Mass transit systems such as commuter rail networks are increasingly important in African cities, supporting workforce mobility in the modern economy. As public transportation becomes ever more popular and busy, we should not forget that the large concentrations of people inherent in rail, bus and subway stations require a heightened alert level when it comes to fire safety.
The technologies and design styles behind transit hubs may have changed over the years, but from a fire safety perspective the threats have remained constant: multiple levels above and/or below ground, unknown numbers of people (many of whom may be unfamiliar with the layout), high powered electrical installations, fuel and other fire hazards. In all cases, the way to mitigate and control these risks lies in detecting the fire early.
Above all, rapid transit and metro systems are about moving large numbers of passengers quickly within a short period of time. When there is a fire emergency, a large number of passengers can be on moving trains, embarking and disembarking from or to platforms, and many more in and around the station areas. A fire poses a highly hazardous condition to rail transit passengers, employees and emergency response personnel.
Detecting early is essential
Early Warning Fire Detection (EWFD) is a critical part for best practices of a fire engineering solution. This is particularly relevant as far as safe evacuation is concerned, even though sprinklers or other forms of suppression are also effective in potentially reducing the fire damage. The simple fact is, the earlier a fire is detected, the less damage it will do, both from asset protection and operations continuity perspectives. Most importantly, it allows time for a calm and orderly evacuation of public and staff; but only a well-designed EWFD system provides risk mitigation to potentially prevent a fire from happening or developing out of control.
One of the biggest challenges for fire safety system designers and consultants is that rail stations can be very complex, both physically and in terms of the regulations that cover them. Typically, they include both enclosed and large open spaces, and act not just as transit areas but also as shopping malls with restaurants and cafes. There may also be offices and workshops within the building, as well as significant power infrastructure such as electricity substations and transformers.
Large stations are mixed occupancies
The building occupancies in rail and other transit hubs are therefore commonly classified as mixed occupancies; separated occupancies with local/national building codes being equivalent to those defined in NFPA 1 and NFPA 101. This means that each portion of a transit hub is classified as to its use or distinct occupancy. Examples include assembly occupancy for ticketing and passenger waiting areas and platforms, or mercantile occupancy where a hub is connected directly to a shopping mall and retail shops. Both inside and outside the actual transit hubs, where support facilities are located, different types of occupancies such as industrial and storage for general purpose and high hazard categories are considered.
Swift evacuation is critical
The most critical fire protection design aspects for transit hubs are to meet building fire and life safety objectives. Foremost is to ensure egress routes are available with assured tenable conditions in the event of a fire emergency evacuation.
NFPA 130 Standard for Fixed Guideway Transit and Passenger Rail Systems is the most widely adopted and referenced standard. Evacuation time is quantitatively specified as the maximum time allowed to evacuate people from a platform (typically 4 minutes) and then to a point of safety (6 minutes).
The required safety evacuation time is calculated based on egress route options, air ventilation operation and escalator movement control against the maximum travel distance to an exit from a platform. Evacuations from a train fire in a tunnel, with cross-passages between the different tunnels at a certain interval and exit capacities at any given time, are also subject to the station occupant load.
Simple fire detectors are a false economy
When designing an EWFD system for such a complex area, the use of special fire detection methods is highly recommended. Standard point-type detectors simply don’t have the sensitivity to detect smouldering fires at their incipient stage. Additionally, large open spaces and complex buildings require so many to be deployed as to make them less cost-efficient than long linear-type heat detectors (LTHD) or powerful aspirating smoke detectors (ASDs). The latter offer the highest sensitivity, and the pipes through which air is sampled can cover hard-to-reach areas. Extra tubing can be included easily and cheaply to allow for an easily accessible test point.
Don’t forget the hidden spaces
When designing special fire detection, extra detecting power and sensitivity can be concentrated in the areas of highest hazard. High-current electrical equipment, with large amounts of equipment and cabling installed in compact spaces, concealed areas or underground tunnels and ducts inside and around rail hubs, present significant risks of fire.
Electrical arcing from a high-voltage rail due to a short circuit could go undetected and unrepaired in non-public areas or inside the rail tunnel, for example. Other invisible hazards include lint and oil build-up in the duct underneath platforms or escalators, which can lead to friction fires caused by mechanical equipment failure. Track fires, particularly dangerous when happening inside rail tunnels or at an underground platform, are also relatively common occurrences due to the mixture of high heat with debris, grease and litter build-up.
Unnoticed incipient fires in enclosed, concealed and dusty spaces increase the chance of a much-delayed emergency evacuation. Linear-type heat detectors or tubular aspirating smoke detector layouts are ideal ways of monitoring these hidden danger areas without requiring regular access for maintenance or testing.
Avoid false alarms
One major additional advantage of using special fire detection is that, despite increased sensitivity, systems such as ASD and LTHD are less prone to triggering false alarms. False alarms, when not verified, will not only interrupt transit system operation, inconvenience the public and slow down productivity, but also potentially lead to unwarranted emergency evacuation – particularly troublesome during the peak hours of operation. Ideally, a detection system with several levels of pre-alarm will be used, allowing trained employees to quickly and discreetly investigate possible fires before the need to sound the general alarm.
Both false alarm avoidance and early fire detection are also hugely beneficial to operators, who will experience fewer and smaller business interruptions as a result. The ability to detect a fire early is therefore very much in the interests of both passengers and transport operating firms.
© Technews Publishing (Pty) Ltd. | All Rights Reserved.
printer friendly version