In discussions with firefighters and fire protection engineers, I 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) standard is actually “buying you” in terms of protection.
Likewise for any other standard. Most of them have an underlying assumption that the fire service will play a role in meeting the overall objective. One source of the disconnect is that the standard writers and fire service don’t understand what the other is thinking or expecting. Many of these issues were written about before; but are refreshed and updated here with insight from continuing discussions.
This series of articles will assume that an NFPA standard is met. If the standard is not met, then worse performance is anticipated, possibly total failure of the system to accomplish its objective. Other articles have discussed how to know if a system meets the standard. An experienced loss prevention engineer can determine how much a deviation matters for the situation at hand. Standards other than NFPA can be evaluated in a similar manner.
Generally, insurance standards will exceed NFPA performance expectations. Sometimes international standards result in lower sprinkler performance than NFPA requires. However, many other factors offset reduced sprinkler performance; for example, very robust fire separations.
We will start with sprinklers, then address other water-based systems, and then cover other systems including passive fire protection in future articles.
Manufacturing facility sprinkler design is derived from experience with what were called pipe schedule sprinkler systems. An ordinary hazard pipe schedule could supply two sprinklers with one-inch pipe, three total (one additional) with inch and a quarter pipe, five total (two additional) with inch and one-half pipe. These were sometimes called 2-3-5 systems based on the number of heads fed from successively increasing pipe sizes. The schedule progressed all the way up to eight-inch pipe. In this context, pipe schedule has nothing to do with pipe wall thickness, as with a Schedule 40 pipe.
There was also a separate pipe schedule for extra hazard occupancies. Performance of these pipe schedule systems was eventually distilled into the density/area curves we use today. For example, an Extra Hazard Group 21 (the most hazardous classification) requires a density of 0.4 gallons per minute per square foot (gpm/ft²) over a 2500 square foot area. This corresponds to an expectation that no more than 25 sprinklers would open. In metric units, this is 16.3 mm/min (like a rainfall rate measurement) over 230 square meters.
Manufacturing sprinkler designs are control mode sprinklers, meaning that the fire will grow according to a heat release curve, typically based on time squared. A time squared curve means that if time doubles, the heat release rate is four times greater. There are many variations of this curve used by fire protection engineers. Once the sprinklers operate, fire growth is expected to stop and then the heat release rate curve will slowly decay over time, from the combined effect of the fuel burning itself out and sprinkler water application.
The actual burning area is expected to be significantly smaller than the design area. 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 how fire spread is limited.
All of this means that although the fire has been controlled, there is still significant manual firefighting to do. For an Extra Hazard Group 2 occupancy, NFPA 13, Automatic Sprinklers, requires that 500 gpm (1900 l/min) be available for fire service hose stream while still meeting the sprinkler demand.
This means that the water supply has to be adequate to supply 25 — assuming a 100-square-foot coverage per sprinkler — sprinklers at 40 gpm (151 l/min) each plus two 2½” (65 mm) hose streams at 250 gpm (950 l/min) each. The duration is required to be 90 to 120 minutes. The shorter duration is permitted when sprinkler water alarms are monitored, resulting in a more rapid notification of the fire service.
In manufacturing occupancies, there is no specific expectation on smoke and heat ventilation like there is in storage occupancies. Nor is there any specific expectation on the need for final extinguishing agents other than water. Those issues are left to the fire service to plan.
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 storage occupancies, 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: Examples of Extra Hazard Group 2 occupancies are asphalt saturating, manufacturing occupancies that spray flammable liquids, mobile home manufacturing and open oil quenching.
John Frank is senior vice president of AXA XL Risk Consulting’s Loss Prevention Center of Excellence where he is involved in loss prevention research and loss prevention training.