Replacement Feedwater Heater Performance Modeling and Rating Calculations

Abstract

It is not uncommon to expect that feedwater heaters will require replacement over their Unit’s service life. In some cases, Unit upgrades, changes in the full range of operation, and the absence of root cause failure analysis can lead to unsuitable replacements. Neglecting these considerations can result in the continuance of similar failures as heaters are either replaced in-kind or not to the extent necessary as dictated by the changes in load and/or how they are operated. The heater technical specification must not only address the obvious issues related to changes in tube material, quality control, and references to the current state-of-the-art heater standards, but also the full range of current and projected modes of operation. An important factor in obtaining a heater that will last reliably for many years is to specify one that will be versatile enough to handle not only the normal base load operation, but will also safely withstand higher loads, higher heat inputs, and other modes of operation reasonably expected. The replacement heater specification must define the full range of projected load impositions to allow the Vendor to consider and adapt his internal layouts and physical geometries to accommodate them safely and conservatively. This paper shall illustrate how performance modeling and predictive rating calculations can be valuable tools in helping to identify the full range of conditions to be considered by the Vendor so as to optimize the specification of the new heater. A variety of hypothetical cases can be examined in order to help determine the optimal design and its effects on the entire heater system as well as projected resultant differences in Unit load and heat rate or efficiency. Constructing a performance mode model, such as the PEPSE example utilized herein, for single heater analysis is quick and relatively easy. The feedwater and drain inlet conditions (temperature, pressure and flow), the heater shell pressure, and the required Terminal Temperature Difference (TTD) and Drains Cooler Approach (DCA) are inputs to the model which then calculates the required steam flow demand and the heater outlet conditions. Using this tool, the initial design, current minimum and full load (from plant historical data or PI data), future full load, abnormal/overload conditions and any other pertinent analysis may be accomplished. The key output data delivered by the model is the required steam and drain flows obtained by the energy balance. This information is then specified to the heater Vendors as it applies to the rating and physical sizing of the replacement. This information can be crucial to ensure that the replacement heater(s) will be capable of providing long, reliable life for even the worst load potentials.