Community Water Scheme With Low Flow and Suspected Pipe Constraint
An engineer is commissioned to troubleshoot a community water scheme. The borehole pump is rated at 8 L/s at 30 m head according to the supplier's curve. The rising main is 100 m of 50 mm internal diameter galvanised steel pipe, with bends and valves equivalent to a further 15 m of pipe. The discharge tank inlet is 25 m above the pump centreline. The measured flow at the tank is only 4.5 L/s, and the tank fills too slowly for the design demand of 120 m³/day over six pumping hours. The contractor says the pump is faulty and proposes replacing it with a higher-head pump. The community has a limited budget and wants a quick fix. The engineer measures the static water level and notes that the pump inlet is submerged, but no NPSH data is available. The pipe is old but appears sound. The engineer must decide whether the problem is the pump, the pipeline or the operating point.
1. Calculate the system head at the rated flow and determine whether the pump or the pipeline is the main constraint.
Read the worked answer
Use Darcy-Weisbach with f = 0.02. At Q = 8 L/s = 0.008 m³/s and D = 0.05 m:
- Area A = πD²/4 = 0.001963 m²
- Velocity V = Q/A = 4.07 m/s
- Velocity head V²/(2g) = 0.846 m
- Equivalent length L = 100 + 15 = 115 m
- L/D = 2300
- Friction head hf = f(L/D)(V²/2g) = 0.02 × 2300 × 0.846 = 38.9 m
Adding the static lift of 25 m gives a system head of about 63.9 m, before any discharge loss. The pump can supply only 30 m at 8 L/s. Therefore the pump is operating far to the left of its rated point; the pipeline, not the pump, is the main constraint. At 4.5 L/s the velocity is 2.29 m/s, the velocity head is 0.267 m and friction is about 12.3 m, giving a system head of about 37.3 m. That is consistent with a pump curve rising at lower flow. A higher-head pump would add pressure but would not fix the high velocity and could worsen cavitation or water hammer.
2. What remedial options should the engineer recommend, and which is technically sound?
Read the worked answer
The best technical option is to replace the 50 mm rising main with a larger diameter, for example 100 mm internal diameter. At 8 L/s, V = 1.02 m/s, velocity head = 0.053 m and hf = 0.02 × (115/0.1) × 0.053 = 1.2 m. The system head becomes about 26.2 m, which is within the pump's 30 m rating. If replacement is too costly, reduce the design flow and pump for longer hours: at 4.5 L/s, 120 m³/day would need about 7.4 hours instead of 6. Another option is a booster or variable-speed pump, but only after checking the pump curve and NPSH. The engineer should also check for clogged valves, scale or a partly closed gate valve. A higher-head pump alone is not recommended because it would push the operating point further out and increase pressure.
3. What further checks and reporting should the engineer carry out before any pump replacement?
Read the worked answer
Before changing the pump, the engineer should collect the pump curve, NPSHr, static water level, suction pipe size and fittings. He should calculate NPSHa at the highest water temperature and lowest water level. If NPSHa is close to NPSHr, a higher-head pump will increase cavitation risk. He should check the pressure rating of the old pipe and the surge pressure if a quick-closing valve is used. He should recommend a proper hydraulic model rather than trial-and-error replacement, and witness a flow test after the fix. If the pipe is replaced, the pump may need impeller trimming or speed reduction to avoid overpressure at low demand. The report should distinguish capital cost from operating cost and state the assumptions used for f, equivalent length and demand hours.