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This report provides guidance for power plant engineers contemplating modernization of their main turbine overspeed trip systems. When a large power plant turbine suddenly loses its output shaft loading due to a generator or power grid problem, the steam flow driving the turbine must be cut off very quickly to prevent an overspeed event. The overspeed trip system protects personnel and plant systems by preventing missiles that can result when turbines disintegrate at higher than normal rotational speeds. It also protects against financial losses of an extended outage and premature turbine replacement. Most power plants still use mechanical systems with moving weights and springs to detect overspeed conditions and initiate steam shutoff. These systems are now obsolete, and updated systems offer significant performance and maintenance improvements. This report explores key technical issues and decisions that should be considered in planning and implementing an updated turbine overspeed trip system.
ObjectivePower plant engineers need to properly manage the aging and obsolescence of turbine instrumentation and control equipment to maintain the safety and operability of the plant. In this case, they need the tools and knowledge to make the transition from obsolete mechanical system technology to updated digital technology in such a way that they realize the benefits and avoid potential pitfalls. Digital technology is relatively new to turbine engineers who maintain mechanical overspeed trip systems, and it can be far more complex in some ways, with potential for new failure modes and unintended functions. Questions also arise in regard to potential insurance implications and manufacturer warranties and recommendations. The objective of this report is to help plant engineers recognize and address pertinent issues and make the important decisions that will arise in planning and implementing a turbine overspeed system upgrade.
ApproachThe project team's goal was to provide practical guidance that will help utilities address specific questions that concern turbine overspeed trip modernization. An EPRI technical advisory group comprised of utility representatives guided the development of the report, and turbine manufacturers and insurers were consulted to ensure that the resulting guidance would be useful and practical.
ResultsReplacing a mechanical overspeed trip system with a modern digital system can eliminate many operational and maintenance problems associated with mechanical systems and also can improve safety and reduce the risk of damage from overspeed events. Turbine manufacturers and insurers are updating their policies accordingly. To ensure high dependability of the new systems, plants may have to update their processes, procedures, and expertise to properly evaluate and apply the new technology.
Application, Value and UseThis report does not attempt to make turbine engineers experts in digital technology, but it will make them aware of key issues and areas where they may need to seek expert help. It also will help them determine important requirements of the new systems, assess merits of various vendor design approaches, and understand utility responsibilities in operating and maintaining the new systems. The report focuses to some extent on nuclear power plant issues, but the guidance can be applied to any turbine-driven large rotating machinery, including steam-driven feedpumps, safety injection turbines, combustion turbines, and diesel engines. This document also provides guidelines for eliminating the mechanical overspeed trip, thereby reducing the mechanical maintenance burden and the potential for serious damage when testing the overspeed trip system. The guidelines are consistent with the approach taken in current installations by major turbine vendors. This document does not provide all the requirements needed to specify a replacement system; it focuses on those features and capabilities needed to achieve specific improvements and ensure high dependability.
EPRI PerspectiveTurbine overspeed trip protection is a clear example of an application where upgrading to digital technology can have clear benefits in improving safety and reliability while reducing maintenance costs. Insurance company data suggest that about half of recent turbine losses occurred during testing of mechanical overspeed trip systems at elevated turbine speeds; digital systems can be tested without overspeeding the turbine, eliminating this risk altogether. Digital systems have better setpoint accuracy as well as hardware fault tolerance and automated diagnostics to increase reliability and reduce the maintenance burden.
Turbine overspeed protection also is a clear example of an application where consequences of a failure can be catastrophic, so users should take great care to properly specify requirements, evaluate products, apply administrative procedures, and maintain configuration control. These are all areas in which power plant engineers will have to be proficient as the industry migrates from obsolete analog technology to modern digital systems. EPRI anticipates that this guideline on modernizing turbine overspeed trip systems will help utilities make the most informed decisions possible as they move forward.
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