Level measurement in conical polymer tank with mixer


Thread Starter

Aaron M. Webster

Measuring liquid polymer level in a 1500 gallon High-density linear polyethylene conical tank with a mixer in the center and pump discharge in the bottom. This level will also be a permissive for the transfer pumps.

The operators mix 1000-1500 gallons of water with the calculated amount of powdered polymer in this tank. The motor powered mixer makes a large vortex in the tank and creates a significant amount of surface disruption including bubbles and foam at times. After the mixing time is complete the operators stop the mixer and start the transfer pumps. They would like to have high and low level switches and a tank level. They do not need a volume measurement, tank distance will be OK. We have tried a drexilbrook us-2x to some success but we have experienced loss of echo. We also tried a outside mounted non-contacting capacitance level switch for the low level. However after some use the tank has a build-up of polymer and the level switch no longer works reliably. There is not much clearance inside of the tank with the huge mixer and the operators are uncomfortable with us adding taps to the sides of the tank. The only tap is also used for pump suction. My next idea was to look at using some type of flow switch to shut down the pumps on low level, but I am open to suggestions.

Please dive into your experience and give me some suggestions.

Aaron M. Webster
You have a real problem, because what you want is the "level in the tank if the mixer is turned off" and that's not an actual measurement. The vortexing in the tank when the mixer is on is probably why you kept losing ultrasonic echo.

Your level measurement device should work when the mixer is off. If you can, keep trying to use the ultrasonic device. Radar level devices are dielectric constant based, and may not work as well as if you were working with a plain, water based liquid like an acid.

Can you tell if the loss of signal corresponds to the point at which the mixer blades are exposed?


Walt Boyes, Life Fellow, ISA; Fellow InstMC
Chartered Measurement and Control Technologist
Spitzer and Boyes LLC

**Spitzer and Boyes LLC publishes the Industrial Instrumentation INSIDER (www.iainsider.com)**