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Hydraulically most demanding area

Er. Parbhakar Dwivedi


The concepts to determine amount of water each sprinkler will need to discharge, the pressure necessary to maintain this flow, the number of sprinklers needed, etc., were explained. Hydraulic calculations were done to determine flow, pressure demand at the most hydraulically demanding single point in the system, which is usually the most distant sprinkler from the water supply.

Now, we must determine what will happen when the water flow splits in two directions so that pressure and flow calculated at a single point may be sufficient to feed two branches. Under such cases, we must adjust the flow of the line demanding less pressure equivalent to high pressure. This will result in greater water flow in this line because greater the amount of pressure, greater the amount of water flow when all other variables remain unchanged. Consider wherein at the point A, water flow splits in two branches, one feeding two sprinkler and another

Branch line water flow adjustment

Feeding one sprinkler. Let's say, hydraulic calculations for first branch at point A indicate a demand of 140 LPMM at 1.033 bar similarly, for branch line with sprinkler 3 operating indicates a demand of 68 LPM at 0.8 bar. At point A, only one pressure can exist, we need to provide 1.033 bar pressure at A so that proper discharge can be maintained from the first line. But this will create an overflow in the second line. Let's calculate by determining the K-factor at point A.

Q = K√p , 68 = K√0.8 , K = 76.

This value of K-factor is now used to determine the flow in second branch line while pressure now becomes 1.033 bar.

Q = K√p = 76 √1.033 = 77.24 LPM.

The total flow demand at A.

Qᴀ = 140 + 77.24 = 217.24 LPM.

Illustrates how to calculate the most demanding area or AMAO.

Let's assume a design area of 135 m2 with sprinkle coverage of 10.8 m²

Number of sprinklers

= 135/10.8 = 12.5. Rounded to = 13 sprinklers.

Number of sprinklers on branch line


Where, A = Design area,

S = Distance between sprinklers on branch line.

Number of sprinklers on branch line

= 1.2√135/3.6 = 3.87. Rounded to = 4 sprinklers.

As shown in the Fig. 17.24, the extra sprinkler after adjusting 4 sprinklers in three branch lines is adjusted in the fourth line from B to E at designer's option.

For looped system, this extra sprinkler shall be placed closest to cross mains because it is closest to the supply and would be hydraulically most demanding on the branch line.

Where the design is based on area/density method, the design area shall be rectangular having a dimension parallel to the branch lines at least 1.2 times the square root of the area of sprinkler operation used, which shall permit the inclusion of sprinklers on both sides of the cross main. Any fractional sprinkler shall be carried to the next higher whole sprinkler.

Some exceptions, where design area may not be rectangular (A in this figure). System B and C shows location of extra sprinkler on the fourth branch line.

The design area for looped systems with various riser location and branch line configuration.

In systems having branch lines with an insufficient number of sprinklers to fulfill the 1.2√A requirement, the design area shall be extended to include sprinklers on adjacent branch lines supplied by same cross mains.


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