Use of sprinklers and aqueous gel for structure protection from wildfire

Stew Walkinshaw, Montane Forest Management Ltd., Canmore, AB

Ray Ault, FPInnovations – Feric Division, Wildland Fire Operations Research Group, Hinton, AB

Video list

Video 1. Sprinkler Water Application

Video 2. Aqueous Gel Application

Video 3. Plot 1 Fire Behaiour

Video 4. Sprinkler Cabin Front Right

Video 5. Sprinkler Cabin Back Left.

Video 6. Aqueous Gel Cabin Front Left

Video 7. Aqueous Gel Cabin Back Right

Abstract

FPInnovations studied the effectiveness of sprinkler systems and aqueous gel for the protection of structures from wildfire. The study results may assist fire suppression personnel when making strategic decisions on wildland–urban interface fires. The time and resources required to set up the systems, water volumes used, structural damage, and structure temperatures were investigated.

Keywords

Structure protection, Fire suppression, Sprinklers, Aqueous gel, ,Wildfires, Wildland - urban interface.

Introduction

Structure protection in the wildland–urban interface involves the use of many different strategies and tactics with the overall goal of protecting the greatest number of structures with the resources and time available. Several wildfire agencies in Canada have successfully used sprinkler systems for structure protection during wildland– urban interface fires. However, sprinklers are not always available on short notice or in adequate numbers, they require adequate lead time for proper setup, and they may cause structural damage depending on setup procedures. Aqueous gel products have been used by some Canadian wildfire agencies and structural suppression agencies during fires with some success. Issues noted with aqueous gel products include product cost and difficulty in cleaning the structure after gel application.

With the help of the Northwest Territories government, FPInnovations–Feric Division conducted test burns near Fort Providence in June 2005 and documented the setup and application, resources required to operate,
and success of structure protection for both sprinkler and aqueous gel systems on test cabins. This report presents results from the first of the test burns; the other burns will be documented in future reports.

Objectives

This study had the following objectives:

· Determine the effectiveness of structure protection equipment that is readily available to homeowners.

· Evaluate the effectiveness of sprinklers and aqueous gel in structure protection under extreme fire behaviour conditions and determine the conditions that influence success.

· Evaluate the time and resources required to treat structures with sprinklers or aqueous gel.

· Evaluate the temperatures at critical points on each structure during and after wildfire passage.

· Evaluate the effect of exterior structural materials to structure survival.

· Test the FireSmart-recommended guidelines for using asphalt-shingle roofing and double-glazed windows, and the recommended practice of skirting decks and open spaces (Partners in Protection 2003).

Methods

Cabins

Cabin dimensions were 2.4 m × 3 m with a 1.2 m × 2.4 m deck on the back (Figures 1 and 2). The cabins were pre-assembled in sections in Alberta and transported to the Northwest Territories site by trailer. Each
cabin was then re-assembled onsite.

Figure 1. Front view of cabin

Figure 2. Side and rear view of cabin

Cabins were wood-frame construction with the following exterior materials:

roofing – asphalt shingle
siding – vinyl (50%) and cedar (50%)
windows – double-glazed
soffit – aluminum
door – hollow-core metal
fascia – aluminum
deck – 51 mm × 102 mm legs with 19 mm plywood decking

Cabin locations were chosen based on the need for extreme fire behaviour to approach and surround the cabin with ease. The only trees removed were those necessary to site the cabin. No modifications were made to
the stand or the fuels surrounding the cabins other than erecting the cabins. Cabins were placed within approximately 20 m of each other to ensure each experienced similar fire behaviour, while not influencing each other
(Figures 3 and 4).

Each cabin was pre-wired with 20 temperature sensors to record temperatures before, during, and after the fire. The sensors were placed at the following locations:

· under cedar siding (4 locations) – right rear and side, left front and side

· outside window (2 locations) – right and left sides

· inside window (2 locations) – right and left sides

· under vinyl siding (4 locations) – right front and side, left rear and side

· in soffit (4 locations) –1 m from right and left front and 1 m from right and left rear

· in peak (2 locations) – 1 m from rear and 1 m from front

· interior of cabin (2 locations) – centre 1 m and 2 m above ground

Each cabin site had four radiant cubes to record radiant heat flux (kW/m²) in the wildfire environment and in-fire video cameras to record the sequence of events before, during, and after the fire.

