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? 

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.

Several years ago there was a book written on this very subject, I have a copy.  The info is as follows:  Optimum Pipe Size Selection, by Claude B. Nolte, Trans Tech Publications, Clausthal, Germany, 1978.  

The chapter titles are as follows:  1.  Fundamentals of optimum pipe size selection, 2.  Least annual cost, 3.  Selection according to pressure drop available, 4.  Velocity allowable, 5.  Two-phase flow, 6.  Fluid mechanics parameters, 7.  Preparations, 8.  Pipe size selection for pump or compressor discharge, 9.  Pipe size selection for pump or compressor suctions, 10.  Pipe size selection for pressure drop allowable, 11.  Steam piping for heating and reciprocating pumps, 12.  Steam piping for turbines, 13.  pressure let-down for gas, 14.  Relief valve discharge headers, 15.  Steam trap piping, 16.  Two-phase flow applications, 17.  Special velocity considerations, 18.  Comparative capacities of pipes, 19.  Mechods of approach, 20.  Conclusion.  

The book contains numerous examples of all types of piping applications, one to fit almost any circumstance.  I highly recommend it! 

As a thumb-rule, vapor velocities are limited to 30 m/s and keeping the dP less than 0.113 bar/ 100 m.
Also, liquid velocities are limited to 3 m/s but to 1.5 m/s for corrosive services.
The above are general guidelines and should be evaluated for the case considered.
For 2-phase flows, erosion velocity criterion needs to be checked (refer API 14E).
However, for 2-phase flow, nobody clearly mentions about phase velocities to be limited apart from erosion criteria. Logically, it should be checked. If anyone has something on this, let me know (especially kstaylor may have something in the book mentioned).

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