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Name: Shankar
Status: Student
Grade: 9-12
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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