Integration of road tunnel systems is essential for maintaining structural integrity and providing safe evacuation to motorists in a fire event.
Fires that occur in road tunnels can grow rapidly and reach very high heat release rates. As a result, road tunnels are designed with mitigation technology and procedures to help reduce the detrimental effects that can occur.
The main goals of the mitigation measures are to:
- Provide a tenable environment for motorist evacuation;
- Assist firefighters with their operations; and
- Maintain the structural integrity of the tunnel.
A fixed fire fighting system (FFFS) is one type of mitigation measure implemented to help achieve these goals. The major components of the FFFS include water delivery infrastructure (pumps, pipes, valves, and nozzles – divided into separate zones for water delivery) and also components for water removal (drainage, pumps, pipes, water treatment).
An FFFS is typically installed to help reduce the fire growth rate and air/smoke temperature, which helps to prolong occupant tenability and provides structural protection. Proper integration of the FFFS with other tunnel fire-life safety systems is essential to achieve the FFFS goals.
The first important question in FFFS integration is whether or not the tunnel has a full-time operator. In many tunnels with FFFS, a full-time operator is present. In this article the integration question is considered in the context of a full-time operator being present, but it is noted that if an operator is not present there will be different integration considerations. Tunnel systems and functions that require particular attention for integration with a FFFS, with full-time operator present, include:
- Closed circuit television (CCTV);
- Ventilation systems;
- Egress provisions;
- Fire alarm systems, control systems, heat detection systems; and
- Traffic and operations.
Poor system integration can lead to a reduction in FFFS performance and fire safety.
System Integration with Fixed Fire Fighting Systems
Closed Circuit Television (CCTV)
Activation of the FFFS at an early stage of a fire incident is the best way to assure optimal performance, and this is typically accomplished through manual activation by the tunnel operator. The tunnel operator relies on the CCTV system to assist in identifying the fire location, as the CCTV system would typically detect smoke or stalled traffic well before a heat detector senses the fire. Once the fire has been located, the operator activates the corresponding FFFS zone. It is imperative that operators can easily and accurately identify the fire locations. Figure 1 provides an example of effective design integration between a CCTV and FFFS system.
The figure shows an example of good systems integration with camera locations relative to their proximity to FFFS zones. Placing a camera within a zone, instead of at zone boundaries, may generate confusion for the operator because, instead of the CCTV image showing the start of a zone, the image would be starting halfway along the zone, requiring the operator to cycle through views to confirm the location.
The ventilation system in a tunnel is used to direct heat and smoke away from the egress path by producing a longitudinal tunnel air velocity flow in one direction (longitudinal ventilation); extracting the heat and smoke through vents along the tunnel (transverse ventilation); or a combination of the two.
The air velocity can cause water in the FFFS’s water delivery region to shift away from the active zones. Computational fluid dynamics (CFD) results in Figure 2 show an example of the extent of water delivery drift for a longitudinal ventilation system. In this example, activation of both the FFFS zone where the fire is located and one zone upstream mitigates drift effects. Careful zone activation can mitigate the effect of drift and provide assurance that water will reach the target. Jet fans near the FFFS zone should be activated only if necessary. In the region near a jet fan’s outlet there will be high velocity relative to the average velocity of the tunnel, which will exacerbate the water delivery drift.
Egress points (e.g., exit doors to escape passages) are generally positioned equidistant from each other along the tunnel and should be placed at the ends of the FFFS zones and not within active FFFS zones where egress may be hindered by visibility reduction, noise (the active FFFS is in fact very loud), physical restriction, and psychological stress. Placing egress points at the ends of a FFFS zone contributes to more streamlined egress. Firefighters using these egress points to enter the tunnel could experience significant disorientation if entering an active FFFS zone, thereby slowing their subsequent response.
Drainage is another aspect to consider when installing an FFFS. In some systems, the very large flow rates of water mean that not all of the FFFS water will be captured at the drains within the zone of discharge, and practically there may be few design options to achieve this. The travelling fuel can create a risk of fire spread since the water can transport the fuel away from the fire site. The fuel draining away from the fire site would be unshielded by vehicles and so it will typically be suppressed, if it is burning, prior to exiting the FFFS zone. Flame traps in the drainage system are sometimes used to prevent a secondary fire moving through the drain pipe network.
Fire Alarm Systems, Control Systems, and Heat Detection
Road tunnels can be fitted with automatic and/or manually activated FFFS. In a manually operated system, operators are provided with a CCTV system to identify the fire location, so that they are able to activate the FFFS in the appropriate zone(s), as described above. In some instances a back-up automatic activation system is provided. This system typically uses a linear heat detector (LHD) to identify the fire location. Once the LHD signal is received at the control panel, a countdown timer activates. If no response is made by the operator within the allotted time, the FFFS is deployed.
The LHD is an addressable sensing cable which can detect absolute temperature or rate-of-rise, with each detection zone coincident with a specific FFFS zone. In the case of an automated response, the following items support good system integration:
- FFFS and LHD zones are to be coincident.
- The FFFS should activate in the first LHD zone to detect heat and the adjacent zone upstream.
- Any further LHD activations must not trigger any additional FFFS zone activations (as explained below).
The system must be programmed such that the operator can override an automated response if necessary. Automated systems are capable of executing ineffective responses, so it is up to the operator to make the final operational decisions. For example, in a tunnel, heat will travel over a large number of FFFS zones and trip the LHD in zones remote from the incident. If all of these zones were to discharge water, there may not be enough water capacity available in the incident zone to suppress the fire (a FFFS can be feasibly designed with enough water supply capacity to feed two or three zones). Conversely, the fire can propagate or the operator may need to correct their choice, which means the operator needs to have the ability to shut zones off and start others.
Traffic and Operations
After a fire is identified, traffic must no longer be allowed to flow into the tunnel. In unidirectional traffic, the vehicles downstream of the fire are expected to exit the tunnel while those upstream are expected to stop (a common assumption in tunnel fire-life safety design).
The system must be designed so that the FFFS is never activated over live traffic. An activated FFFS will reduce motorist visibility and vehicle traction, which increases the chance of a vehicle collision and exacerbates the emergency, or worse still, creates an unsafe situation (see Figure 3).
An FFFS is a useful fire safety tool for a road tunnel. Good integration of the FFFS with other tunnel systems and functions, using the principles outlined above, assists in bringing to fruition its purported benefits for tunnel fire safety. In addition to the engineered systems, it is important that the tunnel operator is well-trained and that tunnel systems are well-maintained to assure good performance.