I need to model some 3/4" thick flanges for 20" and 30" Dia. pipe and I can't find how much they should weigh anywhere. I'm thinking of finding the weight of a section of pipe with the same thickness and length as the flanges and using that. Would that work? I've attaches the flange patterns.

I also have a bit of square duct that I'm not sure if I'm modeling correct... I used a diameter that yielded the same area as the duct ( H*W=pi/4*d^2 or d=([H*W]*[4/pi])^(1/2) Where H=1'-10 3/4" and W=2'-1 1/8" ).