Difficult Level Measurements

“Ask the Experts” is moderated by noted process control authority Béla Lipták (liptakbela@aol.com), editor of the Instrument Engineer’s Handbook. In this column, he and his co-authors welcome questions concerning process measurement or control. Please include P&ID drawings using ISA symbols to illustrate questions where necessary.

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Note: The three questions below are similar, so we are answering them together. The goal in each case is to achieve reliable and accurate level measurement in hard-to-handle, corrosive and boiling applications.

Q: We have a large tank in which sodium hydroxide is mixed into water and the high pH (>13.5) caustic mixture is heated to 200 °F and continuously agitated and recirculated. (It goes to another tank where it flows over extrusion dies, and then back to the main tank.)

Evaporation, other losses and the attendant addition of makeup water and NaOH make it necessary to know the level in the tank. But the fluid in the tank is a hot, dirty, frothy, corrosive nightmare. It is hard to measure level in there, and we have destroyed several types of sensors.
One suggestion is to use pressure sensors at the bottom of the tank to determine level. Is that a good way to measure level in a tank?

William Love
Kredit Automation & Controls


Level Detection of Several Fluid Types
Level detection of sticky, viscous, hard-to-handle, boiling, foaming or corrosive fluids.

Our site manufactures PVC additives. One of our core materials is high-melt paraffin wax, which makes measuring tank levels difficult. We have tried various methods of measuring tank levels (radar, infrared, etc.), but all have been inadequate. Floats or hydrostatic systems get fouled due to heat or wax buildup. Currently we take the measurement manually, but we want to automate the process. Key characteristics for us are 1) The material cannot be touched because it sticks to everything; 2) It must withstand 300 ºF and moderate agitation.Q:

Any suggestions? We’ve heard of nuclear measurement but haven’t found any documentation.

Douglas Dieselberg
Honeywell International

Q: Recently in Control you had an article about drum level median select logic. This got my attention because I’m seeking this information to change our logic (we have a Bailey inf-90) from manual transmitter select to median select.

First you have “less than” blocks with staggered inputs going to a “greater than” block. What is the difference between using “less than” and “greater than” at transmitter inputs?

If a transmitter fails or loop opens, will that drum level be maintained? Is this possible?
Also you mentioned alarms such that the low would be from the lowest, and the high would be from the highest transmitter at any given time. Is this using the same logic as above?

Daniel Sanchez
Colmac Energy

A: The most frequently used level measurement device on slurry and sludge services is the extended-diaphragm  differential pressure transmitter (Figure 1a). The diaphragm extension eliminates the dead-ended cavity in the nozzle where materials accumulate and brings the sensing diaphragm flush with the inside surface of the tank. A Teflon coating on the sensing diaphragm or using corrosion-resistant materials can further minimize material buildup.

One of the best methods of keeping the low-pressure side of the d/p cell clean is to also insert an extended-diaphragm device in the upper nozzle. This can be a pressure repeater, which is capable of repeating both vacuums and pressures if they are within the range of the available vacuum, and plant or instrument air supply pressures. Outside of these pressures, extended-diaphragm chemical seals can be used if they are properly compensated for ambient temperature variations and sun exposure.

D/p measurement detects the mass and not the volume of the tank contents. Therefore, if the density varies, d/p detection does not detect level. When the tank contents are boiling, both the boil rate and the vapor pressure in the vessel will affect the volume percentage taken up by the bubbles. As they vary, bubbles will swell or collapse, and the same amount of fluid will take up different volumes. Therefore, if the level (and not the mass) is to be detected, d/p sensors require density compensation, which can be done with a second d/p located so that both the high and low pressure sensors are in the liquid (Figure 1b). A second choice is to use separate point sensors (level switches) located in the vapor space and provide safety overrides when the level is too high or too low. A third option is to use external devices, such as radiation gauges.

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