The Gas Turbine Repair and Refurbishment Business, Gaining in Importance as the Installed Base Ages and grows, traces its success to the aircraft turbine industry and to the rising level of competition in the power industry that demands high quality at low cost. Understanding the evolution, operation and capabilities of gas turbine repair shops can help end users improve their access to high-quality services for repairs and refurbishments. – Grinding of transition piece. Photo courtesy of Siemens-Westinghouse Power Corp.
Installed gas turbine units increased significantly from the mid 1960s through the 1970s. During this period almost all repairs were done by the OEMs in their factories and later at their authorized repair shops. Actual repairs at this time were of a minor nature, with most damaged parts being replaced with new ones.
Many of the gas turbine users were electric utilities that only operated their units as peakers, so replacing parts that had minor/medium damage with new ones created unacceptably high costs to them. Following the already established aircraft engine component repair philosophy, independent repair shops for large industrial/utility gas turbine components were started to fulfill the gas turbine end users’ needs. The first two independent repair shops formed specifically to refurbish large industrial/utility gas turbine components were Chromalloy in New York and ELBAR in the Netherlands, both in the late 1970s. Today there are more than 15 independent and OEM repair shops in the U.S. (see listings, pages 60-75), most located in Texas and focused at Houston, and an additional 7-10 located in Europe, Middle East and Far East.
Repair shop technical and physical capabilities have grown significantly in the past 20 years, driven by an expanding gas turbine installation base and the complexity of today’s gas turbines (increased firing temperatures, high-tech materials, etc.). Most of the older units used solid turbine section blades/buckets and vanes/nozzles that were uncoated and made from simple forged or cast materials. Today, these same components have 2-4 types of internal cooling, are made from higher strength and complex cast alloys, and have high temperature corrosion plus thermal barrier coatings. Repair shops now need more sophisticated in-house technical capabilities that include laser and other high-tech welding processes, vacuum heat treating, plasma coating, computerized dimensional restoration, laser/EDM hole drilling, and metallurgical and non-destructive evaluation (NDE) methods. Repair shops must be able to prove their repairs or they will not be able to refurbish the higher tech F, G and .3A type parts.
The worldwide population of large frame-size gas turbines is about 6,000, made up of turbines from General Electric, Siemens Westinghouse, ABB Alstom and their affiliates. GE models account for about 60 percent of this total. The many different models and running changes in this population limits parts interchangeability and emphasizes the importance of competitive repair shops. – Vacuum heat treat furnace. Photo courtesy of Wood Group HIT.
For frame-size turbines, the repair procedures and techniques employed by the independents are rarely sanctioned by the OEMs, and there are no overall repair standards like those the Federal Aviation Administration promulgates for aircraft gas turbines. Still, the independent repair shops are popular with end users because of their competitive price and technical skill.
Most components are repaired for 10-30 percent of their new cost, with deliveries of 1-4 weeks and guarantees from 90 days to one year. Very few of the independents offer new replacement parts, exchange parts or maintain a large inventory of used and serviceable parts. In most cases, work is awarded on the basis of firm prices for firm work scopes, and small lots are bid separately with few annual contracts. Operators typically prefer repair of their parts to purchasing new parts whenever technically feasible because of the considerable cost savings and the long lead time for new parts. Baseload operators tend to select repair facilities on the basis of quality and work scope first, then consider price, while peaking operators tend to consider price first, then other factors.
The majority of overhauls of frame-size units take place in the spring and the fall to avoid peak operating months. More than 50 percent of the component repair work is done by independents using their own in-house procedures. The quality of the work is usually good and delivery promises are generally kept. A few baseload operators follow their repairs closely, while most peaking operators never even inspect their repaired parts. Some turbine operators maintain an inventory of refurbished spare parts to be used during the overhaul season and during forced outages.
In the component repair industry, there are actually two levels or standards of repair, one for peaking units and one for baseloaded/cycled units. Much attention has been given to life assessment, full restoration repairs, rejuvenating heat treatments and recoating. All of these techniques and procedures are important for component refurbishment of higher operating hour and higher firing temperature baseloaded units where reliability and durability are the main considerations between 3-5 year overhaul cycles.
Many large U.S. gas turbines, however, are of the older, lower firing temperature design, operating for less than 300 hours per year with less than 12,000 accumulated hours. These peaking units usually do not have any coatings, few or no internally air-cooled parts, and must go 10 years or more between turbine section inspections. For these peakers, life assessment, metallurgical evaluations, reheat treatments, and some dimensional integrity restorations may be unnecessary functions that add cost for the operators without significant added benefits.
When selecting repair shop vendors to bid on a given repair project, the following characteristics should be considered:
- experienced shop, technical and management staff
- proven history of refurbishing the exact parts in need of repair
- documented quality assurance system with procedures, forms and repair specifications
- in-house capabilities for welding, vacuum heat treating, dimensional restoration, NDE and metallurgical evaluation
- past performance in meeting delivery, quality and work scope requirements.
