The differential head is the purely static height difference between the fluid level at the pump inlet and the fluid level at the outlet. It therefore describes only the vertical stroke the medium must overcome and disregards all dynamic components such as pipe losses, friction, or velocity changes.
The delivery head, on the other hand, is a complete energy parameter that indicates the total energy the pump delivers to the pumped medium. It is composed of several components:
- Static component: Differential head (Δz)
- Dynamic component: Velocity changes (v²/2g)
- Pressure component: Pressure differences between the suction and discharge sides
- Loss component: Pipe friction and shape losses in the piping system
While the delivery head is a property of the pump, the differential head results from the system or hydraulic system.
Why the differential head is relevant
In practice, the differential head is a key parameter for pump design because it indicates the static head displacement required by the system. It plays a particularly important role in:
- tall buildings or vertical installations
- well and suction lifting systems
- multi-stage systems for overcoming large height differences
- pressure boosting systems where the static height difference is a significant factor
Since, unlike the delivery head, it is independent of pipe lengths, valves, or flow conditions, it forms the basis for calculating the required total head.
Calculation of the differential delivery head
The differential delivery head is determined purely geometrically:
H diff = z pressure side − z suction side
Where:
- z pressure side: level of the fluid to be pumped
(e.g., tank level, final height of the pipe) - z suction side: level of the medium before it enters the pump
Example:
- Suction tank: 1 m above ground
- Pressure tank: 18 m above ground
→ Differential delivery head = 17 m
Important: This value remains constant as long as the fill levels do not change. Only when reservoir levels fluctuate (e.g., tanks, wells, gauges) must the differential head be dynamically assessed.
Ratio of Differential Head to Delivery Head
The actual total delivery head required by the system is calculated as follows:
H total = H diff + H pressure + H losses + H velocity
The differential head is therefore only one component, but a fundamental one, as it represents the unavoidable and irreducible part of the total delivery head.
For pump selection:
- A pump must always deliver at least the total delivery head.
- The differential head is the static minimum value, which exists regardless of operating conditions.
- 90% of all sizing errors result from confusing static and dynamic delivery head – therefore, a clear distinction is crucial.
Practical Application in Pump Technology
Engineers and operators use differential head to:
- determine the static base load of a system
- determine the minimum head requirement
- estimate the influencing factors during system modifications
- identify energy-saving potential by reducing dynamic losses
The precise evaluation of differential head is particularly important in the context of head optimization, system modernization, and energy inspections according to current technical standards.
