

Water Flow Pressure in Pipe
Name: Shankar
Status: Student
Grade: 912
Location: N/A
Country: United States
Date: December 2007
Question:
Why is the pressure constant when water flows through a
pipe?
Replies:
Actually, the pressure in a fluid as it flows through a pipe is not
constant. In a smooth, straight pipe, the press drops at a constant rate as
it flows through the pipe. If you double the length of the pipe, the
pressure drop doubles.
This is a 'theoretical ideal' case. In the real world, the pressure may drop
more or less at different times as it flows through the pipe. For instance,
if the sides of the pipe are rough, the pressure will drop faster than if
they are smooth. If there are bends or constrictions in the pipe, pressure
will drop more too. Collectively, engineers call these factors "minor
losses".
There are a ton of web sites around that talk about pipe flow  I would
encourage you to look through several of them. Just Google combinations of
"flow" "water" "pipe", etc. When you read the sites, you will see some
additional key words you can use for your search.
Hope this helps,
Burr Zimmerman
Hi, Shankar. The pressure of water (or any fluid that is nearly
incompressible) changes based on speed. The Bernoulli equation
(Wikipedia link: http://en.wikipedia.org/wiki/Bernoulli%27s_equation)
states that the sum of three components remains constant. These
components are the 'hydrostatic pressure' (due to the weight of any
water column above the point of interest), the 'dynamic pressure' (due
to the speed), and the 'static pressure' (the pressure the fluid has
standing still).
The Wikipedia link has the equation itself. Simply stated, as fluid in
a pipe flows faster, the pressure decreases. If the fluid flows at the
same speed through the pipe length, the pressure will not change.
In the real world, there is friction along long lengths of pipe which
converts some of the speed back to static pressure. So if you took
measurements along the length, it would be different. Sometimes in lab
experiments, it is difficult to have a pipe long enough to detect the
change.
In the same way, if the velocity increases too much, the fluid pressure
will decrease so much it will vaporize.
David Brandt
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Update: June 2012

