Steam System Design Considerations for Three Pressure Reheat Cycles With Cascade Bypass System

This paper discusses various issues that should be considered when designing the steam system for a typical three pressure reheat cycle, which is used at many of today’s combined cycled plants. A cascade bypass arrangement in the steam system is commonly used to ensure steam flow is available to the reheat section of the HRSG during startup. For the purposes of this paper, a cascade bypass system will be defined as high pressure steam being bypassed to the cold reheat steam and hot reheat steam being bypassed to the condenser. This arrangement can lead to conflicts between plant operation needs and the steam turbine desire to reduce the HP turbine backpressure as much as possible during startup. Plant operation needs that may dictate keeping the reheat system pressure high include export steam minimum pressure guarantees to customers, cycling operation which forces the plant to restart when equipment is still hot, and hot reheat steam bypass or condenser limitations. The HP turbine and cold reheat steam piping can have temperature limitations which may necessitate keeping the HP turbine exhaust pressure as low as possible during startup.