Speed of water flowing through pipe

  • #1

Homework Statement


The speed of water flowing through the "influent" 4-inch diameter section of the piping system below is 3.0 ft/s. What is the volume flow rate of water in the piping system? Express the volume flow rate in [itex]ft^3/s[/itex]


Homework Equations





The Attempt at a Solution


I really don't know what to do. I know pi*r^2 comes into play here, but I need help. Just tell me how to start and guide me through it please.
2012-11-07%252021.12.49.jpg
 

Answers and Replies

  • #2
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Flow rate = area of the pipe multiplied by the velocity.

You have everything but the flow rate. The area of the pipe is pi*r^2 as you said. Since you're not given a second density we can assume the flow is incompressible, and due to the conservation of mass, the flow rate in the 3 in dimater pipe will be the same. Q1 = Q2. I can't give you anymore without solving it for you.
 
  • #3
SteamKing
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Given that the fluid is water, it's safe to assume that the fluid is incompressible.
 
  • #4
... the flow rate in the 3 in dimater pipe will be the same. Q1 = Q2.
Can the speed of the water be different even though the flow rate is the same? Part two says to find the speed of the water in the 3-inch diameter section in ft/s
 
  • #5
SteamKing
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Yes.
 
  • #6
bigfooted
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Can the speed of the water be different even though the flow rate is the same? Part two says to find the speed of the water in the 3-inch diameter section in ft/s

Yes, this is connected to the equation given by vadar2012: flow rate = area x velocity.
The flow rates are the same everywhere in the pipe (it must be, the water doesn't disappear). Therefore, if the area of the pipe increases, the velocity decreases.

So:
step 1: calculate flow rate = area x velocity or Q1=A1 x V1
step 2: use Q1 = Q2 to calculate V2
 

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