k-factor for a "typical" fire hydrant?

[SprinklerDesigner2]

Doing a small light hazard job in a town where the fire department has a policy of not obtaining a residual pressure when conducting flow tests. Don't ask.

So what they are telling me is we have a 60 psi static and a flow of 1,100 gpm through a 2 1/2" outlet so knowing what a hydrant k-factor is we can work backward to find a residual.

I know this isn't exact but since it's light hazard with a maximum of 250 gpm required any number would be close enough if I design to say 50 psi available at the street. I would never do this for a more demanding Ordinary or storage design.

Anyway, what I need is a K-Factor so I can provide "something" so I can get this job done over the weekend.

I seem to remember FM once figured a k-factor at 260.

 

[GrandmarkFP]

I would not assume anything. Refer the fire department to:
NFPA-291 "Recommended Practice for Fire Flow Testing and Marking of Hydrants"

 

[TravisMack]

Does the city have a mandate that all fire hydrants be capable of providing 1500 gpm @ 20 psi. In the rare cases where I have not been able to get a residual pressure (water dept won't provide, but won't let you run a flow test) for one reason or another, I have taken the static pressure, then used 20 psi @ 1500 gpm as the residual. I also try to leave a very hefty safety margin in those systems as well. Again, these have only been for light hazard systems.

Travis Mack
MFP Design, LLC

 

[firepe]

You're going way way way out on limb!!

I agree with Grandmark. Do not assume anything. You have absolutely no way of knowing anything about the supply piping, such as partially closed valves, corrosion, etc. that may affect the results in any given area.

I would not want to be the engineer of record on this if that's what you're gonna base your calcs on. I am also curious why you are bent on using someone else's information.

I would not put my name on anything that my company did not conduct, or was at least present to witness the test results, period.

Go and do your own test. You're responsible for it, you do it, that's my advice.

 

[cdafd]

there is a site for everything:

http://www.firehydrant.org/

http://www.haestad.com/library/books/awdm/online/wwhelp/wwhimpl/java/html/wwhelp.htm

 

[SprinklerDesigner2]

"I am also curious why you are bent on using someone else's information."

In many areas of North Georgia you couldn't get a flow test right now if your life depended on it until the governor lifts his executive order prohibiting fire departments from flushing mains or conducting flow tests. Last time I checked Atlanta was down to something like 45 days of water left if we didn't get some much needed rain and we haven't got much since.

I've always been one to insist doing my own flow test (I trust my gauges) recognizing I can make a lot of mistakes, run into beams, hit duct work, miss a measurement to name three, and I'll survive all those but the one mistake you don't want to do is calculate a system to water that isn't there.

In the meantime there's a project going up and I need to get pipe up before I am boxed out.

Hey guys, nobody here needs to tell me this isn't the hottest setup in the world but I got to come up with something..

So what do we have to work with?

We got a flow test of a 12" city watermain, a test done a year ago, that yields a 60 psi static and 1,100 gpm flow.

I went through my records finding 5 flow tests in different parts of the country.

#1 78 static, 65 residual @ 1,245 gpm k=154.4
#2 82 static, 77 residual @ 1,385 gpm k=157.8
#3 58 static, 50 residual @ 1,180 gpm k=166.9
#4 72 static, 62 residual @ 1,210 gpm k=153.7
#5 58 static, 48 residual @ 1,035 gpm k=149.4

For all these tests the k-factors are all pretty similar giving us an average k-factor of 156.4 and what is interesting here is all these tests were conducted on rather large (8” to 12”) circulating city water mains so head loss between the test and flowing hydrants would be very little.

I venture to guess if you look back through some old flow tests, these having modern hydrants with large circulating mains, I think you will find the hydrant k-factors are similar to what I came up with.

Modern hydrants are important I checked some older ones and they appear to have a k-factor of around 110. I wouldn't feel comfortable doing this on small dead end mains.

