Technological advancements offer 7FA owners a future for a mature fleet with considerable options.

 

By James Neurohr, Lex Poth and David Maher, PW Power Systems

 

As the technology of the Frame 7FA fleet begins to reach maturity, operators aim to find the best service and product options to optimize their sites. This push to seek the most beneficial offerings has created a robust aftermarket support industry that includes the supply of individual Balance of Plant items as well as full-fledged, multi-year contracts covering gas turbines, steam turbines, generators, and more. This aftermarket is driving down the overall life cycle cost of operating these assets, presenting a more competitive marketplace for owners and operators to find support, and increasing operational flexibility, also.

Maturity Cycles of Gas Turbines

Life cycle costs of power plants drive the power generation industry. This includes competition for aftermarket repairs, new parts, and outage services, and among operational sites to find an offtaker or bid their generated power onto the grid. These costs can keep a natural gas power plant active versus a coal plant in the day-ahead markets throughout the country, or they can make or break a repair facility keeping up with the latest trends. What it all comes down to is cost in $/kW.

These life cycle costs typically follow a “maturity cycle” that moves through three distinct phases (see Figure 1 on pg. 38). The phases can be stretched or shortened on the X-axis representing time in years, or can see costs fluctuate on the Y-axis based upon the innovation of underlying technology; but these curves keep roughly the same shape throughout the industry with each introduction of a new product.

Phase One represents the “Introductory Phase” of any new gas turbine model and covers the first months to years, depending on the underlying technology (e.g., use of new superalloys, casting technology, steam/air cooling, etc.), and its duration in the industry. In this stage, the OEM is the sole source for technology, parts, and repairs. Costs of operation are high, as with any new technology, and cost risks may be somewhat mitigated through Long Term Agreements (LTAs) with the OEM. Risk is also higher during this phase as the operational learning curve is tested. Early design issues are discovered during operation, redesigns are introduced or imminent, and component-life, outage-frequency projections are either met or realized will not meet expectations. There is also a sole source for data; all training and information comes directly from the OEM and there is not an accrual of knowledge in the aftermarket, yet.

Phase Two is the “Turning Point Phase,” an introduction of third-party aftermarket resources and a shift of knowledge from the OEM. More turbines are introduced and commissioned, early design issues are being resolved, and true operating histories establish component life. At this point, new parts are still being sole-sourced from the OEM, and prices reflect a lack of competition. Operating costs start to lower, however, due to the knowledge shift to the aftermarket. Some OEM personnel move to third-party shops, outages are managed by traditional outage-service companies, and competing shops leverage experience from previous turbine models into the repairs of these components. Operators benefit from the new competition and access the aftermarket to drive repair pricing down.

Phase Three is the “Maturity Phase.” The third-party aftermarket gains turbine technology experience comparable to the OEM and begins to comprehensively force operating costs down through the supply of new parts, repairs, and outage services. The OEM still has original LTAs at fixed costs and profits from that stream of revenue, but in the open market, it faces staunch contenders and a realization that price reductions are necessary for viability.

 

In the early stages, the owner’s perspective is that an LTA “caps” their life cycle cost risks. Self-performing is not common in these stages, and risk is perceived throughout the industry due to the uncertainty of component life and outage-cycle length. At this stage, the OEM’s R&D concentrates on reducing internal costs to increase returns. Often, LTAs are required by lenders for nonrecourse debt financing to enforce the “cost cap” and by developers to shift the unknown risks to the OEM. Together, these influences render the aftermarket nearly nonexistent.

Fixed-cost LTAs are typically priced out of the market in the Maturity Phase. Owners and operators are focused more on cash flow and cost reductions than risk mitigation because fleet issues are becoming well known and documented, and multiple sources to which they can turn for support are available. Limiting the risk transfer premium diminishes the value of sole sourcing and increases the viability of the aftermarket options that demonstrate proven technology.

The Frame 7FA fleet is now entrenched in the Maturity Phase of its life cycle. Owners and operators have numerous options for new and used parts, repair facilities, outage crews, controls experts, etc. The interesting development for this fleet, however, is the introduction over the last few years of the 7FA.04 or the Advanced Gas Path solution by the OEM. The solution claims a new combustor and turbine section aimed at increasing output without installing new turbines at established power plants.

This represents an effort by the OEM to move the life cycle of the mature fleet back into the “Introductory Phase,” although there is already a clearly established wealth of knowledge in the third-party aftermarket. This push will hold introductory prices high for the newer solution while the OEM feverishly strives to reduce costs and maintain higher margins. The solution includes new specifications of turbine component alloys that are known to be difficult to weld which result in high scrap rates during the learning stage of the repair cycle. Additionally, minor geometric changes — with a claim of higher performance — were introduced. The OEM has endeavored to lock in certain features to prevent third-party aftermarket companies from providing direct replacement part options exacerbating rising operating costs at the site.

