Can Someone please explain why NYC will require a 3" main for just 33 fixure units on ground fl

[msm15]

I guess my real question is can someone please explain to me what the difference is between "Allowable flow - gpm" and "Theoretical discharge"

I have to admit my experience is a little limited in this area so I am just going based off what I read and charts I found, but the reason I ask is because we have to bring in a new water main into a building for domestic service. We will only be supplying water to a few bathrooms - a total of 2 urinals, 8 toilets, and 5 sinks. I get a total of about 33 fixture units for a demand of about 40 gpm. The building is set back about 160' off the street so we will have to run a decent amount of pipe but I know the street pressure is about 50 psi at 500 gpm residual as measured off the nearest hydrant.

Based on those #'s I get a theoretical discharge of 844 gpm on a 2" pipe @50 psi (NFPA 24 Table c.4.10.1(a). We want to use an 1-1/2" - with a theoretical discharge of 474 gpm
DEP has on file that the "Allowable Flow" for a 2" starts at 100 gpm and goes down as the pipe gets longer. I cant say I have ever seen anyone sitting there regulating the gpm's on these pipes and I know the flow out of the hydrant orifice is a lot more then the "Allowable Flow" so what am I missing - why cant we size the pipe based on theoretical discharge

 

[MiketheEngineer]

Because they can!!

Bring what they want to the house and do what ever you want??

Probably the cheapest solution in the long run!!

 

[msm15]

um no - the difference in cost between a 3" wet connection/service line and an 1-1/2" tap is about $10,000......

 

[MiketheEngineer]

Really?? Didn't think it would be anywhere that much difference. Then go fight with their engineers - show your calcs and methods. If you sign off on it - then it is your responsibility and problem if anything happens. They SHOULD buy off on that!!

 

[zdas04]

"Flow out the hydrant" is going from 50 psig to 0 psig. Guess what, flow rate really is based on differential pressure. The 500 gpm you mentioned is flow rate out an open-ended pipe with no friction losses whatsoever.

So if you use small pipe you get a big pressure drop. If your friction loss is 49.9 psi at 100 gpm, then the dP from the main into your pipe is 0.1 psi and you are not going to get 100 gpm. No one is doing your snarky "regulating gpms", mother nature is doing that. If you don't like it then you are just SOL. You can use slicker pipe. You can use bigger pipe. A flow rate and pipe characteristics (i.e., size, length, and roughness) fully determines the pressure drop. If the friction loss is too great for a flow rate, then you don't get the flow rate. Pretty simple.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[msm15]

Ok, but pressure in the city can vary tremendously like from 85 psi in north manhattan down to 40 psi and lower in other areas, yet DEP has their "allowable flow"

Is the allowable flow a challengeable subject ? Allowable is a funny term - it sounds like they are saying its the max - you mean to tell me that max allowable flow is 100 gpm for a 2" pipe - 10' long as per their charts ?? regardless of inlet pressure ?

Please excuse my lack of knowledge in this subject but it does seem a bit off

 

[zdas04]

I've never built plumbing in NYC so I can't speak for their intentions. I would guess that there is a "contract" minimum pressure on the mains of some value high enough to get some number of floors up a high rise (it takes 540 psi to overcome the hydrostatic head at the top of the Empire State Building, so the contract pressure is not high enough to get to the top floors). If the contract pressure is 30 psig, then if they have to provide 85 psig in one part of their system to reach 30 psig at the end of the pipe that is what they do. You can't rely on always having the 85 psig, but you can rely on the 30 psig.

So if their "contract pressure" is 30 psig and the minimum water pressure at a dishwasher is 20 psig, you have 10 psid to lose to friction. Knowing that dP and the pipe characteristics would allow you to calculate a flow rate. The charts you are talking about have done that using standard fluid dynamics arithmetic.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[btrueblood]

I wonder if the flow is limited to keep the velocity below what might cause noise complaints?

Alternately, NYC has had problems with hard water in the past - perhaps the concern is to provide a reasonable service life before the main constricts due to deposits.

 

[IRstuff]

One other reason is that they probably assume that your construction will wind up low-side compliant and that the as-built flows will be lower than your design values, but the people will complain to the water department, since the contractor and designer will be long gone.

TTFN
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