Flow test results that smell bad.

[SprinklerDesigner2]

Someone made the claim of the following flow test on a 10" city water main.

Standard hydrant with 2 1/2" outlets

Static Pressure 62 psi
Residual Pressure 48 psi
Rate of Flow 1,412 gpm

They did not give a pitot.

This can't be right. I have yet to see a pitot pressure higher than the residual pressure and even if the pitot was 48 psi that would produce 1,163 gpm using a discharge coefficient of 0.90.

So I conducted my own test and got 62 psi static, 48 psi residual with 39 psi pitot for 1,048 gpm using a 0.90 coefficient of discharge. Yeah, it surprised me too that we came out with the exact same static and residual pressures.

Approximately 300' of 8" separated the two hydrants and even if we used a discharge coefficient of 150 a residual pressure of 48 psi would produce 1,040 gpm which is about what I got.

How is it possible someone could have come up with 1,412 gpm? The only answer I could come up with is they used hose monsters on both 2 1/2" outlets and combined them but the fire department who witnessed the test never mentioned two outlets or hose monsters.

And finally (I already know the answer to this) has anyone ever seen a pitot pressure that was higher than the residual upstream hydrant on level ground? Level is it's in the swamp that has alligators, elevation changes is not a factor. (The answer is no, not possible on level ground but I got to ask for someone else).

 

[chicopee]

Since you did a two hydrant flow test, were the two hydrants at the same elevation?

 

[SprinklerDesigner2]

Since you did a two hydrant flow test, were the two hydrants at the same elevation?

Yes, within 1'-0"

 

[cdafd]

Everyone use calibrated Gages???

 

[cdafd]

Plus reading them is it 71, 70, 74????? Depending on the person reading the gage

 

[SprinklerDesigner2]

cdafd,

The most important part of any job is what I did yesterday, the flow test. I don't care how pretty the drawing is if the flow test is inaccurate all you got is garbage so when it comes to calibrated gauges I am probably the biggest stickler on the planet. The bean counters hate it because the calibration cost more than the gauge does.

http://www.dfs-gauges.com/products/35+1009AW+02L+100

But more important than the gauges is the calibration certificate. I get mine done at

http://www.dascosales.com/calibration-and-certification.php

which are tested in accordance with ASME B40.100-2005. The biggest error my last certificate shows (I get these done annually) is 0.27% at 60 psi so it's as accurate as your eyeball.

Unless I am told absolutely not I insist on conducting my own flow test using my gauges with the fire department people watching as a witness. Oftentimes I will suggest they use their gauges to check against mine and when I did this yesterday their gauge read identical to mine.

These gauges are mine and nobody else touches them. I protect them and when not in use they are stored safely in the bottom drawer of my office desk. These gauges due for calibration the beginning of April.

 

[stookeyfpe]

Unless I am missing this, your trying to equate velocity pressure to the hydrodynamic pressures of a system.

 

[SprinklerDesigner2]

Stookey,

No, not trying to it's just that I have never seen results where an obtained pitot pressure was higher than the residual pressure.

 

[rconner]

I'm a little confused of the juxtaposition of the entre' statement,
<<<Someone made the claim of the following flow test on a 10" city water main.>>>

with the later phrase,
<<<Approximately 300' of 8" separated the two hydrants...>>>

While I guess this probably makes some sort of sense, would it be possible to provide maybe a rough sketch of the exact layout of main w/ location of various main size(s?)you say both testers were dealing with, to arrive at these different results?

 

[MGXFP]

Looking through my spreadsheet with years' worth of flow data there are only two instances where the pitot pressure is higher than the residual pressure and those differences are minute (within 10% and on a low pressure main). NFPA 291 will tell you a pressure gage attached to one 2-1/2" outlet on a hydrant flowing the other 2-1/2" outlet will yield close to pitot pressure. Also the listed K factor for a fire hydrant, i read somewhere years ago, is around 148. My statistical data gives me 146 average, median 153 and mode 159. Your 203 is quite an outlier. I think someone simply wrote the results down wrong or made a mistake somewhere else. Your data, SD2, makes much more sense.

 

[SprinklerDesigner2]

Newton,

I've never seen a pitot pressure higher than the residual except where the discharging hydrant was at a significantly lower elevation than the test hydrant.

There's nothing complex about this test; the hydrants are within 1' in elevation of each other, the main is a dead end main with the flowing hydrant at the every end with 300' between the two hydrants.

To clarify the statement to rconner the main is 10" diameter except for the last 300' of pipe between the last two hydrants on the main which is 8".

I've always used a K-factor for a hydrant of 145 only as a double check to see if the results "make sense". With 300' of 8" the k-factor of the hydrant might lower from 145 to 140 but that would be nearly insignificant.

My feeling is someone wrote the wrong number down which is why I prefer doing my own test. Imagine taking someone's word, be they fire department or whoever, only to find out the pump supplying your new 250,000 sq. ft. ESFR warehouse won't work.

 

[MGXFP]

I agree the data from the given flow test is wrong. I do not have the elevation data in my spreadsheet so that may be the culprit to explain the higher pitot vs. residual pressure; in truth I never have sussed out the residual vs. pitot pressure in my data. I use a calibrated diffuser and the ID of the diffuser isn't 2.5" but somewhere near 2.6" where the pitot tube is. I derived the pitot pressures for a 2.5" diameter orifice from the data and compared to residual pressure just out of curiosity.

Like you, I always do my own flow test even if someone else has done the same. I trust my calibrated gages and have documentation that I can rely on to back up whatever I say. Who knows what equipment the other outfit used and in what condition it was in? As you have said; if the flow test is wrong, then everything is wrong.