Sprinkler System Demand Flows Greater than 500gpm?

By Terin Hopkins posted Jun 17, 2022 09:54 AM


To understand when a sprinkler system fire department connection (FDC) needs to be supplemented vs supplied, we must first discuss the widely used term, system demand. This term is not defined in any of the fire sprinkler standards, but National Fire Protection Association (NFPA) 13 Standard for the Installation of Sprinkler Systems does require a system to have a hydraulically calculated water demand flow rate, and this term is defined by NFPA 13. The waterflow rate for a system must be calculated using acceptable engineering practices and is commonly referenced as the system demand.    

3.3.109 Hydraulically Calculated Water Demand Flow Rate. 
The waterflow rate for a system or hose stream that has been calculated using accepted engineering practices  

  System Demand  

 System demand is calculated on an area of a building, based on the occupancy and/or commodities Stored within and does not include all areas of a building. If a fire advances beyond the fire protection system design area, it will eventually exceed the designed system flow or demand. This is a critical transition point when the fire department needs to convert from supplementing the system and will take over supplying the system beyond the hydraulically calculated water demand flow rate.    

 FDC Sizing  

While NFPA 13 requires fire sprinkler systems to have an FDC consisting of two 2 ½ in. inlets, it does not have any requirements for them to be sized for the hydraulically calculated water demand flow rate. The thought behind this is that the system water supply automatically provides the required system demand based on the design area and the FDC is only intended to supplement that required flow. The two 2 ½ in. inlets are more than capable of providing that supplemental flow. The question is what happens when the system design flow is exceeded, and the fire department moves from supplementing the system, to needing to supply the flow above the designed system demand?    
Transitioning from Supplementing to Suppling   

 Transitioning from Supplementing to Suppling 

 While this transition is more prevalent in many of the recent mid-rise buildings fires constructed to NFPA 13R Standard for the Installation of sprinklers in Low-Rise Residential Occupancies, with systems designed for life safety occupant egress, we have also seen large scale fires involving mammoth distribution or processing buildings. These fires can exceed the designed automatic water supply, requiring supplemental water through the FDC, and in some cases supplied to meet the additional flow.   

NFPA 14 Standard for the Installation of Standpipe and Hose Connection gives guidance on the sizing of fire department connections 2 ½ inlets in Section 7.12.3 and requires 250gpm for each inlet. This sizing has been in the standard for many years   

7.12.3 Fire department connection sizes shall be based on the standpipe system demand and shall include one 21∕ 2 in. (65 mm) inlet per every 250gpm.  

 Providing System Flow  

 This concern is not an issue with combined systems, as the standpipe flow requirement is typically greater than that of the sprinkler system and the FDC is sized accordingly so that the system demand can be supplied by the FDC.   

7.7.1 Class I and Class III standpipe systems shall be designed so that the system demand can be supplied by each fire department connection, which is provided in accordance with Section 7.12.  

The issue is when the sprinkler system design flow exceeds that of the maximum flow of the standpipe for combined systems or is greater than 500gpm in sprinkler only systems. In a building protected in accordance with NFPA 13 or NFPA 13R, the water supply for the combined sprinkler and automatic standpipe system shall be based on the sprinkler system demand (including any hose stream demand) or the standpipe demand, whichever is greater.  

Many of these large buildings have very robust automatic sprinkler systems often with calculated demand flows well above the capabilities of that two 2 ½ in inlets or 500gpm inlet standard. While the system is required and designed to provide an automatic water supply and an FDC to supplement the Hydraulically Calculated Water Demand Flow Rate, we now see the benefits of being able to fully supply additional flow rate from the FDC that exceeds the design flow. It might just be time to discuss FDC sizing for larger sprinkler systems with designed flows exceeding 500gpm.