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NOTE: This is an unformatted excerpt from our online fire protection training library.

Hydraulic Calculations

Hydraulic Calculations

Hydraulic Calculation Example

Let's assume that the building occupancy requires a discharge density of 0.20 gpm/sq. ft. for a most hydraulically remote area of application of 3,000 sq. ft.   (0.20/3,000).  Sprinklers are ten (10) feet apart on the branch lines, and the branch lines are twelve (12) feet from each other.  Doing some simple math tells us that each sprinkler has to be able to put its discharge into an area of 120 sq. ft. (10' between sprinklers, 12' between lines). 

In the 3,000 sq. ft. area, there will be 25 sprinklers (3,000 sq. ft./120 sq. ft./sprinkler).  If the first sprinkler is responsible for 120 sq. ft., and must discharge at least 0.20 gpm/sq. ft., then the flow from the first sprinkler is 0.20 gpm/sq. ft. x 120 sq. ft = 24 gpm. 

Knowing that the first sprinkler requires 24 gpm, the pressure can then be calculated by using the formula Q = k (p1/2 ).  The designer now has to specify the sprinkler that will be used to complete the calculation.  In this case, let's assume 1/2-inch orificeThe nominal k-factor for a 1/2-inch sprinkler is 5.6., standard spray sprinklers will be installed throughout the structure.  The pressure for the first sprinkler is calculated as p = (24/5.6)2 , or about 18.4 psi.

So, our first sprinkler requires a minimum flow of 24 gpm, delivered at the sprinkler at a pressure of at least 18.4 psi to meet the discharge requirements.  Each successive sprinkler in the remote area is calculated on the same basis, but there will be additional pressure loss due to the water moving through the piping from sprinkler to sprinkler, throughout the entire most remote area.

Once the sprinklers in the remote area are all accounted for, piping losses continue, as that volume of water has to flow from the source...so pressure loss will occur all the way back to the source of supply.

NOTE: This is an unformatted excerpt from our online fire protection training library.

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