Water Treatment

Latest Steam Turbine Offerings Enhance Plant Performance

Issue 5 and Volume 112.

By Teresa Hansen, Senior Editor

Four OEMs discuss their companies’ latest steam turbine designs, how those designs will improve plant performance and how they plan to meet upcoming market demands.

Low pressure rotor for an Arabelle nuclear turbine-generator after completing a vacuum pit overspeed test at the Alstom Belfort Turbine Works. Photo courtesy Alstom.
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The growing demand for electricity and its increasing price are prompting generators to make more megawatts. Some are planning new plants, others are focused on getting the most out of existing baseload power plants and a few are pursuing both strategies. Whether looking at plant improvements or new plant construction, steam turbines will play a big role in gleaning every megawatt possible. Better design tools and materials, as well as operating experience, have led to improved steam turbine designs.

Anil Gupta, GE Energy
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Power Engineering magazine asked five steam turbine original equipment manufacturers (OEMs) to provide an update on their companies’ steam turbine offerings. Representing Mitsubishi Power Systems Inc. (MHI) is Shinichi Ueki, vice president of commercial operations. Speaking for GE Energy is Anil Gupta, manager of steam turbine and generator product marketing. Alstom is represented by Heinrich Klotz, senior product specialist (Mike Davies, senior product specialist and André Van-Spaandonck, product manager, also contributed). And, responding for Hitachi Power Systems is Udo Zirn, turbine systems engineering manager. (The fifth invited OEM, Siemens, was unable to participate.)

Heinrich Klotz, Alstom
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Power Engineering (PE): Many power generators/utilities in North America face tight capacity margins that are expected to become even tighter. What “new and improved” steam turbine designs is your company offering that can help generators increase efficiency and get more megawatts from their existing plants?

Shinichi Ueki, Mitsubishi
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Mitsubishi: By incorporating the latest blading design and advances in materials, MHI provides complete replacement internals to fit within the existing cylinders of many different types of steam turbines. The replacement rotating and stationary blading, along with advanced design stationary components and sealing technology, result in improved efficiency and therefore increased electrical output at currently operating steam flows. This results in lower fuel cost and additional megawatts without increasing emissions. These features were developed to make Mitsubishi more competitive in the new steam turbine market.

Udo Zirn, Hitachi
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Hitachi: Hitachi is continuously improving its turbine designs to improve efficiency for both new turbine technologies as well as for retrofit applications. Among others, the latest design improvements include reduced clearances at shaft packing with Guardian Post and Vortex Shedder technology. The Guardian Seal is a non-contact seal that can be used at any location where gland rings are typically used and can be installed without the need for any modification of the steam path. In addition, a Vortex Shedding tip seal is installed to reduce the pressure ratio across the seal and, as a consequence, the leakage past it. Other design improvements include increased aspect ratios and reduced root diameters that result in a reduction of end-wall losses and advanced high-load blading with optimized pitch to reduce blade number and profile/end-wall losses. Hitachi’s single forging continuously covered blade (CCB) designs with integral mid-span support structures allow for higher backpressure operation and provide added strength, reduced steam leakage, larger damping and an overall reduction in vibration stress. The axial entry dovetail design of turbine blades reduces the blade axial length and allows for more stages to be installed at a given bearing span. The integral mono-block rotor with no shrunk-on surfaces or key ways reduces the centrifugal stress, eliminates stress corrosion cracking and results in a highly reliable rotor design.

GE Energy: A few of GE’s key service products designed to improve performance and output include Dense Pack technology, which was originally designed for fossil applications. It adds stages to the turbine, increasing reaction, to increase steam path efficiency and output. Dense Packs use advanced aerodynamic designs with optimized vane profiles and improved clearance control technology. GE Energy recently expanded its Dense Pack technology to support nuclear applications, incorporating a moisture-loss prediction model to optimize the design for the specific steam conditions of nuclear applications.

Another improvement is a 26.8-inch active length last stage bucket (LSB) with an axial entry dovetail, which replaces the original 26-inch LSB steampath design. Targeted to increase plant output, the new design offers increased annulus area and reduced exhaust losses.

GE also can provide up to 15 percent uprates for liquid-cooled generators to meet steam turbine performance requirements. While unique to every application, these uprates may include stator core-end redesign, cooling enhancements and use of advanced materials for insulation, laminates and core-end.

