Life-limiting deterioration is frequently created by a detail of a component. A thorough analysis and creative thinking can lead to modifications that result in substantial extensions of turbine lifetime. Sief Mattheij reports.
Gas turbine components are subjected to high temperatures, as well as high stress levels, and are exposed to aggressive gases at the same time. Gas turbines have to be fired to highest possible temperatures to get the best efficiency and the highest output. Fighting degradation of components exposed to high temperature is a continuous challenge.
The steady-state temperature is the first factor. It controls oxidation and corrosion rates, degradation of base material quality, and creep lifetime. Creep lifetime is very dependent on material temperature. Gas turbine hot-section components are made of nickel-base and cobalt-base superalloys.
Excessive temperature can affect the material integrally - sometimes irreversibly. Thermal cycling creates cyclic stress loads, which can be very severe. Most cracking of gas turbine components is a consequence of thermal cycling. Thermal cycling cracks can occur in sound material as well as in aged material.
Steady-state stress levels in materials that are subjected to high temperature will lead to creep, which is a slow continuous plastic deformation of the material.
Cyclic stresses can lead to fatigue, which is the initiation and growth of cracks by cyclic stress. Cyclic temperature changes while starting and stopping can create very high cyclic stresses. As a result, cracks can initiate and grow during even a very low number of stress cycles.
Fatigue by thermal cycling thus is known as 'thermal fatigue' and 'lowcycle fatigue'. When a component is weakened by internal degradation of the base material, it becomes much more sensitive to thermal fatigue. Cyclic stresses can be created by external mechanical excitation, like tip rubbing or rattling of combustion components or by irregularities in the gas flow pattern that create cyclic pressure loads on components.
Usually, the cyclic stress level is low and the vibration frequency is high. It is worth mentioning that the crack growth rate under identical stress and temperature conditions is very similar for most superalloys. Stronger alloys show a longer crack initiation time, but, after initiation, the crack growth rate is nearly the same as for weaker alloys.
Many high-cycle fatigue failures are caused by components that have developed cracks by excitation of a harmonic mode. At the same time, it is very important to understand that pure cyclic mechanical overstressing (by forced vibration) can make a component fail just as easily. In all cases, it is important to determine and address the root cause of the mechanical excitation rather than just focus on the determination and trimming of natural frequencies. Initiation of cracks, growth, and ultimate failure usually take place in the range of thousands to some millions of cycles. Because of the high frequency, this can happen rather quickly.
The excitation of a natural frequency in gas turbine components can lead to severe amplification of the cyclic stress level and to failure in a very brief period of time (seconds to minutes). Especially,long rotating blades can be sensitive to excitation.
Superalloys are a blend of many elements, of which, many are very sensitive to oxidation. Like in stainless steel, the alloys are protected against oxidation and corrosion by the formation of a stable, dense, and tight oxide scale.
Depending on the alloy (or the coating), the is scale can be either chromium oxide or aluminum oxide. Industrial gas turbines with very long operating times benefit from high-chromium alloys;turbines in which the alloys are exposed to the highest temperatures benefit fromhigh aluminum contents. The latter turbines cannot offer the fuel flexibility for which old, 'low' -temperature gas turbines were once known. Only chromium oxides can offer reasonable,protection against attack by sodium sulphate or sodium vanadate salts ('hot corrosion').
All modern gas turbines, as used in public utilities, operate with high metal temperatures. They are protected by high aluminum coatings and need very clean fuel gas and combustion air. Up to about 750°C, the combustion gases only attack the material at its surface or, to some extent, along grain boundaries.
At higher temperatures, diffusion in the alloy becomes increasingly important and, consequently, oxygen and other reactive components from the combustion gases can penetrate deep into the surface of the component. In this surface layer, alloying elements react in order of decreasing reactiveness, leading to depletion and formation of internal oxides, nitrides. etc. Because of the high stability of these compounds, this cannot be restored.
The damage created by the mechanisms above renders the information that is needed to assess the severity of the process creating it. If this damage had been foreseeable in its full extent in the design phase, appropriate design modifications would already have been implemented in that phase.
