# Pressure Drop

## Main Question or Discussion Point

Hello... I can't find the difference between the pressure drop in a pipe due to frictionnal loss and the Pressure difference that cause the flow like in Poiseuille Flow.. Thanks

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They are completely different ideas...what specifically don't you understand about either?

Edit: Oh are you talking about like the losses associated with flows like Poiseuille Flow?

If you are asking how this solution to pressure drop is associated with similar ideas such as the Darcy-Weisbach equation, the answer is that the analytical (experimentally derived) Darcy-Weisbach equation is employable under broader circumstances.

The assumptions taken for Poiseuille Flow equations are (from wikipedia) "...that the fluid is viscous and incompressible; the flow is laminar through a pipe of constant circular cross-section that is substantially longer than its diameter; and there is no acceleration of fluid in the pipe". In theory this is a nice equation to look at to understand where the mechanical energy is being lost to, but in practical applications of pressure drop analysis it is rarely, if ever, employed.

Basically: Poiseuille equation is theoretical, solutions like Hazen-Williams and Darcy-Weisbach are analytical.

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we say that gradient pressure cause flow but in pipe flow the pressure different is due to friction loss

To see how people generally use these equations (Poiseuille is mentioned in there), see http://www.kimberly.uidaho.edu/water/papers/others/Allen_1996_Trans_ASAE_Relating_HazenWilliams_and_DarcyWeisbach.pdf [Broken]

A solution to Poiseuille's equation is used to approximate the D-W friction factor for Laminar, fully developed flow in long pipes.

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Thank you i understand it

Chestermiller
Mentor
If you are asking how this solution to pressure drop is associated with similar ideas such as the Darcy-Weisbach equation, the answer is that the analytical (experimentally derived) Darcy-Weisbach equation is employable under broader circumstances.
In theory this is a nice equation to look at to understand where the mechanical energy is being lost to, but in practical applications of pressure drop analysis it is rarely, if ever, employed.

Basically: Poiseuille equation is theoretical, solutions like Hazen-Williams and Darcy-Weisbach are analytical.
In polymer processing applications, typically involving high viscosity polymer melts (say 1000 Poise), the Poiseuille pressure drop equation is used extensively. This includes the entire man-made fiber industry, polymer granule production industry, and plastics manufacture industry. In addition, it applies to flow of ordinary fluids through capillaries.

Thanks for that, I wasn't aware it was so widely used in that industry. I've never personally had any experience with such high viscosity fluids.