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Repair assessment and modification of gas turbine components and assemblies

21st February 2013


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

 
 
 
 
 
 
 
 


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