×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Pitot tube vs hydrant cap gauge

Pitot tube vs hydrant cap gauge

Pitot tube vs hydrant cap gauge

(OP)
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.

RE: Pitot tube vs hydrant cap gauge

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.

RE: Pitot tube vs hydrant cap gauge

(OP)
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.

RE: Pitot tube vs hydrant cap gauge

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
www.mfpdesign.com
"Follow" us at https://www.facebook.com/pages/MFP-Design-LLC/9221...

RE: Pitot tube vs hydrant cap gauge

(OP)
@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.

RE: Pitot tube vs hydrant cap gauge

(OP)

RE: Pitot tube vs hydrant cap gauge

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

RE: Pitot tube vs hydrant cap gauge

(OP)
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.

RE: Pitot tube vs hydrant cap gauge

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.

RE: Pitot tube vs hydrant cap gauge

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
www.mfpdesign.com
"Follow" us at https://www.facebook.com/pages/MFP-Design-LLC/9221...

RE: Pitot tube vs hydrant cap gauge

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?

RE: Pitot tube vs hydrant cap gauge

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......

RE: Pitot tube vs hydrant cap gauge

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
www.mfpdesign.com
"Follow" us at https://www.facebook.com/pages/MFP-Design-LLC/9221...

RE: Pitot tube vs hydrant cap gauge

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.

RE: Pitot tube vs hydrant cap gauge

(OP)
@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.

RE: Pitot tube vs hydrant cap gauge

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."

RE: Pitot tube vs hydrant cap gauge

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

RE: Pitot tube vs hydrant cap gauge

Matt

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

RE: Pitot tube vs hydrant cap gauge

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

RE: Pitot tube vs hydrant cap gauge

(OP)
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/userguid...
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 smile

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

RE: Pitot tube vs hydrant cap gauge

@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.

RE: Pitot tube vs hydrant cap gauge

(OP)
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

RE: Pitot tube vs hydrant cap gauge

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

RE: Pitot tube vs hydrant cap gauge

(OP)
4.9.2 is talking solely about the flowing hydrant,



elsewhere...

RE: Pitot tube vs hydrant cap gauge

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

RE: Pitot tube vs hydrant cap gauge

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""""""""""

RE: Pitot tube vs hydrant cap gauge

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.

RE: Pitot tube vs hydrant cap gauge


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"?

RE: Pitot tube vs hydrant cap gauge

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.

RE: Pitot tube vs hydrant cap gauge

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

RE: Pitot tube vs hydrant cap gauge

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources