Sewers, Lift Stations, and Pumping Equipment
What the exam expects
This domain covers the physical system: gravity sewers, force mains, lift stations, and pumps. Understand why lift stations exist (to lift flow when gravity grade is insufficient), how wet wells and dry wells differ, and how pumps are controlled by level (float switches, bubbler, ultrasonic, pressure transducers).
Concepts to master
- Pump types: centrifugal (most common), positive displacement, submersible vs. dry-pit.
- Pump curve and system curve intersection; operating point; the effect of a partially closed valve or clogged impeller.
- Net positive suction head (NPSH) and cavitation: symptoms include noise, vibration, and reduced flow.
- Wet well design: storage volume, cycling frequency, and the risk of septic conditions with long detention.
- Force main hydraulics: friction loss, air release valves, and the hazard of surge/water hammer.
- Standby power and alarm systems for lift stations.
Traps
A frequent trap is confusing a pump problem with a system problem: if flow drops, is it the pump (worn impeller, cavitation) or the system (clogged force main, closed valve)? Another is assuming a larger pump always solves capacity; it may overload the motor or cause cycling.
Worked example
A wet well cycles too often. Rule of thumb: increase the operating volume between pump-on and pump-off levels to lengthen cycle time. Cycle time = (wet well volume between levels) ÷ (pump rate − inflow rate). If inflow is 200 gpm and pump rate is 400 gpm with 100 gallons between levels, cycle time = 100 ÷ (400 − 200) = 0.5 min - far too short; enlarge the operating band.