Shouldn't the pressure difference along a pipe be a negative number?

In summary, the question addresses the concept of pressure difference in fluid dynamics, specifically within a pipe. It suggests that the pressure difference should be considered negative, as fluid typically flows from areas of higher pressure to lower pressure. This perspective highlights the directional nature of pressure gradients and the conventional understanding of flow dynamics.
  • #1
lost captain
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TL;DR Summary
We know that across a horizontal pipe, the pressure will become smaller due to friction. So shouldn't the pressure difference be a negative number?
Across a horizontal pipe with fluid flowing inside, the pressure will drop due to the friction between the fluid and the pipe walls
So if for example the fluid flows from left to right and we take 2 points: P1 (pressure on the left) and P2 (pressure on the right) then P1 should be bigger than P2.
And the pressure difference should be
ΔP = P2 - P1 <0
So pressure difference or pressure drop should be negative. Why is it positive?
 
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  • #2
Why is it positive you think? Where did you read that?

It is entirely a matter of definition. If the pipe is along the x-axis and the flow in positive x direction than the pressure gradient (dp/dx) is indeed negative. If on the other hand the flow is in negative x-direction then the gradient is positive.

The way you describe it is however the common way to do this.
 
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  • #3
PipeRoughness-768x355.png



PipePressureDrop-1024x566.png



Pump_pipeline-3-1024x412.png



echanics-hagen-poiseuille-pipe-flow-pressure-drive.jpg
 
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  • #4
lost captain said:
So pressure difference or pressure drop should be negative. Why is it positive?

By the way: if you call it a pressure drop, it should be positive. A negative pressure drop is a rise in pressure.
 
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  • #5
Arjan82 said:
By the way: if you call it a pressure drop, it should be positive. A negative pressure drop is a rise in pressure.
Thank you, very much🙇‍♂️😊
Yes understand that the word "drop" in "pressure drop" basically means subtracting.
 
  • #6
In pipe flow, the entrance pressure is higher than the exit pressure. So the pressure at the exit minus the pressure at the entrance is negative.
 
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  • #7
Chestermiller said:
In pipe flow, the entrance pressure is higher than the exit pressure. So the pressure at the exit minus the pressure at the entrance is negative.
Yeah exactly, but when we calculate ΔP we take the positive value
 
  • #8
lost captain said:
Yeah exactly, but when we calculate ΔP we take the positive value
It’s just terminology.
 
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  • #9
Chestermiller said:
It’s just terminology.
Thank you very much for taking the time to reply 😊🙇‍♂️
 

FAQ: Shouldn't the pressure difference along a pipe be a negative number?

1. What does a negative pressure difference along a pipe indicate?

A negative pressure difference along a pipe indicates that the pressure is lower at the outlet compared to the inlet. This is often a result of friction losses, elevation changes, or other resistive forces acting on the fluid as it flows through the pipe.

2. Why is pressure difference often expressed as a positive value?

Pressure difference is often expressed as a positive value for simplicity and clarity. Engineers and scientists typically refer to the magnitude of the pressure difference, regardless of the direction of flow, to easily compare and analyze pressure drops in various systems.

3. Can a pressure difference ever be zero?

Yes, a pressure difference can be zero when the pressure at both ends of the pipe is equal. This situation often occurs in a system at equilibrium, where there is no net flow of fluid, or in ideal conditions where friction and other losses are negligible.

4. How do pressure differences affect fluid flow in a pipe?

Pressure differences are the driving force for fluid flow in a pipe. A higher pressure at one end compared to the other creates a pressure gradient, causing the fluid to move from the high-pressure area to the low-pressure area. The greater the pressure difference, the faster the fluid will flow, assuming other factors remain constant.

5. What factors can cause a pressure difference in a pipe?

Several factors can cause a pressure difference in a pipe, including friction between the fluid and the pipe walls, changes in elevation (gravitational effects), pipe diameter and length, flow velocity, and the viscosity of the fluid. Additionally, any obstructions or fittings in the pipe can contribute to pressure losses.

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