The diagrams below can be used to indicate vacuum pump exhaust pipe line capacity:
Air flow - scfm
Air flow - nl/s
Example - Vacuum Pump Exhaust Pipe
The exhaust air flow from a vacuum pump is 1200 cfm and the required length of the exhaust piping is 60 ft.
As we can see the range of the diagram above is limited but with half the flow - 600 cfm - the maximum length of a 6" pipe is approximately 120 ft. Or in other words - with two 6" pipes in parallel there will be enough capacity.
Alternatively - if we compare an 8"pipe with a 6" pipe - we see that the internal area of the 8" pipe is twice the area of the 6" pipe - and we can assume that a single 8" pipe provides enough capacity.