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DEMAND OF WATER :
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)
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.
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
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
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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