©Copyright Task Force Tips LLC 2020
LIM-032 February 18, 2020 Rev00
9
6.3 DETERMINING FLOW WITH PRE-PIPED MONITORS
The simplest procedure to determine
fl
ow with automatic nozzles is with a
fl
ow meter. If a
fl
ow meter is unavailable, the
fl
ow may be
estimated using pressure loss data between the nozzle and an in-line pressure gauge at the pump or considerably upstream from
the nozzle. Data is taken with a smooth bore nozzle and handheld pitot gauge. Note: Equations assume no substantial change in
elevation between in-line pressure gauge and nozzle.
Step1: Determine
fl
ow of smooth bore nozzle.
Flow water with a smooth bore nozzle and record the nozzle size, pitot pressure, and in-line pressure gauge reading. The smooth
bore nozzle’s
fl
ow is calculated from the Freeman formula:
Where: F = 29.71 for English units (gpm, inches, psi)
F = .667 for metric units (l/min, mm, bar) Note: 1 bar = 100 kPa
Q
smooth
fl
ow in gpm (or l/min)
D
exit diameter in inches (or mm)
P
pitot
pitot pressure in psi (or bar)
Q
= F x D P
smooth
pitot
2
Step 2: Find pressure loss constant.
Using the results from step 1, use the following equation to calculate the pressure loss constant between the in-line pressure gauge
and the nozzle:
Where: C piping pressure loss constant in gpm2/psi (or l/min2/bar)
P
inline
in-line pressure gauge reading in psi (or bar)
Q
2
smooth
P
- P
in-line
pitot
C =
Step 3: Calculate
fl
ow with automatic nozzle.
Using the pressure loss constant from step 2 and the following equation, the
fl
ow with an automatic nozzle can be calculated for your
particular installation.
Where: Q
auto
automatic nozzle
fl
ow in gpm (or l/min)
P
auto
nominal nozzle operating pressure in psi (or bar)
Q = (P
- P )C
auto
in-line
auto
Mount a graph or table of the results adjacent to the in-line pressure gauge. Deliver any desired
fl
ow by adjustment of pump pressure.