How to choose the right level instrument for applications
There are many level measuring techniques, however they need to be the right fit for the application. See the three key constraints to measuring solids in tanks and vessels.
Companies dealing with bulk solid materials have to measure the contents in vessels and tanks, as do users working with liquids. However, certain obvious differences make the task far more difficult when trying to get an accurate volumetric or mass measurement of solid products. The biggest difference is the way in which solids flow, or don't flow, contrasted with liquids.
Some solids have sufficient granularity to separate into particles. With these, some may agglomerate into chunks, but the underlying assumption is the material can be poured, at least for the most part. (If a vessel is filled with completely solid material, there are bigger problems than simply trying to measure the volume or level.)
Finding a solid level
Liquids, even those with high viscosity, ultimately come to a uniform level in a container. Solids, on the other hand, form piles. If a pipe or chute is sending granules into a vessel, the highest point will be directly under the pipe. The difference between the highest and lowest point in the vessel may be great, or it may be quite uniform, depending on the material and other factors.
This pile-forming characteristic is quantified as the angle of repose, meaning how steep the sides of the pile can become before part of the pile will slide down (see Table 1). Spherical plastic pellets will not pile up very high because they roll down the sides of their own hill. Other products, even if they aren't sticky, can form higher piles due to the particle shape or natural cohesiveness. Under most circumstances, few products have an angle of repose below 30 deg or above 45 deg. Even wet sand, if allowed to move freely, will not pile up steeper than 45 deg.
Picture a round vessel with a conical bottom as shown in Figure 1. It is filled from a pipe mounted from the roof equidistant from the center and outer wall. The outlet at the bottom is centered, and the cone walls are sloped at 45 deg. When the vessel is being filled, the product will pile up under the inlet pipe and move toward the outside. When the filling stops, there will be a conical pile with the sides approximating the angle of repose for the material. The lowest point in the vessel will be near the walls, farthest from the inlet pipe. When the outlet is opened, the material directly over the opening will fall out eventually creating a hole as the material higher up moves in from the sides and above to fill the void.
Three solid measuring technologies
Techniques for measuring solids are usually from the top to the bottom, with the level instrument mounted on the vessel's roof pointed down at the surface of the material. Three of the most common include:
- Guided-wave radar using a probe to direct the pulse travel
- Noncontacting radar
Each of these has its own peculiarities related to how it handles the characteristics of solids. Under normal circumstances, all three calculate measurements based on the elapsed time between an energy pulse being sent down and a reflection from a point on the surface returning to the instrument.
With guided-wave radar, it is a very small point, only 1 or 2 in. in diameter surrounding the instrument's probe. The instrument can't create a picture of the whole surface from just one point, but for some applications, one point may be enough to receive the information. Noncontacting radar and acoustic level instruments can read a larger area, particularly the latter, but may still yield an incomplete picture. Whether it is sufficient or not depends on the process's needs.
Understanding instrument constraints
Guided-wave radar instruments, as shown on the left of Figure 1, use a probe designed to extend down into the material. The reading signal travels down the probe, hits the surface of the material, and returns up the probe to provide a very precise reading of the material height around the probe. Because the signal travels down the probe, it can have advantages over noncontacting radar in low dielectric applications.
Some probes are made from flexible cable while others are rigid rods. Flexible probes are better for solids because when large amounts of material start moving, the forces can bend or even break a rigid probe due to the unevenness of movement, particularly when the vessel is filling or emptying.
Pull force is a characteristic generated because a probe embedded deeply into a large volume of material can have an enormous amount of tensile force applied as the material moves while the vessel is being filled or emptied (see Table 2). Probes can be yanked out of the instrument housing, or worse, a well-reinforced probe can simply pull the top of the vessel down. But the tensile force can be calculated to avoid these situations. Depending on the material, frequent movement close to the probe can also cause abrasion and premature wear. Still, in the right application, a guided-wave radar instrument can be a very accurate and economical choice.
Dielectrics and bulk density
Noncontacting radar and acoustic instruments depend on sending pulses through open air to the surface of the material and the timing of the reflections (see Figure 2). The accuracy of the measurement depends on the strength and repeatability of the return signal. The characteristics able to create a strong signal make for major differences between radar and acoustic methods. Radar instruments depend primarily on the dielectric constant (DC) of the material, while acoustic instruments depend on bulk density.
With a noncontacting radar instrument, a pulse of electromagnetic energy is emitted. When the pulse encounters a boundary where there is a change in the DC, some of the energy is reflected back. The boundary in this case is the interface between the air in the vessel and the surface of the material in the vessel, which can be solid or liquid. The higher the DC of the material, the stronger the reflection is.
