip
Residual width of strip
Total width of strip
É Root : 3.2 mm within
the stellite strip trailing edge
Å Tip : 6.4 mm beyond
the stellite strip trailing edge
Trailing edge of strip
Trailing edge of strip
Figure 5. Blade Replacement Criteria against Erosion.
Figure 6. Crack indication at blade groove.
Figure 7. Welding Repair Procedure for Blade Groove.
Matsuda
758
this material has been applied to our geothermal turbine as a standard rotor material. The concentration of Sulfur and Phosphorous is 0.005%(Max.) and 0.015%(Max.) respectively. In MakBan case, the low sulfur CrMoV rotor was applied.
For highly corrosivc geothermal steam compared with usual geothermal steam, we have developed 12Cr rotor mate-rial. It was verified in the test facility with actual geothermal that susceptibility to SCC was lower and corrosion rate was also lower as shown in Table 3. In addition, the tension test was carried out to verify the strength and the weldability be-tween CrMoV rotor material and 12Cr welding material. The fracture was happened at not the 12Cr welding zone and heat affected zone but also CrMo V material as shown in Figure 8. Therefore, the 12 Cr welding will enable to improve the reli-able operation.
Gland Sealing Part Turbine gland sealing part is exposed to a mixture of
geothermal steam and air (oxygen), i.e. an extremely severe corrosive atmosphere, and could suffer heavy corrosion and erosion. The step parts machined on the rotor surface, to mate with labyrinth seal fins, are completely destroyed beyond the original form.
This could lead to an endless chain reaction of an excess in gland seal clearance, increased air flow leaking-in, further acceleration in corrosion rate, adverse effect on performance (steam consumption) due to increase in leakage steam flow, unless provided with any preventive measures.
The countermeasure and rehabilitation for the gland seal-ing part were introduced in this section, but not applied in MakBan case.
It is recommended to apply anticorrosive coating on the fore and aft gland parts of the geothermal steam turbine ro-tors as a preventive measure every time at periodic inspection, if the step parts machined on the rotor surface have suffered corrosion.
If they have been corroded and eroded heavily, it is recom-mended to apply buildup-welding and machine to the original form, and apply protective coating as above. Inconel alloy steel is used as welding material against the corrosion. The before and after repairing work are shown in Figures 9 & 10.
Performance Improvement Optimized Design
The wellhead in MakBan has energy enough to increase the steam entering into the turbine. However, the turbine nozzle and moving blade were not suitable for the steam condition. Because the orginal plant cycle was planned as double flash
Susceptibilityagainst SCC*
0.0651.0Corrosion Rate*
10325GSR10325MGBProperty
*Corrosion rates are relative values. And the data isbased on the actual geothermal steam circum stance.
Table 3. Comparison of Rotor Material.
Figure 8. Tension Test.
Welding Material HAZ Base Material
Figure 9. Gland Sealing Part (Before Repairing).
Figure 10. Gland Sealing Part (After Repairing).
Matsuda
759
cycle at first. However, the system has not been used for a long time.
In this rehabilitation, the plant cycle has been changed double flash cycle into single flash cycle and main steam pres-sure has been changed 6.68 kg/cm2 abs. into 8.0 kg/cm2 abs. to improve the generator power output as shown in Table 1. The turbine nozzle and moving blade were also optimized m accordance with new steam condition so that the generator capacity was improved.
Bow Nozzle The bow configuration (Figure 11) designed by fully three-
dimensional fluid dynamic method reduces the secondary flow loss which is one of the major blade losses in blade path caused by the generation of a vortex in the boundary layer. The bow blade design was used on all stages stationary blade, thereby increasing turbine efficiency.
Multi Seal Fin & Gland Seal Fin Multi seal fins (Figure 12) have been adopted 1st and 2nd
stage to improve the leakage efficiency by the optimized step-up between nozzle and blade, and installing 5 radial seal fins. In addition, gland seal fins have been replaced due to the deterioration.
Reference Y. Hibara, M. Tahara, 1986, “How to Maintain Geothennal Steam Tur-
bines.” ASME/IEEE Power Generation Conference.
S. Saito, T. Suzuki, J. Ishiguro and T. Suzuki, 1998, “Development of Large Capacity Single-Cylinder Geothermal Turbine.” GRC Trans-actions, Vo1.22.
Y. Nakagawa and S. Saito, “Geothermal Power Plants in Japan Adopting Recent Technologies.” World Geothermal Congress 2000.
Y. Uryu, 2004, “Technology for Reliable Geothermal Turbine.” GRC Transactions, Vol.28.
