Energy-Tech Magazine

Introduction
Midwest Generation Joliet Station in Illinois has two coal-fired units, Unit 7 and Unit 8. Both 550 MW units are located on the Des Plaines River and have been in operation since 1963-1964. The circulating water system is a once-through system comprised of a bar rack, traveling screens, circulating water pumps and reversible condensers. The original condenser tubes on Unit 8 were replaced with similar material in 2005. In 2006, after less than a year of operation, significant microbiological influenced corrosion, or MIC, damage was identified during routine eddy current testing of the tubes.

Later that year, the station decided to test and evaluate new Circulating Water Flow and Fouling (CWFF) instrumentation to determine the cooling water velocities [1] [2]. MIC most likely occurs under stagnant conditions or in operations with low or intermittent flow [3]. The objective was multifold, to measure cooling water velocities and to examine the effect of other variables in the ongoing MIC attack of the SS condenser tubes. In addition to providing

References Condenser Performance Measurement Using New On-Line Condenser Tube Flow Rate and Fouling Instruments, Dr. Timothy J. Harpster and Dr. Joseph W. Harpster, EPRI 10th Balance of Plant Heat Exchanger NDE Symposium. San Antonio, Texas. June 2008. CWFF, a patent pending instrument assigned to Intek Inc. Sourcebook for Microbiologically Influenced Corrosion in Nuclear Power Plants, EPRI, 1988 Rheotherm Technology. USPN 4255968 1981 Intek Inc. Westerville, Ohio. Graphing and initial data analysis conducted by Intek using Intek's Condenser Diagnostics Toolkit, MyCondenser.com

useful information regarding the root cause for MIC, the study unveiled a number of other condenser performance-related issues presented in this article.

Units 7 and 8 share a total of four circulating water pumps. All four pumps are identical in model and rated pumping capacity. The condenser configuration is presented in Figure 1. As mentioned previously, the condenser is equipped with circulating water flow reversing valves, which remove the macrofouling from the tube sheet. Joliet Station experiences rapid debris (macro) fouling and so, as a standard operation, the condenser circulating water is generally reversed on a daily cycle. The primary method for microfouling control is chlorination. Additionally, the station utilizes air drying to remove silt and biofouling by periodically taking a water box out of service, opening the doors and installing hoses connected to an air moving system that circulates warm air through the tubes and dries the fouled material to a point where it is flushed away when the water box is placed back in service. The drying method also is used for a long- and short-term lay-up of the condensers.

Intek's circulating water flow and fouling (CWFF) meter utilizes thermal sensing technology to measure the circulating water flow through an unobstructed flow tube. This technology also enables quantification of heat transfer coefficient. The flow data is single or bi-directional and routed to a process computer for data logging and a range of computations [4]. Important features of the CWFF are that it is accurate, reliable and non-invasive to the flow measurement.  For most steam surface condensers used in the power industry, any flow and fouling instrument must be capable of surviving long term submersion, exposure to unfiltered circulating water and allow ease of on-line or off-line tube cleaning methods; the CWFF is well suited to these conditions.

The flow meters were installed in the upper bundle of one of the four outlet water boxes (82East) of the Unit 8 condenser. A photograph of two of the four installed meters is shown in Figure 2. Brackets anchored at two adjacent tubes were used to provide a mechanical brace from the meter to the tubesheet to overcome a concern that violent flow conditions during reverse flow could stress the adhesive used to attach the meters to the monitored tube. Meter cables were attached to tubesheet with epoxy and routed through a port in the outlet waterbox. The installation locations for the CWFF sensors were carefully selected based on a critical examination of the condenser configuration, so that a very small number of key measuring points provided data for meaningful analysis.

Data analysis
The CWFF meters were built and calibrated for both forward flow and reverse flow, so that flow direction change can be recognized and flow rates in both directions can be measured. Upon first inspection, the data indicated that the circulating water pumps were switched at seemingly random intervals. In reality, pumps are taken off to perform system maintenance at low loads when fewer pumps are needed, at lower circulating water inlet temperatures, for bar rack cleaning and waterbox drying.

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