Pitot tube vs hydrant cap gauge 1

[fpst]

NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants (2013) says:

"4.9.1 If a pitot tube is not available for use to measure the
hydrant discharge, a 50 or 60 psi (3.5 or 4.0 bar) gauge tapped
into a hydrant cap can be used.

4.9.2 The hydrant cap with gauge attached is placed on one
outlet, and the flow is allowed to take place through the other
outlet at the same elevation.

4.9.3 The readings obtained from a gauge so located, and the
readings obtained from a gauge on a pitot tube held in the
stream, are approximately the same."

My question is, what's the advantage to using a pitot tube and holding it in a flow stream? Wouldn't results be more uniform and accurate with the hydrant-cap-gauge method, especially across different individuals. (less prone to holding the pitot in the incorrect place, or turbulent flows, etc.)
I've read from two sources (NFPA 291 and Fire Protection Hydraulics and Water Supply Analysis by Pat Brock that the hydrant-cap-gauge method is virtually identical to the pitot tube, so I'm looking for any drawbacks that might exist for the hydrant-cap-gauge method as it seems to be the better option so far.

 

[fpst]

What I think is going on is that the pitot gauge is measuring the pressure of the water as it accelerates through a known orifice size (the 2.5" hydrant outlet), which gives you the velocity pressure and that subsequently gives you the flow rate. In order to get the actual velocity pressure inside the 2.5" orifice, you have to hit the "sweet spot" in the middle of the water stream. According to Bernoulli's theorem, nonflowing water at the same elevation in the exact same orifice size should be identical.

Imagine a very strange magical hydrant, with one typical pumper outlet, one typical 2.5" outlet, and another 2.5" outlet which is 10 miles long all at the same elevation. Since the trapped 2.5" outlet with its 10 mile long piping is "trapped" dead end water which isn't in the flowing path, the pressure should be instantly the same over the 10 mile distance, so that if you had a gauge at the end of the 10 mile 2.5" outlet, it should also read the same as a pitot reading in the short 2.5" flowing outlet, since it's at the same elevation.

The reason the static/residual pressure gauge on the test hydrant wouldn't be able to do this, is because it's measuring the pressure in the path of the flow stream, therefore the residual pressure is changing due to friction loss. In the trapped 10 mile long 2.5" outlet, the water is theoretically, more or less static.

Another method to all this is the Pitotless nozzle

http://www.hosemonster.com/filebin/images/userguides/Inst_PNTHD_2013.pdf

Which seems to be the same thing as the idea above, except you're measuring the residual pressure in the flow stream just before it reaches atmosphere (but at a distance before the outlet so that water isn't behaving too turbulently). Also, since the distance between the gauge and the outlet is mere inches, the difference in pressure due to friction loss is negligible.

So, it appears there are at least THREE ways to do this flow testing thing

Please someone correct me if I'm going off on a horribly wrong path

 

[MGXFP]

@fpst

I would think (dangerous, I know) that flowing an outlet 90 degrees from the pressure cap would probably have too much turbulence to measure or read the gage accurately. Having never done it myself I can't comment. Next time I do a flow test I'll try an experiment. We use a calibrated diffuser and calibrated pressure gages. I'll try comparing the results of the calibrated diffuser and the pressure gage on the flowing hydrant. My thought is the relationship between the flow and the square root of the pressure will mean little to the final calculation, especially after the hydrant coefficient and the flow rounding per 291 is applied.

@cdafd

The residual reading would be measured on the non flowing hydrant.

 

[fpst]

That is what SprinklerDesigner2 says as well and he says he's seen it happen as he's done it before.

However, even if the water is turbulent, it may be the case that the pressure at that point is the same no matter the turbulence, and the gauge on the hydrant cap is measuring the pressure without caring what direction the water is flowing , a pitot cares about direction and therefore turbulence, but maybe not non-pitot gauges.

However, I've read on the hosemaster website that their pitotless method can have problems with "suction" causing misreadings, this "suction" is apparently caused by turbulence, and the way to correct it is more or less to straighten out the flow by using an extended hose or playpipe before the gauge.

However, we know that the middle of the water stream in a pipe is the fastest moving. On a typical cap-gauge, or the hosemaster pitotless nozzle, it appears the gauge is measuring on the very top surface of the cross-sectional area of the pipe, which would, in theory, not be as "turbulent" as the rest of it, since it's not leaving an orifice or anything.

Who knows.

@Matt

NFPA 291 2013 edition says to use two hydrants at a minimum, not sure if it didn't before

 

[MatthewJWillis]

Hummm

4.9.2 The hydrant cap with gauge attached is placed on one
outlet, and the flow is allowed to take place through the other
outlet at the same elevation.


Seems like this is allowing one. Gauge on one outlet and measure thru the other...

Where in 291 is the 2 minimum specified?

R/
Matt

 

[fpst]

4.9.2 is talking solely about the flowing hydrant,

elsewhere...

 

[MatthewJWillis]

I sit corrected. My apologies.
As penance, I will read 291 again as it has been almost 20 years since I have.
Thank you.

R/
Matt

 

[cdafd]

Matt

Ok, I agree with your post, just thought I was missing something in the process



""""""For hydrant testing you can use one hydrant as evidenced in your 291 post above.

For a fire sprinkler system you are required to use 2 hydrants closet to the building.
You flow the hydrant most distant to the building. You take static and residuals at the hydrant closet to the building. It is a two person operation although one could do it going back and forth.
The way I remember it is that you do not want to get your new building wet.


R/
Matt""""""""""

 

[MGXFP]

To follow up on my previous post we did a flow test today with a hydrant cap pressure gage in one 2-1/2" snoot, a diffuser in the opposite 2-1/2" snoot and both gages read a pressure of 25 psi under flow conditions.

It appears to me this method works just fine.

 

[TravisMack]

Thanks, Newton. That does seem to correlate with what Luke was reading.

This is why I love these forums. You always learn something new.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[cdafd]

I take it was a pitot reading on the diffuser??

It would seem though that you would get a different reading if you were flowing "open butt"?

 

[MGXFP]

Yes, the pitot is in the diffuser. We like the diffuser because it places the pitot where it should be and that is (Radius or Diameter)/2 away from the butt I believe. Everyone using a pitot tube holds it tight to the butt of the snoot and that isn't the ideal way to measure the pressure.

 

[cdafd]

What I was saying was

If they were trying to do a flow test with just a cap gage, and open butt

It seems like with the diffuser in place

It would make the pressure readings different than without the diffuser there


And I am talking not taking a pitot reading, just the cap gage on the other side

 

[MGXFP]

It shouldn't make a difference. The law of continuity should hold and there be no appreciable restriction to the flow emerging from the diffuser versus that of an open snoot. The diffuser increases the area of the water flow and slows the velocity but the flow is the same.

The stagnation pressure would be the same.