Hydrant Flow Test

[KLH]

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.

 

[skoutso]

Its perfectly normal.

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?

 

[KLH]

Thanks skoutso.

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.

 

[jgailla]

If the 850 gpm is the total flow from both outlets, and if there were no significant changes in demand between the two tests, the tests are wrong.
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.

 

[CarlB]

I agree with jgailla. Do you have the mesured data 9pito readings, c value used, etc)? Where was the residual pressure meared, on a nearby htdrant/

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...)

 

[KLH]

Thanks all.

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??

 

[rconner]

Was just curious as ikt was not specifically stated was pressure in the main BEFORE hydrant flow the same in both tests described (also, when hydrants flowed was one flow meter used to come up with the flow value mentioned in the latter test, or is this the sum of two, inividual flow-meter values)?

 

[KLH]

The static pressure before each test was the same, 110 psia.

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.