The owner of the now mature turbine is pressed into another LTA, thus eliminating the benefit of having a strong aftermarket. This has led to a burgeoning form of opposition against a new product line that is being met with an aftermarket repair and outage solutions business, as well as new product launches by non-OEM aftermarket groups, aimed at offering increases in output and flexibility that allow 7FA owners to stay competitive in their markets.

Operators Looking for Aftermarket Solutions

For decades, Pratt &Whitney (“P&W”), a division of United Technologies, has been at the forefront of super alloy and advanced coating development in the aerospace industry. To ensure performance and overcome the challenges of oxidation, corrosion, and creep under extreme operating conditions, P&W engineers have successfully eliminated grain boundaries within super alloys and have implemented new casting methods to create directionally solidified and single crystal components.

PW Power Systems, Inc. (PWPS), formerly Pratt & Whitney Power Systems and now a group company of Mitsubishi Hitachi Power Systems, Ltd., has an exclusive license to these advanced alloy and coating technologies, and continues to utilize the same expertise to revolutionize industrial gas turbine (IGT) performance in the power generation aftermarket. PWPS collaborates with P&W in the development of these advanced alloys for use in the IGT marketplace, providing the PWPS components with an unparalleled level of durability and performance. Stress rupture testing demonstrates that these advanced alloys allow PWPS the advantage of enhanced creep strength as well as oxidation resistance when compared to those alloys used by OEMs and other replacement part manufacturers.

Nearly 20 years ago, the IGT marketplace presented an opportunity for a suite of advanced airfoil bond and thermal barrier coatings and the PWPS San Antonio Service Center addressed this need when approached by the marketplace. With these coatings applied to IGT components, PWPS customers enjoy the benefit of enhanced operational life and lower scrap rates. In the IGT market, the proprietary coatings are able to show improved oxidation resistance through HVOF-applied metallic bond coats and enhanced short- and long-cycle durability with advanced thermal barrier coatings as.

Expanding its niche, the San Antonio facility ventured to enhance component life with improved, high technology repair materials and techniques. Utilizing superior welding and other process advancements, coupled with the strict quality control sustained from its flight engine repair background, the San Antonio shop began to produce high-yield results salvaging otherwise unrepairable parts. In fact, repaired parts often returned to the shop after a second run in better condition than when first inducted.

While the San Antonio IGT group grew its repair business, customers sought a new solution from PWPS to replace scrap parts, which introduced a new opportunity for the engineering team: manufacturing new parts. With years of experience coating and repairing IGT components, PWPS had familiarity with fleet reliability issues and failure modes. Rather than reproduce an existing part with prevailing issues, PWPS seized the chance to employ actual field data and known failure mechanisms to develop corrective solutions for problems experienced across the 7FA fleet.

This was the start of the “Repair-to-Replace” paradigm: build upon repair experience to reengineer a new solution. PWPS could then enhance components through a complete reengineering process by evaluating the base alloy to improve creep strength, increase oxidation and corrosion resistance, and incorporate remedies such as internal cooling enhancements for each component to relieve stress on each part in the turbine section. This model allows PWPS to build a solution based upon customer and fleet-observed experience and is backward compatible with OEM components. Furthermore, this process resulted in the design and manufacture of a suite of replacement hardware for the 7FA.03 fleet that provides enhanced durability, is highly repairable with low scrap rates, and lowers overall life cycle costs.

The development of products and services like those offered by PWPS and other third-party repair facilities or new part providers drove the 7FA market into the Maturity Phase. The competitive marketplace that ensued offered new options to fleet owners and operators; continued innovation and improvements to the technology followed.

Summary of the Key Upgrades Provided by PWPS 7FA.03 Solutions

As a key element of its design solution for the 7FA.03, PWPS incorporated its advanced alloys into the turbine section. Stages 1, 2, and 3 buckets feature an advanced, proprietary nickel alloy required to improve creep strength and low cycle fatigue while providing margin to allow for future growth potential and increased oxidation resistance for extended life.

 

 

PWPS also utilizes advanced thermal barrier coating systems with these components. Improvements made to the Stage 1 and 2 nozzle systems feature a full-throat coverage coating solution not seen in the OEM offering. The Stage 1 shroud block also features a fully abradable thermal barrier coating system offering both performance and durability benefits to the base OEM configuration. This improved, abradable coating is unique to the industry; designed to be truly sacrificial, it does not create wear on the bucket tip and therefore allows for improved sealing.

The final element of the PWPS 7FA.03 is the implementation of feature improvements to the OEM designs. The Stage 1 bucket design has incorporated as-cast trailing edge geometry to eliminate low cycle fatigue cracking modes at the cooling holes. The Stage 1 bucket design also uses additional tip and platform cooling to improve oxidation capability while addressing known durability modes. Stage 2 and Stage 3 buckets feature weight reductions to the airfoil shroud offering improved creep characteristics. In addition to advanced alloy selection, the shroud-tip geometry improvement has attained 72,000 operational hours with two repair intervals on the lead set of Stage 2 buckets with zero scrap buckets in each interval. Finally, Stage 1 and 2 nozzle designs incorporate improved airfoil-to-platform fillet geometry providing enhanced low cycle fatigue and preventing associated cracking.

