Flow rate estimate?

[fish&chips]

Does anyone have an "appoximation" of the flow rate (GPH) from a 1" bottom drilled tank; assuming a gravity drain into a 1" straight PVC line? Thanks.
[ June 13, 2001: Message edited by: Fish&Chips ]
[ June 13, 2001: Message edited by: Fish&Chips ]

 

[broomer5]

Fish&Chips
I can calculate this for you but need some more info.
What is the height of the water in the tank from the bottom hole to the normal water level at top of tank at flowing conditions ? _______________ inches
What is the specific gravity you plan to be running at at flowing conditions ? ________
Is the 1 inch PVC dumping into a sump ?
Yes ______ No _________
If not ... please explain the set up.
If so, then what is the length of the PVC pipe ? _______________
Lastly - if the PVC pipe is going down to a sump, is the water going to be free falling/splashing out the end into the sump, or will there be a length of the pipe that is extended down into the sump that is "surround" by water ( PVC pipe reaches the bottom of the sump and normal sump level would be _________ inches.
Brian
[ June 13, 2001: Message edited by: broomer5 ]

 

[fish&chips]

Thanks Broomer, here's the info you asked for:
What is the height of the water in the tank from the bottom hole to the normal water level at top of tank at flowing conditions ? "23" inches
What is the specific gravity you plan to be running at at flowing conditions ? Gravity as in Salinity level, right? "1.022"
Is the 1 inch PVC dumping into a sump ? "yes, into a sump.
If so, then what is the length of the PVC pipe ? --> 3" down from the bottom of the tank bulkhead, a 90, then 24" to a 90 on the sump.
Also, how much degradation in flow rate would you estimate if I used flexible corregated tubing instead ...say 3' long.

 

[broomer5]

Okay … First off I am not an engineer, and do not do dynamic flow calculations very often, but nevertheless I think I did this right.
Assuming your tank has a hydrostatic head pressure ( distance ) of 23 inches from the top tank water level to the bulkhead at the bottom of the tank, and the additional 3 inch from bulkhead to the first 90 elbow below the tank, there is a total of 26 inches vertical distance, which equals 26 inches of hydrostatic head pressure if it were freshwater.
Since we are talking saltwater – specific gravity of 1.022 – then the actual head pressure is = 26.57 inches of water pressure.
( 26” x 1.022 S.G. = 26.57”H2O )
1 inch schedule 40 PVC pipe has I.D. = 1.029 inches
26 inch spout ( vertical rise ) is what we are using for height in actual inches
Density of freshwater = 62.19 lbs/feet3 = .0359 lbs/inch3
Density of saltwater = 63.56 lbs/feet3 = .0367 lbs/inch3
Calculation based upon 80 degree F water.
Formula:
Pgz (surface) + Patm = 1/2pVjet(2) + pgz(spout) + P(atm)
P = pressure, pounds per square inch gauge,
g = acceleration of gravity ( 32.2 feet per second per second )
Surface = Conditions at surface of tank water
Patm = atmospheric pressure 14.7 psia
Vjet = velocity
Spout = Conditions at the bottom before the 90 degree elbow
The addition of (2) two 90 degree PVC elbows will reduce the flowrate somewhat – but I figured a resistance coefficient (K) of about .60
These 2 elbows would create a friction loss ( flow loss ) nearly the same as say an additional 5 foot section of 1” schedule 40 PVC pipe would induce.
In other words, don’t worry about the flowrate loss from the elbows.
If I did the math right, I think you would have approximately 30 gallons/minute flowrate at the bottom of the 3 inch section of pipe below the tank bottom bulkhead.
Slightly less flowrate at the sump ( not worth worrying about this loss ).
1800 gph assuming you are in a steady state flow condition, pumping water back up to the tank with a return pump, maintaining the level at the 23 water level mark.
Either I have made an error, or you are planning a pretty good size water circulation system on a BIG ASS tank.
Or both
Hope I didn’t steer you wrong, I tried.
Brian

 

[fish&chips]

Thanks Brian, that matches very closely from another source. So, that would be the top flow rate possible w/no restrictions like a tooth overflow. With regard to the overflow box(in-tank version) the size (length and width) of the over flow would determine the flow rate of the box, right? AM I making sense w/ that last question. Thanks again.

 

[broomer5]

Sure that made sense Perfect sense.
The 1800 gph approximation is just the maximum flow for a 1 inch hole at the bottom, with 26 inches of head pressure ( water at the 23 inch normal tank level + 3 inches under tank )constant flow draining to the sump.
1800 gph is max you'd ever expect to see under those conditions.
Now .... Once you put an overflow in to this situation, you now have a notched weir. Each tooth (notch) of the overflow acts as a small weir and will only allow so much water to pass into the overflow and down to the sump. The amount of water allowed through each notch is determined by the it's shape, width and height.
The wider each notch is, the more flow you'll get through it.
The higher the water level rises up against the overflow toothed notch, the more flow you'll get.
Obviously the more notches you have the more flow you'll get through the entire overflow.
I'm unfamiliar with in-tank overflows, but I do have 2 external overflows with syphons, and would imagine that some in-tank overflows behave similarly. On the external ones, not only is the flow limited to the shape, size and number of notches of the "tank-side" box, but after the water passes through the syphon, what really limits the amount of water flow is where the water passes over the upper rim of the PVC pipe in the external box that runs down to the sump box. This upper rim acts as another weir ( normally has a foam sponge prefilter on it ) and will only allow so much water to pass over it as well. The higher the level in the external box of the overflow, the more water passes to the sump. But in this case, now the water flowing down to the sump is not so much influenced by hydrostatic head pressure, but is really just running down the hose to the sump by gravity. In this example, the hose is not full of water, so there's no "top" pressure acting upon the flow. It just runs down internal sides of the hose.
Do in-tank overflows have a "standpipe" running to them. I'd imagine some do, maybe others don't. I'm not sure. If so then this would be sort of the same thing.
The easiest way to figure out all this stuff is to talk to the manufacturer of the overflow / tank. Or is this a DIY project you are getting in to ?
Brian