Pipe Flow Calculations Forum

Hi dear society,
I need your help in very simple calculation, which unfortunately, I cannot perform properly.
I need to calculate a diameter of ideal (no friction) pipe, filled for 100% (so that hydraulic D = D of the pipe)
My capacity (Q) is 10 c.m./hr
Fluid is water at +50ºC
The Re numbers must be 2200 and 15000.
I have used the following equation:

..........Re*(μ/ρ)*A........Re*(μ/ρ)*(πD²/4)
D(m) = ---------------- = -----------------------;
...............Q......................Q

..............4*Q*1000
D(mm) = ----------------
.............Re*(μ/ρ)*π

But I get a very strange results:
= (4*1000*(10/3600))/(2300*(0,0005471/988,1)*3,14) = 2905 mm
= (4*1000*(10/3600))/(15000*(0,0005471/988,1)*3,14) = 426 mm

It means, that to receive a laminar water flow at 10 c.m./hr I need a pipe of almost 3m in diameter !
In the mean time I know for sure that the pipe DN65 will be more than enough.
So my calculation is completely incorrect !

Pls. anybody help me !
I need this calculation badly !
Thank you very much in advance !

Don't change in second line numerator with denominator - just multiply first row with 1000 and you will get result in mm.

Dear Administrator,
Thank you very much for your help !

Unfortunately I do not understand how I can solve the equation in this form:


..........Re*(μ/ρ)*A......Re*(μ/ρ)*(πD²/4)......Re*(μ/ρ)*πD²
D(m) = ---------------- = ----------------------- = -------------------
................Q.....................Q.....................4*Q

At this point I have D on both sides of equation.
So, I cannot solve it.
To solve it, I have to simplify it. ?

.......Re*(μ/ρ)*π*D*D
D = -----------------------
..............4*Q

4*Q*D = Re*(μ/ρ)*π*D*D

4*Q = Re*(μ/ρ)*πD

...............4*Q
D(m) = -----------------
...........Re*(μ/ρ)*π

Where is my mistake ?

Probably I mess with measuring units ?
D - in meters
Q - in cub.m./second, in my case 10 cub.m./hr = 10/3600
μ – in Pa•s, for water at +50⁰C = 0,0005471
ρ – in kg/m³, for water at +50⁰C = 988,1

Is this correct ?

Thank you very much in advance !

Well, just checked your results and they are correct.

Now as you expect to have laminar flow with DN65 pipe, it is just what theory says is not possible.

Dear administration
Now, i would like to have your help for solving this proplem folowing:
I have to calculate the diameter size of steam pipe: Knowing the flow (q) and pressure (p)
Please give me correct formula for this
Thanks you so much, i need your help asap

Here is the formula to calculate pipe diameter for predefined velocity:
http://www.pipeflowcalculations.com/pip ... locity.htm.

If you know volumetric flow rate [m3/h] - you can use first one, but most probably you will know mass flow rate [kg/h].
Anyway, you will need saturated steam density on the actual pressure - you have the table in calculators at:
http://www.pipeflowcalculations.com/flowrate/index.htm. under "steam" on the left.

If the steam is superheated, than other density will be.
It is suggested that steam velocity not exceeds 10 m/s, but you can choose even lower.

Tell me actual pressure and flow rate so I can more help you.