Improving gas turbine reliability

501F gas turbines are employed around the world to provide power and they require regular maintenance to provide continued reliability and performance and this needs to be both managed and delivered efficiently.
By Jennifer Cardillo, Sulzer April 20, 2018

An overhaul project begins with complete disassembly and strip-down. Courtesy: SulzerThe 501F gas turbine was developed and introduced in the early 1990’s as a joint venture between Westinghouse and Mitsubishi Heavy Industries. Since then, the 501F gas turbine has become an extremely popular option for power generation applications world-wide. While most users of the turbine look to their original equipment manufacturer (OEM) for overhaul and maintenance support during the early part of a unit’s life, a growing number are turning to specialist independent providers to deliver these services as their assets get older.

The size and weight of a large turbine like the 501F places particular demands on any maintenance provider. Disassembly, inspection, repair, balancing and reassembly operations all require large and highly specialized equipment and should be down in large turbine overhaul facilities. These facilities, apart from a large capacity crane, also use a vertical pit and scissor lift for rotor stacking and unstacking operations.

The overhaul work scope on a machine as large and complex as the 501F is determined by the customer’s requirements and the operating history of the equipment. Typically, a service will begin with disassembly and inspection of the full rotor unit followed by cleaning, non-destructive testing (NDT) and life-cycle assessment.

Specialist coatings are applied to improve corrosion resistance. Courtesy: SulzerWhen issues are identified during turbine operation or the inspection process, significant engineering work is often required to identify and implement an effective solution. For example, one machine was shut down and brought to a service center after it exhibited significant vibration. Inspection revealed high radial run-out of up to 0.18-in. Upon disassembly, it was found that the register fit on the forward stub had broken off completely.

Another common issue in the 501F is wear or failure of the anti-rotation tabs that hold the air baffles in place. The failures cause the baffles to fret, wear or even break away completely, resulting in severe damage to the turbine. To avoid repetition of this issue, air baffles with anti-rotation features can modify the anti-rotation slots on the turbine to accommodate them. This work can be carried out with the rotor disassembled, or with a different baffle design, that can be installed in the field.

In cases of failure at the forward end of the compressor through-bolt on the 501F, severe damage can occur to the machine caused by the passage of the liberated fastener. Analysis of the failed parts suggests that the issue is most likely the result of loading on the last full thread of the fastener, combined with insufficient loaded surface area between the fastener and hub to mitigate movement. This allows a crack at the end of the bolt to propagate during operation.

In the 501F, a frequent cause of vane failure stems from problems with the flow of cooling air. This can result in the loss of the necessary air dam at the leading edge of the vane, or insufficient flow through the pin hole channels leading to the trailing edge.

Dynamic balancing is completed after each stage of the rebuild. Courtesy: SulzerFor vanes that have been damaged in this, and other, ways, there are several advanced repair processes that can provide significant economic benefits for users. Rather than replacing the entire vane, the "coupon" repair process allows the damaged area of the vane to be cut away and a new section brazed into place. After grinding, testing and surface treatment, cooling holes are cut into the replacement parts and the vane is returned to service.

Elsewhere, where inspection reveals that highly contoured parts of a vane have become too thin for continued use, a slump brazing process can be used to add additional material. This process extends the service life of the vanes and offers a cost-effective repair with minimal downtime.

Ultimately, operators of gas turbines, and their insurers, are concerned about the life expectancy of their rotors. It is widely accepted by user groups, independent consultants and OEMs that the best policy is to thoroughly inspect components, evaluate the risks and to implement mitigation measures.

Jennifer Cardillo, marketing and communications manager americas, rotating equipment services, Sulzer. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.