How to Calculate the Required Capacity of a Fire Suppression Tank? A Practical Guide

A properly sized **fire suppression tank (FP tank)** is the foundation for effective fire protection of a facility. Its capacity cannot be arbitrary – it must guarantee a sufficient water reserve to extinguish a fire in its initial phase. Are you wondering how to calculate how large a tank you need? In this step-by-step guide, we explain what the required capacity depends on and how to estimate it in accordance with regulations.

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What Determines the Capacity of an FP Tank?

The basis for all calculations regarding the capacity of an FP tank is the precise definition of requirements for a given building. The key document relied upon by fire protection system designers in Poland and Europe is the standard **PN-EN 12845**. It defines what **water reserve for fire protection purposes** is necessary. The most important factors influencing the required capacity are:

• **Fire hazard class of the facility** – the greater the risk, the more water is needed.
• **Type and size of the extinguishing system** – e.g., sprinkler system, internal or external hydrants.
• **Required duration of system operation** – i.e., how long the system must be able to supply water under appropriate pressure.

Step 1: Determining the Fire Hazard Class

The first and most important step is the classification of the facility in terms of fire hazard. The PN-EN 12845 standard divides facilities into three main categories that determine further calculations:

• **LH (Light Hazard)** – Low hazard. Applies to facilities with a small amount of flammable materials, e.g., offices, schools, hotels.
• **OH (Ordinary Hazard)** – Medium hazard. The most common category, covering, among others, parking lots, workshops, production, and storage halls. It is divided into 4 subgroups (OH1-OH4) depending on the specifics of the stored materials.
• **HH (High Hazard)** – High hazard. Facilities where highly flammable materials are stored or processed, e.g., chemical plants, paint, and solvent warehouses.

Remember that the correct classification of the fire hazard must be carried out by an authorized designer of sanitary installations or a fire protection expert.

Step 2: Determining the Required Flow Rate and Duration of Operation

For each fire hazard class, the standard assigns specific values regarding the minimum operating time of the extinguishing system and the required flow rate (water output).

• For class **LH**, the operating time is usually **30 minutes**.
• For class **OH**, this time ranges from **60 to 90 minutes**.
• For class **HH**, the installation must be ready to operate for up to **90 minutes**.

The flow rate of the installation depends on its type and the required sprinkler density specified in the standard for a given hazard class.

Step 3: Formula for Calculating FP Tank Capacity

With the above data, we can use a simple formula to calculate the minimum usable tank capacity:

V = Q x t

Where:

• **V** – required usable tank capacity [in m³]
• **Q** – required nominal flow rate of the extinguishing system [in m³/min]
• **t** – required minimum operating time of the installation [in min]

Practical Calculation Example

Let's assume we are designing protection for a warehouse classified as **OH3**. According to the PN-EN 12845 standard for this class, the required operating time is **60 minutes**, and the designed flow rate of the sprinkler system is **2750 l/min**.

1. Convert flow rate from liters to cubic meters: 2750 l/min = 2.75 m³/min.
2. Substitute data into the formula: V = 2.75 m³/min x 60 min
3. The result is: **V = 165 m³**

This means that we need a tank with a usable capacity of at least 165 m³. It is worth noting that the total (geometric) capacity of the tank must be slightly larger to account for the so-called **dead volume** (water remaining below the pump suction nozzle).

Professional Selection of the FP Tank is Key

The above calculations represent a simplified scheme. Precise **selection of the FP tank** must always be performed by a designer and take into account all elements of the installation, including the potential water demand for hydrants. This guarantees that the system will work reliably in a critical moment.