DEMAND OF WATER :
COINCIDENT DRAFT
It is
extremely improbable that a fire may break out when water is being drawn by the
consumers at maximum hourly draft. Therefore for general community purposes,
the total draft is not taken as the sum of maximum hourly demand and fire
demand but is taken as the sum of MDD and fire demand ort the maximum hourly
demand whichever is more. The MDD when added to fire demand for working out TD
(Totaldraft) it is known as coincident draft.
COINCIDENT
DRAFT = MAXM DAILY DEMAND (MDD) + FIRE DEMAND (FD)
Problem
1.A water
supply screen has to be designed for a city having a population of 1,50,000
Estimate the important kinds of drafts which may be required to be recorded for
an Avg. consumption of 250Lpcd. Also record the required capacities of the
major components of the proposed water works system for the city using a river
as the source of supply. Assume suitable figures and data where needed.
Soln:
i.
Average daily demand
ADD =
(per capita Avg consumption in Lpcd) X population
=
37500000 Litres/day
=
37.5 106Litres/day
= 37.5
Million Litres/day
Or
ADD =
37.5 Mld
ii.
M aximum daily draft
May be
assumed as 180% of annual average daily draft.
MDD = 1.8 37.5
MDD =
67.5 Mld
iii.
Maximum hourly demand of maximum day
May be
assumed as 270% of annual average daily draft.
= 2.7 X Annual Avg. hourly demand
= 2.7 X q/24
MHD = 101.25Mld
iv.
Fire demand
Using
national board of fire under writer?s formula, when population is less than or
equal to 2 lakhs we have
Q =4637 rootOf(p) [ 1 -0.01
rootOf(p)]
=71763724.35
Litres / min
COINCIDENT DRAFT = MDD + Fire draft = 67.5 + 71.76
=
139.26Mld.
CAPACITY OF VARIOUS COMPONENTS
1. The
intake structure for fetching water from the stream may be designed for MDD i.e
for 67.5Mld.
2. The pipe
mains carrying the water from the intake to the treatment plant and then to
service reservoir. May be designed for MDD
Required
capacity = MDD (67.5 Mld)
3. The filters and other units at the treatment plant
may be designed for MDD plus some reserve (say twice the ADD)
Required
capacity = 2 x ADD
= 2 x 37.5
= 75 ld.
5. The distribution system including the
pipes carrying water from service reservoir to the distribution system may be
designed for coincident draft with fire or maximum hourly demand, whichever is
more.
The required capacity =
139.26 Mld.
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