Pressure drop in a pipe, wildly different results

Hi all, firstly I apologize if I am asking this in the wrong place. I am thankful for any guidance I can receive on this.

I have been studying over pressure drops in pipes, and in some cases get wild answers in comparison to when I try to use a canned online-calculator. By hand I've checked units and they cancel where they need to. I may be getting the right answer, but it's the results from other sources that make me doubt my own.

Consider a 1" ID stainless steel drawn pipe (roughness epsilon = 0,0015), 2200 feet long, no elevation change.

The fluid being circulated is Cyclopentane, viscosity of 0.44cPs, at 27GPM. Density 746kg/m3.

27GPM is 0.00170343m3/s through a pipe cross-section of 0.001141m2.

I calculate a flow velocity of 1.494m/s.

Reynold's number is calculated to be 144,772.

Using Darcy Weisbach: p2-p1 = friction factor * (length/diameter) * (density*velocity^2/2) :: my result is appx 300psi pressure drop in the line.

I have found "other" Darcy Weisbach equations, but the one I've described seems the most appropriate. Using others can show numbers down to factors of 1-10psi and over 1,000psi. I want to practice these for this reason - I can't get to the bottom of why my results are unconfirmable.

I hope I've explained my problem well - I look forward and appreciate any help that comes from this.

Tucker

I apologize I don't know how to edit a post. I didn't mention the friction factor I used in in the above is is fD = 0.019. It's probably more like 0.0175. The 0.019 is an accidental carryover from using the Moody Chart for the same problem using 1.5" pipe.

But yeah, is it possible to edit your own posts on this board? I don't see a spot I can click to do so! Thank you

tucker240 said:

Consider a 1" ID stainless steel drawn pipe ...

...a pipe cross-section of 0.001141m2.

Click to expand...

Here is one error. For a 1-inch ID, the cross section area is 0.0005067 m^2. This trickles down and gives you the wrong velocity. However, for some reason we get the same value of Reynolds number, ~144,000.

I've attached my calculation using 1-inch ID. I get 391 psi.

However, if I do it again but using 1" nominal pipe (therefore, ID=1.049") I get 320 psi.



This seems consistent with at least one online calculator: here

Man, thank you, Excel is kicking my ass - pi()*D^2/4 is not the same as pi()*(D^2)/4. I had also dropped down to roughness of 5E-6 to try the bottom end of the roughness scale for drawn tubing.

I definitely appreciate that ^. Someone shared with me their answer for the loop is 11psi and this does not pass the sanity-check for me.

But seriously how do you edit a post? I can't find a button for it and it is driving me mad

tucker240 said:

But seriously how do you edit a post? I can't find a button for it and it is driving me mad

Click to expand...

At the bottom of your post it should say “Edit” right next to “Quote”.

Here’s a screenshot of what the bottom of my posts look like on mobile

You need a certain number of posts before you can edit.  I suspect you don't have enough posts for the edit function to be enabled.

tucker240 said:

Man, thank you, Excel is kicking my ass - pi()*D^2/4 is not the same as pi()*(D^2)/4. I had also dropped down to roughness of 5E-6 to try the bottom end of the roughness scale for drawn tubing.

I definitely appreciate that ^. Someone shared with me their answer for the loop is 11psi and this does not pass the sanity-check for me.

Click to expand...

You may want to check how you input those into Excel.  Mathematically they are equivalent.  I typed both into Excel exactly as you have above and for any given diameter the calculation value is the same.

Makes sense, there definitely isn't an edit button next to Quote at the minute.

I do understand the expressions are mathematically equivalent, which is why it I didn't notice the mistake. Somehow Excel is evaluating them differently.

Try using SMath or Mathcad for your calculations.  You will be able to see the mathematical equations.