Fire Pump Flow and Hydrant Test 2

[swoosh172]

I received hydrant flow test results and they are as follows: 92 psi static, 90 psi residual and 750gpm. Will I be able to flow a pump at 1250 gpm off of this main without going under the 20 psi residual requirement? Is there a formula or graph to plot the data on?

 

[TravisMack]

Just a standard n^1.85 flow test graph would give you what you are looking for as far as a graph goes.

Your question has many more variables: What is the elevation change between the flow test and your pump? What is the friction loss in the piping between your test and your pump at 1250 gpm?

Just a rough guess, you are some where around 80 psi at 150% of your pump rating.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[stookeyfpe]

Go forth and graph your results utilizing the attached file.

 

[chillylulu]

Right off the main - yes you will be ok.

The pressure at 1250 GPM is 86.85 PSI (77.15 at 1875 GPM - nice guess TravisMack)

The flow at 20 PSI, based on your data, is 5,194 GPM.

I wouldn't trust the flow at 20 PSI because the test pressures are so close. For instance, if the numbers were rounded to whole numbers, the test could have been 92.4 Static & 89.6 Residual - then the flow at 20 PSI would only be 4,343.65 PSI.

The formula for figuring available flow (Q2) at PSI is:

Q2 = Q1*(((S-P2)/(S-P1))^0.54)
where

Q1 = Flow Test GPM
S = Flow Test Static PSI
P1 = Flow Test Residual PSI
P2 = PSI that you want to know the flow for

& PSI (P2) @ known GPM is

P2 = S-((S-P1)*((Q2/Q1)^1.85))

Where Q2 is the GPM that you want to know the PSI for

 

[swoosh172]

Thank you all for the quick responses! I would love to have this kind of flow and pressure on all jobs!

 

[jgailla]

I would be very suspicious of a flow test with those numbers. Even at 750 gpm, there should be more of a pressure drop.

 

[TravisMack]

Yes, the flow test does not seem to go along with NFPA 291 for a proper flow test. If I recall correctly, you are required to get a 25% drop in static / residual to have a flow test in compliance with NFPA 291.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[MGXFP]

Your numbers sound off for this flow test. There may be a pinched valve somewhere in the water main.

As others have said, you should have more of a pressure drop during the test. This may require you to open two 2 1/2" outlets or the steamer during the test. I have seen test where there is hardly a drop over 3000+ GPM but there is little I can do about it.

 

[SprinklerDesigner2]

Something smells bad on that flow test.

As pointed out perhaps a partially closed valve?

Perhaps they used a restricted orifice, perhaps a UL play pipe, for maximum accuracy? I do this often when quality of water supply is in doubt... two play pipes with tips removed use a coefficient of 0.97.

Something I cobbled together a year or so back.

http://www.sprinklercareer.freewebsitehosting.com/flowtest/flowtest.htm

 

[chicopee]

See my JPEG attachment for graphing your data on a semi-log paper , N^1.85 power. Graph shows no issue with residual pressure when flow is at 1250 gpm. Sounds like that you want to connect a pumper to the hydrant.
Valid points raised by above participants. Was the fire hydrant test conducted with two hydrants, one hydrant, or an hydrant in conjunction with bib or sprinkler valve guage reading?
It seems that further flows should have been conducted by flowing two nozzles and purhaps the fire pump connection for additional flows and lower residual pressure readings.
I

 

[SprinklerDesigner2]

At 90 psi residual you should get somewhere between 1,300 and 1,500 gpm from a 2 1/2" hydrant butt assuming it is operating correctly.

 

[chicopee]

Somehow the attachment did not come thru. So here it is again, hopefully.

 

[rmae]

Recall nfpa 13 recommends nfpa 291 for hydrant flow testing. Nfpa 291 recommends that a flow test to be utilized for a sprinkler system should either be flowed to the design flow of the system or until the residual pressure drops 20%. Neither parameter was met in this case. I would reject this test and request a new one be conducted.

