Fire Water Tank Calculator

How to work it out

1. Set the fire duration (GB 50974-2014): 3 h for hydrant systems in high-rise buildings, Class A/B factories and large public buildings, 2 h for other civil buildings; 1 h for automatic sprinkler systems.
2. Indoor volume V = indoor flow × duration × 3.6, in m³. The 3.6 converts L/s × h to m³ (3600 ÷ 1000).
3. Outdoor volume V = outdoor flow × duration × 3.6. Where the mains cannot guarantee flow and pressure, this volume must also be stored on site.
4. Sprinkler volume V = sprinkler flow × sprinkler duration × 3.6.
5. Credit reliable make-up during the fire: V = make-up flow × duration × 3.6. This is only permissible with two independent mains connections both guaranteed uninterrupted — never with a single connection.
6. Effective volume = indoor + outdoor + sprinkler − make-up; plan area = volume ÷ effective depth, typically 3.5-4.5 m.

Where it goes wrong

• Counting indoor demand only. Where the mains cannot be relied on, the tank must hold indoor and outdoor demand together; omitting outdoor halves the volume and fails fire acceptance outright.
• Assuming make-up is always available and deducting it in full. Only two guaranteed mains connections qualify; deducting against a single connection is effectively falsifying the capacity.
• Taking 2 h for every building. High-rise, large retail and Class A/B factories require 3 h, and one hour short is 50% of the volume — the classic case of discovering the tank is half the required size.
• Forgetting compartmentation. Above 500 m³ the tank must be split into two compartments, each independently maintainable, which increases both footprint and cost over a single-cell estimate.

Material and cost

Reinforced concrete tanks cost 800-1500 CNY/m³ including waterproofing, excavation and base; stainless sectional tanks 1200-2000. A fire tank is a large one-off investment, and remediating an undersized one usually means re-excavating and re-waterproofing at two to three times the original cost.

On site

• Effective volume excludes the suction sump, the freeboard and any portion that must not be drawn by non-fire users. Show both gross and effective figures in the design, or the capacity will be judged insufficient.
• Provide overflow, vent and access manway. The overflow must discharge indirectly and must never connect directly to a sanitary drain.
• Fit anti-vortex devices at pump suction. Insufficient effective depth lets the pump draw air, shown by violently fluctuating pressure gauges and failure to build head.
• In cold regions the tank needs frost protection — earth cover, insulation or an indoor location — with a check that water stays above 5°C in the coldest condition.

Basis

• GB 50974-2014 Technical code for fire protection water supply and hydrant systems, water sources and fire water demand
• GB 50016-2014 (2018 edition) Code for fire protection design of buildings, fire protection installations
• GB 50084-2017 Code for design of sprinkler systems