Borehole Pump Services

A productive borehole is the result of three disciplines, executed in order: hydrogeology to find water, drilling and casing to access it, and pump engineering to lift it. Skipping or compressing any of those phases produces wells that yield little, sand badly, or fail their motors within months. The cost of doing them properly is far below the cost of doing them again.

Site selection starts with regional hydrogeology — granite weathering profiles, basalt vesicles, sedimentary aquifer thickness — interpreted from geological maps, neighbour-well data, and where it pays, geophysics (VES / ERT / TDEM). Targets become drilling locations only after a yield prediction with confidence interval is documented.

Casing design follows soil profile. Surface conductor protects the borehole from collapse during initial drilling. Solid PVC or steel casing isolates aquifers from contamination. Slotted screen sections face the producing aquifer. Gravel pack between screen and formation filters the inflow and stabilises the wall. A poorly chosen slot size or gravel grading sands the pump.

Borehole development is mandatory after drilling — air-lift, surge-block, or jetting cycles remove drilling fluid, fines, and partial cake from the gravel pack. Skipping development hides yield potential and shortens pump life by abrasion. Best practice: develop until the discharge is clear of suspended solids for 30 minutes continuous.

Pump testing is the contract between the driller and the owner. A 24-hour constant-rate test plus a stepped-rate test characterise yield, drawdown, transmissivity, and sustainable abstraction rate. The pump should be sized for sustainable yield, not maximum yield — a 4 m³/h sustainable hole equipped with a 6 m³/h pump runs the pump dry every drought cycle.

Pump selection follows the pump curve, which plots head (m) against flow (m³/h). The system curve plots required head against flow (static lift + friction loss). Operating point is the intersection. A pump operated far from BEP (best efficiency point) wears bearings and impellers, draws more current, and fails earlier.

Submersible motor selection is constrained by pump type, voltage, depth, and water chemistry. 4-inch motors fit boreholes ≥ 100 mm; 6-inch needs 150 mm minimum. Three-phase motors above 5.5 kW are preferred — single-phase has poor starting torque and limited lifecycle.

Cable sizing is the most-skipped pump-design check, and the arithmetic is worth doing rather than eyeballing. Take a 5.5 kW submersible drawing 18 A on a 100 m drop cable. Using the BS 7671 volt-drop figure for 4 mm² three-phase, about 9.5 mV per amp per metre, the drop is 9.5 × 18 × 100 = 17,100 mV, or roughly 17 V. That is 4.3% of 400 V — it fails the 3% limit, and the motor sees about 383 V instead of 400 V. Stepping up to 6 mm² brings the drop to around 11.5 V, or 2.9%, which passes. Undervolting a submersible is not a minor inefficiency: current rises to maintain torque, the winding runs hotter, and the motor is sitting at the bottom of a borehole where nobody will notice until it fails. Specify cable for full-load current × 1.25, verify the drop against the tables before energising, and remember the drop is calculated on the full length of cable including the run down the hole, not just the surface run.

Variable-speed drives transform pump operations. Constant-pressure VSD systems eliminate water-hammer, halve energy at part-load (P ∝ N³), and protect motors from dry-running with current sensing. The premium pays back in 2–3 years for any pump running > 6 hours/day.

Maintenance reality: bore fines erode impellers; iron bacteria foul screens; check-valves fail closed and water-hammer the riser; pressure tanks lose air-charge and force short-cycling. A robust monthly inspection covers all four — it is 2 hours of work that prevents 5-day pull-and-replace operations.

Water chemistry decides materials, and in Kenya it decides them more often than pressure or flow do. Fluoride is elevated across large parts of the Rift Valley and needs to be measured rather than assumed, because it governs whether the water is fit to drink and what treatment the scheme has to carry. Coastal and some sedimentary boreholes carry chloride and salinity that will pit ordinary cast-iron hydraulics, which is where stainless or duplex wet ends earn their premium. Dissolved iron and manganese are the quiet ones: they feed iron-related bacteria that build a slime on the screen and gravel pack, and a well that has lost half its yield in three years has usually been fouled rather than pumped dry. Abrasive fines finish the job on impellers. Take a full chemical and bacteriological analysis at commissioning, and repeat it annually — it is the cheapest test on the site and it explains most of the failures that follow.

Two honest cautions about yield. First, a desktop or geophysical survey narrows where to drill and improves the odds; it does not guarantee a figure, and anyone quoting a precise yield before the hole is drilled and tested is selling confidence rather than data. The number that matters comes from the step test and the constant-rate test, after development, and it should be presented with its assumptions visible. Second, sustainable yield is not the same as the maximum the hole will give on the day of the test. Equipping a borehole at its peak tested rate is the most common cause of dry-running, motor burnout and a collapsed water level a year later. Size the pump to the sustainable abstraction rate, fit dry-run protection regardless, and keep monitoring water level after handover — abstraction in Kenya is permitted and monitored through WRA, and the monitoring record is what protects the licence as much as the pump.