Capacity of 6" schedule 40 pipe at 3 psi. And, where can I find a pressure table for capacities of various size pipes at various inlet pressures

Strait from the engineer tool box.

Natural Gas - Pipe Sizing (http://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html)

For more tables etc.

http://www.google.com/search?hl=en&source=hp&q=gas+pipe+capacity+table&aq=0&oq=Gas+pipe+capacity&aqi=g1g-m1