Ron Astall of United Pumps asks whether Bernoulli’s principle is dead or alive in pumping applications.
Why and when do I need to add velocity head into my calculations? Should I include velocity head in my NASHa* (net absolute suction head) calculations.
Bernoulli’s principle, based on conservation of energy, states that when the speed of a fluid increases, its pressure decreases.
The total available fluid energy in a flowing stream at a particular location is the sum of the pressure energy, potential energy and kinetic energy.
Bernoulli’s principle is based on the sound premise that the total energy in a fluid streamline remains constant excluding losses (see figure 2).
The total available fluid energy at location 1 and 2 will be the same except for the friction losses between those points. If location 2 is at the lower level, the potential energy component will be less, and the pressure reading at this location will be higher by the level difference.
This is obvious in any hydraulic circuit. What is less obvious is that if there are changes in pipe cross-sectional area between points 1 and 2, the revised fluid velocities will create a corresponding change in the kinetic energy component. This will also cause a corresponding change in the pressure readings between the two points.
Why do pressure gauges lie?
They don’t, but they only measure the pressure component of the total available fluid energy. Pressure gauges understate the total head because they do not include the velocity head (kinetic energy) component of the total available fluid energy. This is why all recognised pump test codes specifically include velocity head corrections for all pressure measurements.
Where there are high local velocities, the velocity head correction can be very significant indeed.
In figure 3, the mean fluid velocity in the 52.5mm bore pipe at a flow of 60m³/h is 7.7m/s, and the corresponding velocity head is 3.02m, a significant error if ignored.
Only recently, a colleague was troubleshooting a pump that was “down in head” based on the client’s pressure readings, only to find that with its very small nozzle sizes and high flow, the velocity head correction was around 11m. When this correction was included, the performance was considered ok.
Perhaps they should have measured the pressure further downstream where the piping was much larger. Nevertheless, velocity head correction ought to always be considered. If the inlet and outlet pressure measurement points are located in the same pipe diameter, the corrections will cancel out, but while net differential will be correct, the absolute values will still be wrong.
Suction performance – NASH testing
NASH (previously known as NPSH) measurements will always be incorrect if velocity head corrections are not applied to the gauge readings.
In figure 3, the ISO 9906 formula for H1 specifically includes the velocity head component when calculating the available NASH, after converting gauge pressures to absolute values. This means that pump NPSH/NASH curves, based on performance testing to a recognised code, will have been created based on the total available fluid energy at the pump inlet and hence the NPSH curve will include velocity head.
One example of the importance of including velocity head correction in suction-head measurements is the scenario of high-suction velocities in test loop inlet piping. We have sometimes been able to test down to a theoretically impossible “zero” NASH, based on gauge readings alone.
In these unusual circumstances, the velocity head at the inlet pressure gauge can be around 2m and when added to the gauge reading, a sensible answer is then obtained.
Conclusions
The total available fluid energy in a flowing stream at a particular location is the sum of the pressure energy, potential energy and its kinetic energy. If we are to correctly measure pump performance, we need to always include the kinetic energy component, more commonly known as the velocity head on to our pressure readings.
When assessing NASH, the total energy available must be included, which means that any NASH calculation must include the velocity head.
Where the inlet total head is assessed theoretically, as when using levels and pipe friction loss calculations from a known point in the system, the velocity head is inherent in the calculation and does not need to be added again.
When the inlet total head is arrived at by a direct pressure measurement, then velocity head must be added to the gauge reading.
*NASH is used instead of NPSH throughout this article, following on from the previous Pump Curves instalment in the Summer 2025/26 edition of Pump Industry titled, NPSH, that well-known misprint.
