Application: Assessment and improvement of the hydraulic performance of an existing pump sump with increased capacity.

Customer: Reid Crowther and Partners Ltd, Canada

Service: Physical Model Study

Problem: The proposed upgrade of the City of Thunder Bay's sewage treatment facility involved increasing the capacity of an existing influent pumping station which contained four 164 ML/d pumps. The capacity of the upgraded station was to be 750 ML/d. This would be achieved by adding a fifth pump and replacing two of the existing units with greater capacity pumps. Model testing of the new arrangement was required to ensure that satisfactory conditions would exist within the sump at the new 114% of design flow.

Objectives: To assess the hydraulic conditions generated during the proposed range of operating conditions, to determine the degree of swirl within the pump intakes and vortex activity in the sumps. Recommendations were to be made for any necessary modifications to ensure satisfactory hydraulic performance.

Approach: A physical model of the pump sump was constructed to a scale of 1:8 and operated under steady state conditions. A comprehensive series of tests was undertaken on the initial set-up to quantify the performance of the pump sump for the full range of operating conditions. Pump performance was considered at minimum operating levels, when turbulence and velocities were at their greatest.

Solids settlement problems were considered at the highest operating levels using a solid simulant. Flow patterns were observed using a coloured dye. Modifications were made to the model and the tests repeated to obtain a satisfactory design.

Outcome/Benefits: Initial model tests identified two major problems with the original design. Excessive swirl or pre rotation of flow was generated within the pump intakes. Additionally, turbulence in the inlet section of the pumping station resulted in significant amounts of air being drawn down from the water surface. This passed into the wet wells where some approached the pump intakes. Swirl was suppressed to an accepted value by introduction of flow splitters located on the sump floor upstream of each pump intake. Draw down of air was reduced by structural modifications to the inlet section of the station and by adjustment to station operating levels.

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