Protecting Steam Turbines During Shutdown



Protecting Steam Turbines During Shutdown

A recent EPRI report describes the incipient damage that can be incurred by steam turbines during improper power plant shutdown and reviews techniques for protecting the turbines using dehumidified air.

The report is entitled Shutdown Protection of Steam Turbines Using Dehumidified Air (1014195). Qualified funders may view or download a copy of the report by clicking on the link below.

How to Apply the Results

Improper protection of steam turbines and subsequent corrosion damage are estimated to cost the U.S electric industry in excess of $1 billion annually. This report describes methods for the use of dehumidified air to protect steam turbines during shutdown. These methods are applicable to all steam turbines, regardless of the fuel source. In particular, combined-cycle heat-recovery steam generator (HRSG) and cycling/peaking fossil units stand to realize the greatest benefit as a result of the repeated startup and shutdown periods and highly transitional steam quality conditions.

Background

EPRI research has determined that proper protection of the steam turbine during shutdown periods is essential to the prevention of damage by stress corrosion cracking and corrosion fatigue.

Elimination of moisture following shutdown presents a major challenge to the power plant engineer and operator. The steam environment naturally exceeds the 60% humidity level at ambient conditions, which is frequently identified as required for proper turbine protection. The object is to condition the turbine environment beginning at the initiation of shutdown and control the moisture levels below 40%. Use of nitrogen to eliminate oxygen presence in a turbine is deemed, by most utilities, too hazardous to personnel safety to be a viable alternative.

EPRI Guidelines

For this report, EPRI developed a preliminary roadmap to understanding the damage mechanisms and development of an overall dry or dehumidified air layup methodology. The interface of the operating conditions, residual heat, and operator requirements were considered in the development of appropriate methods for achieving dehumidification.

The EPRI project team noted that pitting of turbine components is initiated through the synergistic action of oxygen and contaminants, such as chlorides, in the moist environment occurring in the turbine during shutdown. Reduction and elimination of moisture on surfaces of the turbine components prevent the formation of concentrated electrolytic films containing oxygen, which are precursors to disruption of the passive films and pit formation.

Use of dehumidified air, a method advocated by many turbine manufacturers, appears to be the preferred approach for protection in fossil plant applications. Dehumidified air rapidly evaporates any residual water left after draining. However, such protection is generally not provided at fossil plants, and better information is needed on how to best assess plant needs so as to implement and apply it.

The report provides information on implementing dehumidified air systems, including industry experience with the systems, equipment requirements, design inputs needed for selecting a system, and plant conditions that need to be considered in system design.

The report also outlines an assessment methodology and methods for developing a design for implementation of dehumidified air equipment for layup of a unit. Two case studies are provided to serve as examples of dry air layup implementation.

For more information contact Jim Mathews, 704-595-2044, jmathews@.

View or download Shutdown Protection of Steam Turbines Using Dehumidified Air (1014195).

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