Figure 3. Site layout of study plot.

Sprinkler installation

Four impact-style garden sprinklers, each with a 3 mm nozzle orifice, were installed an average of 4.2 m from each corner of the cabin (Figure 3). Two were installed at ground level at the southeast and northwest
corners, and two were elevated 1.2 m off the ground on wooden poles at the northeast and southwest corners. The sprinkler arc was set for 90o to wet only the cabin area. Two people installed the sprinklers.

Water supply was provided from an 11,000 L relay tank and pumped with a Honda pump through 38 mm lined fire hoses. Water supply volume and pressure was 68 L/min at 364 kPa. The main supply line was buried 15 cm below ground to avoid burning the hose and losing water supply. Water was supplied to the sprinklers with a 16 mm Wildfire Econoflo® hose, in a closed-loop circuit, from a wye off the northwest corner of the cabin (Figure 5). Following installation, the system was tested and pressures and volumes were recorded. Sprinklers were operated for 22 minutes prior to wildfire impingement (Figure 4).

Figure 4. Cabin locations.

Figure 5. Closed-loop sprinkler setup.

Figure 6. Gel application equipment.

Aqueous gel application

A homeowner application package consisting of a one-gallon plastic jug of gel and a brass eductor nozzle was used to apply aqueous gel. The same relay tank, Honda pump, and hose as used in the sprinkler system were used for water supply. One 15 m length of 16 mm Econoflo hose supplied water to the eductor nozzle for gel application (Figure 6).

Gel was applied by two people—one operating the nozzle and one pulling the hose. Gel was applied to all exterior cabin surfaces (Figure 7) except the exposed underside of the back deck. A gel layer was applied on the wildland surface fuels within 25 cmaround the cabin’s perimeter. Application was complete approximately 53 minutes prior to wildfire encroachment.

Figure 7. Gelled cabin

Figure 8. Fuel type on the study plot

Vegetation

The vegetation consisted of C3 (jack pine) fuel type with a moderate black spruce understory and Cladonia ground cover (Figure 8). Ladder fuels and ground fuels were light to moderate.

Fire weather and ignition technique

Wildfire ignition was accomplished with two people using an all-terrain vehicle and flamethrower (McCulloch 2006). Ignition commenced in the northwest corner and proceeded around the perimeter in a counterclockwise
direction.

The fire was ignited under the Canadian Forest Fire Danger Rating System (CFFDRS) fire weather indices shown in Table 1 (Turner and Lawson 1978). Intense wildfire was anticipated as a result of igniting under these
weather conditions and indices.

Table 1. CFFDRS weather and fire indices at time of ignition

Plot

Date

Temp

(C)

(%)

Wind

(kmh)

FFMC

DMC

ISI

BUI

FWI

June 28/05

25.4

E @ 6

385

Results and discussion

Structure protection strategies - time and resources

Sprinkler cabin

Two people each spent 45 minutes to install the sprinkler for a total installation time of 1.5 person-hours (Table 2).

Sprinklers operated for 22 minutes prior to wildfire impingement for a total of 2000 L of water applied.

Video 1. Sprinkler Water Application. (Click on it to start)

Gel cabin

Two people each spent 12 minutes on the gel system setup and application for a total setup and application time of 0.4 person-hours (Table 2). Gel application time was approximately 6.5 minutes.

A total of 5.7 L of aqueous gel was applied at the recommended application rate of 2% resulting in a total of 335 L of water usage. Application was complete approximately 53 minutes prior to wildfire encroachment.

Video 2. Aqueous Gel Application. (Click on it to start)

Table 2. Installation and application details – sprinkler and aqueous gel systems

	Sprinkler	Aqueous Gel
Installation/Application Manpower	2	2
Installation/Application Time	1.5 person hours	0.4 person hours
Application Water Volume	2000 litres	335 litres

Less time and water were required to set up and apply the aqueous gel than to install and operate the sprinkler system. This may be a benefit where manpower, time, or water supplies are limited for structure protection.

Wildfire passage

The wildfire approached both cabins as an active crown fire with significant airborne firebrand transport landing on the structures ahead of the fire. The sprinkler cabin survived wildfire passage with significant damage and the aqueous gel cabin was destroyed (Figure 9).

