The LWF Blog
Fire Safety Engineering for Design – Pressurisation Systems – Part 205
October 14, 2024 10:44 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 204, LWF discussed the planning for the use of compensated systems. In part 205, we continue to talk about the air release path and leakage.
In situations where air relief is provided via mechanical extraction, the Q value for the pressurised space may be used to calculate the required extract rate.
Where air release is provided by natural ventilation, the following equation applies:
Q is the air flow into a pressurised space (m3 · s–1) and AVA is the area of air/smoke relief vent in m2.
It is worth nothing that the value of Q will be the higher of the values calculated when doors are closed or open, depending on the design of the system and code requirements for the adjoining space.
Leakage
Aside from the obvious sources of leakage – apertures such as windows and doors that are open – leakage may also occur from the building structure. This leakage should be added to the assumed leakage for open doors etc., particularly when assessing the ‘door closed’ scenario.
More air leakage data information is available through BS 9999, although it is accepted that limited data is available and requires updating. Modelling and CFD may be used to demonstrate how the performance requirements are achieved.
Whatever the calculated air leakage is found to be, the level should be increased by 50% to reflect possible increases in leakage found during the commissioning process.
Injection Points
A pressurisation system may provide one or multiple injection points. The provision of injection points is often determined by local codes. NFPA 92 recommends single injection point systems are not provided for stairways in excess of 30 metres in height, however, BS EN 12101-6 suggests that single injection systems are limited to buildings less than 11 metres high and any building above this height should be provided with no more than three storeys between injection points.
In part 206 of LWF’s series on fire engineering we will begin to discuss natural ventilation systems and the three forms available. 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.