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Until now there has been little published information on the behaviour of fires in automated mass storage units. Here Clive Turner reports on research carried out on behalf of MOD Estates. In another article (Warehouse Fire Safety) in this issue of FSE Neil Harrison gives his view on the implications of the tests.
Everyone in the industry knows the devastating cost of fire breaking out in a large warehouse or distribution centre, which is why so many of these buildings are now routinely protected by automatic sprinklers and other fire protection systems. For more than a century, sprinklers have proved effective in controlling or extinguishing fires – these days even in complex ‘hazards’ such as high bay warehouses. But new storage technologies and the introduction of automated systems are always presenting new challenges.
New issues
Throughout the storage and warehouse business, the increasing need to make the best use of space creates particular challenges for fire protection engineers, designers and installers. Recently a division of the Ministry of Defence – MOD Defence Estates – approached Tyco Fire and Integrated Solutions to assess the effectiveness of fire protection systems normally used in carousel or shuttle-type automatic storage units.
These storage carousels are basically long boxes mounted on their sides within an enclosure, and connected to a chain drive. The operator brings a box to the service hatch to insert the items and then the box moves off vertically around the chain. Shuttles have the same hatch entry, but use trays which are lifted up and down a central void and are placed into shelves at either the front or back of the machine. While many of these systems do not necessarily contain highly combustible items, many contain items in combustible packaging. Although not cheap, these units can save between 60%-80% of storage space and also result in savings on manpower to retrieve stored items.
These storage units can be designed to be multi-storey by passing through more than one floor, with retrieval points on each floor. This effectively provides a vertical shaft which can have a high fire load, with the added hazard of breaching compartment floors. From a military point of view, these larger designs are considered to be an unacceptable risk.
Conventional fire detection (linear heat or smoke) and gaseous fire protection are offered by the carousel and shuttle suppliers. While MOD Defence Estates had previously used CO2 fire protection, a risk assessment of these storage facilities brought to light that there was no readily available data on their performance in the event of a fire. This was a particular issue for the Ministry of Defence as it is unable to insure risks on a commercial basis and in any case, the loss of key parts or components could affect the operational capacity of armed forces. So as then assistant chief fire officer of Defence Fire & Rescue Service, I oversaw a series of three trials over six months to establish the relative effectiveness of various fire suppression systems.
Live fire tests
The fire tests were carried out under the auspices of Darchem Engineering. The first two series of trials involved local fires in the motor and control panels. In these tests, the trial carousel unit was provided with a linear heat detection tube threaded around high risk areas, such as the motor and key electrical parts. This would activate a high pressure CO2 cylinder through a single nozzle at the top of the unit. Standard packaging was placed in the storage compartments, and a fire was lit in a lower compartment. In order to simulate the most likely cause of a real fire – including that caused by an electrical fault, friction, discarded smoking materials and arson – no accelerants were used. The fire protection system activated as designed and extinguished the small local motor and control panel fires.
But when it came to extinguishing a fire on the trays within the carousel, the CO2 system had no effect, and the fire burned for five hours until all the flammable material was destroyed. Firefighters were called on to extinguish the fire, but were unable to do so because of the nature of the construction of the enclosure and the potential physical danger to themselves. Applying copious amounts of water using jets to the external envelope was considered, but the force of the water and sudden cooling could cause a potential opening of the panels and intensify the fire. Water sprays carefully applied to the external envelope were finally adopted to provide a cooling effect and to provide some form of control. The problems centred around the lack of sight into the unit and virtually no access to directly tackle the fire. There was also uncertainty as to the intensity and spread of fire within the unit, and the seat of the fire could only be guessed at by external indications of scorching and paint blistering.
A subsequent analysis identified not only why the trial system had failed, but also why any other conventional fire protection was equally unlikely to be effective in such a scenario. It transpired that the configuration of the trays in the unit prevented adequate ingress of extinguishant. The shelves in such units are virtually sealed boxes, packed to capacity. Gaps between panels and shelves, which act as baffles, have a maximum distance of only 45mm, with just a 20mm gap between the service access door and the shelves, and a lack of enclosure integrity in the unit prevented an effective concentration being maintained. We also established that it was necessary to totally extinguish such a fire – rather than just control it – as firefighters would not be able to enter the unit and there would therefore be a very real risk of the fire spreading throughout the building.
A series of options was evaluated before an effective solution was identified, which entailed building up a concentration of extinguishing agent in every storage compartment and then maintaining it for at least 30 minutes. To achieve this, two systems were tested, one delivering watermist, the other the inert gas Inergen.
In spite of various changes in design and configuration to the watermist system, satisfactory penetration could not be achieved and the fire continued to grow after initial partial suppression. Sustained application only prevented some fire spread to the edges of the unit and resulted in saturation of some items.
Inert gas system
A cascade Inergen system using special injection and screening nozzles was then installed, which was designed to establish and sustain the extinguishing concentration over an extended period. Inergen is a mixture of nitrogen, argon and carbon dioxide (all naturally present in air) which extinguishes fire by reducing the oxygen level in the area to below the point at which most combustibles will no longer burn. It is fast, effective, safe, and environmentally friendly. As a clean, clear, non-toxic fire suppressant, it is perfectly safe for exposure to people, and has been successfully tested on human beings. In a room where it has been discharged, people are able to breathe quite normally, yet combustion cannot be supported. This is unique to the special formulation of Inergen, which includes a very small, safe percentage of Carbon Dioxide. This has the effect of allowing people to absorb more oxygen from the ‘thinned’ air within the room, thus bringing the respiratory system back to within normal parameters.
Trials using the cascade system were successful in extinguishing all fires and sustaining the extinguishing concentration for a period in excess of 30 minutes to eliminate the danger of re-ignition. As a result MOD Defence Estates has published a Policy Instruction on the protection and use of these storage systems. Having discussed briefly the findings with the Chief Fire officers Association (CFOA), it is intended to discuss further in detail with them the issues raised and also to carry out jointly with Tyco, seminars to fire services through CFOA and also to industry.
Mass storage units are growing in popularity and are a very efficient and practical way to store goods. During the fire tests conducted, they were found to be robust in construction. But there are problems associated with critical and high value assets, with a risk to business continuity, especially in view of the dangers of manual firefighting. The partnership between Defence Estates, Tyco and Darchem has identified and validated a fire engineered solution using cascaded Inergen.
Clive Turner is former assistant chief fire officer with Defence
Fire & Rescue Service