Portable Robotic FineLine Welding is an enhancement to the GE FineLine Rotor Dovetail Repair. This offering provides portable on-site repair that uses high-tech robotics and automation. This new technology, available for the fall 2008 outage season, reduces cycle time for customers because the repair can be completed on-site. Inspection devices called miniature air gap inspection crawlers (MAGIC) for in-situ inspections on generators are additional robotic product offerings. These crawlers can be used on generators with entrance gaps as narrow as 0.25 inches.

In addition, GE developed an inspection process that streamlines inspection of steam turbine rotors with remote electronic delivery and analysis of ultrasonic inspections. GE has performed a real-time phased array ultrasonic inspection of nuclear low-pressure turbine rotors at a remote nuclear facility, with the entire data analysis for rotors conducted at the company’s Schenectady, N.Y., facility. Engineers on-site were able to submit the data to the New York inspection team electronically.

Finally, GE has improved rewind processes and developed specialized tooling to provide rewinds quickly for emergency needs, potentially resulting in a 50 percent reduction in manufacturing and installation cycle time.

Alstom: By applying many of the design elements and principles Alstom uses in new equipment, performance levels of existing units can approach today’s standards, resulting in improved efficiency, increased output and, thus, reduced specific emissions and extended reliability and lifetime. The heart of each steam turbine retrofit is an up-to-date high-efficiency steam path design. Alstom works to improve the efficiency of its impulse and reaction type blading. Depending on the project specific conditions, Alstom optimizes the overall performance by applying either its 3-D type impulse or reaction type blading or a combination of both, even in one steam path. Alstom offers various low pressure last stage blades (LSBs) in full speed up to 38 inches in steel or 42 inches in titanium, both equipped with integral shrouds to improve efficiency and form a rigid blade ring in combination with snubbers. Besides the standard blades, Alstom has developed LSBs specifically for retrofit solutions.

Alstom can offer various upgrade package options ranging from steam path only to replacement of an inner module, full module or complete shaft line.

PE: What about your company’s latest designs for new power plants, both coal-fired and nuclear? How are the new designs expected to improve efficiency, increase capacity and reduce O&M expenses?

Mitsubishi: MHI has developed five main design features for turbine improvements. We offer a high-temperature HP-IP combined turbine for fossil plants. Generally, large capacity super critical steam turbines have a four-casing design: one single/double-flow HP, one double-flow IP and two double-flow LPs. MHI has developed a three-casing design option with one HP-IP combined turbine and two double-flow LP turbines for a super critical coal-fired unit up to 1,000 MW. Some of the advantages include: 1) easier operation due to simpler vibration characteristics and less thermal differential expansion during the start-up and shut-down periods; 2) easier maintenance due to fewer parts; and, 3) lower construction and maintenance costs due to a shorter turbine foundation and fewer parts.

MHI has made it possible to manufacture dissimilar steel welded rotors. Generally, 12 percent chromium (12 Cr) steel and low-alloy steels are used to manufacture rotors for steam turbine plants. For high- to low-pressure integral steam turbines that need to show sufficient strength at elevated temperatures, integral forged rotors made of high temperature resistant material (12 Cr) are often chosen. Use of 12 Cr steel on the high-pressure side is necessary because of its superiority in high temperature strength. Use of low-alloy steels on the intermediate- and low-pressure sides is preferable because of their superiority in toughness. MHI developed a dissimilar steel welding technique to join 12 Cr steel and low-alloy steel, as well as a common steel welding technique. This technique has made it possible to manufacture highly reliable rotors characterized by both high strength at evaluated temperatures and high toughness.

Another improvement is MHI’s integral shroud blade (ISB) technology for low pressure end blades. The use of longer last-stage blades will reduce the number of low-pressure turbines and will extract more energy from low-pressure steam before it is exhausted to the condenser, thus improving overall steam turbine efficiency. MHI uses 3,600 rpm 40-inch ISB last-stage blades. For ISB design, significant damping occurs because of the contact at the shrouds and snubbers, which is caused by untwisting of the blades under the centrifugal force. This helps reduce the blade’s vibratory stress. Low-pressure end blades are subject to corrosive conditions; therefore, it is also important to reduce the local stress level for these blades. For our ISB design, larger blade root is applied, resulting in a local stress concentration that is well below that experienced with conventional grouped blades. This feature enhances reliability in the corrosive environment. To improve performance, ISBs have been developed by applying a fully 3-D flow analysis method to the design to optimize blade efficiency.