Video 3. Plot 1 Fire Behaiour. (Click to start)

Sprinkler cabin

In-fire video of the event indicates that water application from sprinklers reduced the combustibility of the structural fuels and reduced the fire intensity in the wildland fuels immediately adjacent to the cabin. As a result, the structure survived and surface fuels were unburned for 2 m surrounding the cabin within the sprinkler arc. The cabin exterior was ignited by the initial passage of the flame front but did not sustain combustion once the flame front had passed. Damage to the sprinkler cabin included the following:

Asphalt roofing shingles severely melted but not burned (Figure 10).
All vinyl siding melted and burned (Figure 11).
Cedar siding scorched on front and sides, no damage on back (Figure 11).
Deck surface and legs were undamaged (Figure 11).
Aluminum soffit materials on the front melted (Figure 12).
Metal hollow-core front door severely warped.
Double-glazed windows cracked on left- and right-side exterior panes only (Figure 13).

The sprinkler arc was set at 90^o because water was limited and it was determined more important to wet the structural fuels than to wet forest fuels.

Above-ground sprinkler equipment was damaged and ceased operation upon flame-front passage (Figures 14 and 15). The 16 mm Econoline hose melted or cracked in several locations and water flow to the sprinkler loop ceased. The sprinkler heads were undamaged because the areas surrounding them were moist. The buried 38 mm mainline was not damaged.

Figure 9. Post-fire aerial view.

Figure 10. Post-fire roofing material

Figure 11. Post-fire siding material.

The sprinkler system was operational up to the point of wildfire passage, which appeared to be an extremely important factor in structure survival. Past Feric case studies have shown that if water supply is lost prior to the arrival of the flame front, the probability of structure survival is significantly reduced. Therefore, structure protection personnel should install the water supply, pump, and supply hose in non-combustible areas to ensure that water supply is maintained during flame front passage.

Video 4. Sprinkler Cabin Front Right. (Click to start)

Video 5. Sprinkler Cabin Back Left. (Click to start)

Figure 12. Post-fire soffit material.

Figure 13. Post-fire window.

Figure 14. Post-fire elevated sprinkler.

Figure 15. Post-fire ground sprinkler

Figure 16. Post-fire gel cabin.

Aqueous gel cabin

In-fire video indicates that the flame front completely enveloped the gel cabin and resulted in severe burning of all surface and aerial wildland fuels surrounding the cabin (Figure 16). The cabin was destroyed as a result of both wildland and structural fuels igniting under the untreated underside of the back deck. As with the sprinkler cabin, the gel cabin exterior was ignited by initial passage of the flame front but did not sustain combustion once the flame front had passed, except for the rear deck, under the eaves, and on the joint between the vinyl and cedar siding on the front side of the cabin. The main ignition point for the gel cabin was the underside of the back deck which then supported the combustion on the back wall and under the back eaves. Fire burned on the back wall for approximately 6 minutes before entering the cabin interior through the rear soffit, fracturing the windows, and resulting in cabin collapse approximately 13 minutes after flame-front passage.

The asphalt-shingle roofing material was ignited by the flame front but did not sustain combustion once the flame front had passed. Airborne firebrands ahead of the main flame front did not ignite the roofing material or the deck surface; however, they did ignite the surrounding surface vegetation.

The double-glazed windows remained intact during flame-front passage; however, they were eventually fractured when fire entered the interior of the structure. It is unknown if one or both panes were cracked with flame-front passage.

Video 6. Aqueous Gel Cabin Front Left. (Click to start)

Video 7. Aqueous Gel Cabin Back Right. (Click to start)

Temperatures

The cabin temperature sensors provided valuable information. Table 3 presents the maximum and average temperatures at various locations for both cabins. Table 4 presents temperatures that were taken at or about the same time at various locations for both cabins. Data from the gel cabin sensors should be used with caution and may be inaccurate due to exposure and sensor damage from the structure fire.

Window temperatures

FireSmart – Protecting Your Community from Wildfire (Partners in Protection 2003) recommends that a minimum of double-glazed windows are used for interface structures. The maximum temperature on the outside pane of the sprinkler cabin was 354^oC compared to 73^oC on the inside pane recorded at the same time. The maximum temperature on the outside pane of the gel cabin was 969^oC compared to 188^oC on the inside pane at the same time (Figure 18). The data for both cabins indicate that double-glazed windows significantly reduce radiant heat transfer to the interior of the structure.

