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GRC Transactions, Vol. 35, 2011

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KeywordsOEM products, OEM Services, rotor upgrades, last stage blades, blade path upgrades, reliability, component repair

Mega Rotor and Savannah Machinery Works

On the 10th anniversary of Mitsubishi Power Systems Americas, (MPSA) -the official representative of Mitsubishi Heavy Industries (MHI) in the Western Hemisphere- is proud to announce its expansion and growth via a new manufacturing center in Savannah, Georgia. Savannah Machinery Works (SMW) is now the primary manufacturing and repair center for steam turbine rotor and stationary component repairs and upgrades for Mitsubishi Geothermal and conventional Steam Turbine Units. SMW will provide improved lead times and reduced components costs.

MPSA has established a strong service infrastructure to ad-dress the service needs of its existing geothermal users in the last few years by creating synergies with users and domestic vendors and by expanding its field service and repair capabilities. MPSA acknowledges the specialty and uniqueness of the geothermal in-dustry and is committed to continue to invest and enhance MPSA’s capabilities. In the last few years MPSA has refurbished and sup-plied replacement parts sourced domestically for 60% of its users. In the last year MPSA performed major overhauls, rotor inspections and supplied major components procured domestically for two of the major geothermal producers in the Imperial Valley, CA.

MPSA is privileged to have the experience and continuous support provided by its parent company MHI which has design and manufactured over 100 geothermal turbines and implemented 10 rotor replacements/ upgrades. Today, with the experience and knowledge of MHI, and the new diverse talent that MPSA can provide; MPSA is prepared to design and implement MEGA Rotor upgrades, bladepath enhancements, and reliability improvements to the existing fleet of geothermal users. The enhancement of Mitsubishi’s service infrastructure in the U.S. also provides any new or potential users with the peace of mind that their Original Equipment Manufacturer (OEM) will have Engineered answers and solutions provided domestically. This will eliminate the

owner’s need of using third parties who have to make design assumptions putting the owner at risk. This paper will provide a summary of what MPSA can provide including some recom-mendations for turbine users.

100 K Hour Turbine Inspection

Performing in depth inspection of major turbine parts is criti-cal to ensuring long term durability, and sustained performance of the units. To accomplish this task, MPSA relies on the extensive knowledge and database provided by its parent company, MHI. As a major supplier of geothermal units, MHI is uniquely qualified at understanding the materials and design of the components of the Geothermal Steam Turbine, and how these components behave under long term operation. MPSA with the support of MHI has performed numerous comprehensive turbine rotor and stationary component inspections and provided life assessments and repair rec-ommendations. The scope of the inspection is customized to fit each of the user’s turbine configuration and the quality of field steam. The inspection of spares can now be performed in SMW which has all the technical expertise and latest equipment necessary to provide a comprehensive recommendation of the parts being inspected.

Rotor and Stationary Component Repairs

Having engineering on staff allows MPSA to provide an engineered approach to rotor and stationary component repairs. MPSA is aware of the current repair practices offered by other vendors and is open to utilizing them as long as the repair does not compromise the integrity of the unit.

Fleet Upgrades “Mega Rotor”

After long term operation in geothermal units, steam flow and pressure is reduced due to degradation of the production well. Due to changing conditions, turbine efficiency decreases from the origi-nal optimized design. With degraded conditions, re-optimization of the design is required to achieve a higher level of performance. Mitsubishi’s philosophy for the optimized design is to use the latest

Mitsubishi’s Reliable Solutions and Domestic Service Infrastructure

Muhammad Riaz, PhD1, Marco Sanchez2, and Scott Grace3

1Steam Turbine Engineer 2Service Sales Manager

3Marketing Specialist Msanchez@mpshq.com

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technologies to improve blade path efficiency, increase reliability by using new materials having higher resistance against Stress Corrosion Cracking (SCC), erosion, corrosion, scaling etc. and reducing cost where ever possible.

Mega Rotor is a packaged solution available to customers to improve the performance and reliability of the geothermal steam turbines using state-of-the-art technologies that Mitsubishi has developed after years of extensive testing. These technologies have been validated from simple material testing to full-scale turbine tests in a R&D facility located in Takasago and Nagasaki, Japan. Design features have been applied in multiple units providing improved reliability against common problems observed in geothermal units.

