GPM calculation on 8" pipe...

[mattinirvine]

How would I calculate GPM on an 8"water main. We recently broke a water main and I mean it BROKE! Major flood in our basement. I want to look into sizing up our sump system but want to know the max capabilities of our domestic water output. I am only looking for a rough estimate. Here are the specs...

Water PSI is about 110-115 and that is about where the break happened... so psi loss is not a factor and the pipe is 8"

 

[katmar]

If the source of the 110 psi was 100 feet away from the break you could get close to 10,000 gpm

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[jgailla]

I think it is less than 10,000 gpm. If the total energy of the pressure is converted into velocity ((p/gamma)*2g)^0.5), the velocity is about 37 ft/sec. For an 8" pipe, 37 ft/sec is about 5800 gallons per minute. This would be the upper bound of the flow. The flow would be reduced by the friction loss in the pipe between the pressure measurement and the break.
The conditions of the break and the surrounding materials would also have an effect. For a robust distribution system at 110 psi, I would expect the break to put out somewhere about 5,000 gpm.

 

[cvg]

come on guys, be realistic - unless the water mains in the street are huge, you could never get 10,000 gpm. It's also highly unlikely you will have anywhere near 5,000 gpm. Perhaps half of that.

 

[Artisi]

cvg - I would agree, I think we can assume the OP means 110-115 psi mains pressure not pressure after any failure.

 

[rconner]

To get the highest flow suggested here would require a source capable of delivering that quantity of water close to the break site (like a 24"/600mm pipeline delivering water only to the break and at a flow rate of more than 10 fps 3 m/sec??) If that source is not there, maybe you wouldn't get that level of flow?

 

[rconner]

Oops (I thought someone originally said 17,000 gpm -- @ 10,000 gpm a 24" main would have to supply w/ flow of a little less than I mentioned)

 

[mattinirvine]

Thanks in advance for all the help ... maybe I can clarify...

Under normal operating static pressure at the break, I had a guage that read 110-115. The break was at a PVC flange fitting and broke the entire circumfrence of the fitting. The supply was domestic water from the city and it took me about 20 minutes to turn the water off at the meters. I couldn't see the break while the water was flowing because the room it was in had so much water in it and we couldn't force the water back by pushing on the door with three guys.

good times... and thankful for insurance since my domestic water pumps and controls got submurged.

 

[katmar]

cvg - the numbers may be unrealistic, but then I thought that people that have 8" connections to their basements may have 24" street mains ;-)

More realistic is around 10 fps in the 8" pipe giving 1500 gpm. But this really is a "how long is a piece of string?" question.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[rconner]

I'm sure whatever the flow rate was with that level of pressure was quite a mess! (An 8" pvc fitting sounds like risky business).

 

[mattinirvine]

Mess doesn't begin to cover it.

I understand what you are saying Katmar.

Thanks for all of your help.

As for the PVC fitting... well it is for a Sterling water conditioner. Not sure if I want to put another one in. I haven't been convinced of its performance in the first place.

Thanks to everyone for your help... I kind of have a better idea of what kind of water was lost.

Matt

 

[rmw]

I cut an 8" water main once while demonstrating a tranching machine. I had no way of measuring water flow, but I can tell you it was a whale of a lot of water awful fast. Filled ditches along the road, inundated other trenching machines up and down the line....before they got it shut off. Heck of a mess.

rmw

and PS: the client being demonstrated to didn't buy our machine.

 

[katmar]

rmw, I can't help but think of that classic poster of the train falling off the bridge, and the engineer saying "Oh dear!" (or some similar expression). Pity you lost the sale.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com