1.1 Main Model Test Results: Minneapolis
There were no problems found with the Minneapolis flow meter configuration. The
meters operated as expected in the model and returned flow readings in close agreement
with the measurements.
1.2 Main Model Test Results: Metro Plant
The model of the Metro Plant flow meter configuration reproduced similar variations
(errors) in meter output as were observed in the prototype. It was found that the probable
cause of the variations was an air pocket that formed in the crown of the pipe just
downstream of the meter. The meters are located just downstream of an aerated grit tank
which is a source of entrained air in the wastewater stream. A second source of air is the
entrainment that occurs as the grit channel effluent cascades over the effluent weir. These
two sources of entrained air, would provide a continuous flow of air to the meter conduit.
Bubbles reaching the roof would collect to form an air cavity. Interrupting the
continuous accumulation of air, large volumes of air would periodically "burp" from the
upstream end of the meter conduit; this was observed in the prototype and reproduced in
the model. Such burping would change the flow area and velocity profile at the metering
section as the air passed. However, the errors in meter output are believed to be more a
result of the air pocket and its effect on the velocity profile, rather than the result of the
Small taps were installed in the crown of the model conduit and used to determine the
location of the trapped air. Two vent stacks were installed in the model to release air and
prevent formation of the air pocket. Meter accuracy was improved significantly with the
vents in place, and the meters were shown to accurately represent the measured flow rate,
even with air added to the flow. Thus venting is recommended as a necessary
Thene, John R.; Voigt, Richard L.; Erickson, Benjamin.
Hydraulic Modeling and Testing of Minneapolis and Metro WWTP East Influent Meter Improvements.
St. Anthony Falls Laboratory.
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