We currently are running our digester feed pumps off of the level of the raw sludge blending tank. The current method of receiving the level is a bubble tube system of which the powers to be installed on the inlet side of the circulation chopper pumps causing the water from the bubble tube to be pulled into the pump causing around a 3 to 5 foot error in level. This is a closed cylindrical tank that is vertical. The environment is very caustic with raw sludge, scum from primaries, high strength cheese waste, cheese brine, DAF sludge, salt drippings, aerobic sludge, leachate, and permeate all being mixed together and brought up to 85 degrees so it does not shock the digesters when dumped in. This stuff has eaten a stainless steel capacitance probe before! I need something that can withstand this environment and give me a 4-20mA signal representing a level of 20 feet. Any advice would be greatly appreciated.

 

What about re-locating the bubble tube? $10 of plastic pipe . . .

Did you correct for specific gravity? A 3-5 foot error on 20 foot tank is 15%-25%. Specific gravity could easily account for some of that error, if no such compensation has been applied.

(To find the SG, go grab a gallon of the cocktail and weigh it (tare out the gallon vessel). Water weights 8.76 pounds per gallon. Divide 8.76 into the weight to get an average specific gravity for such a lovely mix.)

Top mounted ultrasonics or radar will shoot down and give you level, if there is not foam. Ultrasonics can't shoot through foam. Radar can choke on foam, too.

 

correction - gallon of water weighs 8.34 pounds

 

This is a classic case of where a non-contacting level transmitter is the answer. If there is foam or steam, ultrasonic transmitters are contraindicated. If there is no steam, try one. If there is steam or foam, use a radar level transmitter, preferably of the FMCW kind. Both kinds of non-contact transmitters can handle the environment (corrosive, heat, etc.). Radar works better when there is foam or a steam-produced thermocline in the headspace between the sensor and the level.

Walt

Walt Boyes, FInstMC, Chartered Measurement and Control Technologist
Life Fellow, International Society of Automation
Editor in Chief, Control and ControlGlobal.com
[email protected]

 

I agree that a contact-less transmitters will probably do the best job. I would suggest using radar or ultrasonic. Check with your local supplier for the suitable of material used on the sensor.

 

I would use a OPTIWAVE 6300 C from Krohne on a application like this. Its antenna is Teflon coated and was originally designed for Cement silo measurements. So if it is suitable to measure that it is suitable to measure through any kind of foam or emulsion in your application and will withstand your process's corosiveness as well.

I will not consider using Ultra Sonic on a application like this.