Fire Sprinkler Design and Installation 

Sprinkler system design starts with water— everything else depends on having enough water pressure ready to control a fire. NFPA 13 requires an automatic water supply for sprinkler systems, meaning that the water will flow through sprinkler heads without any human intervention.

There are many possible sources that can be used, including city water, ponds, rivers, reservoirs, water tanks, pressure tanks, and gravity tanks or water towers. But in most cases, a municipal waterworks is the go-to supply.

Whatever the source, it must have sufficient capacity for fire control. The factors that determine capacity include flow rate (in gallons per minute, GPM), pressure (in pounds per square inch, PSI), and duration (how long it can maintain the required pressure and flow). For a municipal water supply, capacity is determined with a flow test performed at nearby fire hydrants.

A flow test requires at least two hydrants, A and B. First, a static pressure reading is taken at hydrant A while neither hydrant is flowing water. Then, hydrant B is opened fully, and another pressure reading is taken at hydrant A. This residual pressure reading is the amount of pressure that can push water through sprinkler heads (minus some losses). A pitot tool is used to measure the pressure of the water flowing from hydrant B.

Fire Department Connections on Large Sprinkler Systems

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.

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.

The first step of sprinkler system design is knowing the water supply, the second is understanding the building. Sprinkler system designers review the architectural and engineering plans to determine just what the building needs from a sprinkler system. Is it a residential structure? Industrial? How significant is the fuel load? Will it have climate control?

Questions like these determine the requirements needed for the sprinkler system and what type of system is appropriate.

NFPA 13, NFPA 13R, or NFPA 13D—which standard to use?

As mentioned, NFPA 13 is the go-to standard for commercial sprinkler system design. NFPA 13-compliant systems are defined by full sprinkler coverage. This standard is typically used in commercial facilities—offices, mercantile spaces, warehouses, industrial buildings, etc.

Two additional alternatives to NFPA 13 exist:

1. NFPA 13R: Standard for the Installation of Sprinkler Systems in Low-Rise Residential Occupancies

2. NFPA 13D: Standard for the Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes.

NFPA 13R and NFPA 13D both focus on affordable and convenient life safety protection, so they don’t require sprinkler heads in unoccupied spaces like closets and attics. But in some larger residential settings, a full-coverage NFPA 13 system is still used.

In addition to determining the standard, designers also must consider whether a building needs a wet pipe system, dry pipe system, or pre-action sprinkler system.

Most buildings use a wet sprinkler system in which water always fills the pipes. As soon as a sprinkler head operates, water flows.

Structures like parking garages, where freezing is a concern, will need dry pipe systems, so named for the absence of water in the pipes. A dry valve, held shut by a pressurized gas, stops the water in an insulated section of the pipe until it’s needed. When a sprinkler head activates, the gas is released from the piping which allows the dry valve to open, and water flows from the activated sprinkler heads.