Calculating Total Dynamic Head (TDH) ensures your borehole pump delivers adequate pressure for multi-apartment water supply. TDH represents the total resistance the pump must overcome, measured in meters or feet.
TDH Formula Basics
The standard equation is TDH = Static Head + Friction Loss + Pressure Head (if applicable). Static head is the vertical distance from the water surface in the borehole to the delivery point, like a rooftop tank. For apartments, include drawdown—the drop in water level during pumping—and lift to multiple floors.
Step 1: Measure Static Head
Start with dynamic water level: pump intake depth plus drawdown. Add vertical lift to the highest outlet, such as 30m borehole depth + 20m to tank + 15m building height = 65m base static head. In multi-unit setups, factor peak demand drawdown (e.g., 5-10m for 10-20m³/hour flow).
Step 2: Calculate Friction Losses
Friction loss arises from pipe length, diameter, fittings, and flow rate. Use pipe charts: for 50mm PE pipe at 15m³/hour, loss is ~2-3m per 100m horizontal run. Convert fittings to equivalent pipe length (e.g., elbow = 5m pipe). Total: (actual length / 100) × friction factor per manufacturer charts.
Example: 200m pipe + 50m fittings equivalent at 2.5m/100m loss = 6.25m friction.
Step 3: Account for Multi-Apartment Demands
Apartments amplify TDH via branching pipes and pressure needs. Add 10-20% buffer for simultaneous use across 20-50 units. If boosting to 2-3 bar per floor, convert pressure: 1 bar ≈ 10m head. Solar or VFD pumps in Kenya handle variable loads efficiently.
Complete Calculation Example
For a 50-unit Nairobi complex:
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Borehole: 80m total depth, 10m drawdown, intake at 70m.
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Lift: 25m to tank.
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Pipes: 150m horizontal/vertical (40mm main, branches), 10 elbows (50m equiv.), flow 12m³/h.
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Static: 70m + 25m = 95m.
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Friction: (200m total / 100) × 1.8m/100m = 3.6m.
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TDH = 95 + 3.6 = 98.6m (round to 100m).
Tools and Tips for Accuracy
Use apps like Pump Selector or Grundfos tools for precise friction charts. Measure actual flow with a test pump. For Raeli Hydro installs, site surveys confirm drawdown via step-drawdown tests. Oversize by 10-20% for longevity in sandy Kenyan aquifers.
This method matches pumps like Grundfos SQ (up to 150m TDH) to your setup, preventing cavitation or dry-run.