sprinklernotes
Calculating the Largest Water Demand “We have a project where the pipe is existing with just upright sprinklers; it’s a shell space. We’re fitting out the space and due to the ceiling and wall design, we are leaving the uprights and adding pendents throughout. The third-party reviewer is questioning why the calculation won’t be affected with an upright and a pendent coming off of the same outlet. We know that you only calculate the level of uprights or the level of pendents and not both. We looked through NFPA 13 and we couldn’t find anything specific that addresses this situation for calculations. Is there anything in the code that will help our argument?”
We have reviewed NFPA 13, Standard for the Installation of Sprinkler Systems, 2019 edition as the applicable standard. Our informal interpretation is to only calculate the area with the largest water demand. When there is a ceiling separating the upright and pendent sprinklers, it is considered two separate compartments. The driving philosophy of NFPA 13 (as stated in section 1.1.3) is that the sprinkler system is only required to address a single fire. This was assumed to be understood up until the 2010 edition when NFPA decided it needed to be actually stated. Since NFPA 13 only considers a single fire origin, it gives us the requirement in section 27.2.4.6.4 “where sprinklers are installed above and below a ceiling or in a case where more than two areas are supplied from a common set of branch lines, the branch lines and supplies shall be calculated to supply the largest water demand.” This doesn’t mean you combine the demand for the two compartments. It means you consider both compartments and apply the single compartment with the greatest demand. When we have sprinklers in a single compartment, such installation arrangements are used to address obstructions. This issue is addressed by sections 27.2.4.7.3, 27.2.4.7.3.1, and 27.2.4.7.3.2, which also states that only the largest water demand needs to be calculated though, in this case, it’s a comparison of the two individual sprinklers. This is further clarified that the sprinkler at the ceiling is the one that is calculated, as long as the pipe size to the sprinkler below the obstruction follows the sizing pattern of the branch line. The reason for this is that the sprinklers at a lower elevation will have more available pressure than that at the ceiling above (the gain by elevation pressure is greater than the friction loss).
