Determination of Protection Area of Coverage

Determination of Protection Area of Coverage

Predicting the precise sprinkler coverage pattern is difficult because it depends on fire conditions and water discharge pressure.

Although it seems the actual coverage area is almost like a circle at lower discharge pressure and an elliptical shape at higher discharge pressure, NFPA 13 requires that the coverage area be assumed as a rectangle.

This requirement increases the flexibility for locating the sprinkler because the designer is not limited to considering the fixed spacing between the sprinklers along the branch line,

however, in some cases, finding the area coverage of sprinklers can be complicated.
“S × L” is the method for determining the area coverage of standard spray and Residential sprinklers. (9.5.2.1 and 19.3.1.3 – NFPA13,2022)
In accordance with NFPA 13,

The protection area of coverage per sprinkler (As) shall be determined as follows:

(1) Along branch lines as follows:
(a) Determine distance between sprinklers (or to wall or obstruction in the case of the end sprinkler on the branch line) upstream and downstream
(b) Choose the larger of either twice the distance to the wall or the distance to the next sprinkler
(c) Define dimension as S
(2) Between branch lines as follows:
(a) Determine perpendicular distance to the sprinkler on the adjacent branch line (or to a wall or obstruction in the case of the last branch line) on each side of the branch line on which the subject sprinkler is positioned
(b) Choose the larger of either twice the distance to the wall or obstruction or the distance to the next sprinkler
(c) Define dimension as L

Because this method relies on piping routes, it is difficult to find “S” and “L” where the piping is not in regular shapes or sprinkler arrangement is non-uniform (Figure 1).

This paper recommends determining “S” and “L” based on the Cartesian coordinate rather than on the piping routes.

So, we define “S” and “L” as follows:

S: Along horizontal direction:

(a)  Determine distance between sprinklers in horizontal direction (or to wall or obstruction in the case of the end sprinkler on the branch line) upstream and downstream

(b) Choose the larger of either twice the distance to the wall or the distance to the next sprinkler

L: Along vertical direction:

(a) Determine distance between sprinklers in vertical direction (or to wall or obstruction in the case of the end sprinkler on the branch line) upstream and downstream

(b) Choose the larger of either twice the distance to the wall or obstruction or the distance to the next sprinkler

Potential Coverage Area of sprinkler:

For determining a sprinkler’s area coverage,

we must first identify which adjacent sprinkler or wall should be examined to determine the “S” and “L”. To find the adjacent sprinklers and walls,

we should define a new concept as “Potential Coverage Area” or “PCA”. The PCA represents area that one sprinkler can cover.

While drawing the PCA of sprinklers, we should consider the sprinklers whose PCAs intersect for determination of “S” and “L”.

It should be noted, the PCA differs from the “Maximum coverage area” that is defined in NFPA 13,

however, where the maximum spacing is equal to the square root of the maximum coverage area, both definitions will be the same.

(For example, In Light hazard, the maximum spacing is 15 ft and the maximum area of coverage is 225 ft²).

The PCA depends on the classification of the hazard, the maximum area coverage, and the maximum spacing. (In Annex A, PCA of all types of sprinklers are shown)

The PCA is also used to check the location of sprinkler.

In general, we should consider the following terms while placing sprinklers:

• PCA of sprinklers covers the entire floor area.
• The coverage area of sprinklers doesn’t exceed the maximum permitted coverage.

As we’ll see in the following sections, for Upright/Pendent standard spray sprinklers in light hazard occupancies if the first condition is satisfied, we can ignore the second term, but in Ordinary and extra hazard occupancies, both terms must be met at the same time.

For more info : Determination of Protection Area of Coverage-EN-Rev01