During wet weather events, Anne Arundel County, (Annapolis, MD, United States), experiences frequent alarms and costly pump-outs of the main pumping stations at Carr’s Ridge, Mayo Peninsula. Lombardo Associates, Inc. (LAI) was commissioned by the county to investigate the cause of the problem and develop a remedial strategy.
The Carr’s Ridge region is low lying with most of the residential area at sea level, and therefore requires individual house pumps to feed the collectors and interceptor. These pumps are either Gould 1/2 HP (.37 kW) or 1/3 HP (.25 kW) depending on the expected system head at the connection.
The main interceptor runs east-west through Carr’s Ridge and varies from 2 inches (50 mm) to 4 inches (100 mm) in diameter with bolted inspection holes and air release valves (see image below). A section along this interceptor acts as a variable grade gravity sewer during average dry weather conditions. During wet weather conditions, however, the entire interceptor acts as a low pressure force main, (a.k.a. rising main) with the exception of a small length just prior to the interceptor draining into a pump station.
Modeling low pressure force mains is a difficult task because heads are so dependent on the number of pumps operating. This is why LAI turned to SewerCAD.
“Superior pumping algorithms and robust scenario management make SewerCAD the obvious choice for modeling this type of situation,” stated Pio Lombardo, President of LAI. “It was necessary to analyze a plethora of operating scenarios in order to determine the precise problem areas and to develop the most effective remedial strategy.”
SewerCAD was used to model three major scenarios that were identified as Dry Weather, Base Wet Weather, and Flooding Wet Weather, which correspond roughly to average daily flow (ADF), two times the ADF, and four times the ADF, respectively.
Analysis by SewerCAD identified that sections of the main interceptor operated as a gravity main under Dry and Base Wet Weather conditions, and as a low pressure force main under Flooding Wet Weather conditions.
A more detailed study of the Flooding Wet Weather condition revealed that heads in the main could exceed shut off head for all except five of the thirty-six pumps. By dropping the loading in seven incremental stages from the maximum Flooding Wet Weather flow, LAI determined a hierarchy for pump operation.
The first tier represented those pumps that were able to operate even under the Flooding Wet Weather condition. Subsequent tiers progressively stepped down to the Base Wet Weather condition, where all pumps were able to operate. By focusing on the pumps in the more critical locations, LAI was able to develop a comprehensive plan for the replacement of the pumps.
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