Thermal Cycle Evaluation for Feedwater Heater Out of Service Condition
Author: Lindsey L. Dziuba and Robert J. Stakenborghs
Large power plant secondary side thermal cycles typically include multiple parallel trains of condensate and feedwater heating to improve cycle efficiency. These heaters are typically arranged in multiple identical trains and can contain by-pass lines to accommodate removal from service for on-line maintenance. In a closed system, the heaters are typically horizontal u-tube heat exchangers with various combinations of internal and external drains cooling. Heating fluid is provided from steam extraction and drains flow. Drains flow can be piped either in a forwards (pumped forward) or backwards (cascading drains) direction. All of these possible equipment combinations result in a complex system where the impact of taking single or multiple heaters out of service for on-line maintenance can be difficult to properly determine.
This paper presents a description of the various impacts of removing feedwater heaters from service during operation. The impacts can include increased steam flow through the downstream turbine stages, increased extraction flows and drains flows to the remaining heaters, and increased heater tube side flows that can all potentially cause equipment damage. Proper precautions should be taken to protect plant equipment, including reducing plant thermal power output when necessary. This paper provides specific plant examples to illustrate the complexity of the problem and its proper solution. A simplified evaluation methodology is also provided to allow the user to determine the quantitative impact of removing equipment from service. For each particular feedwater heater out of service arrangement, the method uses mass and energy balances around the remaining in-service feedwater heaters to determine the magnitude of the changes in turbine stage, extraction and drain flow rates. The appropriate power level for that condition is determined by applying specific acceptance criteria for the impacted equipment.