Environmental Engineering Reference
In-Depth Information
Fire hydrants are usually used for flushing. In some cases, a
blowoff connection is used if one has been installed. A velocity of at
least 2.5 ft/sec (0.8 m/sec) is needed to remove dirt or other debris
from the pipe. A low velocity flush (1 ft/sec) is adequate to “rinse”
a clean pipe or to refresh the water within a main. Use Table 3-1
to determine the flushing flow rate needed to achieve 2.5 ft/sec or
1 ft/sec velocity within pipelines of various diameters.
Regardless of the velocity, flush long enough to result in two
or three complete changes of the water within the pipeline.
Table 3-2 is useful to determine the volume needed to achieve three
Approximate pipeline flushing rates
*
Table 3-1
Pipe Diameter
Flow for 1 ft/sec velocity
Flow for 2.5 ft/sec velocity
L/sec
†
in
.
mm
gpm
gpm
L/sec
2
50
1,1
10
0.6
11,
25
1.6
4
100
1,1
40
2.5
1,
100
6
.0
6
150
1,190
6.0
200
13.0
8
200
1,
150
10
.0
1,
400
25
.0
10
250
1,
250
16
.0
1,
600
38
.0
12
300
1,
350
22
.0
1,
900
57
.0
16
400
1,
625
39
.0
1,600
100
.0
18
450
1,
800
50
.0
2,000
126
.0
20
500
1,000
63
.0
2,500
158
.0
24
600
1,400
88
.0
3,500
220
.0
* Flushing flow rates are approximate because precise values are not needed for these
operations. Time to flush 100 ft (30.48 m) at 1 ft/sec is about 1.7 min and 40 sec at
2.5 ft/sec.
† The values in this table are calculated for US units and then converted to SI units.
The L/sec values are calculated by converting the approximate gpm values. The
resultant values, therefore, multiply the rounding errors.
Table
Tamer
Use this table to determine the flushing rates for either a
1 ft/sec or 2.5 ft/sec velocity. Example: A 6-in. (152-mm)
pipe needs to be flushed at about 200 gpm (13 L/sec) to
achieve a 2.5 ft/sec velocity.
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