Avoiding obstructions to sprinklers
By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu, Hawaii
A study of sprinkler system reliability (1) included an analysis of cases where a sprinkler system was located in the area of fire origin and did not function effectively. The most common reason for ineffective performance was that water did not reach the fire, accounting for 55% of the total occurrences. One reason that discharge from a sprinkler would not reach a fire is the inability of the discharge to penetrate the fire plume generated by high heat release rate fires as may be expected in high challenge storage occupancies.
More commonly, it is a physical obstruction, which prevents sprinkler discharge from reaching the fire. In addition to blocking sprinkler coverage, obstructions also can serve to delay or prevent sprinkler activation by interfering with the flow of heat to the sprinkler release element. This article will review the rules in the 2007 edition of NFPA 13 (2) for locating sprinklers to avoid obstructions.
A description of typical sprinkler discharge distribution patterns will aid in understanding of the various obstruction rules. Standard spray upright and pendent sprinklers have a discharge pattern that provides an 8' diameter spray pattern at a distance of 18" vertically below the sprinkler deflector and a 16' diameter spray pattern 48" vertically down from the sprinkler deflector. In looking at the FM test criteria (3) for sidewall sprinklers the spray is required to completely wet down all walls at a distance 48" below the elevation of the sprinkler deflector. In a review of NFPA 13, one sees that the obstruction rules for residential sprinklers are more severe than for standard sprinklers. This is because residential sprinkler test criteria (4) requires a thorough wet down of all walls at a level of 28" below the sprinkler deflector. This higher level of wall wet down is needed by residential sprinklers to meet their key design objective of preventing flashover.
Now let’s look at some of the obstruction rules.
NFPA 13 contains a few rules related to obstruction by sprinkler system components. This is a concern primarily for upright sprinklers. Pipe hangers are not permitted to be less than 3" from the centerline of the sprinkler (NFPA 13, 9.2.3.3). Seismic bracing and restraints must be located to avoid obstructing sprinklers (NFPA 13, 9.3.1.4).
To minimize disruption of the spray pattern, upright sprinklers must be installed on branch lines with frame arms parallel with the piping (8.3.1.3). There are also special rules for large drop sprinklers to avoid obstruction due to large diameter pipe (see discussion of the “Three Times Rule” below). Large drop sprinklers installed on branch lines with a diameter of 2-1/2" must be installed on a riser nipple so the deflector is at least 13" above the pipe. For 3" branch lines, the deflector must be located at least 15" above the pipe.
In addition to these sprinkler system component rules NFPA 13 contains obstruction requirements specific to the following types of sprinklers:
1. Upright (SSU) and pendent (SSP) sprinklers
2. Standard sidewall sprinklers
3. Extended coverage upright and pendent sprinklers
4. Extended coverage sidewall sprinklers
5. Residential upright and pendent sprinklers
6. Residential sidewall sprinklers
7. Large drop sprinklers*
8. ESFR Sprinklers
The obstruction rules for specific sprinklers were first introduced in the 1999 edition of NFPA 13.
With the exception of the obstruction rules for ESFR sprinklers, the rules for sprinkler obstructions are arranged similarly.
For each type of sprinkler there is a set of general rules for sprinkler location with respect to continuous obstructions and a set of more specific obstruction rules for 1) noncontinuous obstructions to sprinkler discharge pattern development and 2) obstructions, continuous or noncontinuous, that prevent sprinkler discharge from reaching the hazard.
The general obstruction rules contain a performance objective statement which reads:
“Sprinklers shall be located so as to minimize obstructions to discharge as defined in …, or additional sprinklers shall be provided to ensure adequate coverage of the hazard.”
Then there are the so-called “beam obstruction rules,” which indicate sprinkler deflector location with respect to continuous obstructions either within the room or against to the wall. The rules establish the distance the sprinkler must be from the beam for a given deflector distance above the bottom of the beam. For example, for a standard upright sprinkler located four feet from the side of the beam the sprinkler deflector must be no more than 14" above the bottom of the beam (NFPA 13, Table 8.6.5.1.2), and for a residential sprinkler located 4' from the side of the beam the sprinkler deflector must be no more than five inches above the bottom of the beam (NFPA 13, Table 8.10.6.1.2).
