
Calculators
Friction Loss & Flow
Friction Loss/Flow Calculations
Several methods are available to determine the friction losses and estimated water flow rates in pipes. In this section one of the most widely used formulas for friction loss, the DarcyWeisback equation, will be presented so the user can have an understanding of the use of these formulas and the information they can provide.
The DarcyWeisback formula is:
Friction Loss (h_{f} ) 
= f 
L 
x 
V^{2} 

d 
2g 

h_{f} = 
Friction Loss (ft) 
f = Friction Loss Factor 

L = 
Pipe Length (ft) 
d = Pipe Diameter (ft) 

v = 
Pipe Velocity (ft/sec) 


g = 
32.2 lb m  ft 
(Acceleration of Gravity Factor) 

lb f  sec^{2} 


A simple method to estimate flow rate is the "drum and stopwatch method." Place a drum or container of sufficient size (30 to 50 gallons) at the end of the pipe and, using a stopwatch, estimate the time to fill the container. This will provide a good method to determine the flowrate that can be used in these calculations if a flowmeter is not available.
Sample Calculations:
Determine the friction loss of water pumped through 1,000 ft. of 4inch diameter pipe at a 300 gpm flow rate. Flowrate can be determined using a flowmeter or the "drum and stop watch method."
The first step is to determine the velocity based on the pipe's crosssectional area.
Crosssectional Areas 
= 
π x (pipe radius)² 
Where π = 3.14 
Pipe Diameter 
= 
4inch 
= .33 ft 
Pipe Radius 
= 
2inch 
= .16 ft 
Crosssectional Area 
= 
3.14 x (.16)² 
= .08 ft² 
The second step is to determine the pipe velocity.
Pipe Velocity (V) 
= 
Flow rate 

Cross Sectional Area 

Multiply the 300 gpm flow rate by .002 to convert to ft^{3}/sec 
300 gpm x .002 
= 
0.60 ft^{3} /sec 


Calculate Pipe Velocity 
Velocity (ft/sec) 
= 
0.60 


0.08 


Velocity (ft/sec) 
= 
7.5 ft/sec 


Using a .02 friction factor, calculate the friction loss 
Friction Loss (h f ) 
= f 
L 
x 
V² 
d 
2g 
Friction Loss (ft) 
= 
.02 x 1000 ft 
x 
(7.5 ft/sec)² 
.33 ft 
64.4 
Velocity (ft/sec) 
= 
52.9 ft 









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