Pipe flow characteristics
Pipe flow characteristics
(OP)
Looking for help figuring out how to determine the flow a fire hydrant will deliver. We were recently given the results of a fire flow test as 875 gpm with a 98 psi residual pressure. Unfortunately, there is no information on static pressure, but we do not understand what factors would limit the flow with so much (apparently) available excess pressure, especially when the losses in the 8" supply line are so low (only about 0.6 psi/100 feet). Are turbulent conditions within the 6" hydrant feed and 4.5" steamer outlet creating a ceiling on the velocity, and thus the flow rate, that the pressure simply cannot overcome? If so, this suggests at least this hydrant can never deliver much more than about 900 gpm, no matter what the available head. A suggestion on the equations that govern this condition would be much appreciated.





RE: Pipe flow characteristics
Hydrant info at http://firehydrant.org
In particular flow testing:
http://firehydrant.org/info/ftest1.html
Ask that the hydrant be retested.
RE: Pipe flow characteristics
RE: Pipe flow characteristics
If this was a two-hydrant flow test, you will need to know the static pressure in order to determine the drop in pressure at the gage (test) hydrant. This information is used to evaluate the maximum flow available to draft the hydrant down to a 20 psi residual pressure.
For the single-hydrant test you will want to know the nozzle diameter that was flowed. If this was a single-hydrant test flow a, say, 2.5 inch hose outlet with a 0.9 nozzle coefficient, a pitot gage would read about 27 psi to flow the 875 you mention. If a 4.5 inch outlet was flowed during the test, this equates to a pitot pressure of only about 4 psi to produce a flow of 875 gpm.
The hydrant may possibly deliver more than the 875 gpm, which was flowed to atmospheric pressure. When a fire department pumper truck is attached to a hydrant, it actually sucks more water out of the hydrant since it is creating an outlet pressure lower than atmospheric. But remember, the water department always wants the fire department to follow the minimum 20 psi residual rule on its suction pressure to maintain positive pressure and prevent unintended backsiphonage in the water system.
Post more information about the flow test when you get it. Get the static pressure, the nozzle diameter flowed, and if it was a one- or two-hydrant test.
RE: Pipe flow characteristics
RE: Pipe flow characteristics
There is either a problem with the testing methods or a closed valve at the hydrant. In either case, the crew should be sent out.
RE: Pipe flow characteristics
RE: Pipe flow characteristics
are you sure of the 8 inch supply line, could it be 6 inch?
78 psi pressure differential between two ends of a pressure zone is pushing things. 875 gpm should not cause undue pressure drop in the opposite end of the zone.
RE: Pipe flow characteristics
Everybody's water system and topography is different, so it is not practical to judge what is a good or bad pressure for an 875 gpm flow. Our system maintains pressures all the way from bare minimum 35 psi on the hills to 180 psi in the valley, but our system is broken into pressure service levels to serve customers in the 35 to 90 psi range. We have seen every kind of pressure versus flow situation imaginable.
If you request a repeat flow test, ask them to set a gage or pressure recorder on the highest, nearest hydrant and mind it during the flow test, noting the static and residual pressures. Another hint: ask them to allow the hydrant to flow and steady out for a minute or so if possible before taking the pitot and other pressure measurements so the system has had time to equalize somewhat. This is good extra info to use for for pipe model calibration.
RE: Pipe flow characteristics
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