In the December 2019 issue, this column centered on occupancies and building features not appropriate for early suppression fast response (ESFR) sprinkler protection. Examples included protection of flammable/combustible liquids, open-top containers and buildings greater than 40-feet high.
This month, we’ll look more closely at situations where ESFR sprinklers can be installed but the specific location of each sprinkler is challenged by obstructions and building features.
Obstructions Near the Sprinkler
Because the discharge pattern of ESFR sprinklers differs from standard-spray sprinklers, obstructions near the sprinkler deflector that may be an annoyance for standard-spray sprinklers can defeat an ESFR sprinkler’s water distribution. NFPA 13-2019 edition, Standard for the Installation of Sprinkler Systems, specifies requirements for clearance to obstructions in Chapter 14.
Note that there are differences for clearance for continuous obstructions vs. isolated obstructions. From NFPA 13 definitions, a continuous obstruction affects the discharge pattern of two or more adjacent sprinklers; an isolated obstruction affects a single sprinkler.
Like standard-spray sprinklers, continuous obstructions located at or below the deflector shall not be closer than 1 foot horizontally from the sprinkler. However, unlike standard-spray sprinklers, no “three-times rule” exists. Instead, a series of horizontal clearance conditions apply. While renumbered in the 2019 edition, these clearance requirements have been unchanged in NFPA 13 for several cycles.
• Sprinklers arranged per allowable distances from Table 18.104.22.168.1 for horizontal obstructions below the elevation of sprinklers, or where a row of sprinklers is installed under the obstruction.
• Continuous obstructions 1 foot (300 mm) or less in width and located a minimum of 1 foot (300 mm) horizontally from the sprinkler.
• Continuous obstructions 2 feet (600 mm) or less in width and located a minimum of 2 feet (600 mm) horizontally from the sprinkler.
• Continuous and isolated obstructions 2 inches (50 mm) or less in width and located a minimum of 2 feet (600 mm) below the elevation of the sprinkler deflector or located a minimum of 1 foot (300 mm) horizontally from the sprinkler.
• Isolated obstructions 2 feet (600 mm) or less in width and located a minimum of 1 foot (300 mm) horizontally from the sprinkler.
• Bottom chords of bar joists or open trusses located 1 foot or greater horizontally from sprinkler where the bottom chord is not wider than 1 foot. Upright ESFR sprinklers can be installed over the bottom chords of bar joists or open trusses that are up to 4 inches wide.
In our real-world experience, the most significant obstructions are created by building features such as joist bridging, conduit, ductwork and lights. On some occasions, wayfinding signage and other features installed by the building occupant develop obstructions.
The Fire Protection Research Foundation’s (FPRF) ESFR obstruction testing has shown that obstructions as small as 1 1/2 inches, located directly below and within 8 inches of the sprinkler, resulted in significant water spray disruption. The FPRF’s testing also showed that moving the obstruction horizontally as few as 2 1/2 inches greatly improved the ESFR’s spray distribution.
While the FPRF’s research is not complete and the findings have yet to result in changes to NFPA 13, careful integration of a building’s ESFR sprinkler arrangement with other trades and the building occupant’s equipment is vital to the ultimate success of the life safety system.
Certain building features can challenge fire protection engineers when designing an ESFR sprinkler system.
• High-volume, low-speed (HVLS) fans. Many storage occupancies using ESFR sprinklers also use HVLS fans as an energy-efficient alternative to air cooling systems for personnel comfort. HVLS fans are large fans greater than 6 feet (2.1 m) in diameter that move massive quantities of air at 3 to 5 mph (4.8 to 8.1 kph).
Because this airflow can disrupt a heat plume needed to activate sprinklers, NFPA 13 Chapter 19 specifies the location of HVLS fans as approximately centered between 4 adjacent sprinklers and a minimum of 3 feet below the sprinkler deflector. This clearance applies to both ESFR and standard-spray sprinklers.
• Storage commodities. Building owners and occupants are maximizing their investments by storing products as high as physically possible, often up to the bottom chord of the building joists. In these facilities, the storage commodities can obstruct water spray pattern development. Unless an ESFR sprinkler is specifically listed and approved otherwise, the minimum distance between the sprinkler deflector and the top of the storage shall be 36 inches (900 mm).
Note that because ESFR sprinklers can be installed as much as 18 inches below a ceiling depending on the orientation and K-factor, the commodities could be limited to a storage height of 4 1/2 feet (1.4 m) below the ceiling. This ample clearance is a limitation that the sprinkler designer should communicate to the building occupant early in the design process.
• Mezzanines and walkway floors. Standard features in warehouse storage are walkways, catwalks and mezzanines installed for maintenance or employee access to racking and equipment. These walkways are typically constructed of steel mesh or perforated grating, not solid floors, so they are not always an obvious obstruction to ESFR discharge. NFPA 13 Section 22.214.171.124.1.1 requires protection below open-grate flooring more than 4 feet (1.2 m) wide.
Note that there is not a separate requirement to provide ESFR sprinklers below a narrower width unless the continuous obstruction requirements cannot be met. However, an intermediate-level/rack storage water shield is required by NFPA 13 for ESFR sprinklers below open grating to block water spray from the overhead sprinklers.
• Shelving. ESFR sprinklers are commonly installed to protect warehouse occupancies that use racks with shelving to store and display products. Shelving materials vary from wooden slats to wire mesh to solid plywood. If the shelf material is larger than 20 ft2 (1.86 m2) and is less than 50 percent open, the shelf is considered a solid shelf rack. This is also true if the product is stored in a manner on the shelf that covers more than 20 ft2 (1.86 m2).
For both situations, NFPA 13 does not permit ESFR sprinklers to protect storage on solid-shelf racks unless in-rack sprinklers are provided.
• Conveyors. Another common feature in warehouses is conveyors, material-handling components that transport commodities from one location to another. It is common for conveyors to be added or modified after completing construction of a warehouse protected by ESFR sprinklers.
Like walkway protection requirements, NFPA 13 considers a conveyor wider than 4 feet (1.2 m) to be an obstruction and sprinklers are required below. For this reason, ESFR sprinklers are better suited for storage occupancies than manufacturing facilities.
• Heat-producing equipment. When locating sprinklers near heat-producing equipment such as unit heaters, diffusers and high-temperature piping, ESFR sprinklers are subject to the same restrictions as standard-spray sprinklers. While not an obstruction to pattern development, heat-producing equipment exposes sprinkler designers to unique challenges that can result in unsatisfactory sprinkler system performance.
NFPA 13 Chapter 9 details the clearance requirements, and these can be very challenging to meet, given the strict spacing requirements for ESFR sprinklers.
For example, consider a conventional unit heater installed near the roof of a warehouse. From NFPA 13 section 126.96.36.199, a sprinkler installed within 7 feet (2.1 m) of a unit heater must use a high-temperature fusible element. Additionally, depending on the direction of airflow from the unit heater, sprinklers located between 7 and 20 feet (2.1 m to 6.1 m) from the heater shall use intermediate-temperature elements.
If the unit heater location is changed from the originally specified location, as many as four sprinklers may have the incorrect fusible element temperature installed.
A wide variety of building features and occupant-provided equipment can create obstructions that will degrade an ESFR sprinkler’s ability to suppress a fire. An adequate ESFR sprinkler design is possible only through careful coordination between the building’s architect, building system engineers, building occupant and the sprinkler designer. Also necessary is a thorough knowledge of NFPA 13 and the limitations of the ESFR sprinkler specified for installation.