Pipe sizing question regarding admittable pressure

[cikiste]

Hello group!

I am new member, and of course I need help. For water distribution system (flow 120 lit/min, pressure 25 bars) I need to calculate pipe dimension. The flow velocity calculation is familiar to me, but I am not familiar with calculation regarding material strength, or to put is simple, may I use AISI 1.4301 28x1,5mm pipe (28mm is outter diameter, 1,5 stands for wall thickness)?
Can someone post a good URL link for topic or some other resource information.

Sincerely,
Ivan Lazarevic.

 

[PrinceOfAll]

Hello,

The fluid pipe sizing is usually based on the fluid velocity. For liquid pipes the recommended velocity range is from 1-3 m/sec. If the velocity is much less than 1 m/sec, this will allow the precipitation of the solid particles in the liquid. On the other hand, the very high liquid velocity makes the friction losses too large and hence the pumping power.

According to your problem, 120 lit/min flowrate, the suitable pipe diameter will not be less than 72 inches, API standards. It will give a liquid velocity of 2.74 m/sec.

For the other part of your post; the thickness selection is based on the mechanical design of the pipe considering it as a pressure vessel. The design equation is:

Pressure*Outer Diameter/(2*Thickness) = Material Yield Strength/Factor of Safety

1. Pressure = 25 bar
2. Outer Diameter = 72 inches
3. Thickness: is taken as the minimum standard thickness. From the API standard; Thickness = 0.5 inches
4. Material yield Strength: you have first to select a suitable pipe material; let us say X-80 steel, API standards. This steel grade has a yield strength of 80,000 psi or 5,517 bar
5. You can now calculate the Factor of Safety it will be 3.065
6. Now you have to check this Factor of Safety with the recommended Factor of Safety from the Codes and Standards of a famous organization like the CSA. The CSA Z662-96 recommends to use a Factor of Safety equals 1/(0.64*1*1). That is 1.5625 which is less than our selected thickness.
7. Finally you may repeat the thickness selection process using other materials like X-42 and X-65…etc. The best material and thickness are those who give the least construction cost.

 

[PrinceOfAll]

I am sorry cikiste, I've forgotten to tell you that the pressure you substitute in the design equation should be absolute pressure not gage pressure . So please resolve the problem using 26 bar pressure instead of 25 bar.

 

[TBP]

Are you sure that a 72" line is required for less than 32 GPM?

 

[hydromech]

28 X 1.5 Cold drawn steel tube should be good for about 200 BAR. I can confim Monday when I get into work...no reference at home.

120 LPM in a 72" pipe will be outrun by a asmatic snail with some heavy shopping.

28 X 1.5 will make the water velocity 4 MTRS/SEC.

The smooth bore of the tube will be offset by low viscocity of the water. The flow may well be turbulent but that will not be a problem if there is enough power in hand.

Turbulent also helps to reduced silt build up.

Hope this helps..

Adrian

 

[PrinceOfAll]

OK guys, I have revised my unit conversions; the diameter 28 mm will be suitable. Also a corresponding thickness 1.5 mm will withstand the pressure.