By Jeff Harrington, CEO and Founder of Harrington Group, Inc.

October 1997

Part 1 of this series provided an overview of the confusing and possibly fallible code requirements regarding smoke venting in sprinklered buildings used for storage of goods. I tried to stir up a little controversy for these fire safety solutions and maybe even add to the confusion.

The only thing worse than complaining is not suggesting a solution. The fact is, dozens of large distribution centers are on the drawing board (or CAD station) or under construction across this country as I write these words. These facilities need practical cost-effective, real-world fire safety solutions that perform a meaningful function while satisfying local building and fire officials.

Before designing any fire protection system one should first identify the performance objectives for that system. What should be the performance objectives of a smoke ventilation system in a large warehouse or distribution center? If the building/fire code did not require such a system, would you install one anyway?

The three most widely used model-building codes require smoke ventilation systems for storage areas containing high-piled and high-rack storage. Furthermore, they require the smoke ventilation system be designed based upon the requirements of NFPA 204M1 for both sprinklered and unsprinklered buildings.

However, the design criteria in NFPA 204M are valid only for unsprinklered buildings. So, for unsprinklered buildings there is no controversy. I suggest that a smoke ventilation system be installed using gravity vents and curtain boards in accordance with the model codes and NFPA 204M.

NFPA 204M criteria expressly do not apply to sprinklered buildings (see NFPA 204M; Section 6-1). So, I do have a problem blindly applying NFPA 204M smoke ventilation criteria to a sprinklered building, as the model codes require.

Let’s take the blinders off and consider the performance objectives of a smoke ventilation system in a sprinklered building. What should they be? The Handbook to the Uniform Building Code2 provides one possible answer. According to this Handbook, there are two performance objectives for any smoke ventilation system: The first objective is to prevent large volumes of smoke and heat from descending down from the roof too close to the floor level which could cause manual fire fighting efforts to be ineffective. The second objective is to prevent the accumulation of super-heated unburned gases at the roof level that could possibly ignite violently if air is introduced.

In an unsprinklered building, an automatic smoke ventilation system is the only way to achieve the above two performance objectives until the fire department arrives and begins fire-fighting operations. In this case, an automatic smoke ventilation system is the first and only line of defense.

In a sprinklered building, the situation is quite different. The automatic sprinkler system can, to a large extent, accomplish the two performance objectives all by itself. The first objective is influenced primarily by the total volume of smoke produced by a fire before the fire department arrives on the scene. The volume of smoke produced by a fire is dependent upon the fire’s heat release rate. A fire’s heat release rate grows exponentially over time as long as there is plenty of fuel and oxygen as there is in a warehouse environment.

Water discharging from operating sprinkler heads has a significant effect on a fire. The fire’s heat release rate will immediately be reduced and with it, the volume of smoke produced. Thus, an automatic sprinkler system assists in accomplishing the first performance objective and in some cases can accomplish it entirely by itself.

An automatic sprinkler system also can effectively accomplish the second objective. Sprinkler heads are generally located about 12 inches below the roof deck. Any operating sprinkler heads are, therefore, within and above the hot gas layer. In fact, it is the hot gas layer that causes each sprinkler head to operate. Water from operating sprinkler heads absorbs heat from the hot gas layer and the rising fire plume. The fire’s heat release rate is reduced, which in turn slows the rate at which heat is added to the hot gas layer. An operating sprinkler system can, by itself, prevent the hot gas layer from super-heating and igniting.

Like a smoke ventilation system, an automatic sprinkler system can help prevent large volumes of smoke from developing and descending down from the roof becoming an impediment to manual fire fighting efforts. Also, like a smoke ventilation system, an automatic sprinkler system can prevent super-heating and violent ignition of the hot gas layer at the roof level.

Unlike a smoke ventilation system, an automatic sprinkler system can also extinguish a fire, or substantially reduce a fire’s heat release rate. An automatic sprinkler system can thereby greatly minimize a fire’s threat to life and property. In a sprinklered warehouse, the sprinkler system is the building’s first line of defense and every effort should be made to ensure that it operates effectively as designed.

Testing has shown that smoke ventilation can significantly reduce a sprinkler system’s effectiveness. This is certainly counterproductive, if not dangerous.

The automatic sprinkler system should be allowed to do its job as the first and best line of defense against fire. A smoke ventilation system in a sprinklered building is a secondary line of defense and should not be allowed to conflict in any way with the sprinkler system’s performance.

Therefore, the smoke ventilation system should not operate during the time the sprinkler system is working to control or extinguish the fire. If the smoke ventilation system consists of mechanical exhaust fans, they should be arranged only for manual actuation by the fire department. If gravity vents are used, the fusible elements which open each vent should be chosen such that vent actuation will occur only if the sprinkler system fails to control or extinguish the fire.

In a future article, I will discuss fire safety solutions where curtain boards can be eliminated, ventilation capacity reduced and spacing of vents and air intakes modified. As always, I welcome your comments and ideas.

1 NFPA 204M, Guide for Smoke and Heat Venting, 1991 Ed.; National Fire Protection Assoc., One Batterymarch Park, Quincy, MA.

2 Handbook to the Uniform Building Code: An Illustrated Commentary; International Conference of Building Officials; 5360 Workman Mill Road; Whittier, CA 90601-2298; 1995.

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