Therefore, pure reverse engineering, in which, essentially, the same calculations and estimations are made as in the original design phase, cannot be guaranteedto offer a good analysis and solution.
It is a valuable instrument to support modification initiatives. If a solution to or a mitigation of a problem is possible, often a great deal of additional information can be retrieved from the used and damaged components, especially when there is an opportunity to compare these components with similar components from other turbines and other types and/or brands of turbines. From such experience data, the following comments on generic types of damage can be made.
Creep is slow plastic deformation that occurs in a component under stress at high metal temperature. The creep process gradually exhausts the plastic deformation capability of the component. Up to about two per cent plastic deformation does not usually create serious problems with the alloy. In many cases, however, the accompanying mechanical deformation already exceeds acceptance limits for safe operation.
This creep damage can be repaired - not by addressing the material or the root cause of the problem - but by mechanical correction of dimensions, eg, by cutting back and by rebuilding as required. Since the deformation of straightening actions adds up to the creep damage, the straightening process should not be used for critical components. It should be emphasized that in many cases of creep damage, the base material is still in very good shape, eg, it would be pronounced 'to be in good shape' by an investigating metallurgist.
This is not contradictive to a rejection of that same component because of mechanical non-conformance. Practical experience shows that true metallurgical creep damage is a rare phenomenon.
Most gas turbine components exhibit a metallurgical creep life that is a multiple of the actual lifetime.
When components are produced, the materials structure have the right grain size distribution and size and distribution of different phases like carbides and Gamma-prime phase. These phases have limited stability and can grow, redistribute, disintegrate, or convert into other phases. Unwanted new phases like s-phase can be formed over time as well. Some degradation is completely reversible; some is not.
Fretting is the wear of components that are in low-amplitude vibrating contact. The slight relative motion creates an ongoing process of diffusion welding and tearing apart of these welds on a microscopic scale.
In many alloys, fine particles are thus torn out of the surface, leading to the typical signs of fretting. Hardness of the alloys is only a factor but not an all-determining parameter. Cobalt-base alloys have better resistance to fretting than nickel-base alloys.
In the operation of a gas turbine, good housekeeping is the art of finding the best mcompromise between limited component lifetime and efficient power output of the gas turbine. In other words, gas turbine (hot-section) components are designed for and will be operated to survive an optimised and limited lifetime. The weakest points of designs will determine the lifetime. By repair or modification of these points, lifetime can be extended.
At the same time, these weakest points are usually concentrated in just a single detail of the entire component. Since most life-limiting processes in gas turbine components are controlled by processes that are based on diffusion kinetics, which, in turn, are highly material temperature controlled, small changes in metal temperature can lead to dramatic extensions of component lifetime. Thus, modifications can usually be small changes with big consequences.
A good impression of the stresses and temperatures of gas turbine components can frequently be obtained with a straightforward analysis. Metal temperature distribution can be assessed with fair accuracy by determining the ageing of the alloy in various areas. A simple and straightforward analysis can yield a reliable identification of safe zones for repair or modification, critical danger mzones, and transition zones. It is obvious that this analysis must be carried out and interpreted conservatively.
Thorough knowledge of superalloy metallurgy, properties, processing, coating, etc, is a prerequisite to undertaking any steps in modifying gas turbine components. Despite that fact, most modifications are a result of straightforward sound thinking and acting. Solutions can address a very specific case or a wide array of problems.
Thermal-barrier coatings (TBC) in industrial gas turbines are plasma-sprayed ceramic coatings that act as an insulator between hot gases and cooled components.
A TBC not only reduces average metal temperature, but also reduces steep thermal transients. TBCs were introduced for combustion components many decades ago. Because of their surface roughness, gas turbine original equipment manufacturers (OEM) and users were reluctant to introduce them on airfoil surfaces. When increasing power and efficiency requirements led to higher firing temperatures and internal cooling technology could not keep pace, and when the requirement to achieve acceptable lifetimes from components grew to a nearly unachievable level, TBCs were introduced on airfoils.
