Example of Elevation Pressure Effect on a K-factor
The following example illustrates the effect elevation pressure has on a K-factor calculated to
predict the pressure required or flow required for a typical branch of a system.
Calculate the single sprinkler at Point A
K-factor = 5.6
Density = .2 gpm/sq.ft.
Area = 100 sq.ft.
Use 25’-0” of 2” schedule 40 pipe with 2 elbows and 1 tee
C=120
A
This calculation requires a flow of 20gpm at a pressure of
19.467psi at Point B, the bottom of the 2” pipe.
Use these results to calculate the K-factor at Point B for this
branch.
Q=K √ P
20 = K √ 9.467
K = 4.53
Use this calculated K-factor to determine the pressure required
at Point B in order to flow 40gpm from the sprinkler head at Point
A.
Q=K √ P
40 = (4.53) √ P
P = 77.969 psi
Based on this K-factor a predicted pressure of 77.969 is required
at Point B in order to flow 40gpm from the sprinkler at Point A.
B
Calculate the single sprinkler at Point A. Change only the density.
K-factor = 5.6
Density = .4 gpm/sq.ft.
Area = 100 sq.ft.
Use 25’-0” of 2” schedule 40 pipe with 2 elbows and 1 tee
C=120
The calculation requires a flow of 40gpm at a pressure of 58.293psi at Point B.
This pressure is significantly different from the predicted pressure requirement of 77.969psi.
Use these new results to calculate the K-factor for this branch.
Q=K √
P
40 = K √
58.293
K = 5.24
The new calculated K-factor for this branch is 5.24. This is significantly different from the K-factor
of 4.53 originally calculated for this same branch.
If elevation had no effect on the calculated K-factor at Point B, the predicted pressure required to
flow 40gpm would have been equal to the calculated pressure. Also, the calculated K-factor at
Point B would have been the same for both flows.
This example shows how large elevation differences, which are common in a spray system, can
radically skew the calculated K-factor for a branch. Small elevation differences will have less of
an effect but will also skew the calculated K-factor for a particular branch. Although inaccurate
and hydraulically incorrect, this slight deviation due to elevation has generally been accepted for
sprinkler system calculations in the past.