Water Application During Suppression

During my education as a firefighter, I was taught to be cognizant of the steam produced when applying water inside a burning structure. Water application could create massive amounts of steam. If I wasn’t careful, water application could cause injuries to me, my fellow firefighters and even potentially trapped occupants. Was this concern, emphasized throughout my fire service education, rooted in science?

Testing a fire service concern

In 2013, the Underwriters Laboratories Firefighter Safety Research Institute (UL FSRI) was awarded a grant to evaluate the “Impact of Fire Attack Utilizing Interior and Exterior Streams on Firefighter Safety and Occupant Survival.” One of the goals of the project was to “develop and implement a methodology to measure moisture content in the modern fire environment to answer fire service concerns.” The idea that water application could increase the water vapor in the environment, resulting in firefighter injuries and further injury to trapped occupants, was highlighted as a significant concern of the U.S. fire service. To address this concern, UL FSRI partnered with the Illinois Fire Service Institute (IFSI) to develop a method of measuring water vapor in the fire environment using lasers designed to measure combustion gases in engine exhaust. The IFSI team utilized absorption spectrometry to conduct exploratory measurement of water vapor in a single-family structure test prop.

The 1,620-square-foot single-story test prop was designed to represent a single-story ranch home. The right side of the prop had a living room and kitchen. The four bedrooms were accessed down a long hallway. Three of the bedroom doors were open and one closed.

The experiments were broken into three ventilation cases: No Vent, Single Vent and Two Vent. Varying fire suppression methods were examined using each ventilation case to identify trends in effectiveness. In each of the ventilation cases, water vapor was measured in the open bedroom down the hall from the bedroom fires (Figure 1).

To further understand the results of the measurements, let’s look at the differences between water vapor and steam. Water vapor is the gaseous state of water. The percent concentration of water vapor by volume is unchanged by temperature. One way of reporting water vapor is relative humidity, or the percentage of water vapor in air relative to the maximum potential water vapor the air can hold, up to the boiling point of water or 212 degrees F (100 degrees C). The higher the air temperature, the more water vapor the air can hold.

Steam is one type of water vapor, which is created when water is heated to its boiling point—212 degrees F (100 degrees C). Since steam is only created when water vapor is above 212 degrees F (100 degrees C), steam cannot be reported as relative humidity. This would offer an incomplete picture of the water vapor present. Instead, utilizing the percentage of water vapor in the gas by volume allows for a more direct comparison. Table 1 summarizes the results of the 11 experiments where data was collected in percent by volume of water vapor at ignition, five seconds prior to suppression and 60 seconds after suppression, where suppression was conducted utilizing a 150–165 gpm handline.

In almost all the experiments, the percent by volume of water vapor increased significantly after ignition, as shown by the values measured five seconds prior to the start of suppression. The higher the elevation the water vapor was measured, the more water vapor was detected. At elevations in the smoke layer (five feet), water vapor increased by two to four times the value measured at ignition prior to the application of any suppression water.

This increase in water vapor is likely from the combustion of the fuels within the structure.

What does this mean?

To understand this further, let’s evaluate the combustion of a simple fuel, methane gas. In the presence of oxygen, fuel provided with sufficient heat results in combustion. As seen in Equation 1 the complete combustion of methane (CH4) results in the products of carbon dioxide (CO2) and two molecules of water vapor (H2O).

Although in a house fire fuels are more complex than methane, and sufficient oxygen does not exist for complete combustion, CO2 and H2O are still a portion of the products of combustion. In the fire service, the products of combustion are generalized as the term “smoke.” Smoke contains things such as soot, carbon monoxide, carbon dioxide, hydrogen cyanide and water vapor among many other substances. The water vapor created by the combustion reaction is not trivial, hence the increase of water vapor prior to the initiation of suppression.

Figure 1 – Ventilation Cases and Water Vapor Measurement Location

The water vapor at the higher elevations (three and five feet) increased during the period from five seconds prior to suppression to 60 seconds after the start of suppression. This increase was around 1 percent by volume, a much smaller amount than was seen in the period from ignition to five seconds prior to suppression. This increase could have been due to the application of water or due to transport time for the products of combustion to reach the location of the sensor. At the one-foot level, where a potential victim may be laying on the floor, there was almost no increase in the values from five seconds prior to 60 seconds after suppression. With no appreciable increase in water vapor content at the one-foot level even after suppression, it is highly unlikely the suppression could cause a victim, located in an adjacent bedroom to the fire, further damage due to steam.

Water has an immense ability to cool the environment while suppressing or extinguishing a structure fire, thereby protecting the firefighters and trapped occupants. The practice of delaying or withholding the application of water due to the thought that it could increase water vapor and thus injure firefighter and trapped occupants was not seen in the experiments conducted. In fact, delaying water may actually increase the potential injury of firefighters and trapped occupants. Focusing on early application of water, using the reach of the stream may provide the most benefit.

References

R. Zevotek, K. Stakes & J. Willi. Impact of Fire Attack Utilizing Interior and Exterior Streams on Firefighter Safety and Occupant Survival: Full Scale Experiments, Underwriters Laboratories Firefighter Safety Research Institute. January 2018.