Transmission Line Design Discharge

Question

Hi All, 
 In general, what is being used a the design flow discharge of a pipeline that is transmitting water to a reservoir (tank, treatment plant, etc.; not to customers directly)? During design, we estimate population and target average flow (Qmean - yr:2050) and multiply it with a factor that is the maximum of monthly or daily factor. In other words we use Q max-day or Qmax-month depending on the case. I far as I see, in literature there is not a certain answer to my question, but sometimes Qmax-day (daily maximum flow of year) is advised to be used. However, generally Qmax-day is significantly greater than Qmax-month. So, using Qmax-day instead of Qmax-month makes the design more expensive. And I really wonder, in common practice what kind of method is being applied? 
 Best regards, 
 Gen&ccedil;er Gen&ccedil;oğlu

Tom Walski · Answer

Gencer, 
 
Pipes should be sized to carry the peak flow that will occur during its design life. 
 
However, this is not easy to determine since in looped systems, the peak flow in a pipe is dependent on the size of other pipes. The peak flow may be reduced by the contribution of water held in storage such that some pipes need only provide peak day flow and additional flow can be provided from storage. Then there is the question of reliability and redundancy when an element is placed out of service. Plus there is a great deal of uncertainty in demand projections. 
 
This is why we use pipe network models to analyze demands.

Tom Walski · Answer

The exact wording in the US "Ten State Standards" is 
 
"All water mains, including those not designed to provide fire protection, shall be sized after a hydraulic analysis based on demands and pressure requirements." 
 
Basically it means you need to develop demand estimates and size the piping so that you meet them. Every situation is different depending on type of supply, reliability considerations, growth projections, and available funding. Your engineering judgment is the key.