Hydraulic Modeling and Testing of Minneapolis and Metro WWTP East Influent Meter Improvements

Abstract

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 intermittent burping.

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 improvement.