TDH Components Explained
Total dynamic head has four components:
- Static suction head (Hss): Vertical height from suction fluid surface to pump centreline (negative if pump is above fluid level — suction lift)
- Static discharge head (Hsd): Vertical height from pump centreline to discharge point or fluid surface
- Friction head (Hf): Pressure drop due to fluid friction in all pipes, valves, fittings, and equipment on both suction and discharge sides
- Pressure head (Hp): Pressure difference between discharge and suction vessel (in metres of fluid head)
Where all terms are in metres of fluid head.
Step 1: Calculate Static Head
Static head = discharge elevation − suction elevation (both measured from pump centreline). If the pump draws from a sump 2 m below the pump and discharges to a tank 15 m above pump centreline:
Hstatic = 15 − (−2) = 17 m
Step 2: Calculate Friction Losses
Friction losses are calculated using the Darcy-Weisbach equation for straight pipe and the equivalent length method for fittings.
f = Darcy friction factor (from Moody chart or Swamee-Jain for turbulent flow)
L = pipe length (m), D = internal diameter (m)
v = fluid velocity (m/s), g = 9.81 m/s²
For quick estimates, use these friction loss rules of thumb for water in schedule 40 steel pipe at 1.5 m/s:
| Pipe bore (mm) | Friction loss per 100 m (m head) |
|---|---|
| 25 | ~38 m |
| 50 | ~12 m |
| 80 | ~5 m |
| 100 | ~3 m |
| 150 | ~1.3 m |
Add equivalent lengths for fittings (bends, tees, valves) from manufacturer data or standard tables. Fully open gate valve adds ~10× pipe diameter equivalent length; globe valve adds ~300× pipe diameter.
Worked Example
System: Centrifugal pump draws water from a sump 1.5 m below pump centreline, and delivers to an open tank 12 m above pump centreline. Total pipe length = 45 m of 80 mm diameter steel pipe. Velocity = 1.8 m/s. Fittings equivalent length = 15 m. No pressure differential between vessels.
- Hstatic = 12 − (−1.5) = 13.5 m
- Total equivalent pipe length = 45 + 15 = 60 m
- Friction factor f ≈ 0.019 for turbulent flow in clean steel pipe
- Hf = 0.019 × (60/0.08) × (1.8²/19.62) = 0.019 × 750 × 0.165 = 2.35 m
- TDH = 13.5 + 2.35 = 15.85 m → Apply 10–15% safety margin → Select pump for ~18 m TDH at required flow rate
Plotting the System Curve
The system curve plots TDH against flow rate. Static head is constant; friction losses increase with the square of flow rate. The operating point is where the pump curve intersects the system curve. For the example above, at half flow rate, friction losses drop to 0.59 m (1/4 of full-flow friction), so TDH = 14.09 m at 50% flow.
Plot the system curve and superimpose the pump curve. The intersection gives actual operating flow and head. If the intersection is far from the pump's BEP, consider a different impeller diameter or pump speed. See our pump curve guide for full details.