VARIATIONS IN DEMAND
There are wide variations in the use of water in different seasons, in different months of the year, in different days of the month and in different hours of the day.
Seasonal or monthly variation are prominent in tropical countries like India rate of consumption reaches maximum in summer season due to greater use of water for street and lawn sprinkling etc… It goes down in winter months. The fluctuation in the rate of consumption may be as much as 150% of the average annual consumption.
Avg. Demand on a maxm. Day
TIME IN HOURS.
Daily and hourly Fluctuations depend on the general habits of people, climatic conditions etc… more water demand will be on Sundays and holidays due to more comfortable working, etc…. as compared to other working days. Peak hours may be 6 a.m to 10a.m and 10a.m to 4.p.m minimum flow and between 10.p.m to 4.a.m it is very less. The above graph shows the hourly variation in demand of water or rate of consumption 20% of average hourly demand
ASSESSMENT OF NORMAL VARIATIONS
The maximum demands (monthly, daily or hourly) are generally expressed as ratios of their means. The following figures are generally adopted.
1. MAXIMUM DAILY CONSUMPTION is generally taken as 180% of the average, therefore
Maximum daily demand(MDD) = 1.8 Average daily demand (^DD)
= 1.8 q
2. MAXIMUM HOURLY CONSUMPTION is generally taken as 150% of its average hooray consumption of maximum day, there fore
Maximum hourly consumption of the maximum day or peak demand
= 1.5 (150%) x Average hourly consumption of the maxm. Day.(Litres/day)
= 1.5 [MDD / 24 ] (Litres/hr)
= 1.5 [1.8 q / 24 ] (Litres/hr)
= 2.7 / 24 (Litres/hr)
Maximum hourly consumption of the maximum day = (2.7 Annual Average hourly demand)
The formula given by GOODRICH is also used for finding out the rather of peak demand rates to their corresponding average values.
GOODRICH FORMULA P=180 t 0.10
Where, P = % of annual average draft for the time„t? in days
T = Time in days from 1/ 24 to 365
When t= 1 day (For daily variations)
P = 180 x 1 0.10
P = 180%
MDD/ADD = 180%
When t = 7 days (For weekly variations)
P = 180 x (7)-0.10
P = 148%
MWD/ AWD = 148%
T = 30 days (For monthly variations)
P = 180 (30)-0.10
MMD / AMD = 128%
Maxm monthly Demand / Avg .MonthlyDemand = 128%
1. The design population of a town is 15000 Determine the Average daily, Maximum hourly demand under suitable assumptions
Soln: Assuming Average percapita demand as 270 Lpcd
i. ADD = design population Avg. per capita demand
= 1500 x 270 = 4050000 Litres/day
ADD = 4050 m3/day
ii. MDD = 1`.8 x Average daily demand
= 1.8 x 4050
= 7290 m3/day
iii. Maximum hourly demand of maximum day
= 2.7 x q/24
= 2.7 x (4050/24)
= 455.625 m3/hr or
= 10935 m3/day
EFFECTS OF VARIATION
1. The sources of supplies such as wells etc… may be designed for MDD
2. The pipe mains taking water from the source upto the service reservoirs may be designed for MDD.
3. The filter and OTHER UNITS at water treatment plant may also be designed for MDD. Sometimes an additional provision for reserve is also made for break down and repairs therefore they may be designed for twice the ADD instead of MDD.
4. The pumps may be designed for MDD plus some additional reserve (say twice the ADD)
When the pumps do not work for all the 24 hrs such as in small town supplies, the design draft should be multiplied by
24/ Number of hours in the day foe which the pumps are running
5 The distribution system is generally designed for the maximum hourly demand of the maximum day or coincident draft whichever is more.
6 Service reservoirs are generally designed for 8 days consumption.
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