From:
"Bob Pleski" <[email protected]>
To:
"Bob Pleski" <[email protected]>
Subject: Engineering Analysis - Sprinkler System
Date:
ENGINEERING
ANALYSIS
1. Fast shutoff of the sprinkler heads is the
primary cause of breaks in the laterals
a.
F = M x A, or Force = Mass x
Acceleration
Acceleration = feet per second per
second, or F =
M x Feet divided by second squared
Mass is the weight of the water in
the pipes. Sixty feet of 1-1/2" has about 50
lbs of water.
Velocity of the water when flowing
at 27 gpm in a 1-1/2" pipe is about 4.2 feet per
second.
When this amount of water is
stopped, it pulls the pipe (force) in the direction the water is going.
This Force wants to straighten out the "T" or elbow near the
main line and this can cause breaks on
the inside radii of the fitting. This is
where many of the breaks have occurred.
b. Heads from Toro or Hunter shut off in
1-2 seconds. The Rainbird heads are much faster in
the turn-off. A head that turns off in
1/10th of a second will cause a pulling force 100 times the force of a head
that turns off in one second.
2. High line pressure contributes to the breaks.
Some of the mains are 4" pvc schedule 40 pipe where the maximum recommended
operating pressure is133 psi and the pressure is
routinely going to about 135 psi out of the Pump
Station. This is not good to be
routinely stressing the pipe beyond its recommended pressure rating. Good design practice should only stress the
pipe to about 75% of its rated pressure.
We recently had a break in a 4" main at the 14th tee.
3. Pressure peaks due to water hammer caused by
fast shutoff valves add 100-150% on top of the regular line pressure. (Per NID chief engineer). These pressure transients travel through the
water at about 4,800 ft per second and stress all parts near the source of the
transient.
4. If it is desired to measure this peak
pressure, a Dickson PR-320/325 pressure data logger, or equivalent, with a
one-second sampling rate would be needed.
The purchase price is $526, but we may be able to borrow one of these
pressure gauges.