From zero to Qmax: Jensen Hughes breaks down sprinkler tank sizing

Andrew Cowan, Senior Engineer at Jensen Hughes, explains what Qmax means in the context of BS EN 12845

In fire protection engineering, automatic sprinkler systems are fundamental to safeguarding both life and property. Their effectiveness stems from their ability to detect and control fires rapidly, containing fire growth until emergency responders arrive. To achieve this, two design factors are essential: water density and duration of discharge.

Water density is the volume of water applied to a given area, usually measured in millimetres per minute (mm/min) or litres per minute per square metre (l/min/m²) in the UK. It ensures sufficient water reaches the fire to absorb the heat, cool burning materials and prevent spread. If the density is too low, the water may evaporate before suppressing the fire, reducing the system’s effectiveness.

Equally important is the discharge duration – the length of time the system must maintain adequate flow. This ensures the fire remains under control until it is fully extinguished, either by the fire service or in a controlled burn-out. Insufficient duration risks re-ignition or uncontrolled spread after initial suppression.

Both density and duration are set out in BS EN 12845, the European standard for the design, installation and maintenance of fixed sprinkler systems. Within its hydraulic design requirements, one concept stands out: Qmax, the maximum water flow rate a system must deliver under the most favourable hydraulic conditions. Understanding Qmax is key to sizing an adequate stored water volume.

Methods of hydraulic calculation

To ensure reliable sprinkler performance, designers must conduct hydraulic calculations to confirm that the pipe network can deliver adequate flow and pressure to operating sprinklers. Under BS EN 12845, two calculation methods are permitted:

1. Pre-calculated Method: This method is a solution where the pipework diameters are sized as per tables within BS EN 12845 up until a defined point on the sprinkler array. This is known as a design point. The pipe diameters from the design point back to the Installation Control Valve (ICV) are calculated to ensure the total pressure loss does not exceed 0.5 bar, except where static pressure, between the height of the highest sprinkler on the system and the design point in question can be accounted for. The pressure and flow requirements to be provided to the ICV are prescribed in the standard for the specific risk.
2. Full Hydraulic Calculation Method: This method is required for high-hazard systems but also used where customised pipe sizing or water supplies are needed. The full calculation method requires designers to evaluate two operating scenarios:
3. Most hydraulically unfavourable area: Usually farthest from the water supply, where maintaining pressure and flow is most difficult.
4. Most hydraulically favourable area: Typically closest to the supply, where conditions are least demanding.

Factors considered

To establish pressure and flow requirements, the calculation must consider:

• Pipe lengths/types and diameters
• Elevation differences between sprinklers and the pump or tank
• Friction losses in pipes, fittings and valves
• Hazard classification density requirements
• Application area
• Minimum sprinkler operating pressure

Assumed Maximum Area of Operation (AMAO)

A critical element of hydraulic design is determining the Assumed Maximum Area of Operation (AMAO), which specifies the maximum number of sprinklers likely to operate during a fire. This value depends on the hazard classification and whether the system is wet or dry.

For instance, an Ordinary Hazard Group 3 wet system requires an operating area of 216 m², while the same occupancy with a dry valve necessitates a larger AMAO of 270 m². Designers should consult BS EN 12845 to confirm the correct AMAO for each scenario.

By defining the AMAO, designers ensure the system can handle the worst-case conditions for the specified hazard class. This area is used to model realistic fire scenarios and to correctly size pumps, pipework and water storage.

Tailored design based on actual fire compartment areas

Every fire protection project is unique and should be assessed according to its specific risk profile. While the AMAO provides a standard design reference, actual fire compartment sizes can be used if they are smaller and meet the required fire resistance for the hazard class.

This approach allows for a customised design, optimising sprinkler water demand, pump requirements and tank capacity. By basing calculations on the actual protected area rather than a generic maximum, designers can improve system efficiency without compromising compliance, provided the correct compartment fire resistance is maintained.

Fire pump curve and system demand

After identifying the most favourable and unfavourable demand points, the designer selects an appropriate fire pump that can satisfy both scenarios. This selection process involves analysing the pump curve, which graphically represents the relationship between pressure and flow for a specific pump.

An example of the pump curve and system demand points is illustrated below

The graph illustrates four key lines.

Black Line – 1: The system demand is derived from the pressure-flow relationship of the most favourable area of operation. It is drawn using a hydraulic quadratic formula. This equation helps draw a curve that simulates how the system responds when only the closest sprinklers operate. Since these require less pressure to achieve the required flow, this becomes a conservative estimate for maximum system capacity.

Orange Line – 2: The true pump performance curve. This line shows how the pump’s pressure output decreases as flow increases. Under BS EN 12845, the pump must provide at least 0.5 bar more pressure than what is needed at the most unfavourable point to accommodate design changes during installation.

Green Line – 3: This represents the pump curve plus static pressure. The static pressure comes from the height of water in the storage tank, which contributes additional force through gravity. It essentially reflects the total available pressure at the pump outlet, combining pump energy and gravitational pressure.

Blue Line – 4: This is the Qmax line.

Qmax

Once the system demand curve (Black Line – 1) intersects with the pump plus static pressure curve (Green Line – 3), the intersection point projects vertically to the x-axis. This vertical projection is known as Qmax – the maximum theoretical flow the system could deliver under the most favourable conditions.

In practical terms, Qmax represents the maximum flow demand the fire protection system must be able to supply. It reflects the worst-case scenario of flow in a sprinkler activation and forms the basis for water storage sizing.

Sizing the sprinkler tank

After Qmax is determined, the next step is to size the water storage tank. The volume of water required depends on both Qmax and the required duration of sprinkler operation, which again depends on the hazard class.

The designer would then coordinate with a sprinkler tank manufacturer to select a tank with an effective capacity that meets or exceeds this volume.

Conclusion

Understanding and calculating Qmax is a critical step in the hydraulic design of sprinkler systems governed by codes like BS EN 12845. By accurately determining Qmax, designers ensure that the system is capable of meeting the most demanding suppression scenarios.

Whether you’re a fire protection engineer, designer or building services consultant, understanding the principles behind Qmax gives you an understanding of how a sprinkler tank is sized.

At Jensen Hughes, our fire protection engineers apply these principles to various projects, supporting compliant and efficient sprinkler system designs tailored to each facility’s specific risk profile and operational requirements

This was originally published in the March 2026 Edition of International Fire & Safety Journal. To read your FREE copy, click here.