In many cases, lifetime was doubled or tripled, and, surprisingly, few or no effects were found in internal efficiency or power output.
The leading edge of a gas turbine blade is exposed to the severest risk of overheating because of the high rate of heat exchange that is caused by the impingement of hot gases. Leading edges, thus, are provided with the majority of the cooling air and are frequently designed as a relatively thin-walled pipe that is cooled by internal airflow.
Despite the large amount of cooling air, leading edges are still vulnerable to overheating and thermal cracking, as is demonstrated by many sets of damaged components. It frequently proves that cracking in a set of components is more pronounced in components with thin leading edges than in the ones with thick leading edges.
A thick leading edge will conduct more heat to cooler parts of the airfoil than a thin one. In reverse engineering, component modification to thicker leading edges is more easily incorporated than in repair of existing components although reliable processes are available for the latter as well.
Cooling air is a scarce commodity in gas turbines. Components that lack cooling will deteriorate within an unacceptably short period of time. When lifetime is determined by the corrosion rate of the base material, one option is to simply increase wall thickness of that component.
In the case described here, the modification was more complex than usual. Border conditions were: no changes in total cooling airflow for this component and no changes in the contour of the airfoil were allowed. The original component was produced from uncoated Inconel 939 and had thin walls at the trailing-edge cooling-air exit slot.
The modifications were as follows: internal cavity and the cooling-air exit slot modified to double minimal wall thickness; cooling-air impingement insert modified for cooling-air distribution to the hottest sections; change of material from IN939 to IN738LC with better resistance to internal oxidation and nitridation; application of an oxidation-resistant aluminum diffusion coating.
In these components, lifetime was tripled by this combination of actions.
Sulzer Turbo Services
2010 has been a transformational year for Sulzer Turbo Services with the acquisition of Dowding & Mills, the leading independent electro-mechanical repair business. Since June 2010 the service footprint of the business has expanded greatly, this has been coupled with a doubling of the number of service engineers and technical specialists providing round the clock service, writes Sue Hudson.
The key focus for the business remains the owners and operators however the equipment range that Sulzer Turbo Services can now service has increased to include the ‘driver’ and the ‘driven’. The markets in which Sulzer Turbo Services now operates are oil and gas, refining, petro-chemical, power generation, mining and transportation. The change has not changed the businesses vision, which remains: to be the leading independent, technically advanced and innovative service provider for land-based rotating equipment, to operators and owners, worldwide.
The enlarged service business are offerings are complementary to each other and all based around rotating equipment. Today, you can expect to see Sulzer Turbo Services engineers performing highly skilled services in a number of key oil and gas regions: Alaska, the Gulf of Mexico, Houston, Venezuela, the North Sea, Rotterdam, the Middle East, Russia, Kazakhstan, Indonesia and Australia. The services that they are performing in these important oil and gas regions could be either upstream O&G, on the oil and gas pipelines or within the refinery on anything from condition based maintenance inspections, routine maintenance, planned maintenance or emergency support.
Being an independent service provider on rotating equipment allows Sulzer Turbo Services to offer alternative solutions to the owner or operator; often alternatives that are not available from the OEM. Experience has shown that owners and operators work with Sulzer Turbo Services as they:
* Prefer the technical transparency and partnership approach that is offered
* Benefit from the repair rather than replace approach to problem solving
* Appreciate the superior turnaround speed
Through long-term relationships our customers have found that they can improve their plant reliability whist reducing the cost of maintenance. The ability to help customers to improve their commercial position is based on over 90 years providing services on rotating equipment and by having over 2000 highly trained specialists delivering these services throughout the globe. It is not often that a problem has not been seen before by one of the service engineers and if they have seen something similar in the past they will quickly find a solution.
Enter √ at www.engineerlive.com/ipe
Sue Hudson is with Sulzer Management Ltd, Sulzer Turbo Services, Winterthur, Switzerland. www.sulzerts.com