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.

 

[dssprdesigner]

The advantage for a pitot tube would be when you need to measure a larger volume of flow than one single 2-1/2" hydrant outlet can provide.

 

[fpst]

It was my interpretation that you can open multiple hydrants (each with a hydrant cap-gauge attached) and add the flows or use the hydrant-cap-gauge on a single hydrant attached to the 2.5" outlet, and flow the pumper outlet.

 

[TravisMack]

With the pitot tube, I can measure the pumper outlet or the 2½" or even both 2½" to get flow per NFPA 291.

I would suggest you take a hydrant cap / gauge and a pitot tube to the next couple of flow tests. See if the values are the same. Intuitively, I don't see how they could be the same, but it may be.

Many don't realize that a flow test per NFPA 291 requires a 25% drop in the static/residual, or to flow the minimum amount for fire fighting purposes. I would bet that most minimums for fire fighting purposes is 1500 gpm (based on appendix B of the IFC). So, if you only have a single hydrant cap, you may not be able to get those flows with a single 2½" outlet.



Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[fpst]

@Travis
I was reading NFPA 291 a lot today,
I did not notice it before either,
but technically it says "should" and NFPA 291 is a guideline (uses the word should instead of shall in totality) - another thing I didn't realize until today. Which brings up the question is calibrated gauges are even technically required.. scary.

Art told me the main drawback to the hydrant-cap-gauge is that the reading will jump up and down due to the turbulence inside the hydrant, so I guess I'll see whenever we have our next flow test.

 

[fpst]

 

[TravisMack]

I realize it is titled Recommended, however, it is IMO, good practice to follow the guidelines to the extent possible.

Travis Mack
MFP Design, LLC

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[cdafd]

Have done a number of flows lately where the gages are off .

Did one where neither the cap or pitot would even move.

One where the pitot did not zero out after the test

 

[fpst]

It wasn't obvious to me, even though I read the same title, however I wasn't pointing it out to try and evade any best practices, I merely thought it was peculiar that the most important part of our job has no actual legal enforcement. A shoddy flow test is technically allowed as I understand it, and that would be the most major impairment to the success of a sprinkler system and it represents a major blunder on NFPA's part. I have heard of instances of people (like city representatives) using uncalibrated gauges etc. to do flow tests, and always wondered why it was not stopped - well, the title and language in NFPA 291 is probably the answer.

 

[cdafd]

With just using a cap gage,

And flowing open butt 2 1/2

If you use this method, The reading on the cap gage would be both the residual and pressure to calc the flow.

 

[TravisMack]

CDA:

I believe the initial assumption is that the cap/gauge on the flowing hydrant would be nearly identical to the pitot gauge reading.

What you are implying is that residual pressure and pitot pressure would always be the same. That is not the case. Pitot gauges are measuring the velocity pressure in a flowing orifice which is converted to a flow rate, I believe.

Travis Mack
MFP Design, LLC

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[cdafd]

I understand that just trying to follow this thread

I took it and will look at 291 tomorrow or next day

That it was proposed attach a cap gage, open the opposite 2 1/2 and read the cap gage while the other 2 1/2 is flowing?

 

[LCREP]

CD you got it...

Always amazes me we take a water supply test done by gauges that are jumping around during the test and put the info into a computer hydraulic program that computes the system to .00 psi. We require a 10 psi safety factor on all new systems for just this reason. How manny water flow companies doing the test calibrate the gauges annualy? Heck in some parts of the country they will not let you flow water and provide you with a computer model to design the system too. Trying doing a flow test in NYC, $500, takes 6 months and you get the same results 50-40 500 GPM.

The joys of water flow testing......

 

[TravisMack]

Yeah. Stil a two hydrant test. Hydrant #1 has the static and residual taken. Hydrant #2 has the cap/gauge on one 2½" side while you are flowing out the other side. Apparently, the cap/gauge reading will be the same or close to the pitot gauge reading.

I agree that water flow testing is one of the most important parts of the project, but it is often done without much care given to it. Just send out a couple of rookies with old gauges they can find on the shelf and get it done.

Travis Mack
MFP Design, LLC

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[MGXFP]

Some cities use a max day calculation to use for the design of the fire sprinkler system. The max day is basically the day of the year that uses the most water in the city. This gives pretty large factors of safety on systems which already have large factors of safety if you examine the record of # of sprinklers in the design area vs. # of sprinklers that actually operate in a fire statistically.

The pitot tube measures stagnation pressure i.e. the pressure of the flowing water where the velocity is zero and is derived using Bernoulli's Theorem. It makes sense in that light to use a pressure gage opposite the flowing snoot because you could argue the velocity of the water in the non flowing snoot is zero or close to zero because the water is flowing around the other way to the open snoot (no flow, no velocity). This only works when both 2-1/2" outlets are used; don't think it would be valid for the pumper outlet because the two outlets are at different elevations.

Flow results are suggested in NFPA 291 to be rounded to the nearest 50 gpm if flows are less than 1,000 gpm and to the nearest 100 gpm if flows exceed 1,000 gpm so the method isn't the most precise and we also apply factors to the pitot formula because it is a theoretical and not empirical formula. I wouldn't think you would get any different results using the pressure gage or a pitot tube. The advantage to the pitot tube is one can measure flow from any orifice, but if you're out in the field and forgot your pitot tube the pressure cap works almost as well.

 

[fpst]

@NewtonFP

Some hydrants have the pumper outlet centerline at the same elevation as the 2.5" outlet centerline, I wonder if this would be okay to use a hydrant-cap-gauge with?

However, even on hydrants with pumper outlets slightly below (or maybe even above) the 2.5" outlets, would this minor elevation change significantly impact the results, or just make a negligible differentation?
I ask because it's pretty important here in Georgia. A great deal of hydrants are installed "lopsided" so that the 2.5" outlets aren't even at the same elevation, even though if the hydrant was vertical they would be.

 

[cdafd]

Looks like this was added in the 2010 edition,

wonder why?

Seems like if you cap one side and flow the other side, you are using the "residual reading" to determine the flow?




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."

 

[MatthewJWillis]

Careful, it looks like this post is veering toward water flow testing for sprinkler systems. Which is a totally different animal from Hydrant marking for fire fighting...

R/
Matt

 

[cdafd]

Matt

I will ask what is the difference in the way actual test is done? For sprinkler design and fire hydrant marking?

 

[MatthewJWillis]

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