What Is Pump Head?
Pump head is the amount of energy a pump adds to a fluid, expressed as a height of fluid column. In simple terms, it answers this question: how high can this pump push this liquid when all pressure, elevation, and friction effects are included? Head is usually shown in meters or feet.
Unlike pressure, head is largely independent of liquid density when you compare pump performance curves. This is why pump manufacturers often provide curves in head vs. flow format instead of pressure vs. flow.
Core Formula: Total Dynamic Head (TDH)
The most practical way to calculate pump head in real systems is to use total dynamic head:
For most industrial and building systems, velocity head difference between suction and discharge is relatively small unless diameters are very different. Friction and elevation usually dominate.
Pressure Head
Where P is pressure in pascals, ρ is fluid density (kg/m³), and g is gravity (9.81 m/s²).
Friction Head (Darcy-Weisbach)
Where f is Darcy friction factor, L is equivalent pipe length, D is internal diameter, and v is flow velocity. Equivalent length should include fittings and valves converted into additional length.
How Do You Calculate Pump Head? Step-by-Step
- Measure or estimate suction and discharge pressures at pump nozzles.
- Find elevation difference between discharge and suction liquid reference points.
- Estimate flow rate at operating condition.
- Determine pipe internal diameter and equivalent length.
- Estimate friction factor from pipe roughness and Reynolds number (or use a typical value for preliminary sizing).
- Calculate velocity, then friction head.
- Add pressure head + static head + friction head to get TDH.
- Plot your required duty point (flow, TDH) on pump curves to select the right pump and impeller diameter.
Worked Example
Suppose you pump water with these conditions:
- Discharge pressure = 300 kPa, suction pressure = 0 kPa (gauge)
- Elevation difference = 15 m
- Flow = 30 m³/h
- Pipe ID = 80 mm
- Equivalent length = 120 m
- Darcy friction factor = 0.02
Pressure head is about 300000/(1000×9.81) = 30.6 m. At this flow and diameter, velocity is around 1.66 m/s and friction head is around 4.2 m. So TDH is approximately 30.6 + 15 + 4.2 = 49.8 m.
That means you would choose a pump that can deliver 30 m³/h at about 50 m of head, ideally near its best efficiency point (BEP).
Useful Pump Head Conversions
| Conversion | Approximate Value | Notes |
|---|---|---|
| 1 bar to head (water) | 10.2 m | At SG ≈ 1.0 |
| 1 psi to head (water) | 2.31 ft | Common field shortcut |
| Head (ft) from psi | ft = psi × 2.31 / SG | Adjust for specific gravity |
| Pressure (psi) from head | psi = ft × SG / 2.31 | For quick checks |
Common Mistakes in Pump Head Calculations
- Ignoring friction losses from valves, elbows, strainers, and heat exchangers.
- Using nominal pipe size instead of true internal diameter.
- Mixing gauge pressure and absolute pressure inconsistently.
- Assuming water properties for viscous or high-density fluids without correction.
- Selecting a pump only by maximum head, not by required flow at that head.
- Neglecting future fouling, wear, or process expansion margin.
How TDH Affects Pump Selection
After calculating TDH, the next step is pump selection. You should always match the required duty point to the manufacturer’s performance curve and verify:
- Operating point is near BEP for efficiency and reliability.
- NPSH available exceeds NPSH required with margin.
- Motor power covers expected operating envelope with service factor.
- Materials and seal arrangement are compatible with fluid and temperature.
- Control method (VFD, throttling, staging) fits system behavior.
A correct head calculation reduces energy waste, prevents cavitation problems, and improves pump life.
Frequently Asked Questions
How do you calculate pump head from pressure only?
Use H = ΔP/(ρg). For water, divide pressure rise in kPa by 9.81 to get meters of head approximately, or multiply psi by 2.31 to get feet.
Is pump head the same as pump pressure?
Not exactly. They are related but different representations. Pressure depends on fluid density; head is energy per unit weight and is commonly used for pump curves.
What is a good friction factor to start with?
For clean turbulent flow in commercial steel pipe, a preliminary value around 0.02 is often used. Refine it using Reynolds number and roughness for final design.
Why does required head increase at higher flow?
Because friction losses rise rapidly with velocity (roughly proportional to v²), and velocity rises as flow rises in a fixed pipe size.