Water Online

May 2017

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

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By Dan Gessler, Andy Johansson, and Becca Hall I n today's world of computational fluid dynamics (CFD), there are some applications where physical models remain as the only analysis tool that can provide reliable engineering design results. Pump intake design is one such area. The cost of physical models can be significantly higher than CFD; therefore, the temptation to use CFD is considerable. CFD can provide insight to the flow patterns approaching the intake, but is unable to consistently provide information about the vortex strength and temporal variation. For this reason, the Hydraulic Institute Standards (ANSI/HI 9.8-2012 and ANSI/HI 9.6.6-2009) do not recognize CFD as an acceptable means to predict flow patterns at the pump inlet or to show compliance with the acceptance criteria presented in the pump intake testing section of the standard. The Hydraulic Institute (HI) was founded in 1917 and is a value- adding resource to member companies, engineering consulting firms, and pump users worldwide. The Hydraulic Institute develops and delivers comprehensive industry standards and tools for the effective application, testing, installation, operation, maintenance, and performance optimization of pumps and pumping systems. The Hydraulic Institute Standard for Intake Design (ANSI/HI 9.8-2012) provides guidelines on when pump stations should be tested with a physical model and the model scaling requirements. Some of the criteria for pump stations that require physical model testing include individual pump flows or total station flow greater than 40,000 gpm and 100,000 gpm, respectively, intake or piping geometry that deviates from the standard, pump stations with nonsymmetrical or nonuniform approach flow, and/or pump stations where operation is critical or the cost of a prolonged outage for repairs is significant. For those pump stations that require testing, minimum model scales based on Reynolds number, Webber number, and absolute minimum model dimensions are given. The following paper provides information about the past, present, and future of pump intake modeling. The Past The performance of a pump can be negatively impacted by adverse flow conditions entering the pump. Adverse flow conditions including free and subsurface vortex formation, non- uniform velocity distribution, or preswirl can result in reduced pump efficiency, reduced pumping capacity, or vibrations that can damage the pump or require premature maintenance. Vortex formation has been studied by several investigators, including Alden staff members. An example of vortex identification in a physical model is included in Figure 1. These investigations led to the development of minimum model scale requirements that provide hydraulic similitude between the model and prototype. Standard testing methods were developed to measure the swirl and the velocity distribution approaching the pump 32 wateronline.com n Water Innovations Computational Fluid Dynamics vs. Physical Modeling For Pump Intake Design The past, present, and future of pump intake modeling is examined, highlighting the progress and value of computational fluid dynamics compared to tried-and-true methodology. Figure 1. Vortex identification in a physical model

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