Accurate calculation of Total Dynamic Head (TDH) is one of the most important steps in pump selection. TDH represents the total pressure a pump must overcome to move fluid through a system. An incorrect estimate can lead to undersized pumps (causing low flow and cavitation) or oversized pumps (causing energy waste and higher costs).
This article explains what TDH is, the components that make it up, and how to calculate it step by step.
TDH is the sum of all vertical lifts, pressure requirements, and friction losses in a pumping system. It is expressed in metres (m) or feet (ft) of fluid.
Static Suction Head (or Lift)
Static Discharge Head
Friction Losses
Pressure Head
Components of TDH
Static Suction Head (Hs)
- If liquid source is above the pump, suction head is positive.
- If liquid source is below the pump, suction lift is negative.
Static Discharge Head (Hd)
Vertical distance from the pump centreline to the final discharge point.
Friction Losses (Hf)
- Caused by resistance in pipes, bends, fittings, and valves.
- Can be calculated using Darcy-Weisbach or Hazen-Williams equations.
- Typically estimated with friction loss charts based on pipe diameter, length, and flow rate.
Pressure Head (Hp)
- Additional pressure required at discharge (e.g., sprinkler heads, filters, or process equipment).
- Converted to head using:
Calculation Example
Application: Diesel Pump for Irrigation Sprinklers
- Flow rate required: 40 L/s
- Static suction lift: 4 m (pump above water source)
- Static discharge head: 18 m (pump to sprinkler elevation)
- Friction losses: 10 m (pipes, fittings, valves)
- Pressure at sprinklers: 200 kPa
Convert pressure head:
The pump must deliver 40 L/s at 52 m TDH.
Common Mistakes
Ignoring friction losses – leads to undersized pumps.
Not converting pressure requirements into metres of head.
Forgetting to account for future system expansion.
Selecting pumps far from their BEP (Best Efficiency Point).
Overestimating safety margins, which can lead to costly oversizing.
Best Practices
Always confirm pipe sizes, lengths, and fittings before calculating losses.
Use friction loss charts or software for accuracy.
For large projects, request a hydraulic study to verify calculations.
Double-check NPSH available vs NPSH required to prevent cavitation.
Document assumptions (pipe roughness, flow rates) for future reference.
Summary
Total Dynamic Head (TDH) is the foundation of pump sizing. By correctly accounting for suction lift, discharge head, friction losses, and pressure requirements, engineers can select pumps that operate efficiently, reliably, and with reduced lifecycle costs.