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 of 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/sec2
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. Fluid motions involving vortex formation have been studied by several investigators (Anwar, H.O. et al., 1978; Hecker, G.E., 1981; Padmanabhan, M. and Hecker, G.E., 1984; Knauss, J., 1987)
ANSI/HI 9.8 Pump Intake Design includes additional information on this subject.
Q: During the maintenance of centrifugal pumps, how much wear of the wearing rings is considered normal, and what is the normal wearing ring clearance for a new pump?
A: A simple answer is not available. Normal wear of centrifugal pump clearance depends on many factors, such as cleanliness of the liquid, viscosity of the liquid, the presence and size of abrasive solids, the head developed by the impeller, and the operating speed.
The normal clearance range of a new pump can be obtained from the manufacturer or his representative, but the figure below can be used as a guide. Stainless steel impellers typically require greater clearance than bronze or iron to avoid galling and seizing.
Q: When checking the performance of a new centrifugal pump, the plot of the resulting head versus rate of flow curve appears to be lower than the manufacturer’s rating curve. How much deviation from the manufacturer’s rating curve is normal?
A: The Hydraulic Institute standard contains two performance test acceptance tolerance levels, “A” or “B,” which must be agreed to by both customer and manufacturer. The acceptance tolerance applies to the specified condition point only, not to the entire performance curve. It is recommended that the contractual agreement contain the agreed upon acceptance level. The tolerance for total head has four different categories that depend on the total head and rate of flow. There is an alternate tolerance for rate of flow at the rated total head. All of this is dependent on an agreement with the pump manufacturer to perform a factory test to determine the true performance and take any necessary corrective action before the pump is shipped. Field tests are seldom accepted as reliable due to the difficulty of meeting the proper test procedures in the field and making corrections.
Assuming acceptance level "A" is applicable and the rated conditions are 3,500 GPM at 300 feet, the head variation at the rated flow is +5 percent to -0 percent. An alternate tolerance in rate of flow at rated head is +10 percent to -0 percent.
Additional details are available in ANSI/HI 1.6 Centrifugal Pump Tests.
