Control of Fires Lurking in the Shadows
It seems that, in this era of designing everything to the exact inch and psi, we may be losing track of what the real objective that NFPA 13, Standard for Installation of Sprinkler Systems, is trying to accomplish. The technology of our industry has advanced in leaps and bounds in ways to better accomplish the task of fire control and suppression. We now have very specialized sprinklers with very specific uses and applications, all designed to fit a niche where someone in a laboratory setting says we are possibly failing. And, while I would generally agree that old-fashioned, standard-spray sprinklers may have some shortcomings in certain applications, we need to remember what the real goal is for the vast majority of systems that we design and install. NFPA 13 says that the objective is to design and install automatic fire sprinkler systems to provide a reasonable degree of protection for life and property. We now have certain technologies that imply a purpose of suppressing a fire but, in general, the vast majority of systems are supposed to be designed to control a fire. “Control” being the key word. Fire control is defined as limiting the size of a fire by distribution of water so as to decrease the heat release rate and pre-wet adjacent combustibles, while controlling ceiling gas temperatures and providing protection of the building structure. A good way to define control is stopping flashover. We generally do such a great job of controlling the fire that in many cases we do provide suppression of the fire. Because of this, many Authorities Having Jurisdiction (AHJs) believe that the objective is to suppress the fire in every instance and as a result are becoming excessively conservative with even the smallest amount of obstructions. Keep in mind that, unless you are working with a suppression-mode technology, we are more interested in controlling the fire as the objective. In this article, let’s take a closer look at the standard obstruction rules in NFPA 13 for light and ordinary hazard occupancies, and keep in mind the objective is to provide control of the fire.
Focusing primarily on Chapter 8 of the standard, we have some basic requirements. We are to install sprinklers throughout the premises, as defined by the standard. We mustn’t exceed the maximum or minimum protection area and distance per sprinkler, as allowed either by the standard or by special listing. We are to position sprinklers to provide satisfactory performance with respect to activation time and distribution. Although there are additional basic requirements, these are the main requirements that we will be focusing on. There are many types of obstructions found within buildings that we must account for in the design of the system. Generally, we encounter vertical and horizontal obstructions in the form of walls, ceilings, soffits, columns, and other architectural, mechanical, and electrical elements (and once upon a time we even identified obstructions as either vertical or horizontal). Obstructions may be in the form of continuous obstructions or non-continuous obstructions and they may be located above or below the level of the sprinkler. These are all factors in determining which of the obstruction rules apply. The performance objective is to locate sprinklers so as to minimize obstructions to discharge as defined in the standard, or provide additional sprinklers to ensure adequate coverage of the hazard. When navigating your way through all of the many obstructions and obstruction rules, take note that the committee uses terms such as “reasonable degree,” “satisfactory performance,” and “adequate coverage.” These are very general terms and are not used by mistake. We have to apply some level of common sense and understanding of the actual scope of the standard, which is to control the fire. Again, we are applying this to light and ordinary hazard occupancy, in which the committee defines as having fires with low to moderate rates of heat release or even high rates of heat release but with only 8-ft high plies of contents. This means generally that we are dealing with lower fuel loads and lower rates of heat release than would be found in more hazardous occupancies such as extra hazard and storage occupancies.
There are three main rules that will apply to the majority of obstructions and will be dependent upon whether the obstruction is continuous or non-continuous, and whether the top of the obstruction is located level with or above the plane of the sprinkler deflector. When a horizontal obstruction is located level with or above the plane of the sprinkler deflector, the water spray cannot spray above or around the obstruction and the sprinkler must then be placed far enough away to allow water spray below the obstruction. This is often referred to as the “beam rule.” Generally, this condition will not allow “adequate coverage” and the sprinkler must meet the spacing requirements or an additional sprinkler must be added on the opposite side of the obstruction. An exception would be where the obstruction is greater than 4-ft wide and sprinklers are located beneath the obstruction, such. If the obstruction is less than 4-ft wide and greater than 18 in. below the deflector, it can be ignored. This presents a great deal of shadowing on the floor. Since the objective is to control the fire, this condition provides adequate coverage to meet the objective. Similarly, where a wall-mounted soffit is present, sprinklers aren’t required to comply with the beam rule but can meet the “soffit rules.” In some cases, a soffit or shelf projecting from a wall is allowed to extend 4 ft horizontally without requiring additional protection below the soffit. In this condition, there is a substantial amount of shadowing, or area not directly protected by water spray from the sprinkler. Again, even though we aren’t spraying directly on the floor area, we are accomplishing the objective of “adequate coverage” in controlling the fire. Many obstructions are located below the level of the sprinkler but less than 18 in. below the deflector. This type of obstruction requires the use of the three times rule or in the case of extended coverage sprinklers, the four times rule. This rule requires us to locate the sprinkler at least three times (or four times) the maximum width of the obstruction, up to a maximum dimension of 24 in. (or 36 in.), away from the obstruction. This is to allow water spray to adequately extend beyond the obstruction and limit the shadow area, again providing control of the fire. NFPA 13 gives an exception to this rule, indicating that for light and ordinary hazard occupancies, structural members only shall be considered when applying the three times or four times rules. The purpose to limiting the requirement to apply the three times and four times rules in light and ordinary hazard occupancies goes back to the purpose and objective of the standard. We are pre-wetting adjacent surfaces, controlling gas temperatures, and protecting the structure. Since our heat release and fuel loads are lower, a level of shadowing is acceptable when providing “adequate coverage.” The Appendix material for Chapter 8 provides additional examples of shadow areas that are deemed acceptable with regard to wall corners that partially obstruct sprinkler coverage. This appendix material directly refers to the basic requirement that sprinklers shall be positioned and located so as to provide “satisfactory performance” with respect to activation time and distribution. It becomes apparent that some of these acceptable shadow areas can become relatively large, given the acceptable size of the obstruction and the relative distance of the sprinkler head, and still meet the objective of controlling the fire.
When combining all of these obstruction principals and understanding the purpose and objective of the standard, it becomes more clear that in light and ordinary hazard occupancies, some level of shadowing certainly falls within the definition of “reasonable degree,” “satisfactory performance,” and “adequate coverage.” You will notice that generally, there is not a fully defined quantity of shadowing in terms of square or cubic feet that is provided for the designer and installer to apply. Every condition is different and, as the sprinkler designer and installer, you must always bear in mind what the standard is trying to accomplish – control the fire by pre-wetting the adjacent combustibles and limiting the gas temperatures/stopping flashover until the fire department can arrive to provide full extinguishment. By providing adequate coverage to achieve satisfactory performance with a reasonable degree of accuracy, you have complied with the scope of NFPA 13.
ABOUT THE AUTHOR: Greg Patrick, S.E.T., is vice president, Treasure Valley Fire Protection, Inc., Boise, Idaho. He represents AFSA on the NFPA 22, Standard for Water Tanks for Private Fire Protection, committee and has been in the fire protection industry for 31 years.