Water flowing through a 4ft dia pipe enters from the bottom of a rectangular vessel that is 20ft wide, 5ft high and 8ft long. One side of rectangular pit is open 12ft, all the way up.

If water coming into the pit is equal to water exiting the pit, how can I derive flow rate from level of water in the pit. for instance I measured that level inside the pit was 2ft. Because the level was constant so I know that the incoming and outgoing quantity is same. I don't know how can I guess flowrate without using any kind of flow instrument except the level indicator.

 

Unless there is some relationship between height in the tank and flow in or out of the tank, you can only guess.

I'd recommend that you install a flowmeter on the tank outlet. If the outlet pipe isn't full all the time, you may have further problems.

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Responding to James' 13-Mar-09 (08:12) query... following are several ideas based on weirs, in ascending order of accuracy:

a) Take the height of the outflow as if it were the "head" measurement of a straight weir. Crude, but useful for rough measurement.

b) Construct a weir of 4"x4"x12' timbers, two to four rows high, across the outflow opening, and use the usual straight weir equation.

c) Use the timbers, but cut a "VEE" in the center of the 12' outflow opening, and use the usual "Vee" weir equation.

d) Lastly try what I remember being called the "bump" weir. It consisted of laying down an impediment to flow on the floor of the vessel in front of the outflow opening. Unfortunately I don't remember its formula!

Contact me if you want more info on d) and I will search for it in my old files.

Regards, Phil Corso ([email protected])

 

You say the level inside the pit is 2 ft. and the pipe feeding it is 4 ft in diameter. Do I understand you to mean that the pipe is partially full?

If so, and if the slope of the pipe is known, and the wall roughness can be estimated, you could try using the Manning Flow equation. I will warn you, this equation depends upon estimates of wall roughness and boundary layer behavior. Such measurements are bound to be inaccurate. I would think that if you're getting 10% accuracy, that's as good as you could hope for.

Of course, since you have this "pit", you might think of installing a low head flume in it, such as a Palmer-Bowlus flume. This way, by inducing an hydraulic jump, you can accurately measure flow within a few percent (assuming the pipe doesn't surcharge or flood).

 

Others have given some good ideas, here's my two cents worth, Milltronics (now Siemens) make some good ultrasonic flowmeters that may be adaptable to one of your locations.

If one of your pipes is full can you throw something in and time it over a measured distance to get the velocity?

Perhaps something like a boat speed meter could be lowered into the stream.