Fire protection water supply systems with a storage tank connected to pumps: In this response, I summarize the most useful operational points for design and verification, with reference to systems calculated in their entirety.
Table of Contents
To help you find your way around quickly, here is a summary (which also serves as a Table of Contents).
- Types of Water Reserves
- Project Data to Be Reported
- Useful volume: X and N levels
- Antivortex: Why the Available Volume Changes
- Reliability: Single vs. Superior Single
- Pumps: Types, Motors, Q/H, NPSH
- Under/Over-Betting: Limits and Implications
- Start-up Procedures, Power Supplies, and Controls
- Inspections and Maintenance
What Is a Water Supply System with a Storage Tank and Pumps?
A fire protection water supply system with a storage tank connected to pumps consists of a water reservoir (tank/open-top tank) and pressurization pumps that must ensure the design flow rate (Q) and head (H) for the users.
The Q/H andruntime values are derived from the calculations and verifications required by the applicable rules in each case (e.g., fire hydrant systems or sprinkler systems). When selecting the water supply, the operational reference is to Chapters 8, 9, and 10 of UNI EN 12845.
Full water reserve vs. reduced-capacity water reserve (backup flow rate)
The reserve may be:
- Total water supply: a free-surface tank with sufficient volume to ensurethe requiredautonomy.
- Reduced water reserve: a reservoir with insufficient volume; the supply duration is calculated by adding the current volume to a guaranteed external supply (from the water system), i.e., the backup flow rate.
Herein lies the crux of the matter: if the backup flow rate is essential for achieving self-sufficiency, the water system operator must clearly certify that the flow rate is constantly available. In the absence of such certification, the designer must assume responsibility for declaring that the flow rate is always available and that the water supply is continuous and reliable, as required.
What design data must be reported (pumps and tank)?
Chapter 4 (Design and Documentation) outlines the information that must be provided during the design phase regarding:
- Automatic Pumping Unit
- Storage tank
For pump sets, among the elements mentioned: Q/H curve at minimum level “X,” power consumption curve (based on NPSHr = 16 m.w.c. for centrifugal pumps), required NPSH curve, available motor power, available and required NPSH at maximum flow rate, and submersion depth for above-ground and submersible pumps.
For tanks: total volume, effective capacity and range, backup flow rate (if capacity is reduced), vertical distance between the pump axis and minimum level “X”, maintenance frequency with draining, freeze protection, minimum level “X” and normal level “N”.
Useful Volume and Range: “X” and “N” Levels and Calculation of Effective Capacity
A key point is to distinguish between geometric volume and usable/effective volume. The usable volume is the volume between elevation “X” (minimum level) and elevation “N” (normal level/overflow level or selected maximum level).
The minimum level “X” is determined by referring to Table 12: depending on the diameter d of the suction pipe, the minimum hydraulic head above the inlet is indicated (this applies to both centrifugal pumps and VTPs, considering the minimum suction run).
The purpose of hydraulic coverage is to prevent surface vortices that draw air from the atmosphere into the pump, resulting in a drop in performance and possible mechanical damage.
Practical Example (Unusable Volume): Why “Geometric” Can Be Misleading
In the example shown: with d = 200 mm, the minimum hydraulic cover “A” is 0.62 m and the minimum distance from the bottom “B” is 0.15 m. Total: 0.77 m that cannot be used to calculate the usable volume.
For a rectangular pool measuring 10 × 5 = 50 m² and 4 m high:
- Unusable volume: 0.77 × 50 = 38.5 m³
- geometric volume: 50 × 4 = 200 m³
- Actual usable volume: 161.5 m³
This example explains why, when inspecting existing systems, it may turn out that the actual runtime does not match the expected runtime if the design was based on the geometric volume rather than the usable volume.
Anti-vortex plates: when they’re needed and how they affect the usable volume
In addition to ensuring the minimum coverage, the other solution described is the use of anti-vortex plates: appropriately sized and sufficiently rigid metal plates mounted at the lower end of the inlet pipe. They lengthen the path of the fluid flow and reduce its velocity, thereby limiting vortices.
