The LWF Blog
Fire Safety Engineering for Design – Sprinkler Installation Planning – Part 252
September 15, 2025 10:31 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 251, LWF talked about pre-calculated pipe arrays and fully-hydraulically calculated pipe arrays for sprinkler systems. In part 252, we will look at how to calculate the basis of hydraulic demand for a sprinkler system.
To calculate the minimum rate of flow through each sprinkler, multiply the design density (1.m-2.min-1) by the area covered by each sprinkler. There is a basic minimum running pressure for each sprinkler head, to ensure the correct spray characteristic is established. They vary according to location and hazard as follows:
Light hazard, all types – 0.7 bar
Ordinary hazard, all types – 0.35 bar
High hazard, intermediate rack systems: K80 sprinkler head – 2.0 bar, K115 – 1.0 bar
High hazard, other types – 0.5 bar
ESFR and CMSA – varies according to risk type and sprinkler type
In NFPA 13, the minimum running pressure of standard sprinklers is 0.5 bar.
The calculations performed may include sprinklers below ducts or other obstructions. The final calculations for intermediate rack sprinklers should include both roof and rack systems operating simultaneously, even where the most unfavourable rack location is not in the same area as the most unfavourable roof location, allowing the premises owner to maintain flexibility in the layout of the racks.
The Hazen-Williams formula is the principal formula for establishing friction loss within the calculation process. It’s based on an empirical relationship relating the flow of water in a pipe to the physical properties of the pipe and the pressure drop caused by friction.
Losses or gains resulting from differences in elevation are accounted for using a simplified method, where 1.0m head is taken as 0.1 bar. The balance tolerances to be achieved in calculating are stipulated in the codes, which also state which information has to be provided to any authority having jurisdiction (or for approvals).
Hazen Williams formula is as follows:
where P is pressure loss (millibars), Q is flow rate (l · min–1), C is the roughness coefficient for the type of pipe (contained within design guides or codes) and d is the mean internal pipe diameter (contained within design guides or codes).
The calculation may be performed manually or through using software, which is generally preferred. Output from any software must be checked for validity, by cross-checking with other calibrated software or by carrying out manual cross-checks.
The resulting hydraulic calculations must be plotted on a water supply graph to ensure that the hydraulic demand of the system can be met by the proposed water supply.
In part 253 of LWF’s series on fire engineering we will begin to talk about the water supply for a fire suppression/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.