The LWF Blog
Fire Safety Engineering for Design – Sprinkler Installation Planning – Part 251
September 8, 2025 10:21 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 250, LWF discussed sprinkler protection by considering the three main styles of pipework for sprinkler systems. In part 251, we talk about pre-calculated pipe arrays and fully-hydraulically calculated pipe arrays.
Pre-calculated pipe sizes are most commonly-seen in light and ordinary hazard class sprinkler systems. The pipe configuration influences the pipe tables and pipe diameters used. Full hydraulic calculations may be required for some types of sprinkler system, but can be useful for all system designs.
A fully-hydraulically calculated pipe array is one in which a detailed hydraulic analysis of the entire sprinkler system is performed, with the aim of determining the precise hydraulic characteristics of the system and to balance the water supply capacity.
The calculation establishes the basis of hydraulic demand in the most unfavourable situation for the entire installation. It can be important to also establish the basis for the most favourable situation too, when the water supply is limited and where demand overload could be detrimental or shorten the operational time of the sprinkler system. Limited water supplies tend to refer to systems running with a water storage tank with automatic pumped supply. To get the hydraulic demand of the most and least favourable situations may involve performing multiple calculations, as the situations may not be obvious.
When systems are designed to NFPA 13 standards, they do not require hydraulic calculations for favourable areas. But, the standards relating to the country and area of installation must be adhered to as a priority, unless a fire-engineered solution for the premises is agreed with the authorities having jurisdiction.
The process for calculating the basis of hydraulic demand involves establishing the individual sprinklers in the assumed maximum area of operation (AMAO), which will be as close as possible to rectangular in shape in the case of the most unfavourable location, and square for the favourable location.
The number of sprinklers within the AMAO is calculated by considering the areas covered by individual sprinkler heads and adding until the design area of operation is covered.
In part 252 of LWF’s series on fire engineering we will continue looking at how to calculate the basis of hydraulic demand for a sprinkler system. 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.