Geothermal sites are often located in areas that are difficult to access. In this case, a power station in Indonesia in a mountainous area 780 meters above sea level with narrow access roads needed a steam turbine overhauled. Sulzer was chosen to overhaul the 55 MW steam turbine-generator as a result of its expertise, experience and ability to minimize the project duration by sending its mobile repair equipment to the power station.
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 – especially geothermal steam turbines. To improve the repair process, Sulzer in Indonesia has developed a range of mobile repair machinery that, together with expert engineers, is fully equipped to refurbish steam turbines on-site.
With the world’s largest reserves of geothermal energy, Indonesia is hoping to increase its use of this renewable energy source. At the same time, the power stations that are already in operation need to maintain their reliability with planned maintenance routines.
Kusno Baryadi, Field Engineer for Sulzer in Indonesia, comments: “Geothermal steam turbines operate in a particularly challenging environment, where chemical erosion can have a detrimental effect on their performance. In order to ensure their continued reliability and efficiency, steam turbines should be overhauled every four to five years.”
Sending the steam turbine rotor to the 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. This would also achieve a considerable saving in downtime for the steam turbine, which minimizes costs associated with the refurbishment project.
Effective planning is essential to complete such a project on time. For the power station, the 45-day maintenance window was established, and Sulzer was expected to meet the deadline. Before the generator was taken offline, Sulzer engineers worked with staff at the power plant to organize the most effective method of completing the work.
Sulzer in Indonesia has developed mobile repair equipment that consists of a complete set of portable tools including lathes, balancing machines and welding equipment that can be mobilized efficiently. This reduces transportation costs for the customer as well as potential rotor damage risk, which means insurance costs are also minimized.
All the necessary spare parts were assembled along with the mobile machine tools and balancing equipment, all of which was packed into four trucks for transport to the power station. As soon as the trucks arrived the lathe and the balancing machine were 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. 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, the first step is to remove what remains of the erosion shields that are typically made from 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 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 NDT 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 (OEM).
Kusno concludes:”The overhaul was completed within the 45-day maintenance window organized by the power plant resulting in no unplanned losses. The customer was very satisfied with the results and equally impressed with the mobile repair equipment that made it all possible.”
From rust to robust for major Spanish refinery
Sulzer’s automated welding delivers quality and cost-efficient corrosion protection for distillation columns. The efficient prevention and minimization of corrosion-induced deterioration in distillation columns is a key aspect helping refineries to remain highly productive and competitive. One of the world’s top integrated energy companies was experiencing corrosion issues on a number of units in its refinery complex in Spain. Sulzer’s automated weld overlay capabilities provided an economical and efficient solution to debottleneck the different columns and extend their service lives.
The refinery complex is one of the five largest in Spain. The plant processes crude oil to obtain a broad range of chemicals, such as methyl methacrylate and polypropylene.When the refinery noticed extensive corrosion within the column shells of three main separation towers, it sought to investigate the issue and protect its plant from costly shutdowns or lengthy periods of suspended production.
The initial inspection revealed that the column shells of the atmospheric distillation unit (ADU), the vacuum distillation unit (VDU) and a third fractionator were experiencing corrosion and erosion responsible for cracks, pitting and material losses. In particular, the column showing higher levels of corrosion was the VDU.
The structure of the VDU consisted in one wash bed and three pump-around circuits for the recovery of HVGO (heavy vacuum gas oil), LVGO (light vacuum gas oil) and LLVGO (light light vacuum gas oil or very light vacuum gas oil). The tower had an internal diameter of 9.15 m and its shell was made of carbon steel coupled with a 3 mm bonded plate to prevent corrosion. Over time, the plate started to corrode and lost its ability to protect the underlying column shell. Based on the visible damage present on the bonded plate, the surface area of the damage was estimated around 65 m2.
In order to repair and upgrade its columns, the refinery turned to Sulzer, whose Tower Field Service group has been supporting the company in the past with routine maintenance activities and field services.
Reinforcing fractionating columns
Sulzer’s expert team performed an in-depth visual inspection. To do so, the bonded plate was removed to expose the carbon steel shell. This examination revealed that the extent of column shell corrosion was almost double that of the initial estimate, covering approximately 110 m2. In these situations, refineries are often faced with the choice between replacing the entire column or part of it, which is costly and time consuming, or applying thermal-spray coatings that can prevent column corrosion only for a limited time.
Sulzer offered a third, more time-efficient, economical and long-lasting option: weld overlay. By using this in situ process, it is possible to cover large column surfaces with corrosion-resistant alloys. The company is highly experienced in this technique and has developed a state-of-the-art, fully automated weld overlay equipment to support its operations.
This machine consists of a carriage travelling along a laser-levelled track system fixed to the column shell wall. On the carriage, a robotic index arm moves the welding torch and the oscillator in order to create weld beads. All the relevant process parameters, such as carriage speed or bead thickness, are controlled by a programmable logic controller (PLC), with which human operators can communicate by means of human-machine interfaces (HMIs). One single PLC can monitor multiple welding machines following the same instructions.
The automated process can quickly perform welding with high accuracy. As a result, customers can benefit from a high-quality and consistent process as well as short downtime. In this case, Sulzer could complete the overlay of the 110 m2 VDU corroded surface in the time allocated to 65 m2.
In order to reduce the likelihood of future corrosion, the VDU column shell was overlaid with layers of austenitic stainless steel type 316 alloy, which is widely used in welding processes to avoid carbide precipitation. In addition, the presence of molybdenum and nickel makes the alloy suitable for applications in harsh conditions.
Comprehensive maintenance and revamp services
In addition to repairing the VDU column shell, Sulzer also performed weld overlay on the corroded surfaces of the other two towers, namely the ADU and the third fractionator. Also, column internals, such as trays and packings, were replaced to further improve the performance of the entire oil distillation system. Sulzer performed the weld overlay process in two weeks, while the entire revamp was concluded in 25 days, without any delays despite of the discovery of larger corroded areas within the VDU, increasing the required weld overlay to almost double of the originally estimated area.
Since the revamp, the refinery has operated smoothly at full capacity and the solution from Sulzer could also support the shift towards different crudes without affecting the columns’ corrosion resistance. The customer was so pleased with the work, that it offered to provide positive reference to any potential customers of Sulzer.
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