Seven Metres, Not Ten: The Working Depth Limit of a Suction Pump Once Losses Are Counted

Every introduction to pumps contains the same striking figure: atmospheric pressure can support a column of water 10.33 m high, computed in this study as 10.329 m, from which it is commonly implied that a suction pump at the surface can draw water up from that depth. The implication does not survive an accounting of the losses that stand between the atmosphere and a working pump. This paper computes a corrected limit by deducting, from the barometric head, the vapour pressure of the water at its temperature, a valve pressure-drop allowance of 0.40 m, a leakage allowance of 0.30 m, an acceleration allowance of 1.50 m for the intermittent draw of a reciprocating pump, a working margin of 0.50 m, and the friction and minor losses in the suction pipe at a stated draw rate of 0.50 litres per second. At sea level with water at a temperature of 20 °C, the gross head remaining after vapour pressure alone is 10.09 m, the pipe losses at the resulting lift are 0.166 m, and the corrected working limit is 7.23 m — a total deduction of 3.1 m from the textbook figure, of which the surviving limit is a fraction 0.7. Rounded down to the nearest half metre for practical use, the working limit at that condition is 7.0 m. Warmth and altitude erode it further: at sea level the limit is 6.90 m with water at 35 °C and 6.50 m at 45 °C, while at an altitude of 1000 m it is 6.08 m, at 2000 m it is 5.03 m, and at the warm, high corner of the computed table — water at 45 °C at an altitude of 2500 m — it is 3.82 m. A corrected limit formula and a table over temperature and altitude are provided for direct use.