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Temporary Flow Measurement Solution With An Improperly Installed Flume

Recently one of our customers was contracted to perform a temporary flow study at a commercial bakery.  The purpose of the flow study was to quantify the volume of process wastewater being discharged so a pretreatment system could be designed and installed to satisfy the requirements of the local water control district.  Our customer reached out to us to assist them in finding a way to measure flow at the facility.  Due to the temporary nature of the flow study the customer was interested in renting equipment rather than purchasing and having our technicians install and remove the equipment.

We were informed that there was a flume installed in an existing manhole.  When we opened the manhole cover we found that there was an 8” Palmer-Bowlus flume, but, there was one problem.  Can you identify it in this picture?  I’ll give you a hint…the flow direction is from left to right.

If you guessed that the flume is installed backwards, you are correct!  Now before we go too far let me clarify something.  A Palmer-Bowlus flume is symmetrical so it can work no matter which direction it is installed.  However, many times a Palmer-Bowlus flume includes an approach section, as in this case.  The approach section is a straight section of pipe upstream of the flume that typically includes a staff gage and an attachment for either a bubble tube or ultrasonic sensor bracket so a flow meter can be utilized to measure water level and calculate flow.  In this installation photograph you can see that the ultrasonic sensor mounting bracket is located on the downstream end of the flume indicating that the flume has been installed backwards.  Since a Palmer-Bowlus flume can still operate when backwards we just needed a way to measure the water level somewhere between 4” and 8” upstream of the flume.  What may be difficult to tell in the photo is that this ideal head measurement location has a concrete cover over the pipe so it is not accessible. 

The solution we proposed was to have a contractor cut away the concrete covering the pipe upstream of the flume exposing the proper water level measurement location.  Once this was done a Teledyne Isco 4230 Bubbler Flow Meter could be used to measure the water level and calculate the flow rate.  The Teledyne Isco 4230 Bubbler Flow Meter uses a small internal air compressor and reservoir.  This air is released slowly by a needle valve into a length of bubble tubing.  The other end of this tube is submerged in the flow stream.  Inside the flow meter, the bubble line also connects to one side of a differential pressure transducer.  As air is released slowly into the bubble line by the needle valve, pressure builds inside the line to force the air out of the line into the flow stream.  When there is enough pressure to counteract the hydrostatic pressure of the flow stream, a bubble will be forced from the end of the line.  The amount of pressure required to force the bubble from the end of the line is directly dependent on the hydrostatic pressure of the flow stream over the end of the bubble line.  The pressure transducer inside the flow meter senses this pressure and converts it into an electrical signal that the 4230 converts into level.  From the measured level detected by the bubbler and consulting lookup tables for the primary device being used, the flow meter then calculates flow rate and total flow.  The bubble tubing was installed in the flow stream by affixing it to a Teledyne Isco Street Level Installation Tool which allows an expandable ring to be inserted into the pipe from ground level without having to conduct a confined space entry. 

You can see the expandable ring installed in the pipe upstream of the flume in the photo below.  The bubble line is seen hanging down and is attached along the edge of the downstream end of the ring.

In order to calibrate the flow meter without having to perform a confined space entry, we used an ELAN QuickCal.  The ELAN QuickCal is a 26” tapered aluminum ruler with an inset indicator strip that turns from gray to black when wet.  The QuickCal attaches to an extendable pole so it can be lowered down the manhole into the flow stream then pulled back up and the water level can be read directly from the indicator strip. 

If you have a similar application or need assistance evaluating equipment options, installing equipment, or performing flow studies please contact your local rep or call our main office at 1-800-333-2252

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