Response times of sprinkler glass bulbs in a residential room fire

The response time of fire sprinklers is essential for their performance, especially in applications where life safety protection is desired. The earlier the sprinkler activates, the smaller the size of the fire. The aim of this study was to determine the response time – and the corresponding heat release rate – of different sprinkler glass bulbs in a residential room fire scenario.

Traditionally, most commercial residential sprinklers are fitted with 3mm glass bulbs having a nominal operating temperature of 68°C. However, significantly thinner sprinkler glass bulbs with lower operating temperatures are readily available. The benefit of an earlier activation may provide improved sprinkler effectiveness or allow the use of lower sprinkler discharge densities for residential sprinkler applications.

The test compartment

A series of fire tests were conducted inside a square compartment measuring 3.66m by 3.66m (12ft by 12ft) having a ceiling height of 2.5m. The compartment was either enclosed or had two walls removed to provide a fully ventilated scenario and to mimic a larger sized room. A propane gas burner was positioned at one of the corners of the compartment. The mass flow rate of the gas was controlled such that either ‘slow’, ‘medium’ or ‘fast’ fire growth rate scenarios were simulated. The compartment is shown in Figure 2.

In each test, nine Response Time Index (RTI) and operating temperature glass bulb combinations were tested. The nominal operating temperatures were 47°C, 57°C and 68°C, respectively. The RTI’s were 14(ms)1/2, 24(ms)1/2 and 32 (ms)1/2, respectively, and representative of glass bulbs with a nominal diameter of 1.5mm, 2.5mm and 3mm. Each test was replicated three times and the response times of the glass bulbs were measured by the use of electrical micro switches. In addition to the sprinkler glass bulbs, two commercial multicriteria detectors were installed at the same radius from the compartment corner with the propane gas sand bed burner. If considerably faster than sprinkler glass bulbs, combined heat and smoke detectors may be a viable option for the detection of fire allowing the activation of deluge sprinklers or water mist nozzles within a room. The installation of the sprinkler glass bulbs and fire detectors is shown in Figure 1.

Test results for the sprinkler glass bulbs

As expected, the sprinkler glass bulb having the lowest RTI and the lowest nominal operating temperature activated the earliest (smallest fire size) and the sprinkler bulb with the highest RTI and the highest nominal operating temperature last (largest fire size).

Whether the compartment was fully enclosed or open had a significant influence on the time to activation and the corresponding heat release rate, especially for the sprinkler glass bulbs having the higher RTI’s. For the sprinkler glass bulbs with the lowest RTI, this effect is not as pronounced. With few exceptions, the sprinkler glass bulbs having the same temperature rating and RTI activated earlier (smaller fire size) inside the closed compartment as compared to the open compartment. Intuitively, this seems valid as the combustion gases built a layer of hot gases inside the closed compartment. This is verified by the fact that the measured temperatures at the sprinkler glass bulbs rise faster inside the closed compartment. The measurement data also indicate larger temperature fluctuations for the open scenario, as compared to the closed compartment. It was observed that the activation times of the individual glass bulbs varied more in the open scenario, especially for the ‘medium’ and ‘fast’ fire growth rate scenarios. This was probably a consequence of a more turbulent gas flow at the ceiling.

Another observation is that the nominal operating temperatures of the glass bulbs have a stronger effect on the results than does the RTI. This is true for all three fire growth rate scenarios. In other words, a reduction of the nominal operating temperature of a sprinkler glass bulb has a larger effect on the speed of response than a reduction of its thermal sensitivity. The result for the slow fire growth rate scenario is shown in Figure 3.

The use of sprinklers with 3mm, 57°C sprinkler glass bulbs would not violate the recommendations of NFPA 13D and 13R of using ordinary-temperature rated sprinklers, i.e. sprinklers having a temperature rating between 57°C and 77°C. It appears that heat release rate is in the order of 15% to 30% smaller as compared to the 68°C temperature rating. The reduction in heat release rate is the most noticeable for the open compartment condition. When using 47°C sprinkler glass bulbs, the fire is in the order of 30% to 50% smaller.

By using a specific fluid, some of the glass bulbs with an actual diameter of 3mm had response characteristics similar to 2.5mm glass bulbs. These particular sprinkler glass bulbs are therefore directly interchangeable, e.g. they have the same physical dimensions and mechanical properties as regular 3mm glass bulbs. When using “2.5mm” 57°C sprinkler glass bulbs, the heat release rate is in the order of 25% to 30% smaller as compared to when using regular 3mm, 68°C glass bulbs. When using 47°C sprinkler glass bulbs, the fire is in the order of 40% to 50% smaller.

Sprinkler glass bulbs with a diameter as small as 1.5 mm may typically not be used in regular sprinklers due to their lower mechanical strength, however, these bulbs are commonly used for automatic water mist fire protection nozzles. When using 1.5mm, 57°C sprinkler glass bulbs, the heat release rate is in the order of 30% to 35% smaller as compared to when using regular 3mm, 68°C glass bulbs. When using 47°C sprinkler glass bulbs, the fire is in the order of 45% to 60% smaller.

Test results for the fire detector

The fire detection threshold of the fire detectors was set at a fixed-temperature limit of 40°C. For the activation of a separate sprinkler system using heat detectors, a temperature limit this low need to be combined with additional fire detection criteria, such as the detection of smoke. However, this feature of the fire detectors was not tested as the propane gas burner generated little smoke. The heat sensors of the detectors typically detected the fire prior to the activation of the first sprinkler glass bulb for the slow fire growth rate scenario. However, for the medium and fast fire growth rate scenarios, some of the sprinkler glass bulbs activated prior to fire detection. This is specifically noticeable for the fast fire growth rate scenario. The explanation may be a change of the gas flow profile and velocity of the ceiling jet (at the centerpoint of the compartment) with an increase in the fire growth rate. This may make the position of the glass bulbs relative to the ceiling more exposed to the hot flowing gas as compared to the thermal sensors of the heat detectors. The fire detection times of the heat detectors were barely influenced whether the compartment was closed or open. This is in line with the observation that the sprinkler glass bulbs having a lower RTI was less influenced by the compartment conditions than the glass bulbs with a higher RTI.

From the data, it appears that the largest benefit from sprinkler system activation by a signal from a combined heat and smoke detector is associated with a ‘slow’ or ‘medium’ fire growth rate inside a larger compartment. For the case the fire develops rapidly inside a smaller sized room, the advantage of activating a sprinkler system by a signal from a combined heat and smoke detector as compared to a thermal activation by a sprinkler glass bulb is less. In practice, there may also be a certain time delay associated with signal transmission, the opening of valves(s) and the travel time of water in the empty sprinkler piping. In order to benefit from any reduced fire detection and system response time, it is also essential that the coverage area of the fire detectors does not exceed the coverage area of the sprinklers.

For more information, email magnus.arvidson@sp.se