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Last month’s flow test was conducted as part of the fire protection engineer’s sprinkler design.
Scenario 1: The flow test performed indicated the available water supply was approximately 1620 gpm at a residual pressure 20 psi with a static pressure of 80 psi.
Our sprinkler system design criteria are based on a NFPA 13 occupancy hazard classification of Ordinary Hazard Group 2. The design density is 0.20 gpm/sf over a design area of 1,500 square feet with a hose stream allowance of 250 gpm. The hydraulic calculation indicated a sprinkler demand of 360 gpm at a residual pressure of 50 psi at the base of the sprinkler riser, Point 1 on Figure 1. Adding the 250 gpm hose stream allowance to the sprinkler flow at the same pressure results in a total demand of 610 gpm at 50 psi. This is shown as Point 2 on Figure 1. Is the water supply adequate to support the sprinkler demand? Before we make that evaluation, the sprinkler calculation must be completed.
The sprinkler demand has been calculated to the base of the sprinkler riser, and the fire hydrant flow test represents the water supply at the pressure hydrant. More precisely, the actual location of the water supply is the point on the water main that is flowing water to where the pressure hydrant connects. In a sense, the piping between the pressure hydrant and this point acts as a sensing line. The sprinkler demand calculation is completed by adding the additional pressure loss needed to bring the sprinkler demand out to the water supply data location. Alternatively, one can make the adjustment to the water supply data.
The base of the sprinkler riser is connected to this flow point by an unground sprinkler feed main consisting of 110 feet (equivalent length) of six-inch diameter cement-lined ductile iron pipe with a C = 140. There is also a 6-inch double check valve backflow assembly. We will assume the base of the riser is at the same elevation as the water supply. We will also continue apply the 250 gpm hose allowance to the base of the riser. The Hazen-Williams equation yields a pressure loss of 1.0 psi. The backflow assembly contributes another 5.0 psi of pressure loss.
p = (4.52 Q1.85)/(C1.85d4.87) where:
p = frictional resistance (psi/ft of pipe)
Q = flow (gpm)
C = friction loss coefficient
d = actual internal diameter of pipe (in.),
Curve A, in Figure 1, is adjusted by striking a new Curve B. The first point will be the original static pressure point of 80 psi. The second point will be determined by subtracting the pressure drop of 6 psi from the residual pressure of Curve A at the demand flow of 610 gpm, 71 psi minus 6 psi equals 65 psi, this new point being 610 gpm at 65 psi. The new Curve B can then evaluate the adequacy of the water supply to support the total sprinkler demand. We see that the Total Demand Point 2 is clearly below Curve B with a cushion of 15 psi. Therefore, the water supply is adequate.
Here is a note on the use of a cushion. The normal practice in our office is to require a 10 percent cushion, i.e., the resulting sprinkler demand pressure must be less than or equal to 90 percent of the available residual pressure at the demand flow. NFPA 13 does not contain a requirement to use a cushion. However, paragraph A.24.2.2.2 of NFPA 13 (2013) recommends, “An adjustment to the waterflow test data to account for daily and seasonal fluctuations, possible interruption by flood or ice conditions, large simultaneous industrial use, future demand on the water supply system, or any other condition that could affect the water supply should be made as appropriate.”
Scenario 2: The project is the design and construction of a new warehouse building to be used for rack storage of Class IV commodities to a height of 30 feet. The ceiling height is 40 feet. Our water supply data for this project is 3,200 gpm at a residual pressure 20 psi with a static pressure of 50 psi. This is represented by Curve A in Figure 2.
ESFR sprinklers will be used. From NFPA 13 (2013) Table 16.3.3.1 ESFR Sprinkler Protection of Rack Storage Without Solid Shelves of Class I Through Class IV Commodities Stored Over 25 ft (7.6 m) in Height. The design will use k = 25.2 ESFR sprinkler with a minimum nozzle pressure of 25 psi the design area will contain 12 operating sprinklers. Our hydraulic calculation yields a sprinkler system demand of 1,800 gpm at 70 psi. The hose stream allowance, which is shown in Table 12.8.6.1 Hose Stream Allowance and Water Supply Duration of NFPA 13, is 250 gpm.
As shown in Figure 2, both Point 1 ESFR Sprinkler Demand (1,800 gpm at 70 psi) and Point 2 Total Demand of ESFR Sprinklers and Hose Stream Allowance (2,050 gpm at 70 psi) exceed the available public water supply represented by Curve A.
As is often the case with ESFR sprinklers, we will need to improve the water supply by incorporating a booster fire pump system. Fire pump capacity should be such that the required flow does not exceed 140 percent rated capacity. This 140 percent limit is based on guidance contained in paragraph A-4.8 of NFPA 20 (2013). However, if your system must comply with Factory Mutual criteria or UFC 3-600-01 (DOD projects) the 140 percent limit is a requirement.
We select a fire pump with a rated capacity (flow) of 1500 gpm and a rated pressure of 60 psi. For those of you plumbing engineers that do not do fire protection, the rated flow and pressure defines the three points of the fire pump curve. These three points of the curve being:
Rated Flow and Pressure: 1,500 gpm at 60 psi.
Shutoff head: Max 140 percent of rated pressure (usually for horizontal centrifugal pumps the shut off head is approximately 110 to 120 percent of rated pressure)
Overload Capacity: Minimum 150 percent of rated flow at minimum of 65 percent of rated pressure.
Curve B depicts the fire pump curve with Shutoff pressure of 72 psi, rated flow and pressure 1,500 gpm at 60 psi, and overload at 2,250 gpm at 39 psi.
We combine Curve A and Curve B to determine the water supply curve combined with the booster fire pump system, which is indicated as Curve C. We add Curves B to A by adding the pressures of each of the three fire pump curve points at their given flows. Curve C indicates that at the total demand flow of 2,050 gpm the available pressure is 80 psi can be supported with the addition of booster fire pump, providing a “cushion” of 10 psi over the required 70 psi demand pressure. In a future column, we will evaluate scenarios involving conditions where there are multiple water supply sources.
Figures 1 and 2 have a line called the Demand Curve. This curve sort of tells us the pressure required as sprinklers in the design area begin to operate. Ken Isman, P.E. indicates in “Layout, Detail and Calculation of Fire Sprinkler Systems,” that the demand curve is not required, it has simply been customary to it provide it, and it helps to keep the demand point from getting lost on the graph. The demand curve connects the sprinkler demand point (Point 1) to a point on the y-axis at zero flow. This point is normally the elevation head of the highest sprinkler.
For example, in Figure 2 the sprinklers in our 40-foot-high warehouse are about 39 feet above the floor so this point is at approximately 17 psi. This represents the static pressure of the demand curve. By continuing this line through Point 1 and to the water supply curve, we have an estimate of the actual flow that will occur if the design area fully flows. This gives us a value similar to one obtained when a supply calculation is performed.
Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer with bachelor’s and master’s degrees from the University of Maryland Department of Fire Protection Engineering. He is past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates Inc., a 15-person fire protection engineering firm with offices in Honolulu and Guam. He can be reached via email at SDannaway@ssdafire.com.