Useful Volume of Fire Protection Water Reserves: When discussing water storage for fire protection purposes, “how much it holds” does not always equate to “how much is actually usable.” In this answer, I clarify the difference and the technical reason why part of the volume should not be included in the calculation.*
Table of Contents
Difference Between Geometric Volume and Useful Volume
The geometric volume of the tank is the “total” volume of the container, which can be calculated using basic geometric formulas based on the tank’s shape and dimensions.
The usable volume, on the other hand, is the volume that can actually be counted toward the fire-fighting water supply: in practice, it is the geometric volume minus the portion of the tank that cannot be considered available for achieving the required operating autonomy.
This distinction is crucial because, in practice, the inlet and hydraulic conditions do not allow for the full utilization of all the water contained—”down to the last drop.”
The basis for calculating the unusable portion
To determine the portion (and therefore the volume) that should not be included in the calculation, refer to UNI EN 12845:2020, Chapter 9.3, “Storage Tanks,” specifically:
- Section 9.3.5
- Figure 4
- Table 12
The objective is to estimate the portion of the tank that, for functional reasons and to protect the equipment, is excluded from the usable volume.
Because there is a “non-usable” part: the two functions
The unusable portion of the geometric volume serves a dual purpose.
- Pump protection
This is used to prevent the pumps from sucking in solid bodies or substances that could damage them. For this reason, a minimum distance (B) from the bottom of the tank is indicated. - Prevention of Vortices and Air Ingestion
This is designed to prevent the formation of vortices on the free surface of the tank, which would result in air being drawn into the inlet pipe (or, in the case of VTPs, directly into the pump). This phenomenon is related to the vortex-like movements that form on the surface.
In practice, the goal is to ensure stable inlet flow, without any issues that could compromise operation.
Minimum hydraulic head (A) and minimum distance from the bottom (B)
To control the risk of air ingress, a minimum head (A) is maintained above the pump’s inlet pipe.
The text points out that vortices can be caused by:
- insufficient overlap of the head of the tube and/or
- excessive flow velocity in the pipe.
Assuming that the speed limits specified by the rules have been observed, insufficient hydraulic head remains a possible cause of air ingress. Hence the importance of complying with dimension A.
As indicated, distances A and B depend on the diameter of the inlet pipe, as shown in the reference table.
Practical example: DN 250 and calculation of the non-billable volume
For an inlet pipe diameter of 250 mm, the example states:
- A = 0.75 m
- B = 0.20 m
- A + B = 0.95 m
Hence, the calculation method is as follows:
USEFUL VOLUME = GEOMETRIC VOLUME − BASE AREA × 0.95
Operational interpretation: The volume between the bottom of the tank and a level of 0.95 m cannot be used for firefighting purposes, in order to ensure the two functions described above (pump protection and prevention of vortices/air).
This step is the key to moving from “limited volume” to “actually measurable volume.”
Anti-vortex plate: What changes in Category A
The same table shows that the use of an anti-vortex plate (with the specified minimum dimensions) affects dimension A.
In the example with a DN of 250 mm, it is specified that the plate must have a minimum size of 1.0 × 1.0 m. In this case:
- Height A is no longer a function of the diameter of the inlet pipe;
- A becomes a constant equal to 0.1 m.
In practical terms, this means that the anti-vortex plate is a component that directly affects the determination of the height above the Inlet (A) and, therefore, the non-countable volume, according to the specified configuration.
Conclusion: How to Correctly Use “Geometric Volume” and “Useful Volume”
In summary, the geometric volume describes the total capacity of the tank, while the usable fire-fighting water volume excludes a non-countable lower portion, determined by the distance from the bottom (B) and the minimum head above the Inlet (A). The DN 250 example shows how A and B result in a total elevation (A+B) that must be subtracted—using the base area—to arrive at the usable volume.
