The LWF Blog
Fire Safety Engineering for Design – Sprinkler – Water supplies – Part 255
October 6, 2025 9:43 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 254, LWF talked about the water supplies for sprinkler systems, starting with duplicate supplies. In part 255 we continue to discuss duplicate water supplies and how to ensure your sprinkler system has sufficient water for any foreseen eventuality.
When calculating the necessary water provision for a sprinkler system, it is important to consider the circumstances of a full design fire size in the most hydraulically favourable location. The maximum possible flow rate should be taken into account in terms of the pump driver power and the available water/tank capacity.
The installation demand curves should be extended onto a graph (see below). The point at which the most favourable curve intercepts the pump curve at its highest point is known as Qmax and this value should be used to calculate the tank size and pump duty.
Image from CIBSE Guide E
The tank capacity is determined by allowing for the flow of Qmax for the design duration of demand, and the duration is related to hazard classification:
Light hazard – 30 minutes
Ordinary hazard – 60 minutes
High hazard – 90 minutes
The tank capacity indicated may be negotiable if the refill supply of water to the tank is reliable and suitably fast. The majority of design codes allow for a reduction in tank capacity, providing the shortfall in stored capacity is mitigated by the rate of infill during the discharge period.
Sprinkler pumps must be enabled to automatically start as soon as a drop in trunk main pressure is detected and, once started, must run until switched off manually.
The pump will operate under two possible conditions – flooded suction or suction lift. This depends on the relationship of the pump centre line and low water level. Flooded suction conditions apply when at or below 2 metres depth, or one-third of the effective capacity is reached, whichever is the lesser volume of water, below the centre line of the pump. With natural unlimited supplies, such as lakes, rivers, canals, the pump centre line must be at least 0.85 metres below the lowest known or anticipated water level.
In part 256 of LWF’s series on fire engineering we will continue to talk about water provision for sprinkler systems, starting with the NFPA design guide. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.