Isolated Phase Bus Duct Testing
By Kevin Linfield, Ph.D., P.Eng., P.E.
As nuclear power plants are functioning longer and increasing their output, the efficiency of cooling in the isolated phase bus duct system is becoming more important. An isophase bus duct carries very large currents, typically between a generator and a transformer. Maximum cooling capacity can be achieved by balancing air flow, whether within an existing system, or as part of an upgrade. By measuring the flow in each branch of the ducts, the plant is able to ensure balanced flow and improve the cooling ability, with the added goal of accurately measure the flows in these ducts and perform iterative balancing as necessary.
Much of the testing of isophase bus duct is performed with 3-D velocity probes (shown above) and hot wires with a sophisticated data acquisition system, like the Airflow Sciences Equipment’s 3DDAS also shown above. An overview of the testing and balancing procedure is as follows:
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The field testing crew and plant personnel agree on test port locations, with the number of test points that are selected to exceed EPA Method 2/2F guidelines.
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The bus is then de-energized to fully characterize the flow paths inside the conductor and enclosure.
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The degree of existing flow imbalance is determined, and fan outputs and various damper opening positions are noted.
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The flow is balanced through an iterative process of fan operation and damper position changes.
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The final results include proper damper settings for improved and balanced cooling flow.
For particularly challenging duct geometry, full scale mock-ups of the ducts can be fabricated and tested in the ASC laboratory. The purpose of the physical model is to understand flow patterns and to develop correlation curves for the flow rate versus damper settings in various branches.
One example of a challenging bus duct geometry had the additional complexity of limited test port accessibility. A full scale mock-up of the generator-side ducts was fabricated in the ASC Laboratory with the focus on the need to understand flow patterns and develop correlation curves for the flow rate versus damper setting in various branches. ASC’s engineers were able to use this mock-up to then conduct accurate, repeatable flow measurements in the generator-side ducts, giving the ability to balance flow between the branches.
Airflow Sciences Corporation (ASC) is a fluid dynamics solutions company, specializing in the design and optimization of equipment and processes involving flow, heat transfer, combustion, and mass transfer. Now celebrating 50 years as an industry leader, ASC has focused on testing and simulation of air, gas, liquid, and particulate flows since 1975.
ASC also manufactures standard and custom test equipment, including probes and wind tunnels, enabling customers to collect data accurately and efficiently. ASC’s primary CFD software, Azore®, is also available for customers with in-house CFD personnel. ASC offers comprehensive flow solutions and optimization and serves a wide range of industries including HVAC, power, auto, rail, and food processing.