The art and science of solids level measurement
Granular solids are a collection of macroscopic particles such as snow, sand, rice or coal. Although the form of granular solids is easy to explain, their behavior is very complex. Solids behave differently from liquids and gases so methods of measuring level are specialized and require as much “feel” as technology. There is an art to the techniques described in that a certain amount of experience is required to properly design and apply solids handling systems. Accurately measuring the quantity of solids in a tank or silo is crucial to product management and custody control and transfer. For the purposes of this discussion, it will be assumed the solids levels being monitored are in granular form.
As with liquid level measurement, devices fall into two broad categories: noncontact and contact. Within those categories, devices are broken down further into point-level and continuous-level monitoring. This article describes the principles behind those devices and some applications. Solids level measurement is not as cut and dry as liquid level measurement. Liquids have a characteristic weight that can be translated into level using static pressure devices; solids can vary widely within batches. Liquids also find their own level and present a uniform surface to a measuring device in the absence of mixing or other disturbances. This is one of the challenges of solids measurement.
Solids tend to present an uneven surface to the measuring device, and load or settle unevenly in the container in which they are stored. Finding a level surface to bounce a signal from is challenging.
Noncontact devices used for solids measurement are identical to those used in liquid level measurement with ultrasonic, radar and lasers being the most common devices. Ultrasonic devices have the advantage of being inexpensive and their behavior understood. Unfortunately, this leads to misapplication, which in turn leads to inconsistent results. The same holds true for radar devices. The reason for this is the tendency of solids to resist settling into a uniform and level surface. More often, solids are delivered to a tank or silo by a conveyor that dumps the solids into one spot, causing it to form a cone determined by that solids’ “angle of repose.” When that angle is exceeded, there is a mass settling or sloughing. If a noncontact device is monitoring the top of the cone, this sloughing will result in a sudden change in level. However, as is usually the case, the device is placed adjacent to the delivery area, a true level will never be realized, and the sloughing will again cause a sudden change in level.
The issue is the problem of reflection from an angled surface. Ultrasonic, guided-wave radar (GWR) and laser devices rely on the reflection of a signal from the surface of the material being measured. The surface of a liquid is uniform and presents a good reflecting surface for the signal “bounce.” Solids vary in consistency, granular size and of course, angle of repose. The angle of repose is the angle a solid will naturally settle if it is delivered as a consistent stream. The sloughing previously mentioned is a result of the cone’s height creating an angle that exceeds that repose. Every solid has a characteristic angle of repose. This can be used in a point-level application.
A noncontact measuring device transducer can be mounted at the angle of repose to determine when a solids cone reaches a control point such as a high-level alarm. Continuous level measurement is not so easy, however. The lack of a uniform surface prevents a coherent reflection from being returned to the transducer, and the varying granular size creates a scattering effect—both conditions result in an unreliable signal. Lasers are more reliable point-level devices and are more accurate in determining the presence of a solid at a control point. However, they come with a higher price tag.
A laser can also be mounted in an “over the shoulder” arrangement where the mounting of a device over the container is impractical. A recent application is the filling of a truck with sludge cake. It was impractical to mount a level monitoring device over the truck because of the material handling and delivery equipment.
Other issues affecting noncontact devices are the inherently dusty environment within the container, bulking of solids, and uneven loading. In the first case, it is important to remember dust can combust and explode. Coal dust and grain silo explosions were very common until safety procedures were implemented. This characteristic must be considered when designing a system. Aside from the fuel, oxygen, and ignition required for gas combustion, dust also requires dispersion and confinement to become combustible. This dispersion also will interfere with noncontact types of level-device operation.
Proper equipment enclosures should be designed into the system for proper protection. Bulking of solids occurs when the solid “cakes” and becomes a separate mass from the rest of the silo contents, as in the solids piling up on one side of the silo. This can result in no contact with or by the level device. A common remedy is vibration or “fluidizing” the solids with air to assure it is evenly distributed. Uneven loading can happen for many reasons. However, system design and operator experience, along with mechanical agitation in some cases, can help mitigate this problem.
