Our expert responds #25

18 novembre 2020

by Idroelettrica spa

Geometric volume and useful volume of the fire-fighting water supply storage

QUESTION
What is the difference between the geometric volume and the useful volume of the fire-fighting water supply storage? 

ANSWER
The geometric volume of the tank that contains the fire-fighting water supply storage is obviously the value of the volume of the tank itself as can be calculated with elementary geometry formulas.
While the useful volume is equal to the geometric volume, minus that part of the volume of the tank, which cannot be counted for the purposes of the water storage

Let's see how to calculate this part of the tank that will therefore not be available to achieve the autonomy of operation. The reference is chap. 9.3 Accumulation tanks of UNI EN 12845: 2020 and in particular point 9.3.5, Figure 4 and Table 12.

The part of the geometric volume that cannot be used serves two purposes.
The first is to prevent the pumps from sucking up bodies or solid substances that could damage them.

  • For this purpose, a MINIMUM (B) distance FROM THE BOTTOM OF THE TANK is indicated

The second is to avoid the formation of vortices on the free open water of the tank; with the consequent dragging of air, in the suction pipe (or in the case of VTPs, directly inside the pump) due to the vortex movements that are generated on the surface of the storage tank. These vortices are caused by an insufficient overlap of the head of the initial end of the pipe, and/or by an excessive speed in the pipeline. Having taken care to respect the speed values imposed by the standard, only the insufficiency of the hydraulic head remains as a possible cause of the intake of air.
Figure 1 highlights the phenomenon, whose manifestation we want to avoid.

There is therefore the introduction of a MINIMUM HYDRAULIC HEAD (A) above the pump suction pipe.

As shown in table 12, distances A and B are according to the diameter of the intake pipe

For example, for a diameter of the suction pipe equal to 250 mm we have:

A = 0.75 m ; B = 0.20 m ; A+B = 0.95 m
USEFUL VOL = VOL GEOM – BASE AREA x 0.95

Therefore, the volume between the base of the tank and a level of 0.95 m CANNOT BE USED FOR FIRE-FIGHTING PURPOSES to guarantee what has been analysed above.

From the same table, it can be deduced that: using an ANTI-VORTEX PLATE of the minimum size indicated (for example, in the case we are using, with a pipe of DN 250 mm, the plate must have a minimum size of 1.0 x 1.0 m), the height A is no longer according to the diameter of the intake pipe, but becomes a constant equal to 0.1 m.

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