From a regulatory standpoint, if the plate complies with the dimensions specified in Table 12, dimension “A” can be set to a constant value of 0.1 m for all diameters. In the previous example, the unusable height becomes 0.15 + 0.1 = 0.25, increasing the usable volume (in the example: +26 m³, up to 187.5 m³).
Operational conclusion: Anti-vortex plates can also help in the inspection of existing supply lines, restoring usable volume and operational autonomy.
Power Supply Reliability: “Single” and “Single Plus” (tank + pumps)
The“tank plus pumps”type is found in both single water supply systems and single upper water supply systems, but with substantial differences.
- In the “single” case, the configuration is “a tank with one or more booster pumps.”
- In the “single upper” configuration, the setup consists of “a reservoir and two or more pumps.”
For the higher-class system, the tank must meet specific requirements: total capacity (reduced capacity is prohibited), sealing (no light or material from the outside), use of clean water, and construction such that emptying for maintenance is required only after at least ten years. Internal inspections are also required every 3 years, except for superior-class tanks designed to be serviced after 10 years.
Main Pumps: Permitted Pump Types, Motors, and Rules for Multiple Pumps
Among the points mentioned:
- Acceptable engines for main pumps: electric and diesel.
- The selection of motor power depends on the pump’s performance curve (with reference to conditions corresponding to NPSHd = 16 m for rising curves).
- Main pump types: submersible centrifugal pumps, VTPs (vertical submersible axial-flow pumps); submersible pumps are permitted (for special cases). Horizontal multistage pumps are used for “systems with significant vertical rise.”
- If there are multiple pumps: they must be able to operate in parallel; with two pumps, each must deliver 100% of the design flow rate at the design head; with three pumps, each must deliver 50%.
Key guideline for multiple or dual power supplies: When there is more than one pump, no more than one should be driven by an electric motor.
Under/Over-battle: Main Limitations and Implications for Implementation
The standard distinguishes between under-joist and over-joist plumbing layouts, with limits on elevations, diameters, and inlet velocities.
For the bottom rail (between the marked points):
- at least 2/3 of the effective capacity above the pump shaft
- Pump shaft no more than 2 m above the minimum level
- inlet diameter ≤ 65 mm
- Maximum inlet velocity ≤ 1.8 m/s
For the top panel:
- Distance between minimum level and pump shaft ≤ 3.2 m
- inlet diameter ≤ 80 mm
- Maximum inlet velocity ≤ 1.5 m/s
- Intake manifolds are prohibited; each pump (including the jockey pump) must have an independent inlet
- A fire suppression system with alarms and dedicated connections is required, with a 100- or 500-liter suppression tank (depending on the hazard class).
Starting, Stopping, and Power Supplies (Electric/Diesel): What to Check
Starting systems:
- Each pump is equipped with two pressure switches, each capable of starting
- First pump: automatic start at 80% of maximum Head (Outlet closed); second pump, if any, at 60%
- Once started, the pump can only be stopped manually (with the exception noted above for fire hydrant systems only, where the pump stops automatically 20 minutes after pressure is restored).
For electric pumps: power supply must always be available, via a dedicated and separate circuit, with the socket located upstream of the main circuit breaker; cable sizing criteria (based on 150% of the maximum possible current at full load) and visual monitoring (power available, start request, pump running, failure to start).
For diesel motor pumps: continuous operation at full load (as per UNI ISO 3046), independent power supply for starting, specifications regarding cooling, exhaust, and fuel tank (hours of autonomy by hazard class), dual battery and starting logic (six attempts), indicator light/alarm monitoring, testing and acceptance inspections.
Conclusion: How to Correctly “Read” the Tank and Pumps
Fire protection water supply systems with tanks and pumps are not evaluated solely based on their total “rated” volume: what matters is the usable volume, X/N levels, air ingress prevention, under/over-pressure configuration, the number and type of pumps, and requirements for continuity and reliability. Even when assessing existing systems, measures such as anti-vortex plates can have a tangible impact on the usable volume and, consequently, on the system’s runtime.
