Pipe flow characteristics

[ALeahy]

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.

 

[bimr]

4-Inch outlets should be capable of a minimum flow of 1000 gpm with a pressure loss of not more than 5 psi between the street main and the outlet.

Hydrant info at

http://firehydrant.org

In particular flow testing:

http://firehydrant.org/info/ftest1.html

Ask that the hydrant be retested.

 

[rconner]

(and that the isolation and hydrant valves are fully open during same?)

 

[greggy]

The flow test results you mention are perfectly understandable for an older tuberculated water system that maintains a high static pressure at the hydrant nozzle with no or very low flows, then poops out when the flow is increased due to the extreme head losses through the pipe from the source to the outlet.

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.

 

[ALeahy]

Thanks to all for the assistance, in particular greggy's in depth evaluation. Obviously, we need to prod the system's owner into redoing the test and giving us a static head, at the very least. Hopefully they will cooperate.

 

[bimr]

You did say that you have "875 gpm with a 98 psi residual pressure." I assumed that this is not the static pressure. With that high of a residual pressure, it is not possible to have fouled lines.

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.

 

[77JQX]

Is it possible they're limiting the fire flow to 875 gpm based on pressure at a remote location? In our system, we will report attainable fire flows that are conditioned upon maintaining 20 psi in all parts of the system, not just at the flowing hydrant. In particular, in a pressure zone with varying elevation, pressure at the upper elevations often limit the design fire flows for lower elevations, regardless of the residual pressure at the flowing hydrant. If this is the case, then an 875 gpm flow with 98 psi residual makes sense.

 

[cvg]

was it 98 or 19.8 psi residual?
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.

 

[greggy]

Agree with 77JQX in that water system operators must mind keeping at least 20 psi in ALL portions of the distribution system during a fire or other emergency, particularly if the service area is hilly where pressures in elevated areas drop when someone in the valley opens a hydrant and thinks all is well because the pressure there is adequate, not realizing that water customers at the top of the hill have low or no pressure.

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.

 

[stanier]

You may also like to check the accuracy of the instruments used in the test. Unless they have current calibration certificates the results are questionable.


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