Calculating Wet Well Cycle Time and Pump Starts per Hour

The Problem

Commissioning a lift station, the number that quietly decides whether the pumps live a long life is starts per hour. Every across-the-line start dumps heat into the motor windings that takes minutes to shed. Exceed the manufacturer's allowable starts and you are looking at a warranty argument now and a burned-out winding later. The setpoints between the pump-on and pump-off floats define an effective volume, and that volume — against the inflow and the pump capacity — sets how often the station cycles.

The trap during startup is that you are usually testing with clean water from a hydrant or a temp line at some throttled inflow. If that inflow happens to land near half the pump capacity, you are unknowingly sitting in the worst case for cycling and short-cycling the pump you are trying to prove out.

The Approach

For a single pump cycling between fixed levels, cycle time is fill time plus draw-down time:

t_cycle = V/Q_i + V/(Q_p − Q_i)  (minutes)

Differentiate that and the minimum falls out cleanly: cycle time is shortest when inflow is exactly half of pump capacity. Substitute Q_i = Q_p/2 and you get the worst case directly:

t_min = 4V/Q_p  →  max starts/hr = 15·Q_p/V

Rearranged for design, the minimum effective volume to stay under an allowable N starts per hour, crediting alternation across n pumps, is V_min = 15·Q_p/(N·n). Duplex alternation splits the station's starts between two pumps, which is why alternating effectively doubles the cycling a station can tolerate [1].

Wet well with float switches hung at staggered elevations for pump-off, lead-on, lag-on, and high-level alarm
Figure 1: Float setup in a duplex wet well. The vertical spread between pump-off and lead-on is the effective volume that drives cycle time.

The tool reports cycle time, starts per hour at the entered inflow, and the worst-case starts, and flags when you are near that half-capacity danger zone. It also solves the minimum-volume form for design checks.

One honest caveat it surfaces: if the field float or transducer setpoints differ from the design levels — and they often do after the electrician trims them — the effective volume changes and the whole calculation moves. Verify setpoints against the approved control narrative before you draw conclusions. Single file, vanilla JS, localStorage persistence.

The tool: Wet Well Cycle Time & Pump Starts Calculator.

References

  1. [1] Metcalf & Eddy, Wastewater Engineering: Collection and Pumping of Wastewater — wet well sizing and cycle time derivation.
  2. [2] Hydraulic Institute — intake design and pump cycling guidance. https://www.pumps.org/
  3. [3] Motor manufacturer data sheets — allowable starts per hour for the specific motor on site.