So how much pressure would we need at the hydrant shoe to flow 1,100 gpm?

p=(q/k)^2

On a hydrant having a k-factor of 156.4 we would need 49.5 psi at the hydrant shoe to discharge 1,100 gpm. This sounds reasonable to me and safe enough for now since it is light hazard, ceiling height is only 10' and my area of calculations would only be 900 sq. ft.. To play it on the safe side I'll calculate to 60 static, 40 residual flowing 1,100 gpm leaving myself a good 12 to 17 psi “safety factor”.

I am going to present this to the local fire official with the idea we'll run a flow test to verify what we have when the rains come back.

 

[SprinklerDesigner2]

http://www.haestad.com/library/books/awdm/online/wwhelp/wwhimpl/java/html/wwhelp.htm

Wow, I like that!

 

[FFP1]

To answer your question......I have some FM documentation which indicates the following:

-For a standard hydrant butt with a 2 3/8 in. orifice, the K-Factor is 134.

-For a standard hydrant butt with a 2 1/2 in. orifice, the K-Factor is 149.

-For a standard hydrant butt with a 2 5/8 in. orifice, the K-Factor is 164.

I agree with the conservative approach suggested by others on this blog. My first question would be whether or not you are confident that the 60 psi and 1,100 gpm data is correct. If not find a way to conduct a reliable hydrant flow test or do some research to obtain historical data for the region (the FM Plan review department is a very good resource for water flow tests at FM insured locations in the surrounding area). If you must continue with no reliable hydrant flow test, I would suggest using an even more conservative approach than you have outlined above. If you design to a 60/1,100/30 water supply AND include a 15 psi safety factor, you will sleep like a baby. Given your small operating area and low ceiling height, there should be plenty of pressure available to make everything work out for your light hazard system. So what if you end up using a little more 2 in. pipe than 1 1/2 in. pipe.

I hope this helps and good luck!

 

[SprinklerDesigner2]

-For a standard hydrant butt with a 2 1/2 in. orifice, the K-Factor is 149.

Kind of gratifying to know I wasn't that far off with the 156.4 I managed to cobble up from a series of old flow tests.

In a wierd way I miss the days before computers when we were calcuating grids by hand using the Hardy-Cross method. After working four or five days on a set of calcs you felt like you really accomplished something.

Looking over a rather large database, took me 30 years to collect, it seems the 150 k-factor is pretty consistent where the main is at leastan 8" circulating main but on dead end 6" mains the k-factors seem to run around 110 even after compensating for head loss between the test and flowing hydrants.

I was thinking maybe the c-value of 130 to 140 I was using was way off. One one test logical answer I could come up with was the c-value was around 70 but that sure seems low. Possible I suppose but still seems low. And not confined to one test but seems to be the case with all the dead end/low crummy water flow tests.

Anyone know why?

In this area the terrain is pretty flat and I feel good about the 60 static. I need to do something but I'll play very conservative.

Thanks. Good info.

 

[trashcanman]

Hey, Sprinkler Designer2,
As per your post:

"In a wierd way I miss the days before computers when we were calcuating grids by hand using the Hardy-Cross method. After working four or five days on a set of calcs you felt like you really accomplished something. "

I am curious about how that is done. As far as I know the HC method assumes fixed flows at outlets, and you then calculate the flows around the loop. However, as we know, the flows at the outlets are dependant upon the pressures. So what do you use for the outlet flows?

 

[SprinklerDesigner2]

The last time I did this was 25 years ago and you're asking a guy who can't remember what he had for breakfast?

The first exposure I had was at one of Jack Woods three day hydraulic calculation seminars Viking would have at their plant in Hastings, Michigan. One could follow along pretty well for day one and two but he managed to lose everyone on day three when we left trees and simple loops for ladder grids. Jack called the method Hardy-Cross but not a one of us knew how to do it when we left.

I do remember we assumed all outlets flowed the same and while that is what Jack taught I think most of us developed our own hybrid methods.

Ladder grids were easy we worked hard to keep them symmetrical making the job of calculating easy enough.

It's like that college trig class of so long ago; you remember doing it but couldn't do it today.