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Water Supply - Calculating Demand

Calculating expected demand of water supply in service lines

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

Expected demand in a water supply system can be estimated as

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 minimum expected total water flow can never be less than the demand from the largest fixture. This equation is valid for ordinary systems with consumption patterns like

  • 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  

For these kind of applications, like a wardrobe, it is likely 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

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  • Engineering ToolBox, (2008). Water Supply - Calculating Demand. [online] Available at: https://www.engineeringtoolbox.com/water-supply-expected-demand-formula-d_1076.html [Accessed Day Mo. Year].

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