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

[lost captain]

TL;DR

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?

 

[Arjan82]

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.

 

[Lnewqban]

 

[Arjan82]

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.

 

[lost captain]

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.

 

[Chestermiller]

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.

 

[lost captain]

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

 

[Chestermiller]

Yeah exactly, but when we calculate ΔP we take the positive value

It’s just terminology.

 

[lost captain]

It’s just terminology.

Thank you very much for taking the time to reply