This average daily rate of demand per head is likely to deviate, if period of observation is shortened.
Thus, if average daily rate of demand is say 100, then:
(i) seasonal maximum demand may be 130; monthly maximum demand may be 140; and daily maximum demand may be 180 or so.
(iii) These variations are due to the many factors such as habits of people, climatic conditions, types of industries, etc.
The above figures for deviation from the average for seasonal, monthly , and daily demand, are taken for illustration only.
separately and the variation from the average rate of demand is worked out accordingly.
In practice, the maximum daily rate of consumption is very important.
This maximum daily consumption is to be consumed in 24 hours.
But demand during 24 hours will not be uniform , and it will vary according to hour of day.
A typical graph showing the hourly variation in the rate of demands is shown in fig.
The peak demands occur in the morning , and evening.
The slack periods occur early in the morning , and late at night.
The demand of water from hour to hour is thus variable and the maximum hourly demand will be much higher than the average daily demand. It may be assumed as about 150 percent of the average daily demand
let us assume that the average daily rate of demand 240 litres of water per head.
The rate of consumption on maximum day will about (240 x 1.80) and rate of maximum hourly consumption on that day will work out to be about
(10 × 1.80) X 1.50 or 27 litres of water per head.
In order to meet with the maximum hourly demand, the pumps are either to run at variable speeds or to run at an average speed.
In the former case, the speeds of pumps are changed as per hourly requirements.
This method of working the pumps results in great inconvenience.
In the latter case, the pumps are run at an average speed in such a way that the surplus water stored at slack demand period is used at peak demand period.
In fig. , the average rate of pumping is shown by a dotted line.
The area below the dotted line shows surplus water and the area above the dotted line indicates the shortage of water.
The average rate of pumping should be such that both these areas are nearly equal.
This means that the excess water stored during slack demand period is to be consumed in peak demand period.
The excess water is to be stored in tanks or reservoirs specially constructed for this purpose.
