When the topic of automatic fire sprinklers comes up for building projects, everyone understands and appreciates the benefits to property protection and life safety. However, sprinklers that hang from the ceiling are not always the most pleasing to the eye. This is especially true in public spaces and private residences. To address this aesthetic dilemma, concealed, recessed, and flush mounted (CR&FM) sprinklers were developed.
CR&FM sprinklers are, as their names allude to, located out of sight, and generally above the finished ceiling level. This provides a less obtrusive and more pleasing look. Concealed sprinklers are even provided with a cap that can be colour matched to the ceiling, to allow for an even more discreet look. Property and life safety protection in an unassuming package. This is great, right? Well, that depends. The very nature of the of CR&FM sprinklers being placed above the ceiling could have an impact on the sensitivity of the sprinkler, and thus potentially delay the overall sprinkler operating time.
Sprinkler sensitivity is tested in accordance with Section 33 in ULC/ORD-C199, “Standard for Automatic Sprinklers for Fire-Protection Service” (ULC 199) [1] and is a requirement for sprinklers to be Listed [2]. The sensitivity testing involves the identification of sprinkler operating times and defines sprinklers as either quick response or standard response. Generally speaking the operating time, , is determined based on the oven heat test, which was derived from the ISO 6182, “Standard for Automatic Sprinklers” plunge test, which has been scientifically verified, based on fully exposed, thermal response theory [3]. The oven heat test consists of “plunging” a sprinkler into a heated air stream and recording the time to sprinkler operation. By knowing the operating time of a sprinkler, a Response Time Index can be then calculated.
The Response Time Index (RTI) as developed by Heskestand and Smith to allow for a more practical estimation of a complex time constant [4], accounts for the thermal interaction between the sprinkler and a hot air stream over the sprinkler. Thus, the RTI is effectively the thermal sensitivity of a sprinkler. The RTI is defined as follows:
However, ULC 199 specifically notes that for ceiling sprinkler designs incorporating an integral closure assembly and flush style sprinklers without an air gap between the heat responsive element and sprinkler body, test samples are to be orientated with the heat responsive element exposed to the air flow. Additionally, ceiling sprinklers incorporating removable cups, escutcheons, and removable closure assemblies are to be tested with the closure assemblies removed. Both of these test allowances do not reflect the in situ orientations and/or the two stage operation of the sprinkler (i.e. initial heating needed to cause cover to fall away).
On the other hand, recessed or concealed style sprinkler designs where the heat responsive elements are not exposed, and that incorporate integral escutcheons or closures that are not practically removable, are not to be subjected to the plunge test. As such, these sprinklers cannot be assumed to be covered directly by the RTI concept as the sprinklers are not located directly in the gas steams of the plunge test.
In lieu of this, CR&FM sprinklers need to have specific operating times to be considered as quick response or standard response. Operating times are determined based on larger scale room fire tests. The main issue with the room fire test is similar to that of the plunge test, where there is not corresponding correlation back to the original equation to determine RTI as the heat responsive element may not be directly exposed to the hot gas flow. Due to this limitation, ULC 119 considers these types of sprinklers to be classed as standard response or quick response if the sprinkler operates within a given timeframe and thus, satisfying the requirements for being Listed.
While a sprinkler may be of a listed type and meet the testing requirements in ULC 199, designers should be aware how different sprinklers are tested and the associated limitations. This the general paradoxical issue. Certain types of sprinklers though listed as quick response may not perform as desired given the overall design of the sprinkler even though the design is Listed for use. While potentially less of an issue when dealing with prescriptive codes and standards, this is especially important in performance based scenarios when aiming to determine activation times of sprinklers. Sprinkler activation times are highly reliant on knowing the sprinkler RTI. For example, Alpert’s correlation which is the generally industry standard for estimating the time in which a sprinkler activates when subjected to a design fire condition, requires the RTI for the proposed sprinklers, as seen below:
The gas velocity, , is a function of the heat release rate of a specific design fire and the height of the ceiling, and can be different from the velocity used in the ULC 199 oven heat test. As such, CR&FM sprinklers which are not specifically given an RTI value may not be appropriate for use, unless conservative assumptions are made.
References
[1] ULC, ULC 199 Automatic Sprinklers for FIre-Protection Service, Underwriters Laboratory, 2022.
[2] ULC, "Sprinkler System and Water Spray System Devices," in Guide Information for Canadian Certified Equipment - The Canadian White Book, 2014, p. 170.
[3] R. G. Bill Jr and G. Heskestad, "Thermal Sensitivity Evaluation of Recessed, Flush and Concealed Sprinkelrs," in Fire Safety Science - Proceedings of the Fifth International Symposium, Melbourne, 1997.
[4] G. Heskestad and R. G. Smith, "Quantification of thermal responsiveness of automatic sprinklers including conduction effects," Fire Safety Journal, vol. 14, no. 1-2, pp. 113-125, 1988.
[5] D. Drysdale, An Introduction to Fire Dynamics, 2nd ed., West Sussex: Wiley & Sons Ltd, 1999.