An Optimal System for the Present

Most stations have centrifugal pumps, but a few use air eductors instead. Several handle lifts of more than 20 feet. At some stations, centrifugal pumps and motors are submerged in the wet well that feeds them, being mounted on rails for removal and servicing. Thus, the adjacent dry-well equipment space contains only manual valves. At other stations, centrifugal pumps are mounted at the bottom level of the dry well. Some are submersible units, as shown, and others have vertical shafts extending to motors mounted above maximum floodwater level. Motor controls may be located in a weather-resistant enclosure or a building over the dry well.

Regardless of such variations, five discrete (on-off) signals are monitored at each station:

• Wet well high level from a float switch near the top, indicating that the well may soon overflow.
• Dry well high level from a float switch at the floor of the lowest level, indicating that the space is beginning to flood.
• Open door or hatch, indicating that someone has entered the building or dry well.
• Key-operated switch, indicating that an authorized person has entered the building or dry well.
• AC power failure, indicating that the radio RTU is operating on emergency battery backup.

In addition, two stations pumping combined wastewater and storm water have electrically actuated gate valves to divert the water to the nearby Flint river during a flood emergency. The RTU of each of those stations had to provide two more discrete inputs to monitor gate position switches and two discrete outputs to open and close the gate. However, those gates are no longer used, and combined sewers are being separated.