Water Supply - Calculating Expected Demand

Calculating of expected demand in water supply service lines

The total theoretical demand for a water supply system can easily be calculated by adding all fixtures known maximum demand. Due to the nature of intermittent use the number of fixtures will unfortunate add up to unrealistic demands for the main supply lines to handle. A realistic demand for a supply system will always be far less than the total theoretical demand.

The expected demand in a water supply system can be estimated like

qet = qnl + 0.015 ( Σqn - qnl  ) + 0.17 ( Σqn - qnl  )1/2         (1)

where

qet = expected total water flow (l/s)

qnl = demand of largest consumer (l/s)

Σqn = total theoretical water flow - all fixtures summarized (l/s)

Note that the minimum expected total water flow can never be less than the demand from the largest fixture. The equation is valid for more ordinary systems where the consumption patterns are more continuously as in

  • homes
  • offices
  • nursing homes
  • etc.

Be aware when using the equation for systems serving large groups of people where the use is intermittent, like in 

  • hotels
  • hospitals
  • schools
  • theaters
  • wardrobes in factories
  • etc  

It may be typical for applications like this, like a wardrobe, that all showers are used at the same time. Using the formula blindly would result in insufficient supply lines.

Example - Main Water Supply to a Nursing Home

If the theoretical demand from all fixtures in a nursing home adds up to 50 l/s and the larges fixture requires 0.4 l/s, the expected water supply demand can be estimated like

 qet = (0.4 l/s) + 0.015 ((50 l/s) - (0.4 l/s)) + 0.17 ((50 l/s) - (0.4 l/s))1/2

    = 2.3 (l/s)

Total Theoretical Water Flow and Expected Flow

Expected demand for a supply system at different total theoretical demand can based on the formula above be expressed as

Total Theoretical Demand Summarized
(liter/s)
Expected Demand
(liter/s)
0.2 0.2
0.8 0.4
1.6 0.5
4.0 0.6
8.0 0.85
15 1.1
20 1.5
30 1.8
40 2.1
65 2.8
70 2.9
100 3.7

The maximum fixture load is 0.2 liter/s.

water supply - expected demand

Related Topics

  • Water Systems - Hot and cold water systems - design properties, capacities, sizing and more

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