Common Problems in Geothermal Turbines

In a geothermal plant, geothermal steam is directly supplied to the steam turbine and the turbine material is inevitably subjected to corrosive gases contained in the geothermal steam. Impurities present in the steam are responsible for many issues occurring in geothermal steam turbine. Below are a few of the most common problems geothermal steam turbine users have reported.

Stress Corrosion Cracking (SCC)

Stress corrosion cracking is a failure of the material subjected to tensile stresses in a corrosive environment for an extended period of time. For SCC to occur, stress, material, and corrosive environment are interrelated as illustrated in Figure 1.

Mitsubishi has limited control over the environment pro-vided by the geothermal steam. However, Mitsubishi is working to improve materials, and the stresses in components to reduce SCC. Component design is optimized with features that reduce local and average stresses. For example, large roots and grooves are applied on the rotating blades to reduce stresses and hence improve resistance to SCC. Stress analyses for stationary and rotating components are performed with the latest 3D finite ele-ment analysis tools to calculate stresses accurately.

Along with optimal component design, research in material development has resulted in superior SCC resistant materials. Mitsubishi’s latest 12% Cr steel has shown an improved level of SCC resistance. Rotors made out of 12% Cr material have been in service for more than 10 years without any SCC damage. For repair options, application of welding repair by 12% Cr steel on the cracked rotor grooves is common practice. Figure 2 illustrates a cracked groove weld repair using 12% Cr material.

Corrosion Fatigue

Corrosion fatigue is a failure mode experienced by component subjected to vibration stresses and a corrosive environment. Cor-rosion fatigue is especially found on the rotor grooves of rotating blades. Proper selection of blade material with improved design features such as integral shrouds has shown reduced vibratory stresses and hence improves reliability against corrosion fatigue.

Erosion

Geothermal steam is usually in a saturated condition at the turbine inlet and water droplets start to form as it expands through first few stages. These water droplets cause erosion of components in the steam path (Figure 3). On the stages with higher levels of moisture, Stellite material is used to shield the blade leading edges. Stellite material has shown a high resistance to water erosion and is commonly used for blade protection against erosion.

Other features used for erosion protection include moisture drain grooves and hollow stationary vanes with moisture remov-ing slits. Drain grooves collect the moisture that is collected by centrifugal force between the stages and direct it out of the steam path. Hollow nozzles are applied to long stationary blades to minimize erosion at the last stage blade. Hollow nozzles have slits on both concave and convex surface with holes drilled in Figure 1. Elements responsible for stress corrosion cracking.

Figure 2. Cracked root weld repair using 12% Cr material.

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the profile. These slits catch the water droplets and exhaust them through the holes in the profile and the outer ring of diaphragm to the turbine exhaust.

Furthermore, geothermal turbines utilize impingement type seal plate or the satellite welding on the casing to provide improved protection against erosion in the blade tip area.

Scaling

Generally there are many impurities in geo-thermal steam that can cause serious problems for the steam turbine. Among these problems, a layer of material such as silica and calcium car-bonate can be deposited on the steam path com-ponents. This deposition of scale on the blades reduces the flow area of the turbine thereby re-ducing the output (due to reduction in swallowing capacity) and efficiency of the turbine. The first-stage nozzle is typically most affected by deposi-tion of scale, although scale may be present in other parts of the system (Figure 4). Mitsubishi recommends reducing steam impurities to protect against scaling and continuously monitoring steam purity with steam quality monitoring devices. As a method for scale reduction, water cooled nozzles and water washing are applied to prevent scaling at the 1st stage nozzles.

Blade Path Upgrade

Mitsubishi’s improved blade path utilizes high efficiency fully three-dimensional (F3D) impulse nozzles and blades. These

state-of-the-art blades are designed to achieve a higher level of efficiency by reducing profile and secondary flow losses.

In existing designs with parallel sided nozzles and blades, the boundary layers are formed by the steam flowing along the hub and tip end walls of the blade passage. The pressure differential between airfoil convex and concave surfaces interacts with the hub and tip boundary layers to create cross flow. The resulting vortex, induced by the cross flow, creates a large loss within the blade passage. In general, this vortex is referred to as secondary flow.