Measurement device solutions
Contact measurement devices for this application are unique to solids measurement. These devices rely on either direct contact with the materials or by weight. All solids are different in their basic forms: size, density, moisture content, and weight. These characteristics are used to detect or to infer a level. It should be noted the methods of loading the container or silo are not considered here. Optimally, the operator will load the container or silo as uniformly as possible. Depending on the characteristics of the solid being handled, it will either flow freely, or it will bulk up in a pile until it reaches its critical angle and releases. This is a solids-specific problem for the designer and is beyond the scope of this discussion.
A common point-level detection device is the vibrating reed or “tuning fork” type of sensor. This sensor does exactly what it says: it vibrates. One or two vibrating elements are inserted into the side of a tank or silo. The downside is the sensor is permanently mounted into the tank. This vibration is at a set frequency and transducers monitor the vibrating element. When the element encounters an obstruction, as in being immersed in some solid (or liquid, for that matter), the vibration’s frequency changes. This change in vibration signifies the solid has reached that level. It is a simple and effective way to monitor the fill of a container. Multiple sensors can be applied as needed.
Another common measurement method is a weight and cable system, which is commonly called a “yo-yo.” This device is mechanical, though it contains a slide wire transducer that follows the mechanism as it travels down to the surface of the solid. When the weight contacts the solid’s surface, it stops and reels back in. The slide wire measures the depth to which the paddle has traveled. This device has been used in the bulk solids handling industry since time immemorial. Aside from it being a mechanical device, the other modes of failure result from the buildup of solids on the paddle and the wire. This has a cumulative negative effect on the device’s operation and accuracy.
Strain gauges also are used to retrofit existing containers or silos to allow indication of solids loading within the container. These devices can be bolted onto existing structures, allowing an operator to infer the solid’s level or weight within the container based on the structure’s deformation. These are very simple and can be used on the fly using commercially available containers (“dumpsters”), but calibration and setup can be daunting.
Finally, there are load cells. This method requires that load cells be included in the container or silo’s design. The load cells are an integral part of the structure and must be in place prior to installation. The operating principle is well known. Typically, multiple load cells are placed under the supports for the container or silo to provide averaging due to uneven loading. The system is calibrated for the solid being handled and the level in the container is inferred by the weight observed. Depending on the solid being handled, secondary monitoring of the type described above can be added as high-level detection devices or intermediate point-level monitoring.
There are several other devices regularly used to monitor and measure solids level, which range from relatively simple to sophisticated. At the bottom end there is the “paddle wheel” sensor, which is used to measure the presence of solids in a delivery chute. However, it also is used as a primitive means of point-level measurement; the bulk of the solid moves a paddle when it reaches that point.
On the other end are microwave or gamma radiation sensors, which mount the source on one side of the vessel and the receptor on the other. Being mounted outside the vessel means there is no contact and no fouling or other wear and tear. The choice and application of a solids measuring system is, like any other instrumentation and control system design, highly customized to the materials involved and the physical systems available to handle them.
The application, use, placement, and the method of monitoring and controlling solids levels relies on a thorough understanding of the solids being handled. While liquids are relatively straightforward and predictable in their behavior, solids behave very differently—so differently, in fact, some have characterized granular solids as an entirely new form of matter.
Consider all the characteristics of the solid being handled, including its tendency to produce combustible dusts, which could present a life safety hazard. Generally, the measurement of solids is not a one-size-fits-all like most liquid level measurement applications. It is a matter of experience combined with the proper application of state-of-the-art instrumentation that allows for the proper application of the means and methods available.
Daniel E. Capano is senior project manager, Gannett Fleming Engineers and Architects, P.C. and a Control Engineering Editorial Advisory Board member. Edited by Jack Smith, content manager, Control Engineering, CFE Media, email@example.com
KEYWORDS: process sensors, solid level monitoring
Solids behave very differently from liquids and gases, and methods of measuring level are specialized and require as much “feel” as technology.
As with liquid level measurement, devices fall into two broad categories: noncontact and contact.
The application, use, placement and method of monitoring and controlling solids levels relies on a thorough understanding of the solids being handled.
Consider all of the characteristics of the solid being handled, including its tendency to produce combustible dusts that could present a life safety hazard.