Repair shops should not be on your bidder’s list unless you or someone in your company has visited their shop and reviewed all of the above considerations.
When it is time to make repair and refurbishment decisions pertaining to gas turbine parts, certain definitions must be acknowledged. A repair is defined as work performed to return a part to its physical fit and condition, while a refurbishment is a repair that also rejuvenates the metallurgical properties of the part and restores dimensional integrity. Repairs are mostly performed on compressor parts, cases, bearings, etc., because they are made from less sophisticated materials. Refurbishments, on the other hand, are typically applied to superalloy parts such as combustors, transition pieces, blades/buckets and nozzles/vanes.
The goal of the refurbishment process is to return a part to near its original condition. Before the process begins, however, a determination must be made whether or not it is worth refurbishing the part. This is accomplished by evaluating the price and delivery of a new part versus a refurbished component and should take into account the remaining life and operational mode of the turbine.
The basic process involved in ensuring major gas turbine components are properly refurbished involves five steps: initial inspection, scope of work, bid and evaluation, refurbishment and final inspection.
The initial inspection step begins by identifying the part and gathering as much information about the part history as possible. Part identification includes obtaining information on the part/assembly drawing number, serial number and other visible aspects of the part. Part history data includes total fired hours and fired hours since last refurbishment, gas turbine firing temperature, operational mode and future service requirements.
The part is then cleaned (vapor degreased, grit cleaned) so it can be visually, dimensionally and NDE inspected. On parts where the history is not known, and where visual signs of heavy wear or corrosion attack are evident, it is recommended that a metallurgical evaluation be performed. The metallurgical evaluation verifies the part’s material composition, microstructure, possible diffused coatings, depth of corrosion attack and approximate age. If the remaining life does not meet the future requirements of the part, there is no need to proceed any further. Parts not refurbished because of mode of operation may still be of value to other turbine operators with lesser part requirements.
Scope and Bid
If the expected refurbished remaining life is acceptable, the next step is to determine the refurbishment work scope, price, delivery and repair shop vendor by means of the bid process. Generally, when an open-ended request for quote is submitted, repair shop vendors will respond with a price and their best estimate on what work is required to refurbish the part. Some provide high estimates to be safe, while others estimate low and have adders.
As opposed to the open-ended approach, end users can use the results of the inspection report to generate their own scope of work. The scope of work details the specific tasks to be performed for a given part or parts and can be submitted with each quote request to the repair shops. Since the scope of work is identical, the playing field is leveled, ensuring that all bids include the same amount of work. Bids can be requested for individual items or lump sum for several parts. After all bids are received, a matrix is prepared to determine if any items have been omitted or if additional items have been recommended or added. The award decision is then based on:
- scope of work
- past performance
- shop load
The level of priority given to each of these criteria depends on the unit’s mode of operation and the weighting factors applied by the end user.
The actual refurbishment process relies on the technical skill and capabilities of the repair shop and its employees, but the end user should maintain an active oversight function. Various pieces of documentation should be available for the end user’s review: routing sheet, inspection forms, repair procedures and dimension forms. The routing sheet itemizes step-by-step all of the operations a given part will go through; all steps in the scope of work should appear on the routing sheet. Any steps appearing on the routing sheet that cannot be physically measured later should be reviewed to see if they are appropriate for the material composition and condition of the part, such as: weld filler materials, heat treatment cycles and coating processes.
The final inspection verifies that the original goal has been achieved. First, all of the markings on the part are reviewed and compared to the ones recorded during the initial inspection to verify that it is the correct part. Second, the routing sheet should be reviewed to ensure it has been signed off and dated. Third, the NDE results are reviewed to ensure the part is free of any unacceptable indications. The dimensional inspection results are recorded for all the key areas to ensure that the dimensional integrity was properly restored. Last, the non-visual processes, such as charts for time versus temperature in the heat treat furnace and coating/hardface certifications, are reviewed. Upon confirmation that the scope of work has been properly completed, the part is ready for shipment back to the end user. p
- Know your engine’s components, as well as their past operating and repair history.
- Be aware of your operating goals and how this will affect the extent of your component refurbishment of replacement decisions.
- Plan ahead, have new or refurbished key capital spares available, whether in-house, shared with other users, or from OEM or other stock.
- Get involved with your component or rotor refurbishment or hire someone knowledgeable to help you.
- Insist on documentation and certification even if you don’t have time to read them right away; they can help later and usually mean a better repair.
- Exercise good judgment or get professional help in making repair, replacement, or “alternate source” new replacement part decisions. Don’t base decisions solely on price and delivery.
Ron Natole is president and founder of Natole Turbine Enterprises.