PVC pipe sizing

[joabjim]

I am a HVAC Eng., but now I am navigating unknown waters, hope if somebody could help me with this. I have to size the intake and outfall pipes for a desalination plant. Intake pipe will bring in 55 mgd of sea water to the plant, pipe entrance will be located at 1 mile off shoreline and 50 feet below sea surface. Outfall pipe will carry 30 mgd and pipe diffuser will be 1.5 miles and 100 feet below sea surface, client wants to make directional drilling to place PVC pipes under the sea bed. I need to know a design fluid velocity for this application and friction coefficients to use, also client wants to use relatively small pipes to save in drilling cost regardless of operating cost. Has anybody any suggestion?
Thanks

 

[DeltaCascade]

Friction coefficients are a function of surface roughness, E (actually, E/d), pipe diameter, d, and reynolds number, Re, based on Moody's work. For example, Crane Technical Papter No 410 is an excellent standard reference.

Surface roughness values could be obtained from PVC pipe manufacturer, but I would estimate it to be less than drawn tubing at 0.000005 inch. E/d values would approach that of "smooth pipe", as per above reference, in my estimation.

Limiting factor for suction piping i suspect would be flashing/cavitation, even though you have 50 feet of static head in your favour. Quick calculation of pressure profile for progressively smaller pipe diameters until lowest pressure realized reaches pressure of saturated steam at your highest expected operating temperature (see steam tables and assume pure water)would give minimum diameter based on flashing in pipe. Pump NPSH requirements may also be a limiting factor (nothing new there).

Watch out for system dynamics ... water hammer and the like at start-up or shutdown, etc. But, as a mechanical engineer, you'd likely know more about that than me.

AWWA has some good PVC pipe references eg:

http://www.cpchem.com/performancepipe/Driscopipe/technotes/tech31.pdf

If you have lots of calcs to do, investment in software may be easily justified (eg: PIPENET Standard Module perpetual license is about US$1,500, and works with steady state compressible and incompressible flow, pipes, ducts, pumps, blowers, fittings, control valves, etc; PIPENET Transient module analyzes transient conditions, etc), giving you a good productivity enhancing tool. Contact me if i can help set you up.

Hope that helps...
Wayne at Info@DeltaCascade.com