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57 Tunnel Emergency Ventilation Controls Every tunnel project should have a ventilation control phi- losophy and control design fully developed. The control sys- tem should allow for the response of the ventilation system to a reported incident in accordance with the emergency response plan. This response is based on information retrieved from various sources inside and outside the tunnel. The information is analyzed and validated, and the ventilation response could be activated automatically, semi-automatically, or manually: ⢠In automatic control, there is no intervention of the tunnel operator. However, the tunnel operator could intervene in the automatic process. Fully automatic control should be applied for tunnels with no supervision (unmanned tunnels). ⢠In semi-automatic control, the fire could be automatically detected, and the tunnel operator chooses to implement the preprogrammed procedures of the smoke control system. The semi-automatic control system, when started, controls the components of the smoke control system according to a previously programmed procedure associated with the input objectives defined by the tunnel operator. ⢠In manual control, the operator analyzes the available data and activates each component or groups of components of the smoke control system following a procedure that should be pre-defined for the fire event. Consideration needs to be given to the complexity of the ventilation system and the organization of the operating per- sonnel. For example, for uni-directional tunnels with longi- tudinal ventilation and managed traffic downstream of the fire, knowledge of which is the incident tube could be suffi- cient to start the ventilation, while in tunnels equipped with a single point extraction system, information on the exact fire location and air velocities in the vicinity of the fire is crucial. Experience shows that a complex ventilation system is much more efficiently managed by an automatic or semi-automatic system than by an operator performing under high stress conditions. It has become a common practice to develop mode tables according to pre-defined different fire/egress/ ventilation scenarios. The mode determines the activation of the ventilation and other fire life safety systems including the FFFS and traffic control, which leads to a single push-button preprogrammed for a specific fire scenario event. The opera- tor should also be able to override the control procedure at any time as he/she may have knowledge of false alarms or an inadequate response procedure. The ventilation control should ensure adequate response for all conceivable fire scenarios including scenarios where some equipment or sensors fail to respond. All possible fire scenarios should be modeled using CFD or other approved numeri- cal simulation tools at the design stage of the project and the results should be implemented in the mode tables developed for the tunnel. It also requires training of tunnel operators, first responders, and their interactions. Prior to the activation of the ventilation system, the fire must be âdetected.â The tunnel operating procedures may prescribe that the emergency ventilation is activated before full confirmation of a fire as a precautionary measure. The components of the smoke control system act on the flow inside the tunnel, changing it in an appropriate way, while changing the measured airflow parameters. Therefore, the control method must take into account that the conditions may change over time. This change could be accounted for in an automatic control system, but would be very demanding in the case of manual control. Fire emergency situations require rapid response from the tunnel ventilation system. Consequently, the response time for the entire chain of events, i.e., detection, identification, alarm validation, and intervention, must be reduced to establish a tenable environment during the evacuation phase (Figure 6.1). The best strategy to be adopted depends on the quality and reliability of the available information. PIARC proposes the following fire detection and valida- tion process flow chart in Figure 6.2 [15]. Once the fire alarm is detected (confirmed as needed only) and the ventilation strategy is defined, the control system must be able to achieve C h a p t e r 6
58 Figure 6.1. Overview of the tunnel ventilation control loop [15]. Figure 6.2. Fire detection and validation process flow chart [15]. the pre-defined condition (for example, a particular longitu- dinal velocity). It may be appropriate to place the system in an alert and starting condition even before confirmation of the fire and its location. All steps of the algorithms must be clearly detailed into the specifications. Variable speed drives with controllers have been recently developed for jet fans, which provide the ability to operate the jet fans at prescribed air velocities as part of the control system. The fan speed can be adjusted based on precise fire detection and adequate airflow measurements in the tunnel. In some control systems, the natural ventilation effects can be calculated in real time to control the ventilation system operation. In the Model-based Predictive Ventilation Control (MPVC), signals from the sensors are passed to the PLC with the MPVC software that chooses the fan settings.
59 Ventilation control should be designed along with the con- trol of other systems, such as the FFFS. There is a variety of controls available to fully integrate the FFFS with the venti- lation, operate them separately, fully automate them, auto- mate them with manual override, and manually operate them with auto override. However, each of these options must be carefully evaluated. Different tunnels may require different approaches. Some approaches may consider automatic acti- vation with or without a time delay and override. Others may consider activation by an operator or by the fire department only. The choice of control depends on the objectives of the system. Some tunnels may require a FFFS activation for any tunnel fire event; others may require activation of the FFFS for major fire events only. This is subject to additional research. Typically, a project establishes a time-of-tenability criterion for fire life safety system design. Figure 6.3 provides an exam- ple of such a curve, which shows the fire curve (fire develop- ment), self rescue, and fire life safety (FLS) activation relative to the fire development. This example illustrates that evacu- ation will start while the fire grows and well before the venti- lation mode and FFSS can be fully activated. The only means to get fire growth under control for this example is to activate the FFSS prior to the ventilation system. Ventilation should remove smoke and toxic gases once activated. Considering that the amount of water required for the fire suppression is directly related to the fire size, it becomes critical to activate the deluge FFSS no later than when the design fire heat release rate is achieved on the curve. Appendix B illustrates the effect of the FFSS suppression on the heat release rate. With timely activation of the suppression system, the heat release rate is reduced. With delayed activa- tion, the fire overwhelms the system and it is not effective. Some typical problems with ventilation control must be considered, such as the following: ⢠Multiple automatic alarms may occur, some of which could be upwind of the fire event far from the incident. This may happen due to the airflow in the tunnel at the time of the incident. Some alarms can be triggered from the gas monitoring systems and a signal to activate normal ventilation can be sent. ⢠Excessive demand of information from the operator must be avoided, and the priority criteria must be established. ⢠Nuisance alarms. ⢠Lack of operator training for rare fire situations. Training simulators are useful tools for ventilation controls training. SELF RESCUE FLS SYSTEMS ACTIVATION A. Make a decision to evacuate B. Disembark the bus C. Walk away from the fire affected zone D. Reach cross passage 1. Detection Time 2. Operator Reaction Time (alarm) 3. FFSS Systems Activation 4. Ventilation (All Fans) Activated 5. Ventilation Mode in Full Operation 0 2 4 6 8 10 12 14 16 18 20 22 24 26 3028 Time (min) Figure 6.3. Example of project established time-of-tenability curve [3].