Interior cabin temperatures

The maximum temperature in the sprinkler cabin at 1 m above ground level (AGL) was 47oC compared to 80oC at 2 m AGL and 154oC at 3 m AGL (Figure 18). The maximum temperature in the gel cabin at 1 m AGL was 162oC compared to 170oC at 2 m AGL and 358oC at 3 m AGL. The results confirm that interior structure temperatures are significantly lower than exterior temperatures during flame front passage but they increase with distance from the ground.

Comparison of interior temperatures between the sprinkler and gel cabins indicates significantly lower temperatures in the sprinkler cabin, which may be attributed to the flame front being much closer to the gel cabin and the cooling effect of the water on the sprinkler cabin.

Under-siding temperatures

The average temperature under the cedar siding of the gel cabin was 288oC compared to 838oC under the vinyl siding. The temperatures under the vinyl siding were significantly higher due to the early melting of the
vinyl siding from the sheathing, resulting in exposure of the temperature sensors. The under-siding temperatures for the gel cabin should be used with caution as the temperature sensors lose accuracy once they are exposed to flame from the burning structure.

The average under-siding temperatures were significantly higher for the gel cabin than the sprinkler cabin. This is attributed to the flame front being much closer to the gel cabin and the continued cooling provided by the sprinklers.

Conclusions

The structure protection materials (i.e., water storage, pumps, hose, and sprinklers) that are readily available to the homeowner from local hardware stores and retail outlets can be successful in providing structure protection.

Sprinkler systems work well for structure protection under extreme fire behaviour conditions providing the water supply continues up to and during the time of fire front passage. Therefore, the water supply lines should be protected from radiant heat and direct flame impingement, and sprinkler arc patterns should be set to wet as large an area surrounding the structure as possible.

Aqueous gel systems may be effective in reducing ignition of structural fuels. However, all areas of the structure at risk to ignition must be treated. It may also be important to treat adjacent wildland fuels with gel to reduce fire intensity immediately adjacent to the structure.

Sprinkler systems require more time, manpower, and water to install and operate than aqueous gel set-up and application. Structure protection personnel must consider several factors including fire behaviour; time to arrival; and manpower, equipment, and water available to implement the appropriate strategy for the situation.

Overall, structure temperatures were significantly lower for the sprinkler cabin than for the aqueous gel cabin. This is likely due to the application of water to wildland and structural fuels and the resulting reduction in fire intensity immediately adjacent to the sprinkler structure. Interior structure temperatures were lowest at 1 m above ground level and increased significantly with distance above ground level. This supports the importance of staying as low as possible if using the structure for protection during an entrapment survival situation.

This study examined the effectiveness of certain structural materials to withstand extreme fire behaviour. The results of this study support the information presented in FireSmart – Protecting Your Community from Wildfire (Partners in Protection 2003) regarding the recommended use of asphaltshingle roofing materials and double-glazed windows, and the recommended practice of skirting decks and open spaces.

Recommendations

Property owners can improve the probability of their homes surviving a wildfire event by developing a structure protection plan that includes an independent water supply, pump, hose and sprinklers.
Water supply lines for sprinklers must be protected from the extreme heat of the fire front.
Under-building and deck openings need to be protected by skirting that will prevent the fire and embers from reaching under the structure as the flame front passes.
Set sprinkler patterns to wet as large an area around the house as possible if an adequate water supply exists, as this will substantially reduce the thermal intensity applied to the house as the fire front passes. When the water supply is limited, water application directly on the structure is preferred.

References

McCulloch, W. 2006. Evaluating the effectiveness of a fire torch system for community protection prescribed f ire operations. Feric, Vancouver, B.C. http://f ire.feric.ca/36112001/ TerraTorchReport.pdf

Partners in Protection 2003. FireSmart– protecting your community from wildfire (2nd ed.). Partners in Protection, Edmonton, Alta.

Turner, J.A.; Lawson, B.D. 1978. Weather in the Canadian forest fire danger rating system: A user guide to national standards and practices. Canadian Forestry Service, Pacific Forest Research Centre, Victoria, B.C. BC-X-177.

Acknowledgements

The author would like to thank the Northwest Territories and Alberta governments for their support of this project, and Mark Ackerman from the University of Alberta’s Faculty of Engineering for his time and effort spent working on the project and for producing the heat flux and temperature data in this report.