We are going to a new gas main system. The existing system is 3" black pipe at 15" WG. I am looking at changing pressure from the existing 15" WG to 7" WG, 2psi or higher. I would like your opinions for 2 reasons; 1 I am studying for my PE and wish to know the "test" method for calculating this issue and 2 real world calculations and expectations.

I chose a conservative approach for sizing in as simple a method as possible. So I chose to use each "group" as a point load at the furthest distance from the source even though all will come off the branches at specified intervals. The volume requirement falls off faster than the piping losses allowing for additional capacity or fluctuation in pressure.

I used tables from:

http://www.gpta.net/classes/gas%20sizing/gaspipe%2...
I chose the following for the 2 psi sizing.

2" main @ 30 psi from Gas company

2" line teed to 2 - 2" branches;

1st run 100' 2" line BTU/hr requirement ~7 million; 2" continuing on additional 400'with a ~400K BTU/hr requirement.

2nd run 200' 2" line BTU/hr requirement ~4 million.

The machinery is assumed to be operating at 100% flow at all times.

I used the following link for calculation of initial low pressure system 7" WG:

http://www.engineeringtoolbox.com/natural-gas-pipe...
My calculation came up to ~5" line to support the required flow. Cost wise the 5" is ~2x the cost of existing 3" line.

In the calculation the pressure drop on 7" WG is listed typical at 0.5"/100'. I don't understand how higher pressure drop actually increases the flow volume? I presume the pressure drop would increase the loss and reduce overall flow?

Can the Bernoulli equation be used for this calculation to get accurate results?

Is there any reason to avoid a higher pressure system? It seems the higher pressure yields lower overall cost in material. I have tables going to 5 psi and am comfortable extrapolating to higher pressure if needed. At 5psi I can use 1.5" pipe and achieve similar flow requirements. At 35psi it appears I am safe at 1" pipe.

I wish to use PE plastic after the heating units for transmission to office heaters to reduce cost. Is there any reason to avoid PE in this application?

Thanks

Rich