Thar She Blows! Application of Modeling and Desktop Tools to Determine the Cause of a Chronic Force Main Overflow

A large diameter force main has occasional overflows that occur at the high point in the force main when there has been a loss of power to the pump station. Structural modifications to the vault have not fixed the problem. Hydraulic modeling and desktop tools were used to evaluate four scenarios: Obstructions in the pressure sewer, rapid pump ramp up, sudden increase in flow meeting a downstream mass of “standing” water, and “Air binding” resulting from a full pipe condition. Hydraulic modeling software could not replicate the overflows. Desktop tools led authors to believe the overflows are caused by trapped air resulting from transitional open channel hydraulics. Additional hydraulic modeling proved that overflows can be eliminated by removing the open air stack and replacing it with an air/vacuum valve and installing additional vacuum breaking valves between the pump station and the high point.