I walked through an attic this afternoon where all of the existing
mains need to be replaced. The piping is 3" schedule 10 black steel,
that is developing leaks after 17 years (we found another one on our
tour). The proposal is out there to replace the existing with 3"
schedule 40 galvanized (and provide proper pitch). I did a couple of
quick calculations and at 250 gpm, the friction loss per foot for the
two pipes is very close to the same. (.077 for the sch 10 black v.
.075 for the sch 40 galv). My guess is that the difference in
pressure would be about 0.75 psi. The calc has about 12 psi
remaining. If this was strictly a pipe replacement (no other
modifications), would you recommend new hydraulic calculations?
Todd G. Williams, PE
Fire Protection Design/Consulting
Stonington, Connecticut
www.fpdc.com
860.535.2080
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