This article was printed in CONTROL's August 2009 edition.
By Walt Boyes
Most level applications are for liquids and interfaces between two liquids. Yet there are many applications where it's useful to know the level of a powder or other bulk solid. But, there are some special issues with powders and bulk solids that have to be understood to make those measurements.
Angle of Repose
The first problem is that, while liquids have a flat surface, powders and other solids do not. Depending on how the solid is fed into the vessel, there will be a little mound. So measuring the level needs to account for the mound's angle of repose. What you're really trying to measure is what the level in the vessel would be if there was a flat surface. That's an inferential measurement, not a physical measurement, and this introduces error into the data.
Clumping, Bridging and Ratholing
Finely divided powders and particles that are subject to static electricity, such as plastic pellets, don't flow like liquids either. They clump together, and voids form around the clumps. This makes the apparent level higher than the actual level would be without the voids. Sometimes, when a vessel is emptied from the bottom, the voids form tunnels called ratholes. Bridging is the inverse of ratholing—instead of a void, a bridge of material forms and refuses to flow or move. Sometimes, slow mixing paddles can keep bridging and ratholes under control. In most cases, the vessel or bin can be attached to a vibrator, which shakes the vessel to pack the powder together. However, packing risks causing very large clumps, and vibration makes it hard to measure level.
Sometimes it's necessary to measure the level of materials more properly considered chunks. These too are subject to angle of repose and ratholing issues, but their added problem is that it's impossible to eliminate the voids between chunks. They also can cause extreme bridging, which must often be broken up manually.
In some cases, corrosion and, more commonly, abrasion can be an issue, and can dictate the level measurement method needed.
Measurement Strategies and Techniques
Because it's difficult to make solids level measurements, there are many measurement methods. Choosing the right one often depends on how frequently measurements need to be made. Many applications were traditionally considered so difficult that automation professionals only applied point level measurements for high and low level and overflow alarms. Sometimes, when the value of the measurement is high, a combination of techniques and technologies is used.
The basic measurement techniques used for powders and bulk solids are mechanical point level, minimal- or non-contact point level, continuous mechanical level, non-contact continuous level and weighing. Each technique has features to recommend it, as well as problems that make it difficult or impossible to use in some applications. Mechanical point level systems are susceptible to damage. Minimal- or non-contact point level systems are susceptible to blocking, as well as problems induced by coating and buildup on the sides of the vessel and even on the level probe. Continuous level techniques have issues with angle of repose, ratholing, bridging and buildup. Both mechanical and non-mechanical continuous level technologies in contact with the material can be coated or damaged.
Because of the inherent problems in measuring the level of powders and bulk solids, expected measurement accuracy should be considerably less than that expected with liquid levels. And installed accuracy of a level measurement system may deviate wildly from the specified accuracy of the device performing the measurement.
For example, an ultrasonic or radar level transmitter may have a specified accuracy of ±0.5 in. (approx. 25 mm) or even better when calibrated on the manufacturer's test range, aimed at a solid target with nothing between the transmitter's face and the target. In a 30-ft tall (9.14 m) vessel, with a 60˚ angle of repose, and solids with an average diameter of 3 in. (~75 mm), the accuracy of the installed system might be ±2 in. (~50 mm). This same logic applies to any measurement technology. In fact, vendors regularly get calls from users who complain that the device they're using isn't as accurate as advertised. However, if vendors tried to specify installed accuracy, then nobody would buy such an inaccurate device!
Point Level Measurement Technologies
There are a variety of technologies used in solids level applications, from mechanical flop switches to gamma nuclear point level switches.
Diaphragm switches sense the pressure of the material as it rises to the level of the switch, and then presses on the diaphragm to actuate the switch. If there is ratholing near the switch, however, or bridging above it, the material will not press on the diaphragm, even though the level has risen above it.
Tilt or flop switches are also common and can have extended switch arms able to partially compensate for angle of repose issues. However, tilt switches can be easily damaged during filling, so they must be located away from the filling port.
For many years, the most commonly used point level technology in powders and bulk solids applications was the rotary paddle switch, or "bindicator." When material impedes the free rotation of the paddle, a switch actuates, signaling that the level has risen to its level. The paddle again rotates freely when the switch changes state, indicating the level has fallen.