Hydraulics & Pump Selection Guide — Last Updated: 8 August 2026
Total Dynamic Head (TDH) represents the total work a pump must perform to move water from a suction source to a discharge destination under operating conditions. It is measured in metres of water column (m). For complementary hydraulic calculations, explore our Water Flow Rate Calculator and Pipe Volume Calculator.
On site, plumbers often make the mistake of looking only at static height — for example, thinking a 5-metre lift only needs a pump rated for 5m head. In reality, water moving through long pipe runs and tight elbows encounters significant friction resistance. Add in the 200 kPa (20.4m head equivalent) needed to operate a modern shower head comfortably, and that 5-metre physical lift easily becomes a 30-metre Total Dynamic Head requirement.
The Four TDH Head Components
The complete TDH equation combines four distinct hydraulic head components:
- Static Vertical Lift: The vertical distance from the pump discharge outlet up to the highest fixture in the building.
- Suction Head: The vertical lift required to pull water up from an underground tank or sump into the pump inlet. For rainwater catchment storage sizing, see our Rainwater Tank Size Calculator.
- Friction Head: Hydraulic head lost due to friction between flowing water and internal pipe walls over straight pipe runs and fittings.
- Pressure Head: The remaining residual pressure required at the point of use (e.g. 200 kPa for residential fixtures = 20.4m head).
Suction Pipe Sizing & Avoiding Pump Cavitation (NPSH)
More pumps fail prematurely from improper suction design than any other installation defect. When water velocity in the suction line is too high, or vertical suction lift is too deep, hydraulic pressure at the pump inlet drops below the vapor pressure of water. Microscopic vapor bubbles form and violently collapse against the impeller, causing cavitation—heard as loud gravel-like rattling, loss of prime, vibration, and pitted impeller damage.
To design a reliable suction pipe setup according to Australian hydraulic standards (AS/NZS 3500.1):
| Design Parameter | Recommended Standard | Why It Matters |
|---|---|---|
| Suction Pipe Velocity | 0.8 m/s to 1.2 m/s (Max 1.5 m/s) | Prevents friction drop and vortex turbulence in suction line |
| Suction Pipe Diameter | ≥ Pump inlet port (typically 1 size larger) | E.g. Use DN32 (1¼") suction pipe for a DN25 (1") pump intake |
| Maximum Practical Lift | ≤ 6.0m to 7.0m practical limit | Atmospheric pressure limit; beyond 7m use a submersible pump |
| Suction Pipe Type | Rigid PVC (Class 12/18) or reinforced suction hose | Prevents pipe collapse under continuous vacuum/suction pressure |
| Foot Valve & Strainer | Full-flow spring-loaded foot valve | Maintains pump prime; prevents foreign debris intake from tank floor |
Hazen-Williams Friction Loss Formula
To calculate pipe friction head loss accurately, Australian plumbing hydraulics relies on the empirical Hazen-Williams equation:
- Friction Loss Formula: h_f = (10.67 × L × Q^1.852) / (C^1.852 × d^4.87)
- Where L is total equivalent pipe length (m), Q is flow rate (m³/s), C is pipe smoothness factor, and d is internal pipe bore diameter (m).
| Pipe Material | Hazen-Williams C-Factor | Friction Resistance |
|---|---|---|
| PEX & Poly (HDPE) | 150 | Lowest friction loss |
| PVC Pressure (Class 12/18) | 150 | Lowest friction loss |
| Copper (Type A & B) | 140 | Low friction loss |
| Galvanised Iron | 120 | Higher friction loss |
Fitting Equivalent Lengths Table
Elbows, tees, and check valves disrupt smooth laminar flow, creating localized turbulence and head loss. Plumbers account for this by converting each fitting into an equivalent length of straight pipe:
| Fitting Type | Equivalent Length (DN25 Baseline) | Hydraulic Impact |
|---|---|---|
| Check Valve (Non-Return) | 2.5 metres | High head loss (spring resistance) |
| Tee (Branch Flow) | 1.5 metres | Moderate-high loss |
| 90° Standard Elbow | 0.8 metres | Moderate loss |
| Tee (Line Run) | 0.5 metres | Low loss |
| 45° Elbow | 0.4 metres | Low loss |
| Gate / Ball Valve (Full Open) | 0.15 - 0.2 metres | Minimal loss |
Australian Pump Selection Reference Guide
Matches calculated Total Dynamic Head against standard Australian pump types and trusted local models:
| Pump Type | Head Range | Popular AU Brands & Models | Typical Application |
|---|---|---|---|
| Sump/Sewage Submersible | 0m – 15m | Davey D Series, Grundfos Unilift, Zenit | Basement pits, stormwater sumps |
| Submersible Rainwater | 15m – 35m | Davey RainBank, Grundfos SBA, Claytech | Rainwater tank to house / garden |
| Centrifugal Surface / Jet | 20m – 50m | Davey XP / HS Series, Onga JSP, Bianco | Surface transfer, shallow bore, garden |
| Multistage Centrifugal / VSD | 30m – 80m | Grundfos SCALA2 / CM, Davey BT, Onga Subflo | Multi-storey boosting, high-pressure irrigation |
Real-World Worked Example: Victorian Rainwater Pump
Step-by-step walkthrough for an automatic submersible rainwater pump for a 2-storey home in Ballarat, VIC supplying 30 L/min to upstairs toilets and laundry:
- 1. System Layout: Static vertical lift = 5.5 metres, 0m suction lift (submersible), 35 metres of DN25 Copper Type B pipe run.
- 2. Fittings Installed: 4 × 90° elbows, 1 × check valve, 1 × ball valve. Total fitting equivalent length = (4 × 0.8) + 2.5 + 0.2 = 5.9 metres.
- 3. Total Equivalent Length: 35m + 5.9m = 40.9 metres.
- 4. Friction Head Loss: At 30 L/min through 23.03mm internal bore copper, Hazen-Williams yields 2.8 metres friction loss.
- 5. Required Outlet Pressure: 200 kPa target pressure = 200 ÷ 9.81 = 20.4 metres.
- 6. Total Dynamic Head (TDH): 5.5m (lift) + 2.8m (friction) + 20.4m (pressure) = 28.7 metres TDH.
- 7. Pump Selection Result: Select a Davey RainBank or Grundfos SBA 3-45 rated for at least 30 L/min at 29 metres head. For sanitary drainage connections, see our Drainage Pipe Sizing Calculator.
Common Australian Pump Types & Selection Mistakes
- Submersible Rainwater Pumps: Installed directly inside the tank. Silent operation, no prime loss, ideal for urban rainwater reuse.
- Centrifugal Surface Pumps: Mounted externally beside tanks or bores. Reliable and easy to service for agricultural water transfer.
- Multistage Pumps: Feature multiple impellers in series. Essential for high-head requirements and multi-storey buildings.
- Variable Speed Drive (VSD) Pumps: Automatically adjust motor RPM to maintain constant pressure regardless of how many taps are open.
- Common Sizing Mistake: Ignoring check valve friction loss or using pipe outer diameter instead of internal bore diameter in Hazen-Williams calculations.