Thursday, April 23, 2020

Pipe Flow/Friction Factor Calculations : (U.S. units)

Workbook Contents :-
Click on tabs at the bottom of the screen to access the following:
  • Tab 1. Contents (current tab)
  • Tab 2. Head Loss/Frictional Pressure Drop
  • Tab 3. Pipe Flow Rate
  • Tab 4. Required Diameter

This workbook is provided "as is", without warranty of any kind, express or 


Tab 3. Pipe Flow Rate :-
NOTE: This is an iterative calculation, because an assumed value of Q is used to start the calculations.  If the final calculated value of Q is different from the assumed value, then the assumed value of Q should be replaced with the calculated value of Q, leading to a new calculated value for Q.  This should be repeated as many times as necessary to get the calculated value for Q to be the same as the assumed value.  This iteration typically converges rather rapidly.

This spreadsheet shows only the final assumed and calculated values of Q (when they are equal).If the initial assumed value of Q was 1.5 cfs, then subsequent calculated and assumed values for Q, leading to the final solution are as follows:

Assumed Q, cfs Calculated Q, cfs
1.5 0.85
0.85 0.83
0.83 0.83

Tab 4. Required Diameter :-
NOTE:  This iterative procedure doesn't converge smoothly to a solution.  If the calculated pipe diameter in this step is larger than the assumed pipe diameter above, then replace the assumed pipe diameter value with the next larger standard pipe size.  Repeat until you find the smallest standard pipe diameter that gives a smaller calculated required pipe diameter.  That is your solution.

With the example values given here, an assumed pipe diameter of 3 inches gives a calculated pipe 
diameter requirement of 3.4 inches, increasing the assumed pipe diameter to the next standard pipe size ( 3.5 inches ) gives a calculated pipe diameter requirement of 1.8 inches, so a 3.5  inch size is the minimum standard pipe diameter that will do the job.

*Standard nominal U.S. pipe sizes in inches: 
1/8, 1/4, 3/8, 1/2, 3/4, 1, 1 1/4, 1 1/2, 2, 2 1/2, 3, 3 1/2, 4, 5, 6, 8, 10, 12,  14, 16, 18, 20, 24,30, 42, 48, 54, 60.

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