Sept. 26, 2010

Insights From the Hydraulic Institute

 Liquid Vapor Pressure, PSIA 80 40 20 10 NPSHR Reduction, Feet 10 5 2 1

Q: We are designing a circular sump for the installation of three sewage pumps. Because of local restrictions on the size, we are concerned the pump intake structure may not provide a good hydraulic design and are considering whether to build and test a model of the intake. How do we select the appropriate rate of flow for the model test?

A: Models involving a free surface are operated using Froude similarity since the flow process is controlled by gravity and inertial forces. The Froude number, representing the ratio of inertial to gravitational forces, can be defined for pump intakes as:

F = u/(gL) 0.5

Where:
u = average axial velocity (such as in the suction bell entrance) in ft/sec
g = gravitational acceleration, 32.2 ft/sec 2
L = a characteristic length (usually bell diameter or submergence) in ft.

The choice of the parameter that is used for velocity and length is not critical, but the same parameter must be used for the model and prototype when determining the Froude number. For similarity of flow patterns, the Froude number shall be equal in both the model and prototype, and solving for the velocity in the model will answer your question.

In modeling a pump intake to study the potential formation of vortices, it is important to select a reasonably large geometric scale to minimize viscous and surface tension scale effects and to reproduce the flow pattern in the vicinity of the intake. In addition, the model shall be large enough to allow visual observations of flow patterns, accurate measurements of swirl and velocity distribution, and sufficient dimensional control. Realizing that larger models, though more accurate and reliable, are more expensive, a balancing of these factors is used in selecting a reasonable model scale. However, the scale selection based on vortex similitude considerations is a requirement to avoid scale effects and unreliable test results. ANSI/HI 9.8 Pump Intake Design includes additional information on this subject.

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