Minimum cooling flow rate: This is the minimum flow rate that must be maintained for a main fire pump even when the main flow rate is zero, in order to prevent overheating. In this answer, I’ll explain what this means, what is required, and how it applies in practice.
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Minimum cooling flow rate: operational definition
The minimum cooling flow rate is a continuous flow of water through the pump, designed to prevent overheating when the pump is operating at the outlet.
In practical terms, this means that for each main pump, it must be possible to ensure that qmin ≠ 0 even when Q = 0, in order to avoid problems related to the water heating up inside the pump housing.
What do the rules (section 10.5) require regarding the minimum flow rate?
The requirement is clearly stated: devices must be provided to ensure a continuous flow of water through the pump, sufficient to prevent overheating when it operates with the Outlet line closed.
This flow is not a “minor detail”; it must be taken into account:
- in the hydraulic analysis of the system;
- when choosing a pump.
In addition, the following is required:
- the circuit outlets are clearly visible;
- If there is more than one pump, the circuit outlets must be separate.
Why is a qmin ≠ 0 required when Q = 0?
When the pump operates with the outlet line closed, the main flow rate can be Q = 0. Under these conditions, without a dedicated water passage, problems may arise due to the water overheating inside the pump housing.
The rationale behind this requirement is therefore twofold:
- ensure a dedicated minimum continuous flow rate (qmin);
- to prevent overheating by maintaining flow through the pump even when there is no demand for a flow rate to the system.
Objectives to be ensured: continuity and verifiability of the flow
To be truly effective, the minimum cooling flow rate must not only “exist on paper,” but must also be controllable.
The stated objectives are:
- Ensure that, for each main pump, there is a minimum flow rate qmin ≠ 0 when Q = 0, to prevent problems caused by overheating.
- Check that the flow rate is proper within the minimum flow rate circuit.
This aspect is also directly linked to the requirement that the exhaust be clearly visible: visibility is an integral part of the ability to verify compliance.
Common technical solution: diaphragm valve (calibrated nozzle)
One of the most common technical solutions is the creation of a diaphragm-type socket—that is, one equipped with a calibrated nozzle—that ensures the minimum flow rate qmin under all conditions.
The approach described provides that:
- The dedicated socket for powering the minimum flow rate circuit must be installed on the outlet line of each main pump;
- The socket must be installed upstream of the check valve.
This creates a dedicated path that ensures the minimum flow rate and supports the required flow continuity.
Where to indicate the minimum flow rate: below the flap vs. above the flap
Under SOTTOBATTENTE conditions
Under SOTTOBATTENTE conditions, the minimum flow rate must be recorded inside the WATER RESERVOIR.
To make the flow visible (and thus easier to verify), you can use:
- a propeller-type display;
- a transparent tube.
Under OVERHANG CONDITIONS
Under OVERFLOW conditions, the minimum flow rate must be directed into the PRIME TANK.
Here, too, to make the flow visible, you can use:
- a propeller-type display;
- a transparent tube.
How to “check” in practice that the circuit is working
The required test concerns the proper flow rate in the minimum flow rate circuit. For this reason, the solution adopted should allow for clear and repeatable observation of the discharge.
In practice, the combination of:
- clearly visible exhaust pipe;
- instrument/display element (propeller-type display or transparent tube);
- separate drains when there are multiple pumps;
It helps make the verification process more straightforward, avoiding any ambiguity about which pump is actually recirculating the minimum flow rate.
Conclusion: What to Remember
The minimum cooling flow rate is the continuous flow that must be maintained through each main fire pump when it is operating in outlet mode, to prevent overheating. It must be taken into account in pump calculations and selection, with a visible discharge (and a separate discharge if there are multiple pumps) and a circuit that also allows for verification of the correct flow rate.