For nuclear steam turbine generators, MHI is developing 74-inch ISB last-stage blades, which will be used with the

In addition, MHI has made steady efforts to improve the accuracy of predicting efficiency and internal turbine flows by using 3-D multi-stage flow analysis that considers viscosity. The results of these analyses
were confirmed with air turbine verification tests. In addition to conventional steady state analysis, MHI has established an unsteady flow analysis method capable of predicting loss producing mechanisms even more precisely.

As a result, MHI has developed new high-performance reaction blades using these methods. The degree of reaction and the 3-D stacking of profile are further optimized, compared with the flow patterns of conventional blades. Production of the secondary flow vortexes is controlled and the vortex zones are shifted toward the inside and outside endwall of each blade, reducing losses due to the secondary flows in the rotating blades. In addition, a new profile to reduce unsteady losses produced by the interaction between the rotating blades and stationary blades is applied to the middle zone of the blade height where profile losses are dominant.

Unsteady flow analysis has also quantitatively clarified that vortexes produced in the vicinities of the inside and outside endwall of each blade are due to leakage from and inflow into the spaces between each rotating blade and stationary blade. This flow interacts with secondary flow vortexes and increases the secondary flow losses. Accordingly, MHI is optimizing the shape of the flow paths including those between the inside circumferences of the stationary blades and the rotor disks and those around the rotating blade shrouds.

Finally, MHI has incorporated active clearance control (ACC). The ACC seal is a seal in which segments of labyrinth seal rings are made to be movable in the radial direction. When the turbine is starting or stopping and during turning operation after stopping, the segments are raised by a spring force to keep the clearance between the rotor and the seal fins large. On the other hand, when the turbine load is increasing, the seal segments are shifted until the proper clearance is maintained radially toward the center by using the pressure difference for sealing. Then, the narrowest clearance is kept during the loaded operation.

Hitachi: For coal-fired applications, raising the steam conditions is the most effective way to improve plant efficiency and to minimize greenhouse gases emitted to the atmosphere. Hitachi has developed new materials and structures for critical components that are exposed to high steam temperatures. HR 1200, Hitachi’s most advanced material, allows steam temperatures up to 1,200 F. Moreover, a sophisticated steam cooling system uses low-temperature steam to cool rotor, blades and nozzles that are exposed to high temperatures and eliminates creep damage on dissimilar welds. Hitachi is also developing further material advancements to allow steam temperatures up to 1,300 F. To date, six Hitachi steam turbines with ultra-super critical steam conditions are in operation worldwide.

To address the need for larger turbine capacities, Hitachi offers 33.5-inch, 40-inch and 45-inch last stage blades for the 60 Hz market. Shorter blades are available. All blades are capable of operation at elevated back-pressure associated with air cooled condenser applications, which has become a more frequent customer requirement for the purpose of plant water consumption minimization.

For the nuclear market, Hitachi offers its mature ABWR turbine design with an advanced and standardized modular construction that, in conjunction with modular construction of the nuclear power island, can reduce the total plant construction man-hours by nearly 40 percent. In addition, Hitachi offers steam turbine designs for PWR nuclear applications and for GE Hitachi Nuclear Energy’s ESBWR plant designs. Hitachi also offers its own turbine design in lieu of a GE design. To address the need for larger capacities, Hitachi’s mature 52-inch last stage blade is available for the 60 Hz
market and even longer blade lengths are under development.

GE Energy: GE Energy’s award for eight, 900 MW super critical steam turbines in Texas spurred a number of innovative design features. Although this order was cancelled, the developments were integrated across GE’s product portfolio. Some key features include: GE’s latest steampath technology with steam conditions of 3,770 psig/1,080 F/1,080 F; advanced 10 Cr and 12 Cr materials capable of providing improved thermal performance and long component life (the HP turbine section first stage buckets now use these advanced materials); advanced bush seals in the HP and IP turbine sections; enhanced physics-based clearance, which allows for improved turbine control and operability; and, improved LP section performance, achieved through improved inlet and exhaust designs and last stage group aerodynamics.

