Pit Dewatering Total Head, Staging, and Suction Limit

A pit lifting water 180 ft with 42 ft of friction is 222 ft of total head, four fifths of it static before a single foot of pipe. Pumps developing 120 ft each mean two stages, each lifting about 111 ft with the intermediate pump on a bench roughly halfway up, and at 500 gpm that duty is about 28 water horsepower and 43 at the shaft. THE CONSTRAINT THAT SURPRISES PEOPLE IS ON THE SUCTION SIDE. A pump sitting above the water can only lift water to itself by atmospheric pressure -- about 34 ft in theory and 20 to 25 ft in practice once friction, vapour pressure, and NPSH margin are accounted for, and less at altitude, roughly 17 to 20 ft at 5,000 ft of elevation. Put that second-stage pump on a bench 28 ft above its catch sump and it will cavitate or fail to prime REGARDLESS OF ITS RATING. The fix is to move the pump down to the sump or use a submersible, not to buy a bigger pump, and a plan showing a pump on the rim drawing from the bottom does not work at any horsepower. Head and staging arithmetic for a dewatering system. It does not size the pump, select the impeller, or evaluate the pump's curve against the system curve, which is where the actual operating point is found; it does not compute NPSH available in full, which requires the water temperature, altitude, and suction line details and which is the real limit rather than the rule of thumb used here. It does not address the inflow the pit actually produces -- groundwater inflow and storm response determine the required capacity and come from a hydrogeological assessment -- and it does not address discharge permitting, sediment control, or water quality, all of which are regulated. The pump manufacturer's curves, the site hydrogeologist, and the discharge permit govern.

total head = static lift + friction + discharge pressure; stages = ceil(total head / the head one pump develops); head per stage = total / stages; water horsepower = gpm x head / 3,960, divided by the pump efficiency for brake horsepower.

The staged-pumping head relation by name, with the practical suction lift of roughly 20 to 25 ft at sea level (less at altitude) named as a rule of thumb rather than a net-positive-suction-head calculation. The pump manufacturer's curves, the site hydrogeologist, and the discharge permit govern.

Addition and division on the user's own lift and pipe data; no pump curve is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: static_lift_ft, friction_head_ft, discharge_pressure_ft, head_per_pump_ft, suction_lift_ft, practical_suction_limit_ft, flow_gpm, pump_efficiency_pct, total_head_ft, stages, head_per_stage_ft, suction_excess_ft, water_hp, brake_hp, static_share_pct

• Static lift dominates a deep pit friction is the small term, not the large onedewatering practice
• Suction lift is a hard physical limit about 20 to 25 ft at sea level and less at altitude, at any horsepowerpump practice
• The rule of thumb is not NPSH the real limit needs water temperature, altitude, and the suction linepump manufacturer data