Effective volume, pump cycle time, and worst-case starts per hour for lift station commissioning. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: This is a field arithmetic aid for a single lead pump cycling between fixed on/off levels with constant pump output. VFD-driven stations that modulate speed to track inflow, and stagger/lag pump interaction, are outside this simple model. The motor manufacturer's allowable starts govern.
Effective Volume
Volume between ON and OFF levels
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Worst Case (inflow = ½ pump capacity)
Minimum cycle time
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Maximum starts per hour (station)
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Maximum starts per hour (per pump, alternating)
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At Entered Inflow
Fill time
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Pump-down time
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Cycle time
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Starts per hour (station / per pump)
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Sizing Check
Minimum effective volume for allowable starts
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How the Numbers Are Derived
Cycle time
For a lead pump cycling between fixed levels with effective volume V (gal), pump capacity Q_p (GPM), and inflow Q_i (GPM):
t_cycle = V/Q_i + V/(Q_p − Q_i) (minutes)
Fill time is the first term; pump-down time is the second. Starts per hour = 60 / t_cycle.
Worst case and minimum volume
Cycle time is shortest when inflow equals exactly half the pump capacity. Substituting Q_i = Q_p/2:
t_min = 4V/Q_p (minutes) → max starts/hr = 15·Q_p/V
Rearranged for design, the minimum effective volume to keep the station at or below N allowable starts per hour (with alternation credit for n pumps):
V_min = 15·Q_p/(N·n) (gal)
With duplex alternation each pump sees half the station starts, which is why alternation effectively doubles the allowable station cycling.
Why this matters at startup
Short-cycling overheats motor windings — each across-the-line start dumps heat into the rotor and stator that takes minutes to dissipate. Exceeding the manufacturer's starts/hr is a warranty problem and eventually a winding failure.
During commissioning the wet well is often tested with clean water at artificially low inflow from a hydrant or temp line. If the fill valve happens to sit near half of pump capacity, you are unknowingly running the worst-case cycling scenario — watch the starts.
If actual float/transducer setpoints in the field differ from the design levels, the effective volume changes and this whole calculation shifts. Verify setpoints against the approved control narrative before drawing conclusions.
References
Metcalf & Eddy, Wastewater Engineering: Collection and Pumping of Wastewater — wet well sizing and cycle time derivation.
Hydraulic Institute — intake design and pump cycling guidance. https://www.pumps.org/
Motor manufacturer data sheets — allowable starts per hour for the specific motor on site.