GE Energy’s next-generation ultra super critical 1,000 MW steam turbine incorporates several new key features. It is manufactured with advanced 10 Cr and 12 Cr materials and designed to operate at 3,770 psig/1,130 F/1,150 F. It also includes the Dense Pack steam path technology described earlier. The turbine design incorporates integral cover buckets (ICBs), which provide advanced sealing technology. The improved ICB tip sealing reduces tip leakage flow, improves radial clearances and reduces maintenance costs. Another feature is optimized exhaust hood aerodynamics, which lower exhaust pressure and optimize annulus area of the G3 turbine, improving heat rate and performance. The turbine has direct actuated valves that incorporate advanced hydraulic technologies and eliminate multiple moving parts, providing improved controllability and reduced maintenance. In addition, the turbine’s 45-inch last stage buckets (LSB) provide application flexibility, improved performance in hot temperatures and an increased annulus area for a reduced number of LP sections.

GE Hitachi Nuclear Energy’s next evolution of advanced BWR technology is the Economic Simplified Boiling Water Reactor (ESBWR). Key features of this 1,550 MW product will include: 52-inch LSB, Dense Pack steam path, main stop and control valves and combined intercept valves, advanced controls architecture, and the world’s largest generator–a four-pole generator with an estimated rating of
1,913 MVA.

Alstom: Both our fossil and nuclear steam turbine designs are characterized by high efficiency, simple and robust design and good accessibility for inspections and maintenance work. Both turbine types have unique design features leading to high and sustained efficiencies and outstanding reliability and availability. For fossil applications, the shrink ring HP design of the Alstom STF series is notable, as is the top-performing high-strength steel or lightweight titanium last-stage blades. For 60 Hz, the maximum blade length for steel is 38 inches and the maximum for titanium is 42 inches. For 50Hz, the maximum steel blade is 45 inches long and the maximum titanium length is 49 inches. The steam turbines can have up to six low-pressure exhaust flows. The adaptation to high-temperature applications is maintained by selective material substitution for casings and rotor sections. For high-temperature sections, Alstom uses 9 percent Cr materials developed in the European material development programs COST501 and COST522. The materials can cope with steam temperatures up to 1,150 F. Alstom is currently building a 1,100 MW single shaft unit that can operate at steam temperatures up to 1,112 F and up to 1,150 F for reheat steam.

Alstom’s ARABELLE steam turbine for nuclear plants features a combined HP/IP design with single flows, leading to long blades with increased efficiency compared to more conventional arrangements. It has proven efficiency and reliability records (large units: forced outage rates less than 0.03 percent). The two last-stage blades for 60 Hz machines are 48 inches and 56 inches long, while the two last-stage blades for 50 Hz machines are 57 inches and 69 inches long. The large units feature two or three low-pressure turbines.

PE: Discuss how the retrofit turbine market in North America compares to the new plant turbine market?

Mitsubishi: Our retrofit market is a small percentage of our new unit market business in the United States. The new unit market for coal-fired plants has seen many political challenges and some difficult technical hurdles affecting siting and investment decisions. As a result, the number of new coal-fired units being built has dropped dramatically from only a few years ago.

However, with the number of operating coal-fired units where improved flue gas cleaning has been installed, the ability to improve the steam turbine efficiency for more output has become a strong driver in our increasingly active market. The interest in recovering parasitic losses while avoiding triggering New Source Review has stimulated the steam turbine retrofit market.

Without exception, the need to avoid triggering New Source Review has utilities cautious about making sure that the heat input prior to steam path improvements is not increased as a result of the steam path efficiency improvements. Many owners are taking extreme measures to confirm the steam flow and heat input before and after any retrofits. With improved efficiency and utilizing the same steam flow, the total heat input to the boiler should not increase. Modifications to the boiler may be needed to shift the heat load but careful attention is paid to avoiding increasing the heat input.

Hitachi: To better support the U.S. retrofit market and customer after-sales support for new equipment, Hitachi acquired Mechanical Dynamics & Analysis Ltd. (MD&A) in 2005. MD&A’s capabilities enable us to support our own retrofit and new unit fleets in addition to providing turnkey overhauls, generator field rewinds, complete liquid cooled stator rewinds, liquid cooled stator bar clip replacements, installation of modern designed Hitachi L-0 buckets and centerline installations on a variety of other OEM equipment.