With these designs, the PWPS 7FA.03 replacement solution represents a fleet upgrade relative to the OEM offering when used at the same standard operating conditions. This solution is not strictly limited to the standard 72,000-hour life cycle, allowing an opportunity for PWPS Engineering to assess further component life extension.

Evolution Path to the PWPS 7FA.03+ Solution and Operating Parameters

As the OEM introduced its .04 solution and new operating parameters (i.e., higher firing temperatures, increased output, and extended maintenance intervals), the logical next step for PWPS offering advanced 7FA.03 replacement parts was to allow 7FA users to run at a higher firing temperature. Operators could receive measurable value from the already upgraded PWPS designs. PWPS employed the repair history of its 1,000-plus sets of repaired OEM and PWPS-designed 7FA components to re-baseline the finite element analysis and predict new part failure modes. Performance models were updated to simulate higher firing temperatures, and the hardware that required redesign was identified. The result of this study prompted a redesign of the Stage 1 components and a confirmation of the existing life margin of the Stage 2 and Stage 3 PWPS 7FA.03 hardware to allow higher output and longer intervals.

The initial 7FA.03+ study determined that a simple over-fire of the unit would yield higher NOx due to the unbalanced combustion system. The solution was to redesign the Stage 1 nozzle to cut cooling and leakage flow and allow extra air to enter the fuel nozzles during combustion. By balancing the air-to-fuel ratios, the operation of the combustion system – reduction of NOx emissions, turndown, and tunability – is maintained. In addition, the Stage 1 nozzle throat was retuned for extra flow to retain the pressure ratio. The result is higher output without the risk that would result from operation of the combustion and compressor systems at off-design points. To implement the new 7FA.03+ program, a simple exhaust temperature constant is updated in the control system allowing the upgrade to be used in conjunction with standard Mark IV controls or any of the other aftermarket auto-tune control systems available.

The 7FA.03+ Stage 1 nozzle with improved cooling can withstand the hotter firing temperatures. The key to the 7FA.03+ Stage 1 nozzle redesign is a change in the configuration from a doublet (two airfoils per part) to a singlet design (one airfoil per part). The singlet design affords better line of sight for drilling additional cooling holes, which permit more efficient film cooling and allow for optimized oxidation and spallation-resistant TBC coating to be applied. The geometry of the outer and inner sidewall hook is optimized for strength and flexibility. This reduces the thermal interaction that causes low cycle fatigue (LCF) cracking, an issue that affects the doublet 7FA.03 design. Compliant w-seals are also added to the chordal hinge, thereby reducing loss of cooling air past the Stage 1 nozzle.

The redesign of the 7FA.03+ Stage 1 bucket augments the improvements of the existing PWPS 7FA.03 Stage 1 bucket. The 7FA fleet platform has been troubled by LCF and start-based cracking. The new 7FA.03+ bucket implements a channel that actively cools the underside of the platform resulting in significant life extension. The existing tip design, with a plate to close the internal passage, has been redesigned to be integrally cast with the blade eliminating the risk of plate liberation as well as allowing for more and smaller cooling holes to increase the oxidation life.

“7FA owners must educate themselves on marketplace offerings. Those who take advantage of the flexible technology… will remain competitive.”

 

 

The 7FA.03+ Stage 1 shroud resembles the existing 7FA.03 shroud tile and employs the same upgraded alloy, but is redesigned to better utilize the cooling flow that is delivered by the outer shroud block. A compliant seal reduces cooling flow leakage and permits more flow to cool the edges of the tile. The shroud tile utilizes the same abradable coating as the current PWPS solution that can be custom sprayed to reduce first bucket tip clearances.

As described earlier, the 7FA.03+ utilizes the same upgraded Stage 2 and Stage 3 components that PWPS offered originally. Each component has been reanalyzed at the updated 7FA.03+ temperatures and meets program goals. This approach allows PWPS to use existing inventory and affords an even greater customer value at the higher temperatures. Rather than an all-encompassing redesign of the turbine and combustion systems, this new system allows for flexibility within an owner’s fleet and lowers customer cost, and enables a simpler path for upgrade and a faster pace to market. The reparability of the .03+ components is already proven with well-defined and high yielding processes due to the high level of commonality among existing PWPS-designed parts.

Mature Market Flexibility

The maturity of the 7FA marketplace brings an opportunity to any owner/operator within the fleet. A profusion of knowledge, resources, and vendors exist within the aftermarket space compelling innovations and solutions that result in lower operating life cycle costs. As these costs continue to be driven down, 7FA owners must educate themselves on marketplace offerings, and those owners who take advantage of the flexible technology being offered will remain competitive. These technological advancements ensure a future for a mature fleet with considerable options.