Click here for a printable version of this article.

May 2011: Automatic Sprinklers, Heat/Smoke Vents and Draft Curtains

Our previous articles in October 2004 and August 2006 touched on the subject of the need for heat and smoke vents in fully sprinklered facilities.

We wrote about engineering and fire testing research versus Building Code requirements for smoke and heat venting in fully sprinklered warehouses and how roof vents were originally used as an aid to fire fighting in unsprinklered buildings.

FM Global Property Loss Prevention Data Sheet (DS) 1-10, Interaction of Sprinklers, Smoke and Heat Vents, and Draft Curtains, has been totally revised to further address this issue. Among other changes, the guidelines on draft curtains from FM Global DS 1-19, Fire Walls, Subdivisions and Draft Curtains, are now included in DS 1-10 and they have been revised to reflect testing and loss experience. The DS also added more detailed guidance on acceptable types of materials for draft curtains, as well as test standards that draft curtains should meet (from a combustibility standpoint). Furthermore, more information has been added regarding the interaction of sprinklers, smoke and heat vents, and draft curtains.

In most buildings where smoke venting is provided, it was installed to comply with local codes based on the building's size and/or occupancy. Codes usually require smoke venting in an effort to aid occupants' egress from the building and to support manual firefighting efforts. In some cases, the amount of venting may be based on a "rule of thumb" and may not be required to be scientifically calculated considering the size of the design fire or the exact height of the building. Required venting can vary widely from 1 sq. ft. of venting for every 30 sq. ft. of floor area to 1 sq. ft. of venting for every 100 sq. ft. of floor area.

Draft curtains are also usually recommended by Code to help improve the potential effectiveness of the vents, but may also be recommended in some FM Global data sheets and NFPA standards to protect against specific hazards. Heat/smoke venting is not considered a substitute for automatic sprinkler protection, as the vents will obviously not control a fire.

It should be noted that FM Global, and NFPA for that matter, would prefer that heat/smoke vents not be provided in fully sprinklered facilities except for certain occupancies. This is based on fire testing and loss history that shows that, for sprinklered storage fires, vents are at best of limited value and in many cases they can be detrimental (to be explained further).

In FM Global Property Loss Prevention Data Sheet 2-0, Installation Guidelines for Automatic Sprinklers (January 2011 edition), Section 2.1.1.7.1 explicitly recommends that automatic smoke and heat vents should not be installed in facilities equipped with sprinkler protection. It does provide an acceptable alternative for manual heat and smoke vents. Furthermore, in cases where local codes require the installation of automatic smoke and heat vents, it provides other alternatives (use of FM Approved vents for occupancies protected by quick-response storage sprinklers, use of FM Approved vents equipped with a standard-response 360°F (182°C) nominal thermal activating device or installing quick-response sprinklers directly under the vent openings).

If vents are arranged for manual operation, or automatic with higher temperature rated thermal activating device (normally a fusible link) as noted above, this will allow the sprinklers to gain control of the fire. The provision for manual operation of the vents should be in place to allow the fire department to operate them once the combination of sprinklers and manual fire fighting has gained control of the fire.

As noted in our previous articles, roof vents were originally used as an aid to fire fighting in unsprinklered buildings. During a fire, the smoke and hot gases rise until they reach the ceiling and then they spread outward and down towards the floor level, making manual fire fighting difficult. Roof vents, helped by draft curtains, can sometimes relieve smoke accumulation in unsprinklered building by containing the spread along the ceiling and by permitting the smoke to escape. The smoke rises through the vent because it is much hotter than the outside air. If this difference in temperature decreases, then the rate of smoke removal will also decrease.

Vents in sprinklered buildings, however, can have a neutral or negative effect. Although vents may delay the time of smoke obscuration, their premature operation may result in increased fuel consumption by increasing burning intensity and spread. Passage of hot air and smoke through the vents causes fresh air to enter the building through other available openings, resulting in greater fuel consumption and an increased water demand. Sprinkler water also absorbs much of the heat from the gases, cooling them and thereby decreasing the rate of smoke through the vents. For this reason, gravity venting (non-mechanical) is even more inefficient than mechanical smoke ventilation in sprinklered buildings.

Hence, venting a sprinklered storage fire is not always effective or desirable. During the initial stages of a fire, it is usually best to keep the building closed to limit the supply of fresh air while sprinklers gain control.

Section 2.1.1.8 of the DS 2-0 recommends against installing draft curtains in buildings protected by sprinklers unless they are specifically used to separate areas protected by quick-response ceiling-level storage sprinklers from areas protected by standard response ceiling-level sprinklers. Exceptions do apply in some cases as noted in the data sheet and in some occupancy-specific data sheets.

DS 1-10 reaffirms that draft curtains should be used to separate quick-response sprinklers from other types of sprinklers to prevent a fire originating in the adjacent area from opening the quick-response sprinklers.

However, if the draft curtains are not properly installed and aisle spaces are inadequate (as noted in Section 2.3.2), there could be a detrimental effect on automatic sprinkler operation. It can obstruct sprinkler water flow needed to control the fire and channel heat away from the fire origin, thus opening sprinklers well beyond the fire, which could in turn deplete the fire protection water supply. This is evidenced by loss experience (NFPA, Bulk Retail Store Fire: Tempe, Arizona, March 19, 1998) and testing by FM Global (Troup, 1994).

Since smoke tends to work its way downward from the roof/ceiling in a fairly even manner, it is important to keep the bottom edge of the draft curtain relatively level. In buildings with peaked roofs, it is important to keep the bottom edge of the draft curtain at a constant height above floor, while the elevation of the upper edge varies with the roof height. Section 2.3.4 gives general guidelines outlining the minimum recommended depth of draft curtains.

If you would like further information regarding the use of smoke/heat vents and draft curtains in sprinklered buildings, please contact Risk Logic Inc.