R M Arsenault Engineering Inc.

http://www.rmae.ca

 

[TravisMack]

I just looked this up for some one else on another project. You are actually supposed to get system demand or a 25% drop.

It is actually funny when I get flow tests from local jurisdictions where you are not allowed to test it yourself and their tests do not meet NFPA 291 criteria. I also see tests done on the 4" outlet, but they don't make the modification based on using the pumper connection. However, if I modify the flow test results to be per NFPA 291 about the pumper connection, I get the plans rejected. So, I just calc it with the correct flow test, then change it to submit with the "approved" flow test and just show a bigger safety margin.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[Kap61]

NFPA 291 does not list any modifications for higher pressures.

Does anybody know where these would be listed or a formula to figure the proper modification to use?

I only have one down stream hydrant and opening 2-2-1/2" outlets did not drop the residual enough and did not flow my required 3000 GPM, I opened the 4.5" pumper connection and dropped from 80 psi to 60 psi but had a 32 PSI pitot.

 

[TravisMack]

NFPA 291 does have modifications when you use the pumper connection. Look at Table 4.8.2 for use when you use the pumper connection.

With a 32 psi pitot out of a pumper connection, you have to multiply your gpm by 0.83.

Assuming a 0.9 coefficient hydrant, your gpm on that test is 29.83*.9*4.5*4.5*32^.5*.83 = 2552 gpm. If you don't apply the Table 4.8.2, then you would report a 3075 gpm flow test, but this would not be correct per NFPA 291.

If you are needing a 3000 gpm demand for the flow test, you are going to need to perform that test again and open a second hydrant.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[Kap61]

NFPA 291:"the best results are obtained with the pitot pressure (velocity head) maintained between 5 psi and 10 psi"

The chart covers 2 PSI through 7(and over)with a coefficient ranging from 0.97 down to 0.83. I was questioning whether the higher 32 psi would require a lower coefficient than 0.83 since it was over the recommended 10 psi.

I know "Hose Monster" which is FM approved; is rated from 10-75 psi and has a coefficent of 0.548. (Attachement - "Calculating Flow Rates"

 

[TravisMack]

Your pumper modification gets worse as the pitot increases. A pitot of 2 is 0.97 and a pitot of 6 is 0.84. For 7 and over, it is 0.83. So, maybe it is even worse as you get to a pitot of 32. I have no idea. I just know that to be safe, when NFPA 291 says to use the 0.83 for a pitot of 7 or greater, then I will always apply it.

The funny thing is that I have yet to see a water authority or FD apply the pumper modification when supplying the water information for a site.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[chillylulu]

The coefficient of discharge is really a factor that is used to determine the discharge of water from an outlet based on the velocity pressure measured at middle of the water stream, or from the highest velocity in the stream. Over 10 psi the turbulance on the hydrant steamer outlet is too high to simply factor, so a pitot reading doesn't work. The variables from hydrant to hydrant are even more significant.

The turbulance from a steamer outlet can be high for several reasons, two of which are that the water flow has just turned 90 degrees and the steamer outlet is close in size to the barrel (here the steamer outlets are 4-1/2" and I think that the hydrant barrels are 5-1/4" inside.)

Think of water flowing in a river where it makes a 90 degree bend. Fast flowing water would be at the far side of the bend, and at the inside corner the water might be barely moving. If you took measurements of the water flow across the river shortly after the bend, you would expect the water near the far side to be moving faster than water closer to the near side. The flow in the middle of the river would be somewhere in between the inside and outside measured flow. The difference between the inside and outside is greater as the water velocity increases.

If you need higher water flow to get a good flow curve from a hydrant, you need to have a way to accurately determine the gallons flowing from the outlet. A properly installed flow meter would work. A straight piece of pipe could work. I suppose you could attach a large pipe with multiple (stream straightened) outlets. If you measure the stream from all the outlets, you would commulatively get an accurate measurement of flowing gallons per minute.