Formed Suction Inlet Design for Mixed Flow Pump Applications

The use of formed suction inlets (FSI) on large vertical pumps has been common in the flood control field for many years. The US Army Corps of Engineers Type 10 inlet is a common example that has been used extensively throughout the south eastern United States for flood control as well as on numerous projects as part of the broader Everglades Restoration Project. The inlet is gaining popularity in higher head applications such as circulating water system for power plants. Extensive design development and research in the performance of the Type 10 Inlet has been undertaken over the years. However, nearly all of this research has focused on Type 10 inlets for flood control applications which typically utilize axial flow pumps. Recently the Type 10 has been used or considered for a wider variety of projects including cooling water applications which typically utilize a radial or mixed flow pump. This type of pump typically has a smaller throat diameter than an equivalent capacity axial flow pump. Given that the design of the FSI is based on the throat diameter there is some question regarding the impact of the smaller resulting FSI design and corresponding increase in velocity. In several recent applications of Type 10 inlets installed on mixed flow pumps the inlet was designed for a larger pump impeller inlet diameter to increase the size of the FSI and reduce the inlet velocity. A larger than normal reducing cone was then used to transition into the smaller pump throat. This paper presents a comparison of inlet conditions for a variety of mixed flow and flood control applications which were evaluated during physical model studies. Several different "inlet eye diameter" to flow rate or velocity ratios are investigated with an attempt to identify the ideal ratio to use for determining an appropriate diameter for the FSI design. These results can be used by design engineers when considering FSI for mixed flow pump applications and reduce the likelihood of inlet related problems associated with increased inlet velocities.