Minimum flow rate of a centrifugal pump: This is a topic that often comes up when a pump is operating at partial load or is at risk of approaching very low flow conditions. Here is a practical guide to estimating the minimum “safety” flow rate (Qric) and, above all, to understanding why operating at a flow rate (Q) close to zero for many hours is never a good idea.*
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Minimum Flow Rate of a Centrifugal Pump: What Is It Really For?
When the flow rate decreases too much, the energy transferred to the machine can result in an increase in the fluid temperature in the volute area. For small pumps operating at a temperature sufficiently far from the vapor pressure, it is possible to estimate a minimum flow rate using a simplified method based on energy parameters.
This estimate yields a reference value (Qric) that is useful for preventing unwanted temperature increases. However, it is essential not to focus solely on temperature: at low flow rates, critical mechanical phenomena also come into play.
What data is needed to estimate Qric?
To determine the minimum flow rate (Qric) using the approach described, at least the following data and parameters are required:
- P = power of the centrifugal pump motor [kW]
- ρ = density [kg/m³]
- c = specific heat of the fluid
- An acceptable temperature rise value
The permitted temperature increase is related to:
- ts: temperatures at the inlet
- te: Permissible temperature behind the pump balancing device
In other words: you’re not choosing a “random” number, but rather setting the temperature range you can “accept” between the inlet and the area downstream of the balance point.
How to Choose the Permissible Temperature Rise (te – ts)
As a rough estimate, we can assume a temperature difference of about 20°C . This choice is intended to provide a quick estimate, consistent with the practical goal of preventing the machine from operating under thermally critical conditions when the flow rate is low.
For water, the text uses:
- c = 4.18 kJ/kg·K
This information is important because, given the same power and density, specific heat affects the minimum flow rate required to limit the temperature rise.
Practical example: 75-kW pump
Applying the proposed configuration (with the parameters listed above and a reference temperature increment), the following results are obtained for a 75-kW pump:
- Qric=3.23 m³/h
This is the “base” minimum recirculation/guaranteed flow rate value shown in the example. It is a useful figure for orientation, but it does not yet represent the final recommended value for operation, because an additional criterion related to the pump’s internal mechanics comes into play.
Why Apply a Coefficient of 1.5 (and What Does Axial Balance Have to Do With It)?
A coefficient of 1.5 must be applied to the calculated value, with the following practical objective: to prevent Qric from falling below the flow rate required to achieve axial balance of the impeller.
So, in the example:
- base value: Qric=3.23 m³/h
- Applying a coefficient of 1.5: the operating value becomes approximately Qric = 5–6 m³/h
This passage directly links the topic of “temperature” to a “mechanical” issue: it is not enough to prevent overheating; one must also avoid conditions in which the machine is unable to maintain an acceptable axial balance.
PLEASE NOTE: At low Q, the problems are not just thermal
The text clearly points out that when the flow rate decreases and the pump operates at minimum flow for long periods, mechanical risks increase. Specifically:
- Increasing axial thrust: This must be compensated for by suitable bearings.
- Single-volute pumps under partial loads: They generate high radial thrust, which requires precautions such as:
- shaft reinforcement
- Pay attention to the bearings
And there’s another critical issue: operating at reduced Q can cause damage due to cavitation. So, even if “thermally” you might think you’re in control, the machine can still be damaged.
Conclusions: Never stay near Q = 0 for many hours (limit the time at Qric)
It is not advisable to operate centrifugal pumps at flow rates close to zero for extended periods of time: this type of operation causes serious mechanical damage. And this is a key point: such damage is unrelated to the increase in water temperature inside the volute.
Consequently, even when you set the device to Qric mode, this should be limited to a few dozen minutes and not continued for long periods.