GE Energy: The North American market has been active across all applications from new units to services. Coal-based power generation continues to be a key factor in the North American generation mix, although environmental concerns and rising capital costs have led to suspension or cancellation of some new projects. Delays in these projects have made continuing investment in the existing fleet of coal-fired assets more important to our customers. The market is requiring the existing fleet to operate at higher capacity factors with more flexible dispatch capabilities. GE has a range of solutions to enhance the economic value of our installed fleet. Depending on the unit’s age, solutions include: steam path uprates, life extension solutions, generator rewind and in-situ monitoring services. GE uses common technologies and repair solutions to apply the most cost-effective, high-technology solutions to the customer’s need. In addition to core technology, GE is also investing in a regional service network to provide field support and repair solutions. GE continues to broaden its steam trubine applications expertise. For example, GE recently secured an order for the Duke IGCC project and will provide a steam turbine to accommodate the increased steam flow from this IGCC facility.

Alstom: The turbine retrofit market in North America has traditionally been entirely separate from new equipment. It is fair to say, however, that North America has been a leading market for steam turbine retrofit. For the last decade, it has yielded a substantial proportion of Alstom’s annual turnover. The business for new equipment is characterized by much greater annual fluctuations.

Retrofit does not aim to compete with new business. Each is aimed at a different market segment. For example, a retrofit might be attractive to a small single station utility if it has a market for the additional 5 percent output (perhaps 50 MW) that a retrofit could provide. A new station, on the other hand, might produce 2,400 MW. The question is rarely “new build or retrofit.” The strategy may include a combination of both, such as that currently taking place in South Africa.

A decision about whether or not to retrofit usually includes several drivers, including financial, regulatory and technical factors. Often some of the financial issues are shared with new plant builds. The differentiators are usually technical and regulatory issues, such as plant life extension and environmental regulation. Retrofit can, for example, produce power to the grid much more quickly than a new station and, by performance improvements, without increasing emissions, thus avoiding New Source Review restrictions. Also, planning constraints usually are not a problem with retrofit, but can be with a new station.

PE: Some major turbine manufacturers have steam turbine manufacturing facilities in North America to meet demand. Discuss your company’s manufacturing strategy for supplying turbines and related components to the North American market.

Mitsubishi: MHI will open its new manufacturing facility in Orlando, Fla., in June 2008. This new facility will double our current factory space in Orlando and will manufacture state-of-the-art gas turbine F- and G-class blades and vanes for both the Western Hemisphere and Japanese markets.

Hitachi: Hitachi’s steam turbine manufacturing facilities are in Hitachi City, Japan. This factory has manufactured steam turbines since 1933. To control costs, many of Hitachi’s steam turbine materials are procured worldwide. Most of the steam turbine auxiliaries are currently procured from U.S. suppliers.

GE Energy: GE Energy has steam turbine manufacturing facilities in Schenectady, N.Y., and Bangor, Maine, as well as component factories in Latin America and Asia. GE plans to make investments to increase supply chain and engineering capability on a global basis to meet increasing demand for steam turbines around the world. Expansions for steam turbine production and engineering centers are planned in North America, Europe and Asia.

Alstom: Alstom has a worldwide manufacturing network for steam turbines, including Birr, Switzerland; Belfort, France; Elblag, Poland; Beijing; Mannheim, Germany; Morelia, Mexico; and Chattanooga, Tenn. Chattanooga was inaugurated in December 2007 and will become Altom’s largest steam turbine manufacturing shop, including a rotor
weld shop.

PE: Although several coal-fired power plants have been cancelled or put on hold in the United States recently, coal-fired plants are still being built in other parts of the world. How has this affected your company’s ability to meet demand for large turbines worldwide?

Mitsubishi: As a global manufacturer, MHI has to balance the delivery requirements of our customers early in the discussion of any new steam paths or new units. The challenge has increased greatly as a function of the continued global new unit market and the very active steam path replacement market in the United States.

We are also challenged to meet the schedule requirements of our U.S. customers. For example, a discussion on steam turbine upgrades that may have begun two years ago with a planned delivery in 2010 (based on an 18-month lead time from order placement), may now require more than 24 months to deliver. The lead time is determined mostly by the forging lead time from our two main suppliers.

