NFPA Journal

BY BRIAN O'CONNOR

A topic that has appeared numerous times recently on the NFPA Today blog has to do with obstructions and early suppression fast response (ESFR) sprinklers—more specifically, how ESFR sprinklers cannot tolerate obstructions that might not affect, or minimally affect, other types of sprinklers. Because ESFR sprinklers are designed to suppress a fire versus controlling a fire’s growth, any blockage of the ESFR’s discharge may impact its ability to achieve rapid suppression.

ESFR sprinklers were developed in the 1980s and have been hailed as one of the greatest fire protection innovations. The current edition of NFPA 13, Standard for the Installation of Sprinkler Systems, defines ESFR sprinklers as “a type of fast response sprinkler … listed for its capability to provide fire suppression of specific high-challenge fire hazards.” They come with a plethora of benefits, as well as an array of incentives to use them—one of the most important benefits of ESFR technology is that they can be installed over higher piles of storage, often without having to use in-rack sprinklers, which is why ESFR sprinklers are predominately used in storage occupancies.

The amount of water required to suppress a fire increases as the fire grows, meaning if you can get water to the burning fuel early in the growth phase, suppression can be achieved with less water. ESFR sprinklers address the need to suppress a fire quickly; they have a quick response time and a discharge pattern that creates droplets that can penetrate the fire plume. They accomplish this with an increased sprinkler sensitivity and a special distribution pattern. A sprinkler’s response time is a function of thermal sensitivity of the operating element, its temperature rating, and its distance relative to the fire. The earlier the sprinkler activates, the smaller the fire. The smaller the fire, the more likely water will reach the burning fuel since the heat plume of a large fire could either evaporate or blow away incoming water droplets.

Because of how ESRF sprinklers work, however, many obstructions that can be tolerated for other types of sprinklers are unacceptable where ESFR sprinklers are used. For this reason, ESFR sprinklers include unique obstruction requirements, which are located in Chapter 14 of the 2019 edition of NFPA 13. A common obstruction for ESFR sprinklers in warehouses, for example, are high volume low speed fans; NFPA 13 has several requirements addressing these obstructions, including maximum diameter of fan blades, sprinkler positioning in relation to the fans, and automatically shutdown of the fans upon sprinkler activation.

The obstruction rules in NFPA 13 are broken down into three categories: obstructions at the ceiling, noncontinuous obstructions below sprinklers, and continuous obstructions below sprinklers. NFPA 13 has specific definitions located in Chapter 3 for continuous and non-continuous obstructions that explain how a continuous obstruction affects multiple sprinklers, while a non-continuous obstruction affects only one.

There are a few rules that can be applied to obstructions in any one of these categories. One such rule comes in the form of a table and a figure in NFPA 13 (14.2.11.1.1) that requires obstructions to be a certain distance away from the sprinkler based on how far the sprinkler deflector is located above the bottom of the obstruction. There are additional rules that allow obstructions of a certain width and location to be permitted, as well as rules regarding when you can install sprinklers beneath an obstruction.

The clearance between the sprinkler deflector and storage it is protecting is required to be at least 36 inches (900mm). If the storage is any closer, it could be considered an obstruction, since the adjacent sprinklers wouldn’t be able to provide sufficient overlapping of their discharge patterns.

Overall, these provisions are more restrictive than those for other types of sprinklers, but they are necessary for successful performance of the ESFR sprinkler system. Even with the additional obstruction restrictions, the use of ESFR sprinklers is still a great option that many designers choose for the protection of storage occupancies. 

Brian O’Connor is a Fire Protection engineer at NFPA. NFPA members and AHJs can use the Technical Questions tab to post queries on NFPA 13 at nfpa.org/13.