Gravity flow from circular pipe out of tank
Gravity flow from circular pipe out of tank
(OP)
I am trying to determine the gravity flow out of an oil imterceptor tank. Water is flowing out of the tank through a 50mm diameter pipe and the pipe is about half full. Is there acalculation I can do along the lines of that for a v-notch wier to calculate the approximete rate of flow.





RE: Gravity flow from circular pipe out of tank
You can calculate pipeflow using mannings equation (as this is essentially open channel flow), there are published tables for flow rates and velocities (HR Wallingford Tables), you can determine full bore discharge using colebrook-white and modify the diameter using the Butler pinkerton shape correction factor to account for pipes part full. However, these are only valid if you do not have significant head behind the pipe and assume steady non-uniform flow.
As you have a tank with an inlet and an outlet it might be more complicated. If the tank is completely full all the flow that enters will ultimately leave, subject to the buffering capacity of the tank itself. It is also likely to flow in surges instead of a constant flow. You basically have a length small diameter pipe, widening to a very large length or pipe, narrowing back down to a small length of pipe. This is an non uniform flow situation. And if the flow entering is changing with respect to time it also becomes an unsteady flow situation.
As an approximatation, I would do a rough calc for a number of different top water levels, use that to calculate a hydraulic gradient across the tank and use that values as gradient (slope) then plug that into mannings equation.
RE: Gravity flow from circular pipe out of tank
I have an open topped tank with pipe inlet at half bore and the outlet pipe is acting as an overflow so is flowing out under gravity at about half bore.
I have got Manning's formula but can you help me with the following:
Roughness - In a reference I have this can vary from 0.017 to 0.067, 0.17 being a smooth earth channel, but this is for channels dug in natural ground (I am a geotechnical engineer). I have found an internet link http:
Hydraulic Radius / Crosss sectional area - Is this the cross sectional area of the whole channel or just the area of the channel which is under water, i.e. corresponding to the wet perimeter.
Thanks
RE: Gravity flow from circular pipe out of tank
Mannings equation should provide a reasonable estimation of flow rate based on the depth of water in the pipe.
RE: Gravity flow from circular pipe out of tank
RE: Gravity flow from circular pipe out of tank
Clean, uncoated cast iron 0.013-0.015
Clean, coated cast iron 0.012-0.014
dirty, tuberculed cast iron 0.015-0.035
There is a theoretical formula for n found in Ven T Chow's book "Open Channel Hydraulics"
n={[(R/k)^0.167]*[1/(21.9log(12.2R/k)]}*{(R/k)k^0.167)
Where R is the Hydraulic Radius
and k is the roughness height
RE: Gravity flow from circular pipe out of tank
RE: Gravity flow from circular pipe out of tank
Do you have a slope on the exit pipe? (sounds like yes)
if so, does the flow according to the Mannings equation greater or less than critical? (probably greater)
if greater, then the tank exit-pipe entrance is a critical point therefor the level in the tank will depend solely upon the critical equation and the downstream pipe will have no effect upon the tank level,
besides the critical losses, you should include the entrance loss which is dependant upon if the pipe is reentrant, square, or champered as it connects to the tank.
Hydrae
RE: Gravity flow from circular pipe out of tank
If you wanted to get really precise, assuming it is only a pipe without any other flow controls,I'd analze the pipe as a culvet. You'd have to look at whether it was inlet, outlet, or barrel control. Then you get into whether the inlets and outlets are submerged ect. ect. Not the easiest thing to do for someone not familiar with hydraulics.
Kwalton, If you gave a more detailed description of the system, everyone may be able to provide you with a better answer.