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Hydraulic Calculations for Fire Fighting System

We designing a fire protection system, hydraulic calculations are very important to ensure that the flow rate in piping network will be enough to control fires effectively. In NFPA, the calculation procedures are established and verified by the three basic elements of firefighting system:

  1. Water delivery requirements of fire sprinkler system

  2. Available water supply

  3. The piping network system and its related friction losses.

Occupancy Classification

Any hydraulic design of sprinkler systems must begin with an analysis of the commodity being protected. The National Fire Protection Association, Standard No. 13 “Installation of Sprinkler Systems” (NFPA-13) defines three basic commodity classification: Light Hazard, Ordinary Hazard and Extra Hazard. These classifications are defined in NFPA-13 and examples are given in that document. Ordinary Hazard occupancies are divided into two categories: Ordinary Hazard Group 1 and Ordinary Hazard Group 2. Extra Hazard occupancies are also divided into two categories: Extra Hazard Group 1 and Extra Hazard Group 2. Once a hazard classification is chosen for a known occupancy, the water requirements may be established from NFPA-13, (2007) Figure 11.2.3.1.1

Density/ Area Curve for water requirements

NFPA-13, (2007) Figure 11.2.3.1.1 displays five density/area curves overlain on a graph. These five curves correspond to the previously discussed hazard classifications: Light Hazard, Ordinary Group 1 Hazard, Ordinary Group 2 Hazard, Extra Hazard Group 1 and Extra Hazard Group 2. These curves stipulate the required minimum densities and remote areas that establish minimum water requirements for sprinkler systems. 

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Densities and Remote Areas

These curves stipulate the required minimum densities and remote areas that establish minimum water requirements for sprinkler systems.

Density - The amount of water that must be delivered every minute for every square foot of floor space. The English units for density is (gallons per minute per square foot (gpm/sq.ft.) For example, a Light Hazard occupancy could be designed for a density of 0.1-gpm/sq.ft. This means that 0.1-gallons must discharge every minute for every square foot of floor space over a specified area.

Remote Area – the minimum area of floor space over which the density must discharge.

As an example, Figure 11.2.3.1.1 indicates that an acceptable design for an Ordinary Hazard Group 1 occupancy is 0.15-gpm/sq.ft. over 1500-sq.ft.

This means the sprinkler system designer will choose the most hydraulically demanding 1500-sq.ft. of the sprinkler system layout and perform a calculation of all sprinklers within that 1500-sq.ft. area. Therefore, only a portion of sprinklers on a sprinkler system are expected to operate during a typical fire, not all. If all sprinklers on a sprinkler system operate the water supply most likely will not be adequate to control the fire.

Calculating number of sprinklers in Remote Area

Total number of sprinklers = Remote Area (Design Area) / Area covered by one sprinkler.

For example, if Remote area = 1500 sq-ft and area covered by one sprinkler = 130 sq -ft

 

Total number odd sprinklers = 1500/130 = 11.538.

Therefore, we shall consider 12 Nos. of sprinklers which the water tank must provide for sprinklers to operate in case of fire.

End sprinkler start conditions

Once the hydraulic requirements are selected for the occupancy, the hydraulic calculation of the sprinkler system may begin. In modern times sprinkler system hydraulics are performed by computer programs. However, in the early days of sprinkler system hydraulic design all systems were calculated with a calculator as we will do here. A thorough understanding of these principles is not required to perform calculations on a hydraulic calculation program but these are the principles on which the program operates. A better understanding of these principles will allow the designer to better understand how the sprinkler system will function hydraulically. The density/area curves will tell the designer the starting point of the design. Our example is an Ordinary Hazard Group 1 occupancy and we will choose the point on the density/area curve of 0.15- gpm/sq.ft. over 1500-sq.ft.

This means that every square foot of the 1500-sq.ft. remote area must be covered with at least 0.15- gpm. By extension, this means that the floor area under each sprinkler, called the Protection Area of Coverage, must be provided with 0.15-gpm/sq.ft. In other words, each sprinkler must discharge enough water to provide a minimum of 0.15-gallons per minute for every square foot it protects.

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The end sprinkler must discharge 19.5-gallons every minute over the 130-sq.ft. it protects in order to meet the minimum density requirement of NFPA-13. We might start the calculation process at this point, however there is one other item that must be verified. NFPA-13, (2007) section 22.4.4.10.1 states that no sprinkler may operate at less than 7-psi. This means that we must verify that our end sprinkler satisfies this condition. For our example, we have chosen a sprinkler with a k-factor of 8.0.

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Formula used in Hydraulic Calculation process

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Simple Example for Manual Calculation

Consider the following remote area. 

Hazard Classification : Light Hazard

This is three story building (Ground Floor +1st Floor + 2nd Floor + Roof). Fire water tank and Fire Pump set is installed on roof. Therefore, we will select remote area on second floor since it will have less pressure.

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Remote Area

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Calculation Summary in Remote Area_Page_

Calculation Summary 

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Sample Calculation 2

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