T H E R E P U B L I C O F U G A N D A - M I N I S T R Y O F WAT E R A N D E N V I R O N M E N T
This shows were
a cleanout or wash
out valve should be
located. See next
page for description
where.
Figure 7‑4: Siting of an Air Valve.
7.6 Washouts
7.6.1 General
Washouts are installed on pipelines to drain the pipe section especially during cleaning out of sediments in
the pipe. They are provided at low points or valleys in the pipeline and should be kept to a minimum.
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WA T E R S U P P L Y D E S I G N M A N U A L S E C O N D E D I T I O N
T H E R E P U B L I C O F U G A N D A - M I N I S T R Y O F WAT E R A N D E N V I R O N M E N T
7.6.2 Location
Washouts should be placed only at “pronounced” low points or valleys on a pipeline. In this context, a
low point is considered to be “pronounced” if the high points immediately upstream or downstream of
that point stand more than 10 m higher than that point.
7.6.3 Washout Sizes
In a major pipeline, primary washouts may be installed to drain the majority of the length between
section valves; secondary washouts of smaller diameter can then be used to empty un-drained low
points. Sizes, particularly of primary washouts should be calculated according to the required drain
down time, which should typically not be longer than one working shift. Factors for consideration are
the number of washouts, head available and limits on discharge, access and resources. The drain down
time is dominated by the low head available during the later stages. The washout diameters given below
should allow the last 200 m length of a pipeline to be emptied in about one hour in typical situations.
Table 7‑6 Washout Diameters
Size of the main
Size of the Wash out diameter
Up to 300 mm
80 mm
400 to 600 mm
100 mm
700 to 1000 mm
150 mm
1100 to 1400 mm
200 mm
1500 to 1800 mm
250 mm
If the normal shear stress of 10N/m2 on the walls of a main pipeline, and the available pressure of 0.1 to
0.2 MPa are assumed, then the required sizes of washouts should be determined as follows.
d = 0.6 D (If the upstream and downstream sides of the pipeline are washed simultaneously.)
d = 0.4 D (If only one side of the pipeline is washed at a time)
Where:
d = diameter of the washout pipe in mm
D = diameter of the main pipeline in mm.
7.6.4 Washout Valves and Drains
Washout valves should be installed only on washout outlet pipes and not on the main pipelines. Open
drains conveying water from the washouts away to suitable outfalls should be provided.
7.7 Firefighting
7.7.1
General
In fighting fires the flow rates from fire hydrants depend not on the water system design constraints,
but on the type of equipment and the number of people involved in fighting the fire (DOH, 2009). As
water system pressure decreases, the pump in the fire truck eventually begins to cavitate and is unable
to deliver any substantial flow rate. At that point, the pump turns off. Design engineers should evaluate
the potential that firefighting equipment may cause very low water system pressure. These low pressures
may present a public health concern due to an increased risk for contamination from cross-connections
and pathogen intrusion. Guidelines concerning provisions for fire-fighting have been presented in
Chapter 2 – “Water Demand”.
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