# Understanding Pressure Drop & Flow in Pipes

• tonyjk
In summary, the conversation discusses the difference between frictional loss and pressure difference in pipe flow, specifically in relation to the Poiseuille equation and its practical applications. The Darcy-Weisbach equation is mentioned as a more widely used analytical solution, while the Poiseuille equation is primarily used in polymer processing applications involving high viscosity fluids.
tonyjk
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

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

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

Travis_King said:
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.

## 1. What is pressure drop in pipes?

Pressure drop in pipes refers to the decrease in pressure as fluid flows through a pipe due to factors such as friction, changes in pipe diameter, and elevation changes. It is an important concept in fluid dynamics and can impact the efficiency and performance of a piping system.

## 2. How is pressure drop calculated in pipes?

Pressure drop in pipes can be calculated using the Darcy-Weisbach equation, which takes into account factors such as fluid velocity, pipe diameter, and roughness of the pipe surface. Other methods, such as the Hazen-Williams equation, can also be used depending on the specific characteristics of the piping system.

## 3. What factors can affect pressure drop in pipes?

Several factors can affect pressure drop in pipes, including fluid properties (such as viscosity and density), pipe characteristics (such as diameter and roughness), and flow conditions (such as velocity and turbulence). Changes in these factors can result in changes in pressure drop.

## 4. How does pressure drop affect flow in pipes?

Pressure drop and flow in pipes are directly related. As pressure drop increases, flow rate decreases, and vice versa. This is because pressure drop represents the resistance to flow in a pipe, and a higher pressure drop requires more energy to maintain the flow rate.

## 5. How can pressure drop be reduced in pipes?

There are several ways to reduce pressure drop in pipes, including increasing pipe diameter, smoothing out rough surfaces, and optimizing flow conditions. Additionally, selecting the appropriate type of pipe and using efficient pumping systems can also help minimize pressure drop and improve overall performance.

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