F3D Rateau nozzles and rotating blades are designed to sup-press the secondary flow by altering the pressure distribution within the blade passage. Twisting and bowing the blade profile, in accordance with MHI aerodynamic and 3-Dimensional analy-ses, creates a downward force towards the hub and tip end walls, thereby significantly reducing the size of the vortex and contribut-ing to higher efficiency.

The secondary flow losses of the F3D nozzle and blade are considerably reduced in comparison with the previous generation parallel sided nozzle and blade. By utilizing a bowed F3D nozzle/blade, the overall efficiency is increased significantly compared to that of the parallel sided blade. Mitsubishi has performed extensive efficiency verification tests in air model turbine and test steam turbine at the Mitsubishi R&D center located in Takasago, Japan.

Figure 4. First stage nozzle with heavy scaling.

Figure 3. Erosion observed on last stage blades.

Figure 5. Losses are reduced with latest twisted and bowed rotating and stationary F3D blades.

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Rotating BladesThe new generation of first few stages of rotating blades is

comprised of an integral shroud and a T-root or side entry type root, machined from 17-4PH or 12% Cr stainless steel material. The integral shroud feature eliminates the stress concentrations associated with the tennon of a riveted shroud blade. The ISB blade is designed so that the shroud contact between adjacent blades offers direct vibration suppression. It also offers improved per-formance through better tip sealing by allowing more sealing fins.

The Mitsubishi design utilizes 17-4PH or 12% Cr material even for the stopper or the last blade in a row. The stopper blades are designed to have proper margin relative to the subjected stresses. Therefore it is not necessary to apply titanium material blades. In contrast, employing titanium stopper blades may introduce a balancing issue due to the difference in weight relative to the remainder of the blade row. Eliminating the titanium blades helps cut cost and provides an economical solution for the customers.

Stationary ComponentStationary blades are also designed using state-of-the-art CFD

and FEA tools. These bowed and twisted blades are designed to compliment high efficiency rotating blades to provide an efficient blade path by reducing losses. The first few stages of stationary nozzles are high efficiency F3D nozzle diaphragms. Individual blades are machined from 12% Cr stainless steel and can be welded to optional 12% Cr inner and outer rings to form complete 12% Cr diaphragms. The diaphragms are split into upper and lower halves. The complete 12% Cr material stationary component structure offers higher resistance against erosion and corrosion.

Rotor Upgrade

Developments in steel manufacturing and forging technologies have allowed development of fully integral turbine rotors with no-bore. During ingot formation, ladle refining and vacuum degas-sing processes have reduced the presence of Silica, Potassium and Sulfur which are responsible for material segregation. Exclusion of these elements enhances the integrity and uniformity of steel ingots. For rotor forgings, higher compressions ratios are achieved by using screw presses with high load capacity. To achieve preci-sion grain size, strict temperature control is applied during heat treatment that adds strength to the resultant rotor forgings.

Rotor MaterialGeothermal rotors are susceptible to stress corrosion cracking

due to the corrosive environment. Mitsubishi is actively looking into new materials that can provide higher resistance to SCC to improve product reliability. CrMoV rotor material forgings have been used in geothermal applications. Mitsubishi has verified the use of this material through years of operating experience. To further improve resistance to SCC, Mitsubishi has developed a 12% Cr steel for geothermal applications. The 12% Cr rotor provides high strength and excellent corrosion and stress corrosion cracking resistance. Mitsubishi has implemented 12% Cr rotors for geothermal applications since 2002 without any occurrence of SCC and offers this as an ideal material for highly corrosive geothermal environment.

Mitsubishi has performed extensive testing during the develop-ment of the 12% Cr rotor material. Accelerated SCC comparative testing of CrMoV and 12% Cr materials was performed by ex-posing test specimens to actual geothermal steam. While crack initiation can be observed after 3 months, and SCC readily ob-served after 6 month in the CrMoV test specimen; there is no crack initiation after 12 months in the 12% Cr test specimen. The ac-celerated comparative testing indicates a substantial improvement in the time before the onset of crack initiation, a 4X improvement using the 12% Cr material relative to the CrMoV material.

