I am wondering what the best diameter of pipe is for a given volume flow rate.  I understand that conductors are sized based on flow velocity, surface roughness etc. but I have read that fluid friction is reduced at the point of transition from laminar to turbulent...Is there a critical velocity for each given pipe size?

Also, if you look at a Moody diagram you'll see that Moody friction factor decreases with an increase in Reynolds number.  How does this all relate to design and pressure drop?

The best pipe diameter is one that minimizes the life cycle cost of the system.  The installation cost and operating costs are considered using time value of money concepts.  Since there is a tradeoff between large pipes (high initial cost, low pumping cost) and small pipes (low initial cost, high pumping cost) there exists an optimum diameter.

If you are looking for "rule of thumb" information as a starting point, consider 5-8 ft/s velocity as a first pass to start an itterative analysis of varying pipes sizes.

Its simple folks:  for single phase liquid flow (non-viscous, non-abrasive, etc.) use 7 to 10 feet per second (minimum of 3, maximum of 15).  for single phase gas flow, use 45 to 100 feet per second; but only after first checking pressure drop (use .25 to .50 psi/100 feet).  Then, depending upon future expansion, short term peaking, available space, equipment nozzles, etc. choose a diameter that is one larger or smaller.  

If you are calculating life-cycle cost effectiveness, you have too much time on your hands.

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