We frequently hear disconnects about what is expected from fire protection systems. It is important to understand what complying with a standard, such as a National Fire Protection Association (NFPA) or insurance company standard is actually “buying you” in terms of protection.
We started with sprinklers in manufacturing operations (please see part one) and continued with sprinklers in warehouse operations (please see part two). One topic regarding warehouses that needs to be reinforced due to continual misunderstanding of performance expectations, is that ESFR (Early Suppression Fast Response) sprinklers are not designed to extinguish fires. The S means suppression. Suppression means driving the heat release rate to near zero. The 2019 edition of the NFPA Automatic Sprinkler System Handbook (page 777) says that suppression means to nearly extinguish a fire.
Although “nearly” represents outstanding sprinkler performance, final 100 percent extinguishment must still be accomplished. That takes a hose team, which is not necessarily the public fire service, but a hose team provided by someone is still needed. Although 250 GPM is reserved for hose streams with ESFR sprinklers, that much is rarely needed if suppression is achieved. A more realistic number might be 100 GPM. Stream reach is the main thing that is needed, not volume.
Part three was intended to address other water-based systems such as water mist; however, considering the ITC terminal fire in Deer Park, Texas (covered extensively in the Spring 2019 edition of this publication), passive protection, which includes tank spacing, will be addressed instead. Fire walls and barriers will also be discussed.
Tank-to-tank fire spread occurs from spreading ground-level liquid fires or radiant heat transfer. Ground-level spread is controlled with dikes (bunds) and is relatively easy to understand and prevent through proper diking.
Spread from radiant heat transfer seems to be less commonly understood because tank spacing requirements vary so much among standards such as NFPA, API, and insurance companies. My company has among the most stringent spacing standard. They were based on loss experience of one of our predecessors, the Oil Insurance Association. They were developed long before fire protection engineering calculations could validate them scientifically. Usually we recommend a spacing of one tank diameter. That takes real estate and therefore is frequently not applied.
Fire protection engineering calculations typically assume that spread from tank-to-tank will eventually occur if the radiant heat flux exceeds 8-12 kW/m2 on uncooled tanks. To put this in perspective, the heat flux from the sun on a hot summer day is around 1 kW/m2. Just as sunburn does not occur in the minute you walk out into the sun, the time to ignition of adjacent tanks is a function of incident heat flux and time. If a fire can be extinguished quickly, a higher heat flux for that short time can be tolerated.
There are simple engineering correlations to estimate heat flux on exposed tanks. Advanced computer models that can account for wind and time are also available. Details of these calculations are beyond the scope of this article. The point is that they can be readily calculated by fire protection engineers (and in many cases probably already have been) and therefore tank-to-tank spread can be predicted, expected, and planned for.
Meeting minimum spacing standards, including NFPA 30 spacing, is not necessarily adequate to prevent tank-to-tank spread due to radiant heat transfer. Multi-tank fires such as the ITC terminal are foreseeable as are the resources needed to handle them (as described in the Spring 2019 edition of this magazine).
Besides diking, a common method of preventing ground level fire spread for other than bulk tank fires (whether outdoors or indoors), is drainage to an area that does not threaten important facilities. Simple impounding, such as with 4-inch (100 mm) curbs in an indoor flammable liquids rooms is not enough to prevent fire spread beyond the room. That’s because fire protection water is typically discharged in much greater quantities than can be contained by the curbing. Scaled up, this can also apply to outdoor chemical processing areas.
Drainage does not necessarily eliminate the risk of fire. It just moves the liquid to an area where the effect of a fire would be acceptable. Drains are just as critical to fire protection as fire suppression, but they are frequently ignored in the design process, undersized, not maintained once installed, and sometimes intentionally blocked for environmental reasons.
Drains are hydraulically designed just like water based fire suppression. They must be of sufficient capacity to remove the combined spill volume and fire suppression water. During preplanning, the ability of the drainage system to do this should be known. If the drainage is found to be inadequate, then ground level liquid fire spread should be expected.
In my experience, especially in flammable liquids warehouses, drains as required by NFPA 30, Flammable Liquids Code, are often completely ignored. When evaluating drainage for adequacy, large piping akin to stormwater drainage should be anticipated, not a 2-inch (50 mm) pipe going to a small holding tank. A new system known as an Ignitable Liquid Drainage Floor Assemblies has recently gained FM Approval and is expected to significantly reduce drainage needs by reducing sprinkler water demand.
Fire Walls and Fire Barriers
In structures, fire barriers of various types are a form of passive protection that might serve these roles:
Prevent fire spread in the event of failure of all other forms of fire protection. This type of wall is uncommon.
Working in conjunction with other fire protection, especially sprinklers. prevent the spread of smoke or heat to a protected area. This is by far the most common wall prescribed in various codes.
The first type is the most important for the industrial facility and also the least common. They are intended to allow part of a facility to survive with a total loss of other fire protection such as sprinklers and manual firefighting. They are known in the insurance and risk management community as Maximum Foreseeable Loss (MFL) walls.
NFPA calls them High Challenge Fire Walls. They are structurally freestanding so that even if the building collapses on one side or the other, the wall remains standing. The reason they are uncommon is that they are typically only required by insurers for extremely large facilities. They are not typically code required. And finally, they are very expensive to build and maintain.
All other walls are more properly called fire (or possibly smoke) barriers. If the sprinklers control or suppress the fire, these walls are expected to keep heat and smoke from the protected side. In smaller unsprinklered buildings, they may give the municipal fire service time to control the fire in the compartment of origin and prevent further spread.
They are not free standing. In a warehouse, if the roof collapses because of inadequate or impaired sprinklers, the barrier wall may collapse as well. If the sprinklers fail, the wall serves some value as a potential line of defense. Collapse must be anticipated. At several fires, aerial streams have been proven to work in conjunction with such a wall to limit damage to the protected side.
Unlike active fire protection, such as foam systems, sprinklers, and manual firefighting, passive systems are extremely difficult, if not impossible to upgrade once installed. Rarely, an MFL wall will be added to an existing facility, but this is extremely costly. Likewise, drainage is almost never retrofitted if it was omitted in the first place. Bulk tanks, once installed are there until dismantled. The spacing is not going to change. The responder, therefore is left to adapt to whatever they inherit with almost no chance for improvement.
John Frank is the senior vice president of the AXA XL Risk Consulting's Loss Prevention Center of Excellence, where he is involved in loss prevention research and loss prevention training.
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