Rotor Shot PeeningIn addition to material development, Mitsubishi also provides

additional techniques to reduce rotor stress and SCC. Shot peening imparts a compressive stress of approximately 30 kg/mm2 to the rotor surface. This compressive stress counteracts the tensile stress arising from centrifugal force to reduce the overall stress field at the rotor surface. This reduction in rotor surface stress is especially beneficial in im-proving the resistance to stress corrosion cracking. Figure 6 below identifies the general areas of the rotor where shot peening is applied.

Coating on Gland Area (Option for CrMoV Rotor)

The rotor gland areas are particularly susceptible to corrosion due to the severe corrosive environment of geothermal steam and air (oxygen). The corrosion results in an increase in gland seal clearance which further exacerbates the adverse situation by increasing the amount of air ingress. It is not uncommon that all rotor gland seal steps are completely destroyed beyond recognition due to heavy corrosive thinning.

To alleviate this problem, Mitsubishi offers anticorrosive Inconel 625 welding overlay or thermal spray coating applied to the gland areas of the CrMoV rotor.

Last Stage Blade Technology

Along with improvement in other parts of the steam turbine, multiple improvements have been made in last stage blade (LSB) design. LSBs are the longest blades in the steam path and are ex-posed to very severe mechanical conditions. Earlier generations of LSBs are generally grouped blades with one or more lashing lugs that are welded together to form groups for increased stiff-ness. Figure 8 shows a typical last stage blade with lashing lugs.

Figure 6. Red lines showing rotor locations that are shot peened.

Figure 7. A typical damaged gland area and coating with Inconel 612.

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Mitsubishi’s next generation of blades are designed with shrouds at the blade tips that are integral to the blade and with a mid-span snubber connection. The blade is designed to untwist during operation so that the shroud and snubber contact is main-tained and the blade row is continuously coupled.

The shroud contact between neighboring blades provides significant damping and improved fatigue life through drastic reduction in vibratory stresses. Tests have shown a significant damping improvement over existing designs.

Along with improvement in aerodynamic design of the blade, a larger side entry root is applied to the blade to reduce local and average stresses in both blade root and rotor groove. This reduction in rotor groove stresses in conjunction with a reduction in stress concentration provides high resistance to stress corrosion cracking.

Sealing Upgrade

Replaceable seals are offered for ease of maintenance and keeping turbine performance on track. Most existing turbines utilize impingement type or caulked seal design. Instead of directly caulking-in seal strips into the diaphragm inner ring, replaceable

seals are applied. Blade tip seals are also optimized to provide improved protection against erosion in the tip area.

Benefits of OEM Products and Services

At the planning stage of outages or upgrades, the turbine own-ers have multiple options for the parts and services to be provided. Many 3rd party companies offer packages for these services but they lack the in-depth engineering knowledge of the product and design details. Identification of the component material is a significant challenge, and incorrect material substitutions can lead to catastrophic failure. Many 3rd party companies take risks by using similar material for components. Mitsubishi develops its materials based on years of research and extensive testing to ensure all the design goals are satisfied.

Typically 3rd party providers rely on reverse engineering for their replacement parts. The challenge is that small design details ranging from tolerances specified on the drawing, to details of the assembly process cannot be properly identified. As an example, a tight tolerance specified for a rotating blade vane may be required to keep a certain vibration mode under control. But without the design information, the blade could be built and operated at, or close to a resonant frequency leading to blade failure caused by high cycle fatigue.

A further benefit that OEMs provide is fleet experience. A 3rd party provider may have limited exposure to similar turbines but typically they lack the experience and knowledge of the problems a similar fleet of turbines is experiencing. An OEM, based on previous experience can recommend a prompt solution that may have been validated in the test lab or in the field.

Mitsubishi is always looking for new technologies to improve their product line. A lot of resources are dedicated to research and development to build products with higher level of efficiency and reliability. When turbine owners use the OEM for spare parts and services, it ensures not only the original design conditions are met; they also get parts and services that can improve the turbine performance.

Figure 8. Last stage blade with lashing lugs.

Figure 9. A typical last stage blade with an integral shroud and side entry root.

Figure 10. A comparison between old and new last stage blade design features. ISB blade has larger root.

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