Hitachi: Hitachi supplies steam turbines worldwide. Despite a large factory capacity, the worldwide demand does impact the ability to meet U.S. demand, and vice versa. In many cases, our ability to meet demand is not limited to our factory capacity, but by critical material suppliers’ limitations. In particular, suppliers of advanced materials required for ultra-super critical steam conditions are limited.

GE Energy: Construction of pulverized coal-fired plants is occurring around the world. However, in China and India, this capacity is being supplied primarily through state-owned manufacturing enterprises serving their domestic needs. GE Energy continues to provide pulverized coal and nuclear steam turbines in select areas. Orders have recently been secured in South Korea for super critical pulverized coal and nuclear steam turbines and in Chile for sub-critical pulverized coal steam turbines. GE is addressing strong demands for combined cycle power plants in Europe and independent water and power plant applications in the Middle East.

Alstom: The Chattanooga manufacturing and engineering site will enable Alstom to serve its U.S. customers from a domestic base more rapidly and more cost-effectively by reducing supply chain/logistics costs and time. It will also minimize currency risks for Alstom’s U.S.-based customers. The site will benefit from its location on the Tennessee River which gives it access to 80 percent of existing or planned nuclear sites. The engineering office will be operational this year. The staff will adapt designs for the U.S. market and support NAFTA sourcing. The actual manufacturing facility will begin operations in 2010.

PE: Although several nuclear plants are planned for the United States, none are yet under construction. How has worldwide demand already affected, or how do you expect it will affect, your company’s ability to meet future demand in the United States?

Mitsubishi: The primary impact of the continuing new unit global demand has been to utilize the existing global manufacturing capacity, especially in the area of large high-quality forged steel items. While manufacturing time on these replacement steam turbine rotors and cylinders has remained essentially unchanged, the lead time to obtain a high-quality forging specific to the application has increased two-fold in the last several years. Mitsubishi is in the process of expanding its Takasago factory to accommodate the largest anticipated nuclear LP rotors (74-inch last row blades) and provide the blade forging capability for these extremely long blades.

Hitachi: Hitachi has continuously been building nuclear power plants since the 1970s. We have executed several nuclear power projects on a turnkey basis and are currently involved in multiple nuclear power plant projects worldwide. Hitachi is prepared to supply nuclear steam turbines to the U.S. market, either as part of GE Hitachi Nuclear or as a supplier to other entities. Hitachi has allocated future factory capacity and raw materials for nuclear steam turbines to be supplied to the U.S. market.

GE Energy: GE Energy has secured manufacturing capacity for major long-lead components for ESBWR projects. We are also involved in the design of a new steam turbine for a PWR application in South Korea with an expected COD of 2013.

Alstom: Alstom is expanding its manufacturing capacity worldwide in regions where nuclear is developing. For the U.S. market, Alstom is reequipping and investing in the manufacturing workshop in Chattanooga for turbines and generators. In addition, Alstom is expanding capacity in its Morelia factory in Mexico. With these investments (more than $200 million in the Chattanooga facility), Alstom will be able to fully meet future U.S. demand.

PE: Discuss the major supply chain issues you face and what steps your company is taking to control costs, which generally have escalated dramatically in recent months.

Mitsubihsi: MHI has begun expanding its factory manufacturing capabilities to improve its throughput. However, the current constraint on production is the capacity of high quality steel forging suppliers. These facilities (suppliers) are increasing their capacity to address the anticipated global new nuclear steam turbine market. Materials cost, unfortunately, are not an area where we can compromise on the specific requirements of our alloy steel composition. One of the key factors in providing steam turbines that avoid stress-corrosion cracking is the carefully controlled material composition of our alloy steel from the forging supplier. This area cannot be compromised and so we are at the mercy of the global market for vanadium, chrome, nickel and other alloy steel key materials.

Hitachi: The recent turbine price increase is due mainly to material cost escalation and exchange rate fluctuations. To minimize cost, Hitachi is sourcing material worldwide.

GE Energy: In response to challenging material cost escalations, GE Energy is looking to expand and diversify its supply chain base and operations on a global basis.

Alstom: The industry is currently experiencing a period of very strong demand, which is putting a lot of stress on the forging capacity in particular. Alstom has secured a worldwide supplier network that allows it to secure its needs and meet the needed capacity.