Flow rate calculation? 1

[ContractorDave]

If I have my static and residual flow pressures and I know my pipe sizes and equvelant pipe lengths for fittings, how do I solve for gpm? For example a 2" main drain test with 75 psi static and 65 psi residual; with 10' of sch 40 steel c factor 120, (2) 90s and (1) angle valve. Can I use a variation of the Hazen Williams formula to do this? I have it attached wher I solve for pressure loss. Can I change this around and if so how?

 

[LCREP]

We tried this back in the 1980's, ran many field tests and followed up with a fire hydrant test to verify the results. The results, apple and oranges, the 2" drain was not in the same ball park as the hydrant flow test. You ever look inside of a ball valve of a 2" drain, about 50% of the flow because of the restriction caused by the valve stem , gasket, etc, you never get the full flow, that is the problem.

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Fire Sprinklers Save Firefighters’ Lives Too!

 

[ContractorDave]

I'm doing forward flow testing for backflow preventers. Main drain piping is not usually arranged in a fashion conducive to proper pitot tube testing. Any thoughts on this? I need to confirm that a backflow device flows in gpm the system demand.

 

[TravisMack]

You can always drain the system, remove the check on the fdc and reverse. Then, flow out the FDC as long as it is not a double clapper FDC.

Good luck!

Travis Mack
MFP Design, LLC

 

[LCREP]

Ultrasonic flow meter will work. We did some testing and based on pressures of 60-150 psi we got flows from 150-400 gpm. What is the occupancy and or demand?

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Fire Sprinklers Save Firefighters’ Lives Too!

 

[ContractorDave]

Thanks Travis.
Draining down the system could more than double the amount of time required to test the DCVA, hence my interest in a mathematical solution. I would simply pitot the main drain (for light, OH1 and OH2) but 99% of the time (my guestimate) one would not have a piping arrangement that will give a good laminar flow.

Thanks LCREP
I had thought of this and wondered how accurate they were. Pretty expensive, but it would pay for itself eventually. Would you have a little more information on the make and model you used, if you recall, and your experiences with it?

Regards all

 

[ContractorDave]

Further to this, in the NFPA Pocket Guide to Sprinkler System Installation, 2nd edition (2004) on page 389 there are 2 tables given (VII.5 and .6) which gives figures for Actual Flow Rates based on "Service Pressure" (it doesn't specifiy if these are static or residual). These would most likely be the average readings after many flow tests of a typical type of drain piping arrangement (those similiar to Short Discharge or Long Discharge as described). As an inspector who had to approve a device based on flow rates, would anyone here use these figures and qualify the report by referencing this handbook? Do these figures appear accurate?

Regards

 

[LCREP]

Light,

It was 5-7 years ago and do recall all the details, just the flows. I know we have still have the meter in the Training Center, next time I am at the center, I will get the name of the unit.

Tom

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Fire Sprinklers Save Firefighters’ Lives Too!

 

[cdafd]

I could see possibly doing the forward flow through the 2 inch, but the problem is most systems you are going to run into will not be able to achieve the flow rate required.

not an engineer that drives a train is there some reason that table 5-2A out of the fire protection handbook 7th edition cannot be used????

 

[ContractorDave]

gads man!! 7th edition?? I hope we are speaking of two seperate references because mine is the 19th edition. Flow rates for the system demand of a hydraullically claculated system including hose stream can usually be achieved through a 2" main drain for light hazard and OH1 and 2.

 

[cdafd]

HAY watch it we are not the train engineers so do not use FPH much since it is in other stuff also.
I have been trying to get a company to give me the hydraulic clacs to prove one system they are installing, to see if they can use the two inch drain.

 

[ContractorDave]

Lol..
I have to admit that your reference to train engineers slips by me... sprinkler engineers are like train engineers: not much going on? (don't want to start a war here though! )
Thanks for your replies cdafd.

 

[Bhhowie]

Why can't you get a flow test from the system outside your facility and then do the math to bring it to the base of the riser?