Ventilation is an important part of managing any structure fire. Procedures on how and when to conduct ventilation operations have varied globally. In December 2010, Underwriters Laboratories released a major study that challenges traditional assumptions at residential fires in the United States. With regards to large sprinklered warehouses, there is no global consensus for smoke and heat venting. An additional variable was created with the introduction of large truck or trailer mounted ventilation fans, often called mobile ventilation units. These units can move as much as 750,000 cubic feet per minute (21,250 m3/ min). They are marketed for ventilating large warehouses.
In all occupancies, ventilation must be carefully controlled. Ventilation performed by the fire service needs to be coordinated with suppression. At a residential fire, a ladder company cannot just start breaking glass without coordinating with the hose line attack. At a large sprinklered warehouse, uncoordinated ventilation could lead to loss of fire control. Very large openings can be quickly created by opening truck or rail siding doors. Building climate control fans and even zoned engineered smoke exhaust fans can cause excessive ventilation. Mobile ventilation units can also cause excessive ventilation if used indiscriminately.
This author is aware of three very large storage fires where excessive ventilation by the fire service was a significant factor in the loss of the facility. At all of these fires, the sprinklers were providing control of the fire. In one case, the sprinklers had nearly extinguished the fire. In all three cases, employees had safely evacuated so rescue was not a factor.
The warehouse sprinkler system designs found in NFPA 13, Standard for the Installation of Sprinkler Systems (and its storage predecessors, the NFPA 231 series) and FM Data Sheet, 8-9 Storage of Class 1, 2, 3, 4 and Plastic Commodities, as well as other 8 series data sheets, assume that no ventilation is present. The original tests on which these standards were based had no ventilation whatsoever. Later tests had some ventilation for pollution control and enhanced visibility for test observers but not enough to influence the fire.
Of course, smoke has to be removed. Today, many insurance companies advocate manual ventilation by the fire service rather than automatic smoke removal systems. This is because:
1. Automatic smoke and heat vents can delay sprinkler operation by channeling the ceiling jet away from the sprinklers early in the fire. Convective heat transfer from the ceiling jet is what activates the fusible link.
2. The fire service can make a judgment as to when the fire is controlled by the sprinklers and how ventilation should be carried out. The three aforementioned fires show that this is not always accomplished. It is all too tempting to open every possible opening and start every possible fan (building and mobile). It is also tempting to make additional openings in the building with power saws or by other means.
3. If sprinklers are doing their job, automatic “gravity” vents may not be very effective because the smoke is cooled by sprinkler spray and is less buoyant.
Limiting discussion to fires that have been controlled by the sprinklers and where rescue is not needed. Hose line operation may still be needed for final extinguishment. (Remember, sprinklers control or suppress a fire but are not expected to extinguish it even though sometimes they do.)
We suggest a controlled, methodical and reversible approach. This methodology was first discussed in NFPA 231D, Standard for Storage of Rubber Tires, last published in 1998. It was then moved to NFPA 230, Fire Protection of Storage, last published in 2003. NFPA 230 was eventually rolled into NFPA 1, Fire Code; however, the methodology was not carried over to NFPA 1. At the time the methodology in NFPA 231D was developed, mobile ventilation units were not in common use. The methodology can certainly be extended to this mobile ventilation technology and also applied to all kinds of storage. NFPA 230 refers to the “critical stage,” which is after sprinkler control has been achieved and when ventilation begins. It is during this stage that sprinkler control can be lost, followed by potential loss of the building.
Cutting large holes or removing large sections of walls is not reversible and therefore not recommended for smoke removal. We are therefore talking about building and mobile fans that can be shut off and openings that can be closed.
1. Sprinklers should not be shut off.
2. Check pressure gauges on the apparatus supplying the fire department connection (FDC) to ensure that additional sprinklers are not opening. If sprinklers are still opening, the fire is not yet controlled. If the fire pump house can be safely accessed, those gauges can be checked as well.
3. Monitor roof conditions from an aerial platform. Special call a platform if one is not assigned initially.
4. Charged, staffed hose lines should be in place to hit areas that may be shielded from sprinkler discharge or to complete extinguishment. Remember that the typical warehouse hose stream allowance figured in the sprinkler system design is 250 gpm (950 l/min) for ESFR (early suppression fast response) protected warehouses and 500 gpm for other warehouses. Excessive hose use can rob water from the sprinkler system. This allowance equates to one or two 2½-inch (65 mm hose streams) flowing 250 gpm each. More lines can be deployed for tactical coverage but the design assumes that no more than the allowance is flowing at one time.
5. Slowly begin ventilation efforts and monitor the impact visually (including changes in smoke color, density, volume and the velocity at which it exits the building) using thermal imaging cameras and apparatus pressure gauges. For mobile ventilation units, this means to bring them up to speed slowly.
6. If the fire starts to accelerate, be prepared to use prepositioned hose lines to rapidly regain control. Also be ready to withdraw firefighters, turn off fans or reclose doors (preposition people with radios to do this). Of course, this needs to be carefully coordinated with the interior and incident commanders. Turning off ceiling fans could cause the smoke layer to drop and disorient firefighters inside the building. This is the point where coordination and judgment become critical.
7. Reevaluate and repeat the process as needed.
8. Eventually sprinklers will need to be turned off once the incident commander is positive that the fire is extinguished. Keep a firefighter with a radio stationed at the sprinkler control valve so that it can be reopened if needed.
Manual roof ventilation is not addressed here. Most large sprinklered warehouses in the United States are of lightweight bar joist construction. If ventilation is needed to relieve hot smoke conditions for firefighters, then it is probably too dangerous to operate on the roof or even inside the building. Such conditions may indicate that sprinklers have not controlled the fire. If the smoke has been significantly cooled by the sprinkler discharge it will have lost buoyancy and will probably not vent very well through holes cut in the roof or manually opened gravity vents.
If horizontal ventilation is used, questions often arise about spreading smoke to areas of the warehouse that have not yet suffered smoke damage. This is a potential that is best avoided by using ceiling mounted manual fans under the guidance discussed above. However, if there are no ceiling mounted fans, or if they have been damaged by the fire, there is little choice but to ventilate horizontally. Efforts should be made to direct the smoke over the shortest and least damaging possible path.
These are general guidelines that may have to be altered to meet the specific needs of the situation. The key point is that uncontrolled and uncoordinated ventilation can cause the loss of sprinkler control. It might not be possible to regain control and the facility could be lost.
Feel free to contact the author at [email protected].
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