In the case of beams/soffits less than 30" wide against walls, the beam rules for upright and pendent sprinklers provide conditions where coverage by sprinklers adjacent to the beam are permitted. Of course, if the beam/soffit obstruction is greater than 30" wide, then a sprinkler must be provided below the obstruction.
Obstructions to sprinkler discharge pattern development include the “Three Times Rule,” which can be applied to fixed items, columns, and pipes that are 24" or less from sprinklers. The rule states that if the clearance from the sprinkler is greater than three times the largest dimension of the obstruction (the diameter of a pipe column for instance), then no obstruction exists. This is based on the premise that the ability of the sprinkler to throw water on either side of the obstruction or over and under the obstruction overcomes any potential dry shadow area caused by the obstruction Also, the “Three Times Rule” does not apply to sprinkler piping that is less than 3" in diameter. It is noted that for extended coverage spray sprinklers this rule becomes the “Four Times Rule” as the clearance is increased to four times the largest obstruction dimension.
There are also some rules that apply to open trusses, bar joists and beams with limited member widths.
The next set of rules for obstructions to the discharge pattern apply to the location of sprinklers with respect to suspended or floor mounted obstructions. This would include privacy curtains commonly found in patient rooms in health care facilities or free-standing partial height partitions.
The rules for ensuring obstructions do not prevent water from reaching the hazard include the “Wide Obstructions Rule,” which requires additional sprinklers under fixed horizontal obstructions over 4' wide such as ducts, decks and cutting tables that are fixed in place. Another common occurrence of a wide obstruction is with overhead doors when in the open position.
Though not specifically identified as obstruction rules, provisions for clearance of sprinkler deflectors above storage for certain sprinklers are intended to ensure that the discharge pattern development is not unobstructed. The clearance is at least 18" for standard and extended coverage upright and pendent spray sprinklers and standard sidewall spray sprinklers. For large drop and ESFR sprinklers the clearance is 36". An annex note in NFPA 13 reminds us that the clearance requirement should not be applied to storage on shelving that is either against the wall or mounted on the wall.
Large drop and ESFR sprinklers have additional rules that are needed to assure that the sprinkler discharge reaches protected commodities with minimum interference from obstructions.
A few more items on obstructions. In-rack sprinklers are not required to observe the obstruction rules. Also, in motion picture and television production studio soundstages and approved production facilities, where obstruction issues are common, the sprinkler discharge obstruction rules do not apply if “approved mitigation techniques are employed” or if the sprinkler system meets design criteria for extra hazard Group 2.
Sprinklers have a documented success rate of approximately 89%1. Considering all the things that can impair a sprinkler system’s performance, this is an extraordinary rate.
Knowledge of NFPA 13 obstructions rule should help improve the performance and reliability of fire suppression systems with which you are involved.
References:
1. John R. Hall, Jr., Ph.D., An Analysis of Automatic Sprinkler System Reliability Using Current Data, National Fire Protection Association, February 2, 2006
2. NFPA 13, Standard for Installation of Sprinkler Systems, National Fire Protection Association, 2007
3. FM Standard 2000, Approval Standard for Automatic Control Mode Sprinklers for Fire Protection, FM Global, March 2006
4. FM Standard 2030, Approval Standard for Residential Automatic Sprinklers for Fire Protection, FM Global, August 2009
* In the soon to be released 2010 edition of NFPA 13 the requirements for large drop sprinklers will be grouped in the larger category of Control Mode Specific Application sprinklers, (see next month’s article “Changes in the 2010 Edition of NFPA 13”).
Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer with bachelors and masters degrees from the University of Maryland Department of Fire Protection Engineering. He is past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates, Inc., a 15-person fire protection engineering consulting firm with offices in Honolulu, Hawaii and Guam.