Life Extension Evaluations - Most of the heavy Duty gas turbines installed since they were first installed in 1950, are still in operation today. The earliest models had a design life of 100,000 fired hours. Starting in the mid 1950’s the design life increased to 200,000 to 240,000 fired hours. Assuming the 240,000 hour design life (approximately 30 years when operating at base load), the earliest turbines are now over twice their design life, and still going strong. This article will address all GE design gas turbines up thru the “E” class design (MS3000, MS5000, MS6000, MS7000, & MS9000). At present, there are a large number of gas turbines in all frame sizes that currently exceed the 240,000 hour design life. It is recommended that all turbines have a life cycle review between 200,000 and 240,000 fired hours, so as to endure continued successful operation. A life cycle evaluation would address the following areas:
• Compressor and Turbine casings. Compressor casings are all very rugged and are generally designed to last indefinitely. The biggest issue for all casings is cracks along the vertical and/or horizontal flanges. Cracks can be the result of repeated thermal expansion/contraction, but are most commonly due to repeated maintenance cycles and over-torqueing the casing bolts. Smaller cracks that do not grow over time are not usually an issue. But, any crack that grows over time with repeated maintenance cycles should be addressed. All casing cracks should be well documented during each maintenance outage with measurements and pictures. A life evaluation study would evaluate the actual casings during an outage, and a thorough review of the maintenance reports.
• Turbine Rotor. All turbine rotors are subject to very high temperatures during operation. They are also subjected to considerable strain during start-up (fast warm up under considerable centrifugal stress), and during shut-down (fast cool down). A thorough turbine rotor inspection is recommended during each HGP/MI inspection. For customers near the 240,000 fired hour mark, who also intend to keep their turbines in operation for many additional years, it is recommended that they replace the turbine rotors at this operation interval. At the 240,000 fired hour mark, the wheels & shafts are well beyond the materials charts for design life. Any turbine wheel burst would be a major catastrophe. So, the best option for continued operation life a thorough review of all operating records and all maintenance reports should be made. Also, it is highly recommended that all customers consider replacing their turbine rotors.
• Compressor Rotors and blades. Compressor rotors are not subject to the very high temperatures that turbine rotors see, but, customers should be very careful in evaluating the compressor rotors during each Major Inspection, and to keep very good records in the maintenance reports. It is recommended a complete compressor rotor break down inspection be done at the 240,000 fired hour interval and a thorough inspection of all wheels and shafts be down and documented. This would be a good time to also replace all rotating blades, depending on condition. It is recommended that a thorough life cycle evaluation be done on the compressor rotor parts to ensure continued reliable operation.
• Accessories and instrumentation. All these items are typically inspected and repaired at each normal maintenance interval. Similar to the design life for the actual gas turbine engine, all the accessories and instrumentation will likely be beyond their design life. Thus it is recommended that all these items be considered for replacement at the 240,000 hour operating interval. At a minimum, all pumps and motors should be sent out for a complete rebuild; replace seals, gaskets, bearings, windings, etc. it is also recommended that additional spares be kept in stock, as failure rates of all these items will likely be higher due to the age of these items.
• Packaging and Wiring. These items usually see a lot of wear and tear due to weather, maintenance, and operations issues. All these items should be inspected during each maintenance interval, and repaired as needed. It is recommended that all these items be thoroughly reviewed during the life cycle review and replaced during the next maintenance interval as needed.
• Controls. All vintages of control systems have a high reliability when first installed, and remain so, today, even though they may now be very old. The biggest problem with older control systems is the availability of replacement parts. Frequently OEM’s will only stock replacement controls parts for 15 or 20 years. Thus, it is recommended that all customers keep a large stock of replacement parts on hand, especially when their control panels approach the 15/20 year mark. Third party suppliers will generally stock many of these items longer than the OEM’s, but as the state of design for electronics progresses, many of the smaller components of the control cards are no longer available, so, even these 3rd party suppliers cannot stock most of the controls replacement parts. A thorough tune-up of the controls system is recommended at each maintenance interval. Upgrading to a newer vintage control system is recommended, if not already implemented, at the 240,000 operating interval, as it will give the asset owner considerable additional reliability, operability and flexibility needed for the best operating reliability going forward.
In our hearts we all know that our operating gas turbines will last forever. But, only if we keep them well maintained and replace/repair parts as needed. All customers with older vintage gas turbines that have been in operation for longer operating intervals have had a great staff of operating and maintenance personnel at their plants. But, just as the gas turbines age, the long term experienced station personnel retire, and are frequently replaced with less experienced personnel, who at no fault of their own, do not have the experience and operating background of those who have seen the gas turbines thru their first 30 years of operation. Thus, owner/operators frequently need the help of experienced OEM or 3rd party service provider’s personnel, when doing a life cycle review, especially at the 240,000 hour/30 year interval. The Keck Group International (KGI) is available to help any and all customers who need help with their Life Extension Evaluation. We have several engineers on our staff that have helped many other customers with these evaluations.