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 (Part 1). We’ll continue with sprinklers in warehouse operations. Unlike manufacturing sprinkler designs, which are derived largely from historical performance, sprinkler designs for storage are derived mostly from full scale fire tests at UL, FM Global, and international labs such as CNPP (France) or RISE (Sweden).
A full-scale test usually consists of two forty-foot-long double row racks with single row tacks on each side as a target. The test fire is a robust single point ignition source. This means that it is not based on multiple ignitions in separate areas as might be the case with employee arson. Key requirements for the sprinkler design to pass the test:
- Not spread to the ends of the racks (or pile)
- Not jump an aisle and burn through to the opposite face of the target rack (or pile)
- Steel supports (e.g. trusses) cannot reach their collapse temperature
- No more than a certain number of sprinklers can open. This number varies with the test objectives.
The basic idea is that if the fire is contained to a small section of the rack, and if the steel stays cool enough so that it does not collapse, the fire service can then affect final extinguishment. All testing is done without automatic smoke and heat vents. The expectation is that the fire service will manually ventilate the building.
In recent years, warehouse sprinklers were classified as control mode or suppression mode. Control mode means that the fire growth rate is stopped and then held constant by the sprinklers, but that the fire service will still have active firefighting left to do. Over an extended period of time, the heat release rate will slowly decline.
Suppression mode sprinklers not only stop the fire growth, but quickly (typically before the fire service arrives) drive the heat release rate down to near zero. The fire service still needs to affect final extinguishment, but the expectation is that they will not need to perform aggressive firefighting. It is important to note that suppression does not mean extinguishment.
FM Global Data Sheets no longer differentiate between suppression mode and control mode sprinklers. Instead, they simply refer to storage sprinklers. This is because the lines between control and suppression are blurred with modern warehouse sprinklers.
In some cases, such as flammable liquids storage, the fire service may need to deploy foam lines inside the warehouse and even use dry chemical on three dimensional fires in order to affect final extinguishment.
The actual burning area is expected to be significantly smaller than the design area (frequently between 1200 and 3000 square feet). Sprinklers beyond the burning area operate because of hot convective gases spreading under the ceiling. The heads that operate outside of the burning area pre-wet the surrounding fuel, which is part of how fire spread is limited.
What is important is to know the occupancy, know what to expect from the systems, and know what is needed from the fire service.
In future articles we will address occupancies protected by other water-based systems (foam, water spray, water mist, and water additives, occupancies protected by gas and powder systems, and passive protection such a fire barriers and bulk storage tank separation.
Editor's note: Manual ventilation has been discussed in past articles. If done improperly, the fire can redevelop, and control can be lost. Superventilator trucks and other forms of excess ventilation have caused control to be lost at warehouse fires.
John Frank is 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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