Testing Water Mist Systems Against Large Fires in Tunnels

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SUPDET Extended Abstract Mawhinney & Trelles, January 15, 2009.

TESTING WATER MIST SYSTEMS AGAINST LARGE FIRES IN TUNNELS: INTEGRATING

TEST DATA WITH CFD SIMULATIONS.

J. R. Mawhinney, P.Eng., FSFPE; [email protected]. Trelles, Ph.D.; [email protected] Associates, Inc., #817 3610 Commerce Drive, Baltimore, MD 21227-1652

For presentation at the NFPA Suppression and Detection (SUP-DET) Technical Working Conference,Orlando, Florida, February 25-27, 2009.

EXTENDED ABSTRACT

As a result of a number of multiple-death fires that have occurred in the last few years in highway

tunnels in Europe, for example the Mont Blanc and St. Gotthard tunnel fires, there is a growing demand

on public highway officials to meet a minimum standard for life safety in tunnels [1, 2]. In addition, the

huge economic losses (measured in billions of Euros) associated with the extended loss of use of a major

transportation tunnel due to fire are of major concern to national governments. Although long viewed as

unnecessary and even undesirable by tunnel authorities in Europe and North America [3, 4], active water-

based fire suppression systems now appear to be critically important to meeting life safety and propertyprotection objectives of public highway authorities, as well as national security objectives.

As a result of full-scale fire tests conducted in Runehamar in Norway in 2003 [5], the tunnel

community has recognized that unsuppressed fires in tunnels can be much larger than previously

thought. Water mist systems have been shown to be effective at mitigating the effects of even extremely

large fires in tunnels. The finely atomized water mist maximizes the cooling efficiency of the spray at

lower water flow rates than conventional sprinkler, deluge or water spray systems. Although other active

suppression technologies have high potential, such as compressed air foam systems, water mist systems

are presently the most cost effective technology accepted for active fire suppression systems in tunnels.

In the last few years there have been a number of full-scale fire test programs to approve water mist

systems for tunnels, including in Norway, the Netherlands, Germany, Austria, Switzerland, France andSpain. These tests have been supported by manufacturers of water mist system hardware, and

witnessed by national approval agencies and transport authorities. The various tunnel authorities have

independently developed the fire scenarios to be addressed, and the minimum level of performance to be

deemed acceptable. Some tests involve hydrocarbon pool fires (Netherlands); some involve groups of

passenger automobiles [6]; and others involve heavy goods vehicles (HGVs) loaded with normal

cellulosic fuels or combined with plastics. As shown in the Runehamar test tunnel [5], normal HGV fuel

loads (furniture) may generate fires ranging between 70 and 200 MW depending on plastic content.

These fires are 10 to 30 times larger than, for example, the Factory Mutual water mist test protocols for

machinery spaces for which the largest test fire is 6 MW [7]. Transportation authorities require that the

test fires for evaluating the performance of water mist systems be in the same scale as the potential HGV

fires, i.e., potentially 100 MW. Even the purchase of materials for such large fuel packages posesproblems for achieving consistent and repeatable fire conditions.

Tunnel safety authorities have developed performance criteria that must be met for a water mist

system to obtain their approval. Based on experience with moderate scale fire testing traditionally used

for approval testing, authorities typically select a temperature or other measurable condition that must be

met at a specified point relative to the fire in a specified period of time. Interestingly, analysis of recent

fire tests in tunnels suggests that single point measurements in the turbulent conditions associated with


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large fires are not accurate or consistent measures of performance. As an example, a performance

criterion was set such that the temperature at mid-height in the tunnel, at a distance of 5-m from the end

of the fuel load, must be 350oC or less within 10 minutes of activation of the system. The evident

assumption was that the temperature at that point should be initially high, and decrease with time as the

suppression agent gradually reduced the intensity of the fire. However, because the fuel load was 14-m

long and 3.5-m high, and the fire started at the upwind end, the unburned fuel shielded the downwindthermocouple so that it saw no heat for the first 15 minutes of the test. The hot gases flowed along the

ceiling and did not descend to mid height of the tunnel until approximately 10-m from the end of the fuel

load. The result was that temperatures at mid-height farther from the fire were higher than those closer to

the fire, contrary to the logical expectations. Although the fire was controlled, it was not extinguished.

When the fire burned to the end of the fuel load, heat was directed onto the critical thermocouple so that it

suddenly became very hot 30 minutes into the test. The performance criteria, which had been selected

based on the authoritys experience with smaller fires, did not reflect the actual performance of the water

mist system.

Several full scale fire test programs involving heavy goods vehicle (HGV) fires were conducted in the

San Pedro de Anes Test Tunnel facility in Spain in 2006. Hughes Associates, Inc. acted as a third party

witness and reporting agent for tests conducted by Marioff Corporation Oy of Finland [8]. The tests

evaluated the performance of a Marioff Hi-Fog high pressure water mist system against fires in stacks of

standardized wood Euro-pallets, intended to represent standard severity truck fires and in stacks of

wood pallets interspersed with high density polyethylene (HDPE) plastic pallets, intended to represent

high severity truck fires. The tests were instrumented to measure the heat release rate of the fires as

well as temperatures at various positions in the tunnel. The tests confirmed the outstanding effectiveness

of the water mist systems in controlling temperatures in the tunnel. Although the water mist system did

not extinguish the fires, it reduced the Heat Release Rate (HRR) by 40 percent or more for even the most

extreme fires. However, the degree of the reduction was not equal for all levels of fire severity. The

finding suggests there is a need to differentiate between performance against standard severity fires

involving normal cellulosic combustibles, and high severity" fires. It should not be required that the water

mist system achieve the same temperature at a point at the same time for both levels of fire severity, but

it is reasonable to expect that catastrophic damage to the tunnel be prevented even for the more severe

fires.

