“Pump prevalence”:

what it means, how it is calculated, and practical applications

In fluid handling, head is one of the most important parameters for choosing and operating a pump correctly, both in centrifugal pumps and (in equivalent terms) in diaphragm pumps, where pressure is often considered more directly.

Understanding what head is, how it affects performance, and how it is calculated allows you to select the most suitable machine and ensure efficient, safe, and long-lasting operation of the system.

Flow rate and head: an essential balance

Flow rate (Q) and head (H) are closely related, especially in centrifugal pumps: in general, as the flow rate increases, the available head decreases, and vice versa.
This behavior is described by the manufacturer’s characteristic curves (flow rate on the horizontal axis and head on the vertical axis) and is used to identify the correct operating point, ideally close to the point of maximum efficiency (BEP).

What does “prevalence” mean (in simple terms)?

The head indicates how much energy per unit of weight the pump must supply to the fluid to move it from point A to point B in the system.
It is expressed in meters of fluid column (mH₂O when referring to water).

In practice, head is used to:

overcome the difference in height between suction and delivery,
overcome pressure losses due to pipes, bends, valves, and accessories,
(if present) manage pressure differences between tanks/lines.

Useful note: head and pressure are related but are not the same thing. The conversion depends on the density of the fluid.

How to calculate total prevalence

To calculate the head correctly, the system must be considered as a whole. The main contributions are:

Height difference (geodetic/static head): difference in height between the suction level and the delivery point.
Pressure losses: energy lost due to friction in pipes, bends, valves, filters, etc. (both on the suction and delivery sides).
Pressure difference (if any): for example, pressurized tanks or lines with different pressures.
Velocity term: often small, but appears in the complete formula.

Formula:

H = (p₂ − p₁)/(ρ·g) + (z₂ − z₁) + (v₂² − v₁²)/(2g) + Σh_losses

Where:

p = pressure, ρ = density, g = gravity
z₂ − z₁ = difference in elevation (geodetic height)
Σh_losses = total pressure losses

Typical practical case (system with open tanks and negligible velocity):
H ≈ (z₂ − z₁) + Σh_losses

In practice, it is reasonable to add a margin of 10–15% for variations (temperature, density, scaling, pipe wear).

Clarification of terms:

In technical language, some terms are misused or used as synonyms. Here is a clear definition:

Height difference / geodetic head (static): only difference in height.
Pressure losses: friction + localized resistance (bends/valves/fittings).
Total head: sum of height + losses + any pressure + any velocity.

Therefore: the geodetic height does NOT include losses (these are another factor).

How to choose the pump based on the head

A correct selection usually follows these steps:

Define the required flow rate (Q)
Calculate the total head (H) including height difference + losses + (possible) pressure
Estimate the pressure losses accurately, which depend on:
- length/diameter and roughness of the pipes
- number of bends and fittings
- type/condition of the valves
- viscosity and temperature of the fluid
Compare the point (Q,H) with the flow rate/head curves of the available models

Check NPSH: make sure that the available NPSH is always greater than the NPSH required by the pump, otherwise you risk cavitation (noise, vibrations, damage).

Prevalence and pressure: rapid conversion

For water at room temperature (ρ ≈ 1000 kg/m³):

1 bar ≈ 10.2 mH₂O

For denser or lighter fluids, the ratio changes, so this conversion is useful as a quick estimate, but should not be taken as a “universal law.”

Why prevalence is crucial

Prevalence is not just a number: it determines whether the system will operate with:

stable flow rate,
reasonable consumption,
fewer breakdowns and less downtime,
longer pump life.

At Fluimac, these parameters are central to every project: each pump is designed and tested to ensure consistent performance over time, even in the most complex applications.