The surface angle presents a problem with measuring the level of solids. If it is flat, the reflection goes straight back to the instrument, but if the pulse hits a slope, part of it can be reflected to the side of the vessel and not captured (see Figure 3). In most situations, enough of the signal is returned to get a usable measurement, but if the material has a low DC and a high angle of repose, it makes for a difficult combination. Special solids algorithms in the instruments and parabolic antennas can help with measurements in solids applications.
Acoustic instruments have a similar, but different constraint. An acoustic pulse or sound wave produced by the instrument passes through the air space until it encounters the surface of the solid contents of a vessel or the vessel walls. Reflection strength is determined by the bulk density of the material, which is the mass of the substance in a given volume. So if the material surface stays fluffy, it absorbs some of the sound energy, and the reflected waves are not as strong.
Again, situations where this becomes a serious issue are rare, but it is something to keep in mind with some particularly troublesome products. Since the beam is much wider than noncontact radar instruments, the instrument can capture information for average level calculations over the entire surface it sees. On some vessels, the level information can be enough to calculate the volume within 3% accuracy, although larger vessels may need multiple devices to get these results.
Other level-measuring variables
When getting down to specific individual application cases, there are more subtle characteristics in different technologies and instruments. Other considerations capable of influencing method and instrument selection include:
- Dust levels
- Product abrasiveness
- Product corrosiveness
- Moisture and condensation
- Inlet and outlet positions
- Internal obstacles
- Ambient noise and EMI
- Frequency of filling and emptying cycles
- Instrument mounting constraints
- Minimum and maximum distance.
These are all things that should be discussed with a measurement solution provider when choosing the right level instrument for a specific application.
Creating a topographical model
For many applications, the plant operator may only need a general sense of how much material is in a vessel. If experience suggests a clear picture of how the material tends to pile with low and high spots in the same areas, one point measurement may be sufficient. Other situations may not be as simple.
When the vessel has a large diameter, or where inlets and outlets cause uneven loading due to awkward locations, one or two measuring points may not be enough, particularly when a high degree of accuracy is needed. In those cases, acoustic instruments used alone or in coordinated groups offer a capability to create a more detailed picture.
Since the acoustic instrument information is so complete over the area it sees, it can create a topographical map of the surface inside the vessel. In very large vessels, individual pictures from multiple instruments can be knit together into a very precise larger image covering the surface of the material from wall-to-wall (see Figure 4). Software external to the instruments averages high and low spots when multiple instruments are needed to calculate total volume very accurately. The complete visual image also can warn operators if excessively high or low points are forming, which can indicate moisture contamination or poor material flow within the vessel.
The number of instruments needed to create such a detailed picture is determined by a combination of factors:
- Vessel diameter—Larger vessels need more instruments
- Head space—When the material surface is close to the top, one instrument cannot read as large an area
- Precision requirements—Increasing the number of instruments helps create a more precise picture.
This approach can be an expensive solution since it may require a large number of instruments supplemented by the necessary processing power to do the calculations. However where the material is very expensive, has movement or flow problems, or a very accurate picture is needed, this type of solution can deliver whatever level of precision is needed.
Level measuring factors
Measuring level with solids is more complicated than with liquids because so many measuring techniques simply don't work, leaving a shorter list of options. The three technologies that have been discussed can cover the vast majority of applications if the process's needs are understood along with the technology's limitations.
When a small-area-spot reading is enough, or the material is especially problematic due to low dielectrics, guided-wave radar is accurate and economical, provided the mechanical constraints can be overcome. Noncontacting radar also can provide a slightly larger single-point level reading without worrying about a probe.
When a larger area needs to be read and averaged, acoustic can do the job. The determination of which technology is best for a given application will depend on a number of factors including:
- Specific material characteristics
- Installation considerations
- Measurement needs.
For applications that require precision volume measurements or where actually seeing into the tank can protect the product or provide additional safety, multiple acoustic instruments working together through a supporting software program can draw a map of the material surface from wall-to-wall and may be worth the additional investment.
Lydia Miller is the senior marketing engineer working in the Rosemount Process Level group at Emerson Process Management. Edited by Emily Guenther, associate content manager, Control Engineering, CFE Media, firstname.lastname@example.org.
- Different solid measuring technologies
- The operations of guided-wave radar instruments
- The restraints between radar and acoustic instruments.
How can multiple instruments work together to increase efficiency for particular applications?