Parallel tanks as a water reserve: the answer isn’t a simple “yes” or “no.” The key is to ensure the performance, continuity, and reliability of the water supply throughout its expected service life, even in the event of a fire.
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The question (basically) is why it’s a sensitive issue
When it comes to multiple tanks connected in parallel, the key issue is not “how many tanks,” but how the system behaves during operation.
This is a sensitive issue because, if the splitting is not managed properly, it can lead to a malfunction of the entire fuel supply system. For this reason, before adopting a solution with multiple tanks, a rigorous approach is needed: the goal is not simply to “connect tanks in parallel,” but to ensure that the reserve remains truly available to the system.
What UNI/TR 11438 Says: Water Supply as a “Single Reserve”
The key reference is UNI/TR 11438 (4.2.3), which clarifies a fundamental concept: “The water reserve must be understood as a single reserve.”
Consequently:
- Splitting the water supply into multiple parts should be avoided, as it can cause service disruptions.
- Situations other than those involving a single water supply must be engineered to ensure that the system meets the required performance standards in all cases.
- The design must ensure the continuity and reliability of the water supply throughout its expected service life, even in the event of a fire.
- Availability must also be ensured in accordance with the provisions of section 4.1.1.
In other words: the rules promote a “single” reserve not out of abstract preference, but to reduce the operational risks typical of a fragmented system.
Parallel Tanks and Design Flow Rate: The Non-Negotiable Point
The “single reserve” principle gives rise to a very clear practical requirement: even at the minimum level, the tanks must be capable of delivering the design flow rate.
This point is crucial because it shifts the focus from “total” capacity to the capacity that can actually be delivered under the worst-case conditions (i.e., when levels are at their lowest). Therefore, it is not enough for the sum of the volumes to be adequate: it must be verified that the system, as a whole, delivers the required flow rate without any loss of performance that would compromise the supply.
What to size: tank outlets and inlet manifolds
If you decide to use parallel tanks, the text emphasizes that at least the following components must be properly sized:
- the diameters of the tank outlets
- the diameter of the inlet manifolds
The reason is directly related to the required performance: if the outlets and manifolds are not properly sized, the design flow rate may not be guaranteed, especially under critical conditions (for example, at the minimum level).
The text indicates that, through the following illustrations, it aims to provide basic calculation methods for determining the sizing of these components. The key point, however, is already clear: the “parallel” solution cannot be improvised and requires careful sizing of the connection’s key components.
Scope of Application: When Elementary Methods Are Sufficient
The “elementary” calculation methods mentioned above apply within specific limits. In particular:
- No more than two tanks connected in parallel to the same manifold
- Collector length <, 10 m
These conditions are not a mere detail: they define the scope within which the approach remains reasonable. If these limits are exceeded, the degree of approximation increases, and there is a risk of making design decisions that are not appropriate for the primary objective: ensuring the performance and continuity of the water supply.
When an “Engineered” Approach Is Needed (and Why)
The text is clear: in the case of:
- > -2 Tanks 2
- >, and 10 m manifold lengths
The approach described above (i.e., the elementary approach) would become too approximate.
This is consistent with the principle cited in UNI/TR 11438: situations other than a single water supply must be engineered. Here, “engineered” means designing the solution in such a way as to ensure:
- performance requirements of the system;
- continuity and reliability of the water supply;
- availability for the entire expected duration;
- resistance to the effects of fire.
It is therefore not a matter of “design style,” but rather of consistency with the safety objective and the reliability expected of the reserve.
Conclusion
Using tanks in parallel as a water reserve is a matter that requires caution: the reserve should be treated as a single unit, and splitting it up should be avoided if it could cause service disruptions. If this approach is taken, the non-negotiable requirement is to ensure the design flow rate even at the minimum level by correctly sizing the tank outlets and inlet manifolds, paying particular attention to the specified applicability limits.
