determining standard flowrate of pvc water pipes

quick estimating Q=3600*((pi*D2)/4)*V Q-flowrate m3/h pi-3.1415 D-diameter,m. V-flow rate,m/sec (for gravity pipe(PN1)=1-1.5m/sec; for pressure pipe=2-3m/sec)

or.. if you don't like quick estimating ;-) must-see http://www.flsnet.it/ => engineering info => Velocity / Flowrate Conversion

What exactly are you asking?

1. What is the recommended flowrate through different sizes of pipe to minimize pressure drops?
2. Are you designing/building to some type of code? Such as International Plumbing Code?
   
3. Do you have an existing piping system and you are trying to determine what flowrate you have?
4. ......?..........?..........?........

Does this website help you at all?

Do a google search and look for something like "economic speed fluid flow".

For each fluid to be carried out, the guys already figured out what's the better approach to use for the fluid speed, considering the required pipe size to be installed, general pressure losses, etc.

Besides of that, one can choose any speed for fluid movement. But it will be more and more expensive as the speed grows, up to approximately the speed of sound in the fluid being conveyed, when it's sad to be choked, and any increase in pressure will not reflect an increase in flow.