Weir-in-a-box: Keeps pipe full, but can it measure flowrate? 1

[BobMaron]

I've encountered an application where a customer used a large 'box' with an internal Weir to insure that the upstream pipe is always full. The pipe is 12" diameter, and the 'box' is around 1x1x1 meters in outside dimension. The internal Weir divides the box in half and is about 3/4 of the height. Flow enters at the bottom ofn one side of the Weir, flows over the Weir, and flows out the bottom of the other side. The purpose of this device is to always keep the upstream pipe full. But now would be very convenient to get an approximate flowrate from it by measuring the height of the fluid (slurry in this case) flowing over the Weir.
Has anyone had experience with such a device? If so, please comment on your experience of this "Weir-in-a-box" flowmeter? Thanks very much.
Bob Maron

 

[roydm]

You should be able to get some idea of the flow but may find it hard to calibrate. I don't think you will be able to use any of the standard wier formulas since it's slurry.
What's the reason for keepinfg the pipeline full, I have experience with mining slurry, it sounds like it would sand up.
Roy

 

[itsmoked]

Could be a siphon Roy.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Bob I would think you could use that 'weir in a box' quite well. It would depend on the accuracy you need. It might be many percent off. But that may still be very useful. You could also use it as a "relative" value.

If you could block the exit briefly and catch the resulting overflow you could calibrate the weir.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BobMaron]

Thanks for the reply Roy. The reason for this device is to keep the upstream pipe 100% full so a flowmeter can make a proper measurement. To help keep the pipe full, just upstream of the 'box' there is also a 45 degree upward bend (about 2-4 diameters of vertical rise) to further insure full-pipe condition. Problem is that the flowmeter appears to be working well, but plant people think it's reading high based on some 'assumptions' about the process. We tried using 'standard' Weir formulas and get an answer that is somewhat closer to the flowmeter than to the readings based on plant 'assumptions'. But because the geometry is significantly differnt than that used by the formulas, we're not very confident in the 'Weir' flowrate. I'm hoping somebody might have some experience to share.

 

[BobMaron]

Keith, I don't think it's working as a siphon, based on piping and box geometry. Regarding accuracy, since the plant's estimated flowrate is ~1/2 the flowmeter value, then accuracy of ~10% or so would be quite acceptable. That relatively low accuracy requirement is why I think this Weir-in-a-box measuremt could be very useful. Calibration IS apparently the issue. Catching and measuring the discharge is certainly an option, but not a convenient one considering the pipe size (12") and the rigid piping that is integral to this continuous process. Has anyone tried to calibrate such a box??? Seems like it could be a useful device (maintains full pipe for whatever reason, and also serves to check flowrate occasionally). Await with interest your replies. Thanks in advance.

 

[MikeHalloran]

I think you could calibrate a simple straight weir with a bucket and a sluice, traversed across the weir.

For a sluice, fabricate a thin channel, with wide flanges, say a 1" web and 4" flanges (or whatever you need to capture the depth of the overflow) in say 18 gage metal. Lay the channel on top of the weir, web down, and redirect a fraction of the weir's flow into the bucket. Traverse the channel across the weir a web at a time and sum the individual flowrates.

If it's a notch weir, you can find formulas in old M.E. handbooks.



Mike Halloran
Pembroke Pines, FL, USA

 

[roydm]

I assume the flowmeter upstream is a mag flow. Since it's a slurry is it possible that the line is sanded out at the bottom thus giving a higher velocity (higher flow reading)through the meter.
If the line is open at both ends another way of checking flow would be to put something in and time it from one end to the other.
What sort of slurry?
Roy

 

[BobMaron]

Roy, your comments are right on. This is iron ore slurry in a concentrator plant. We also think sanding is a real possibility. The upstream meter is a sonar-type meter. It's relatively new technology in mining; about 5 years old. See

http://www.cidra.com

for more info. Anyway, we're going to check for sanding, although it's hard to do without opening the line. This is rubber-lined steel pipe, so checking for sanding non-invasively is difficuly. Suggestions?
I know about using 'tracers' to check flowrate, but that's only a velocity check, and if there are sanded sections of significant length, then the procedure will not accurately the volumetric flowrate. Right?
Roy, do you work with slurry flows?
Any other comments Roy?
Thanks in advance.
Bob

 

[roydm]

Bob, you might be able to detect a different sound, tapping with a hammer top to bottom.
Yes,I worked in mining, mainly copper & gold for many years but not so much now.
I assume you don't have a vertical section to place your meter, on a horizontal line you will get a flow profile with the heavier portion traveling slower along the bottom.
Interesting new meter, have you tried it at different locations on the pipe, 3 o'clock, 6 o'clock etc?
Have you compared it with a mag flow on vertical line, e.g. cyclone feed?
I assume it allows for the two layers, steel and rubber.
Regards
Roy roy_matson at yahoo dot ca

 

[waross]

How about changing the shape of the weir to a shape for which a formula is available?

Bill
--------------------
"Why not the best?"
Jimmy Carter