The LWF Blog

Fire Safety Engineering for Design – Fire Growth – Part 86

June 20, 2022 10:47 am

LWF’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 85, LWF discussed fire growth rates for post-flashover fires and began to look at the effects of sprinklers on a fire. In part 86, we continue to look at sprinklers and smoke detector sensitivity.

A sprinkler system becomes operational when the temperature in the area of the sprinkler head reaches the set operating temperature, which is commonly around 70 °C. At this point, a fusible element is designed to fail, a solder link melts or a glass bulb breaks.

The rate at which the element heats is based on its shape and mass. A short, heavy bulb will take longer to reach the designated temperature than a slimmer bulb, for instance.

This type of sprinkler sensitivity is known as the response time index (RTI). A sprinkler system which is rated with an RTI of less than 50 m1/2 · s1/2 is considered a quick response system. Other systems may be rated up to 350 and are considered a standard response time system.

Concealed sprinkler heads do not have a designated RTI by manufacturers due to the nature of the sprinkler assembly. Some research was undertaken in the past which determined RTI values for some sprinkler head types and it was ascertained that for a concealed sprinkler with a temperature-sensing element with a quick response, the RTI for the overall arrangement could not be classed in the same range as a quick response (less than 50) but could be within the expected range of a standard response sprinkler head.

The nature of a concealed sprinkler head means that its suitability and response time must be established and agreed upon with the approving authorities.

Some software programs may be used to assess fire growth, sprinkler location and sprinkler sensitivity on sprinkler activation, as an alternative to calculating the variables manually. One example is B-RISK which can be used to predict sprinkler activation times and corresponding fire sizes.

B-Risk is able to use the differential equation describing the temperature of the sprinkler sensing element, based on the RTI concept of Heskestad and Bill (1988) incorporating gas temperature and velocity in the vicinity of the sprinkler sensing element, using the unconfined ceiling jet correlation by Alpert in Chapter 14 of the SFPE Handbook.

In part 87 of LWF’s series on fire engineering, we will discuss sprinklers effect on fire size. 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 for over 25 years 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.

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