Relation between pipe velocity, pressure loss, pipe roughness

[Bernoulli31]

Don't you hate it when you have an excel calculation sheet, done by others, and there's no way for you to tell how the values were calculated?

Case in point, I have a calculation that uses VBA functions. I want to back engineer the calculations, so that I can validate the results. Unfortunately, the owner of the calculation locked the VBA developer, and I have no way to look at the code without a password.

So here's what I'm trying to figure out:

Function calculates the velocity in a pipe with certain diameter. Here's the thing: flow is not considered!
Here's a copy of the table, for thermoplastic pipe:

Size fps psid/100ft GPM
2 4.17 1.2 51.4
3 5.30 1.2 133
4 6.06 1.2 227

Size and pressure loss are input values. fps and GPM are calculated values.
When I look at the in-cell formula (function) for the velocity (fps), I can see the following parameters are used:

Pipe roughness
inside diameter
fluid density
dynamic viscosity
psid/100ft
max velocity (if calculated velocity is higher than max, then max velocity overrides result).

The flow GPM is then calculated based on the fps values, in effect giving us the capacity of the pipe.

So my question is, how can the velocity in a pipe, based on its diameter, be calculated, when the flow is not known, and using the parameters given?

I've looked inside Crane TP, Nayyar handbook, and several other sources. Also did a search in this forum. I must be missing something.






I design aqueducts in a parallel universe.

 

[katmar]

If you have all that information then you can calculate the flow rate, and if you know the flow rate then you can calculate the velocity.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Bernoulli31]

it appears the velocity is calculated first. Then the flow.

I design aqueducts in a parallel universe.

 

[cvg]

its not uncommon to calculate the velocity first. For instance a standard form of both Manning and Chezy equation is written to solve for velocity. there is no flow rate in the equations.

https://i.ytimg.com/vi/uM9nPHS7GSQ/maxresdefault.jpg

If this spreadsheet was given to you for your use or review, than ask for an explanation of the calculation procedure. If you are pirating this spreadsheet, than you may have to generate your own code.

 

[quark]

I think they are using the maximum velocity value to check friction factor and then calculating velocity based on pressure drop using D-W Equation.

 

[Bernoulli31]

Manning's for open channel flows. Not applicable here.

Fyi, I have this spreadsheet from a company I used to work for. Since I don't want to use it in my current firm, I'm just trying to figure out the engineering formulas behind the calcs. I'm not interested in the code itself as that would be proprietary. It's more of an intellectual exercise I'm keeping myself busy with during my downtime.

What I also hope to understand, eventually, is what this velocity means, compared to max or design velocity that is used everywhere to calculate the capacity of a pipe.

 

[cvg]

What I also hope to understand, eventually, is what this velocity means, compared to max or design velocity that is used everywhere to calculate the capacity of a pipe.

velocity is related to the available head and to the head losses through the pipe or channel.
these would include friction, elevation change, entrance and exit losses, momentum losses etc.

by the way mannings, chezy and Darcy Weisbach all can be used for pipes as well as open channels.

 

[Bernoulli31]

I think I was able to crack it, thanks to Crane TP410. Sometimes it helps to sleep over it and take a fresh look at the formulas.

Step 1: calculate Reynolds # based on the given maximum velocity )(Equation 6-3)
Step 2: calculate friction factor using Colebrook equation 6-5
Step 3: Calculate the velocity in the pipe using Darcy equation 6-8 for delta_p

Voila.

I'm making an assumption they used maximum velocity to calculate the Reynolds # in Step 1. My manual calculation results are very similar to the function values. I'm guessing they used a function just to calculate the pesky Colebrook equation.


I design aqueducts in a parallel universe.