Ventilation and Suppression Systems in Road Tunnels: Some Issues regarding Their Appropriate Use in a Fire Emergency

Two important tunnel safety technologies are addressed. The majority of long road tunnels have ventilation systems. In the event of a fire in a tunnel, such systems will influence fire development in a number of different ways. Recent research and observations on these influences are presented. The effects discussed are critical ventilation velocity for smoke control and the influence of ventilation on fire size, fire spread and fire growth. There is no well defined ‘best’ approach to operate tunnel ventilation in a fire emergency. Another technology of growing importance is sprinklers and water mist systems, which are being installed in an increasing number of tunnels. There are some concerns regarding the integration of ventilation and suppression systems, these are discussed. Of particular concern is the interaction between water mist droplets and ventilation flow which may result in the suppression agent being carried long distances downstream, away from the fire. Ventilation and suppression systems should not be considered to be separate entities, but two parts of an integrated fire safety system. The paper closes with an opinion on how ventilation and suppression systems ought to be controlled to work together for fire safety. INTRODUCTION The majority of road tunnels of significant length have some form of ventilation system for various reasons including smoke control in a fire emergency. These fall broadly into two categories, transverse systems and longitudinal systems, although an increasing number of tunnels, for example the refurbished Mont Blanc tunnel [1], have elements of both types installed. Transverse ventilation systems use air ducts, generally either above a false ceiling within the tunnel or below the road deck, to supply and extract air at periodic locations along the length of the tunnel. Sometimes, while the supply duct may extend along the entire length of the tunnel, extraction is only carried out at a small number of locations, such systems are known as semi-transverse systems. Longitudinal systems use jet fans, generally mounted on the ceiling, to move air along the main tunnel void. In the event of a fire the primary function of any ventilation system is to maintain a smoke free egress path for escaping tunnel users and to allow smoke free access to the fire location for the fire brigade. In fully transverse systems, the strategy is generally to provide maximum extraction in the vicinity of the fire, while air supplies are generally reduced. In semi-transverse systems, the strategy is often to provide maximum extraction on one side of the fire, to allow safe egress on the other side. While the strategy used with longitudinal systems is to blow all the smoke to one side of the fire, once again allowing safe egress on the upwind side. However, any movement of air in the vicinity of the fire will have an impact on the fire development, smoke production, peak fire size and propensity for fire spread to other vehicles. This paper reviews research onto these aspects of the interaction between ventilation systems and fire behaviour. In addition to ventilation systems, an increasing number of tunnels are being fitted with fixed fire fighting systems (FFFS). Many of these are conventional deluge sprinkler systems, but many new FFFS are water mist systems (WMS), which produce mush smaller droplets than conventional systems and therefore use significantly less water. The majority of ventilation systems produce fairly high velocities during emergency operation. Yet most FFFS have only been demonstrated to work at low ventilation velocities. This paper discusses some of the unresolved issues of such systems, particularly with regard to integration with existing ventilation systems. 2nd International TUNNEL SAFETY FORUM for ROAD AND RAIL, 20-22 April 2009. pp. 375-382 Organised and Sponsored by Tunnel Management International, ISBN: 1 901808 27 0