Building a head of steam

Image credit: 123rf

Why geothermal energy is vital to Indonesia and how a challenging plant overhaul was carried out.

Routine maintenance is essential for all pieces of rotating equipment, but steam turbines are far from small, making them difficult to transport to a workshop for repairs.

This is especially true of geothermal steam turbines. And a further complication for engineers is that geothermal sites are often located in areas that are difficult to access.

This article was originally published as part of the PEI print edition in 
Smart Energy International Issue 4-2020
Read the full PEI section, the full combined digimag or subscribe to receive a print copy.

This perfect storm of challenges was realised recently when a steam turbine needed an onsite overhaul in a power plant in a mountainous area 780 meters above sea level in Indonesia where access was restricted to narrow roads.

The repair work was important, not just for the plant itself but for Indonesia’s wider energy mix. Thanks to its volcanic geography, the country has the largest reserves of geothermal energy in the world and the government is keen to tap into them to fulfil its renewable energy ambitions.

Indonesia is currently a net importer of oil and continues to rely heavily on fossil fuels for power generation. Of the total installed national power capacity at the end of last year, 88% was sourced from fossil fuels while 12% came from renewables. Indonesia has almost 2 GW of installed geothermal power and plans to develop an additional 4.6 GW to help meet the government’s renewables target.

Last year, the World Bank handed Indonesia a $150 million loan to scale up investments in geothermal energy, and that funding was accompanied by $127.5 million in grants from the Green Climate Fund and the Clean Technology Fund.

“Indonesia’s geothermal sector has vast potential and our current installed geothermal power capacity is already the second-largest in the world,” says the country’s finance minister, Sri Mulyani Indrawati.

She said developing the geothermal sector “is an integral part of Indonesia’s overall energy security, as well as making us less dependent on imported fuels.”

The government has a target of 23% of renewables in its energy mix by 2025 and it is estimated that this will require contributions from geothermal development of about 7% ” the equivalent of 7000 MW. While unlocking the full potential of Indonesia’s geothermal reserves will be key in the next five years, there is equal importance attached to ensuring that those geothermal plants already operational are boosting their reliability with regular maintenance.

Which brings us back to 780 meters above sea level and the onsite overhaul of the steam turbine. The work was contracted to Swiss engineering firm Sulzer, which at the end of last year opened a new service centre in Balikpapan, Indonesia ” its 20th such facility in the Asia-Pacific.

The company’s Indonesia field engineer Kusno Baryadi explains: “Geothermal steam turbines operate in a particularly challenging environment, where chemical erosion can have a detrimental effect on their performance. To ensure their continued reliability and efficiency, steam turbines should be overhauled every four to five years.”

Baryadi said that sending the steam turbine rotor in this particular case to the company’s workshop was “quite risky due to the power plant’s remote location and unsuitable roads.” It was therefore decided to perform the overhaul and repair on site, which he added would also “achieve a considerable saving in downtime for the turbine, which minimizes costs associated with the refurbishment project.”

Baryadi stresses that “effective planning is essential to complete such a project on time.” Firstly, a 45-day maintenance window was established. Before the generator was taken offline Sulzer’s engineers worked with staff at the plant to organize the most effective method of completing the work.

“Geothermal steam turbines operate in a particularly challenging environment”

Sulzer’s Indonesia team have developed mobile repair equipment that consists of a complete set of portable tools including lathes, balancing machines and welding equipment that can be swiftly mobilized.

Baryadi says this reduces transportation costs for the customer “as well as potential rotor damage risk, which means insurance costs are also minimized.”

All the spare parts were assembled along with the mobile machine tools and balancing equipment and packed into four trucks for transport to the plant. Once the convoy arrived, the lathe and balancing machine were the first to be set up, while the rest of the team started to disassemble the upper casing of the turbine.

Once the rotor was removed from the casing it was set up on the lathe, where the dimensional inspection, runout checks and non-destructive testing could be performed.

Preparing the turbine rotor for balancing

Part of this inspection showed that the L-0 and L-1 blades would need to have their erosion shields replaced, which would be possible as their design allows them to be brazed into position.

Erosion shields are typically attached on the leading edges of steam turbine blades in the final rows of the low-pressure section that protect the airfoils from erosion. They reduce wear on the blades caused by cavitation erosion from condensed water particles in the steam.

For repairs where only the original erosion shield has been worn and not the blade material, Baryadi explains that the first step is to remove what remains of the erosion shields, which are typically made of cobalt base material. Having sand-blasted and cleaned the blade recess, a replacement can be installed using a special jig and heating elements. The specific pressure and temperature applied during the installation process is determined by the bonding material in use.

The turbine rotor was balanced before being reassembled

The Indonesia inspection also revealed that the labyrinth seal strips needed replacing on one turbine-side stage and four generator-side stages. All of these seals and the erosion shields underwent further non-destructive testing procedures to ensure that all replacement parts conformed with required specifications.

With all the repairs complete, the final low-speed balancing of the rotor was performed and the turbine reassembled, before being recommissioned and put back into service. The temperature and vibration sensors all indicated values within the specifications recommended by the original equipment manufacturer.

The overhaul was completed within the 45-day maintenance window organized by the power plant and resulted in no unplanned losses.

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