That is a very legitimate concern. My guess is that you could very
easily have a problem, especially if your static head is less that
4.6 psi. In my experience, there are very few good uses for 4" pipe
prior to the system riser. With 6" pipe, your friction loss would be
about 2 psi.
At 01:32 PM 10/31/2008, you wrote:
400 gpm fire pump supplied from tanks with suction flanges higher than pump
suction flange. Approximately 100 feet of 4" piping (including fittings)
from tanks to pump. NFPA 20 permits the gauge pressure at the pump suction
flange to drop to -3 psi, with the pump flowing at 150% (600 gpm in this
case). The friction loss in the piping for 600 gpm flow is about 7.6 psi. Is
there a concern for the pressure available at the pump suction, when the
water level drops in the tanks? I have been told that it will, so checking
the opinion of the forum.
Tony
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Todd G. Williams, PE
Fire Protection Design/Consulting
Stonington, Connecticut
www.fpdc.com
860.535.2080
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