Pitot tube size differences for standpipe testing.

[RainbowFS]

Some time back I made some alterations to a standpipe system. This forum proved invaluable for hydraulic calculations, and now I return with a question regarding flow testing of it.
Flow verification testing will consist of flowing three (2 1/2") ports simultaneously. Each port needs to flow 250 gpm while maintaining 100 psi residual.
A pitot at each port will be used to measure the 250 gpm.
My question is- what size pitot orifice do I need to accurately measure this?
I have a (handheld)pitot tube that I use for hydrant testing. Its gauge has an effective range from 10-30 psi, at 10 psi the flow is 530 gpm, at 30 it's 920 gpm. A reading of 250 gpm would be around 2 psi on my gauge, way outside the effective measuring range.
In all my searching online for various pitot tubes- I can never find anything regarding what range and flow a given tube will measure, at least in a manner somewhat like I described of my own.
Can anyone give me some insight on what kind/size of pitot tube I need to measure 250 gpm out of a 2 1/2" port?

Alex Traw
Rainbow Fire Sprinklers
Albany, Oregon

 

[cdafd]

Put a Gage with lower scale on?? 0- 20

 

[cdafd]

The other thing is as long as you are hitting above 2 on all your flows to me you are good to go

 

[skdesigner]

Sounds like you're using a standard pitot gauge. What flow apparatus are you using? Is it possible to screw on playpipes and pitot them? I've seen manifolds built to attach 2.5" hoses to playpipes on a solid mount, but it may not apply if you're on the roof...

 

[LCREP]

Are you going to flow this off a roof, or onto the roof? Do you have a test header on the roof? Do you have a pressure gauge at the top of the standpipe riser?

If you plan to flow off or on the roof try looking at hose monster.

http://www.hosemonster.com/

They have pitots built into the test device so you can flow water safely. They can flow 250-1500 gpm depending on the size of the outlet. Also with hose monster they stay put and will not lift you and the 2 guys off the roof. This is the safe way to flow water. Down side is they are $$$.

Make sure if you are flowing water onto the roof it is in good condition and it can take 750 gpm. Check roof drains to make sure they are open and the roof does not have cracks and is in poor condition.

The flow is 750 gpm at 100 psi. 3 lines at 250 gpm each or one at 750 gpm, does not matter. Just have to meet 100 psi at the top gauge while flowing 750 gpm, if that is your design.

Hope this helps.

****************************************
Fire Sprinklers Save Firefighters’ Lives Too!

 

[FFP1]

Pitot tube readings below 10 psi are considered unreliable and the test would not be valid. You also cannot test directly from the 2.5 in. diameter test header valves.......you will have a turbulent water flow pattern and there is no suitable coefficient for this arrangement.

You will need to use playpipes, hose monsters or some other attachment to obtain the laminar water flow for a reliable flow test. I suggest fire hoses & playpipes with the 1 1/8 in. diameter ends attached. You will need 46 psi from each playpipe to flow 248 gpm. Make sure the playpipes are properly secured!

 

[RainbowFS]

Thanks for the responses. Two of the ports are on the roof of the hospital and the third is in the basement just outside surgery!
I'm hip to the turbulance issue and while I do have a beautiful set of 2.475 aluminum playpipes, using them in this situation would be a bit violent. I have inline couplings with pressure gauges and a possible outlet to borrow a Hose Monster, but not three.
[Rant- adds little to the topic at hand.]
The local fire chief, who came from the city (his words- "When I was in The City we did..."...big city things presumably) expects all of these ports flowing and reading at the same time, which puts a strain on this two man outfit. I am paying to rent the pumper truck from the Fire Dept with two men to the tune of $280 per hour, then when I asked if we could use some fire hose to run the flow up out of the basement I was refused- "With budgets these days we're pretty thin on hoses, so I doubt it." I asked what I was getting with the pumper truck if it has no hoses?, and I got very little in the way of response.
Turns out "the city" is Redmond, Washington, a town which no one is actually from. Better known as the home of Microsoft. Here is what we are having to do, almost word for word-

http://www.redmond.gov/insidecityhall/fire/prevention/PDF/flowtestguidelines.pdf

[Rant off]
I managed to put a note in my bid regarding extraordinary costs relating to testing and passing those costs on to the hospital. So I got that on my side, but I still see little sense in spending a pile of money on things I'll likely never use again. I've already spent a good bit getting certified gauges, and now I need to figure out this pitot bit.

Alex Traw
Rainbow Fire Sprinklers
Albany, Oregon

 

[cdafd]

Have you do e a standpipe before???

Does not seem out if the ordinary and seems Richmond has done standpipes before