Lift Station Wet-Well Volume and Cycle Time

The relation is short -- active volume is the minimum cycle time times the pump rate divided by four -- and the four is the interesting part. THE GOVERNING CASE IS INFLOW AT EXACTLY HALF THE PUMP RATE. At a low inflow the well takes a long time to fill and the cycle is long; at an inflow approaching the pump rate the drawdown takes a long time and the cycle is long again. Halfway between, both halves are short at once, and that is where a fixed-speed station cycles fastest. Sizing on average inflow rather than that worst case produces a station that short-cycles at a flow it will certainly see, usually within its first year. THE CYCLE LIMIT IS A MOTOR RATING, NOT A HYDRAULIC ONE. Starting current is several times running current, and the heat it produces has to be shed before the next start -- so the constraint is the manufacturer's permitted starts per hour, and it falls as motors get larger. Short-cycling gives no immediate symptom. It shortens motor life and burns contactors quietly, and the station gets diagnosed when something fails rather than when the cycling starts, which is why the calculation belongs at design rather than at the first callout. AND A BIGGER WELL IS NOT A FREE FIX, which is the tension the design actually sits in. Active volume buys cycle time, and the same volume buys detention time: wastewater held in a wet well goes septic, generating hydrogen sulphide that corrodes the structure above the waterline and produces an atmosphere that has killed people who entered without testing. So the well is bounded below by short-cycling and above by septicity, and the resolution on a station with a wide flow range is usually variable speed or a second smaller pump rather than more volume. This sizes active volume from an entered cycle limit. It does not size the pump or select it against a system curve, determine the inflow or its diurnal pattern, set float or transducer elevations, address the minimum submergence a pump needs or the clearance below the stop level, evaluate the force main, its velocity, or the surge and air release it needs, model a duplex or alternating arrangement or a variable speed control, or address the ventilation, confined space entry, and corrosion protection a wet well requires. Ten States Standards or the applicable design standard, the pump manufacturer's permitted starts, and the design engineer govern.

active volume V = t x Q / 4 for a minimum cycle time t and pump rate Q; the level differential is V over the well's gallons per foot (pi/4 x D^2 x 7.481).

Lift station wet-well sizing as Ten States Standards and standard practice state it. The permitted starts per hour is a MOTOR rating from the manufacturer.

One published sizing relation and one geometric conversion.

Estimate. AHJ and licensed professional govern.

Field names used by the API: pump_gpm, well_diameter_ft, min_cycle_minutes, max_starts_per_hour, inflow_gpm, active_volume_gal, area_ft2, gal_per_ft, level_differential_ft, worst_case_inflow_gpm, cycle_at_inflow_min

• Worst case is half the pump rate not the average inflowstandard lift station practice
• Starts per hour is a motor rating and falls as motors get largerthe pump manufacturer
• Bounded above by septicity detention time is the competing constraintTen States Standards