Relative to historical industry experiences, D11 steam turbines have come of age at an early stage in their lifecycle and are strong candidates for retrofit solutions. The most common reasons reported to MD&A for prompting retrofit consideration include (1) reliability issues, (2) rotor and shell end of life and (3) the need for increased power. Historically, reliability and end of life considerations occurred after several decades of operation. However; the experience of progressive major reliability and operational issues coupled with outdated technology warrants the evaluation of a retrofit solution much earlier for the D11 to address continuously increasing maintenance costs. As Owners invest in upgrades and rotor life extensions on their combustion turbines, many recognize that the time has come to address the poor reliability and end of life considerations on the D11 steam turbine to ensure the plant will reliably operate until the targeted retirement date. Currently, many power producers are being asked to supply more power, which is addressed by the enhanced efficiency and opportunity to increase flow with the retrofit.
In MD&A’s experience, significant mechanical failures, particularly in the High and Intermediate Pressure section (HP/IP), such as outer shell cracking and creep, continue to worsen in both severity and frequency of occurrence. In addition, other reliability issues such as poor vibration characteristics, bowed rotors, and diaphragm dishing further contribute to increasing losses in unit’s thermal performance.
The underlying reasons for early maturation of the D11 fleet are varied. Application of low-alloy, single-shell configurations operating at 1050 F is a major factor in the reliability issues of the D11. However; the evolution of operational demands for combined cycle power plants also plays a major role in minimizing life expectancy. Early industry expectations were that combined cycle plants would be primarily base loaded. However; as the industry matured, combined cycle plants were eventually required to be capable of sustained, frequent load cycling, and start/stop operational profiles. By itself frequent cycling can be sufficient cause for early onset mechanical failures. The coupling of operational cycling with single-shell, low-alloy components of the D11 has a multiplying effect. Based on reported feedback, the severity and range of mechanical issues for the D11 fleet continue to progress with time.
In circumstances where it is determined that the best approach is to repair and maintain existing turbine hardware, MD&A offers a full complement of turn-key field services, repairs, and parts. MD&A has a 20-year history of supporting the D11 fleet including such services as weld repair of cracked shells and packing heads, straightening of bowed rotors, diaphragm repairs, installation of advanced sealing packages, and steam valve internal component manufacturing and replacements. MD&A also offers replacement valve body casings and last stage buckets. MD&A is recognized as an industry vanguard in the area of fossil steam turbine shell weld repairs for over many decades.
For units that experience progressive reliability issues, spiraling maintenance costs, lost thermal performance and electrical generation, a 15-20 year life-cycle Cost of Electricity (COE) evaluation suggests a retrofit which, in comparison to continuous maintenance expense of the existing design, significantly out performs the later on a net present value basis (NPV). Additionally, many units are nearing end-of-life for the rotor and shell which becomes a another driver for the retrofit. The magnitude of NPV spread is primarily dictated by unit/customer specific data. As an example, shell cracking and advanced creep at the main fit ultimately necessitates shell replacement for long term resolution as a weld repair is considered only a temporary solution to return the unit to service. Shell cracking is also common at other locations such as the HP and IP inlet areas. Discovery of shell cracking during maintenance inspections or as a root cause for forced outages results in high repair costs and disruption to the owner’s operations. Additionally, the HP/IP rotor life is limited by low cycle fatigue driven by the number of starts. For plants that require longterm operation (15+ years), the shell and rotor end of life must be addressed. An important question for D11 owners is; How close to end of life is their rotor and shell? How much further along will these components be at their next major? A proactive approach provides the opportunity for significant additional value via upgrades. A reactive approach will result in limited options and possibly extended time off line.
MD&A, in conjunction with its parent company Mitsubishi Heavy Industries (MHI), has developed a “Drop-In” retrofit solution for the D11 HP/IP section which addresses fleet reliability concerns, providing excellent availability, and enhanced thermal performance. The MD&A/MHI design was created by leveraging proven steam turbine technologies from MHI. Given MHI’ extensive combined cycle fleet experience, our design team was able to develop a retrofit solution which maintains plant mechanical and thermodynamic interfaces. Each retrofit offered by MD&A/MHI is custom designed and optimized for its unique application and includes replacement of the existing flexible, low alloy single shell with a robust, double shell construction utilizing high chrome material within the high temperature locations. The new shell facilitates implementation of other improvements such as a larger diameter rotor with excellent vibration characteristics and incorporation of a high efficiency steam path. Similar to the double shell, these components also utilize a robust mechanical design including high chrome material for superior creep characteristics.
Inherent with the retrofit, an opportunity exists for re-optimization of the steam bottoming cycle including an increase to duct firing. In situations where plant operating missions are modified, combustion turbines are upgraded, or duct firing profiles are altered, resetting the unit design point can provide the end user with significant value. Design re-optimization and/or increasing flow can yield a much shorter Return On Investment (ROI) for the retrofit project beyond resolution of reliability and thermal efficiency deficiencies. Some owners have verified their BOP can support an additional +10% flow without reaching an significant mechanical or electrical breakpoints.
Through its services experience and a multi-year detailed engineering study of the D11 configuration, MD&A/MHI have developed an HP/IP section “Drop-In” retrofit solution based upon proven steam turbine technology with a strong project payback. For more information, contact MD&A at (518) 399-3616 or use our contact form.
Also, read how MD&A has expertise in innovative D11 repair solutions while delivering consistent quality and value along with superior communications.
