Hydrant Flow Test
Hydrant Flow Test
(OP)
Hello,
A flow test was performed on a hydrant with one 2-1/2" butt opened. Flow was measured at 990 gpm at 65 psig residual pressure.
A second test was performed on the same hydrant with two, 2-1/2" butts opened. Flow was measured at 850 gpm at 48 psig residual pressure.
Can anyone explain why the flow would decrease?
I do know this:
The tests were performed within minutes of each other using high quality equipment and trained personnel.
The water supply is a municipal, networked system in a large city.
The city's fire marshal was also present and I know there were no firefighting activities taking place during the time of the test.
Thanks.
A flow test was performed on a hydrant with one 2-1/2" butt opened. Flow was measured at 990 gpm at 65 psig residual pressure.
A second test was performed on the same hydrant with two, 2-1/2" butts opened. Flow was measured at 850 gpm at 48 psig residual pressure.
Can anyone explain why the flow would decrease?
I do know this:
The tests were performed within minutes of each other using high quality equipment and trained personnel.
The water supply is a municipal, networked system in a large city.
The city's fire marshal was also present and I know there were no firefighting activities taking place during the time of the test.
Thanks.





RE: Hydrant Flow Test
Your system obeys the hydraulic balance law. The available pressure balances exactly the friction losses including the pressure drop at outlets. Friction losses are a function of flow. The flow and pressure at the outlets obey the following formula
Flow = k * sqrt(pressure), k=constant
In the first case you system exhibits a
k factor=990/sqrt(65)=122.8
In the second case, you system would therefore deliver
flow = 122.8 * sqrt (48) = 850 gpm
Amazing accuracy, isn't it?
RE: Hydrant Flow Test
I failed to mention that the flow of 850 gpm was the total of both butts' flow rate.
The hydrant's k-factor would change as soon as the second butt was opened.
Opening the second butt would be similar to further opening a faucet, allowing more flow with a drop in residual pressure.
It's really not making sense to me.
RE: Hydrant Flow Test
If you have 990 gpm at 65 psi, you must have more flow at a lower psi. The water system, given some assumptions, works the same way as a pump curve.
You have a certain static pressure at no flow and the higher the flow the lower the pressure will be.
I'm not sure why a test would be run with two butts open. I don't have the NFPA standard with me, but I believe it specifies to use only one outlet.
I would rerun the test and/or test a nearby hydrant. The two hydrant flows should be similar in a networked and robust system.
RE: Hydrant Flow Test
If data were all good, I'd look for an explanation such as the water utillity noticed the high flow and shut down a booster pump or some control valve (but normally the opposite should happen...)
RE: Hydrant Flow Test
I am now getting more information regarding the test with two butts open.
The pitot gauge (Scale = 1 to 100 psia) readings were 5 psia for one butt and 8 psia for the other butt.
Perhaps these readings are too low to be accurate??
RE: Hydrant Flow Test
RE: Hydrant Flow Test
In the first test, one diffuser type pitot tube was used.
In the second test, the same diffuser type pitot tube was used for one butt and a hand held blade type pitot was used for the second butt. The sum of both measurements was used to calculate flow.