Pipe sizeing problem

[Engineerator]

Hello...

I am struggling with calculating the pipe size diameter for real life problem, suppose I want to supply water to a premises from munsipality main line and the data I have:

Water demand= 90 cubic meter/day
Avilable pressure at tie-in = 1 bar
Distance from tie-in point to the premises ground water tank = 20 meter
the pipe line runs horizontally on the ground and goes vertically about 3 meter to the tank.

The equations I am working on are Energy and Darcy equations, but I am not sure how to fit the scenario I have described above into those equations, also I am comparing my result with Pipe Flow Wizard software but with no success!

Any help would be appreciated

Thank you.

 

[quark]

First check what instantaneous flow rate you require based on peak demand. You can also work out on average demand as you have an intermediate tank. Consider elevation of 3 meters and back calculate pipe diameter required from Darcy's equation for a pressure drop of 10 meters (since available pressure is 1 bar and your tank is at atmospheric pressure, you can afford a pressure drop of 1 bar or 10 meters).

 

[gerhardl]

Qark is of course right.

90m3 a day is however not much. If you do a rough calculation, even putting in considerable less pressure drop to overcome any unforeseen factors, you will probably have more than sufficient capacity with a one inch pipeline.

Please check necessary peaks as stated by Quark.

Any dimension below one inch have to be more exact calculated. Going up or down in piping size is more a question of what is normally done or required (also by any regulations and demands, also including considerations for, if any, firewater demand) in the area, and what is best suited and economical over time.

 

[BigInch]

You need to use an outlet pressure at the tank corresponding to the maximum possible height of water in that tank.

You should be getting something like a 2" or 2.5" diameter pipe, which will lose somewhere around 150mm of head in the 20 meters.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Engineerator]

Quark and Gerhard thank you for your response

Just want to correct one thing!

The total demand is 2160m3/day = 90m3/h, and we are going to fill the tank within 8 hours; therefore the flow rate would be 270m3/h.

My most concerned is how to do the calculation rather than logic.

 

[BigInch]

That makes a lot of difference!

Logic. Use outlet pressure = max tank fluid level
Use flowrate = 270 m3/h
Assume pipe diameter
calculate inlet pressure (online pipe flow calc)
If inlet pressure is greater than available,
increase pipe size
or add pump
Return
Else
Reduce pipe diameter
End



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[katmar]

The Darcy equation is the right one to use. But because Darcy calculates pressure drop as a function of flow and pipe diameter you have to use trial and error. Guess a pipe size and calculate the pressure drop. If the calculated pressure drop is more than you have available then try a larger pipe and vice versa.

For Darcy use length = 23 m, pressure available = 0.7 bar (= 1 bar - 0.3 bar). I would add in a K value of about 10 to cover valves, bends, entrance and exit losses.

My software says you need an 8" pipe for 270 m3/h.

If you are still having problems with the calculation you will have to be more specific about exactly what is troubling you.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[quark]

Considering 5 bends and a ball valve, I get about 7 meters drop including elevation, for a 6" sch40 pipe. 5" is just about the size but non standard.

Harvey,

What are your assumptions?

 

[katmar]

Hi Quark, I had made a more conservative assumption than you on the fittings and I put a K of 10. Also, I don't like designing for average flowrates - you need a bit of capacity up your sleeve. This turns out to be slightly bigger than a 6" pipe, so I said 8". You might get away with 6", but this is a high flowrate for a municipal main and who knows whether it will maintain the 1 bar supply when this extra flow is taken.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[quark]

Thanks, that makes much sense and you are closer than I am.

 

[Engineerator]

ok guys!

applying "Energy equation" and "Darcy equation"

assuming p1=1 bar, p2= 0.5,Z1=0, Z2=3m,Vsqr.1=Vsqr.2(removed from both side of Equation)

solve for hf,=2.1m

Darcy Equ.

D=(8(fL/hf)(Qsqr/PisqrXg))1/5

assume f=0.001, given: L=20, Q= 0.075m3/s and hf=2.1

Therefor, D= 85mm

using Moody Diagram,

Ks/D----assume Ks=0.001

we get Resistanse coeff., f=0.0095

Therefor,D =133mm=5in, so I can go with the next commercial pipe size avilable wich is 6in

"Pipe Flow Wizard" software gives me almost the same result, also same as Quark result

 

[katmar]

I misinterpreted your original problem statement, but with the extra information you have given now I would still use an 8" pipe. Previously I assumed 7m was available for friction. Now you are using only 2.1m for friction. Where does the other 4.9m get lost? Is this the height to the tank inlet?

You cannot ignore the velocity change. Effectively V₁ is 0 m/s and all of V₂ is lost in the target tank. You have also ignored any valves and bends. There must be at least one valve because you need to control the flow, and there will probably be an isolation valve at the municipal tie-in.

How did you calculate your friction factor? My calcs make it 0.016. Can you upload a screen shot of the Pipe Flow Wizard input data and results - I would be most surprised if commercial software confirmed this answer.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Engineerator]

Katmar, this is a screen shot of my software, and I may discuss the other issues you have raised later on.

 

[katmar]

Thanks for posting the screenshot. As would be expected, Pipe Flow Wizard has calculated the correct answer for the information it was given.

I see you are using copper pipe. Under these circumstances copper will give about 30% less friction loss than commercial steel pipe because copper is smoother than steel. That explains my query about the friction factor. Copper pipe is more expensive than steel, and I am not sure how easily it is available in this size.

The screenshot confirms that you have ignored the fittings, valves, entrance and exit losses. Pipe Flow Wizard seems to have a reasonable selection of fittings and valves, but I cannot find any allowance for entrance and exit losses. This will lead to real errors, especially if you are trying to use a velocity of 8 m/s. This is a much higher velocity than what would typically be used.

I still do not see how you got to 2.1m for the allowable friction loss.

My impression of this job is that you need more help than what is available via Eng-Tips. Eng-Tips is not intended as a replacement for sound engineering help and I would advise you to engage a local professional engineer to do this design. At the very least run your intended design past the municipal engineers who will probably have a wealth of practical knowledge that cannot be given in a forum like this.


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Engineerator]

Katmar, thank you for your time,

I believe it won't make big deferent if you choose a deferent pipe material or adding fittings and valves, in term of pipe size selection, calculations and software confirm that. I have chosen copper just as an example; otherwise I can go with HDPE or MDPE.

Regarding head loss,hf=2.1m I got that from Energy Equation:

P1/?g+V2/2g+Z1=P2/?g+V2/2g+Z2+hf

P1=100KPa/m2,---------->(100KPa= 1bar)
Assume P2=50Kpa/m2(minimum factor that we can expect from system to be in the safe way! And we may use it in the software as available pressure )

V1=V2 since the pipe sectional area is same at all the pipe line system, and since it is energy equation the total energy in the left side will be equal to the right side, so V's cancel each other.

Z1= 0, because the entry pipe line is running on the ground, and Z2 =3m (targeted tank height).

?g=9.81x1000

Solving for hf, hf = 2.1

What do you think?