The San Pedro de Anes tests showed that very large fires (20 to 60 MW) create variable conditions,

even for apparently similar test conditions. The complex interaction of the fire, proximity to the tunnel

walls, ventilation air, water mist, and shadowing created by the fuel package and other obstructions to

airflow resulted in fairly large variations in readings over small distances. There is a motivation to seek

global performance criteria for evaluating suppression systems that are not fixed to single point

parameters.

In the San Pedro de Anes fire tests, HRR, temperature distributions, and tunnel ventilation air

speeds were measured. There are many physical phenomena associated with fire suppression that were

not measured, but which are indicators of the benefits of a suppression system. Computational fluid

dynamics (CFD) modelling provides a means to visualize/understand the phenomena that cannot be

easily observed or measured. Following the full-scale HGV testing conducted by Marioff, Hughes

Associates, Inc. developed a CFD model of the test tunnel, and simulated the fire tests using the NIST

Fire Dynamics Simulator FDS4 [9]. Data from the HGV fire tests were used to validate the simulations.

The incentive for simulating the HGV fire tests was to provide a means of analyzing the performance of

the water mist systems in terms of global performance criteria, and to permit one to explore a variety of


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what if scenarios to determine how a water mist system might perform under different conditions. The

simulation of an unsuppressed fire, which could not be conducted because it would have damaged the

test tunnel, provided a point of comparison to illustrate the benefits of having a water mist system. The

benefits of the suppression system can be characterized by parameters such as reduction in the surface

area of ceiling exposed to damaging temperatures (compared to an unsuppressed fire), reduced region of

high heat fluxes, elimination of back-layering and preventing propagation to targets. Furthermore,performance criteria should differentiate between standardversus high severity fuel packages, in

recognition of the fact that the probabilities of occurrence of all fire scenarios are not equal, provided that

catastrophic loss is prevented even in the worst case.

Computational fluid dynamics (CFD) modeling is a powerful tool for extending the value of a limited

number of fire test results, without incurring the cost of additional fire tests. If the CFD model can predict

the degree of temperature management measured by the instrumentation in a real test, confidence in the

accuracy of the model increases. This presentation will compare results of a CFD simulation using FDS4

with the measured results of full scale fire tests data and show how the model can be used to evaluate

the performance of water mist systems in global terms. Examination of the simulation results reveals

aspects of the conditions occurring in the tunnel that cannot be deciphered from the limited

instrumentation results. It is possible to simulate configurations not tested and make comparisons that

aid in understanding the benefits and limits of the suppression system. It is proposed that CFD modelling

validated using fire test data, ought to become a requirement for performance evaluations of fire

suppression systems.

Keywords: computational fluid dynamics, fire testing, FDS, heavy goods vehicle, HGV fires, heat release

rates, HRR, performance criteria, tunnels, water mist.

REFERENCES

1. Beard, A.N., and Carvel, R.O., (Eds.), The Handbook of Tunnel Fire Safety, Thomas TelfordPublishing, London, U.K, 2005.

2. Arvidson, M., Fixed Fire Suppression Concept for Highway Tunnels, International Conference onTunnel Fires and Escape from Tunnels, Lyon, 5-7 May, 1999.

3. NFPA 502, Standard for Road Tunnels, Bridges, and Other Limited Access Highways, 2004,National Fire Protection Association, Quincy, MA, U.S.A.

4. World Road Association. Fire and Smoke Control in Road Tunnels, PIARC Committee on RoadTunnels, (C5), 1999.

5. Ingason, H., & Lnnermark, A., Heat Release Rates from Heavy Goods Vehicle Trailer Fires inTunnels, Fire Safety Journal, 40, No. 7, pp. 648-668, 2005.

6. Mawhinney, J. R., Approval Testing of a Hi-Fog Water Mist System for Protection of a PassengerVehicle Tunnel, Paris, France, Client Report HAI-5022-010-1, for Marioff Oy, Finland, October 7,2005.

7. FM Approvals, Approval Standard for Water Mist Systems, Class Number 5560, Appendix G, FireTests for the Protection of Machinery Spaces and Special Hazard Machinery Spaces with volumesExceeding 9,175 ft

3(260 m

3), FM Global, Norwood, MA, May 2005.

8. Mawhinney, J. R., and Ingason, H. Full-Scale Fire Testing of Suppressed Heavy Goods VehicleFires in Road Tunnels, Client Report HAI 5022-010-2 for Marioff Oy, Finland, San Pedro de AnesTest Tunnel, Asturias, Spain, February, 2006.

9. Mawhinney, J. R., and Trelles, J., The Use of CFD-FDS Modeling for Establishing PerformanceCriteria for Water Mist Systems in Very Large Fires in Tunnels, in Proceedings of ISTSS 2008, 3

rd

International Symposium on Tunnel Safety and Security, .Stockholm, Sweden, March 12 14, 2008.

Testing Water Mist Systems Against Large Fires in Tunnels

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