Level detection in case of bulk solids or powders in a bin/silo can be problematic due to following reasons:
- It is necessary to transform the level into volume and mass for measurement purpose.
- The characteristics of the material under measurement can also pose problem, for example, the material can be either too heavy or very light in weight.
- The solids being measured can be in the form of either minute size powders or outsized having pointed edges.
- Also some materials tend to generate huge quantities of dust while filling and expulsion.
- Sometimes, the materials used can be hygroscopic in nature means they have the tendency to easily absorb the moisture in the air. This moisture then mixes with solids due to which they get clustered inside the vessel resulting in material blockage.
- Unlike all virtual liquids, the bulk solids contained in a vessel do not possess a plane horizontal surface. They generally have an angular surface shape known as the angle of repose which can change according to the fill and discharge settings.
- Besides, the coarser the process material, the more is its probability to cluster, bridge, leave voids and stack up.
Level Measurement Methods
Various technologies available for the measurement of level (or quantity) of bulk solids stored in silos, bins or hoppers are listed below:
- Weight & Cable Systems
- Ultrasonic Systems
- Guided Wave Radar Systems
- Thru-Air Radar Systems
- Laser Detectors
- Load Cells
- Strain Gages
Among the above mentioned technologies, the first four operate by determining the distance between sensor and the material surface whereas techniques like load cells and strain gages operate by calculating the mass of material via detection of force exercised on the sensor.
Each of these techniques is more responsive than others towards its particular application. The choice of an unsuitable measurement method can lead to wastage of time, money and manpower. Hence, all the factors of above mentioned technologies must be considered carefully before implementation.
Weight & Cable systems
These systems provide very cost effective and simple level measurement solution in case of bulk solids. This method of level measurement is generally applied in fields are used in applications where level of the material changes very frequently.
They determine level by converting it into volumes and weights. “Converting a distance into a volume requires an assumption of the relationship between the point on the surface of the material where the level measurement sensor will make its’ measurement and (given the angle of repose) the “theoretical” surface height for that same volume of material if the surface were flat and horizontal.”1
These systems provide extremely accurate results for distances below 30 feet. For longer distances and higher accuracies, other techniques are required to be employed though with an increase in cost. These level detectors mostly consist of fixed components and hence free from maintenance and part wear.
These continuous level detection systems are almost equivalent to weight & cable systems in technology. They also propose highly enhanced results and reliability owing to the recent developments in technology. Latest techniques and software have brought in improvements mainly in the field of transducer design and signal processing systems integrated in the device. However, there are various problems which can affect the efficiency of ultrasonic sensing systems such as:
- Dusty environments
- Variations in pressure
- Shift in angle of repose
- Bulky size particles
- Obstructions inside the process vessel
- Material build-ups on the surface of the vessel
Out of these factors, few can influence the reflection of sound waves and result in production of false echoes which can disguise the correct level signal.
Guided-wave Radar & Thru-air Radar Systems
Two types of radar systems can be put into use for measurement of level in case of bulk solids and powders which include: GWR i.e. Guided-wave radar systems and TAR i.e. Thru-air radar systems.
As opposed to weight and cable systems, GWR systems are strictly continuous. Their use is preferred over weight & cable systems only in situations where processes requiring repeated continuous measurements are concerned. These systems prove to be very beneficial in areas which were once considered to be unmanageable.
Thru-air radar systems are considered to be pioneers in the field of radar level measurement. Earlier these systems were employed for level detection in liquid and slurry applications only but nowadays, they have been increasingly used for complex powder and bulk solids applications. “The TAR energy diverges as it shoots down into the vessel in order to reflect off the solids surface. As with ultrasonic level systems, internal vessel obstructions, changing angle of repose, clumps of material adhering to the vessel walls, etc. can affect the reflected level signal and create reflections inside the vessel.”2
TAR systems are available in two major types: Pulsed radar and FMCW i.e. frequency modulated continuous wave radar. In order to get better results, these systems should be properly installed and calibrated. These systems tend to consume extra power than GWR systems. As compared to TAR systems, GWR systems offer following major benefits:
- “GWR systems are more accurate because the radar pulse does not disperse, the signal-to-noise ratio of the reflected pulse is higher, and GWR must account for only one signal reflection.”3
- GWR systems are flexible and simple to install. Besides, they are proficient to work easier with extensive range of vessels and in constricted locations.
- GWR systems provide fast response which is helpful for fast filling and discharging requirements.
- The performance of GWR systems is not affected much due to material coatings and deposits.
- Unlike TAR systems, Guided-wave radar systems work well with materials having low dielectric constants
- Also, in case of longer distances, GWR systems provide fine resolution.
- GWR systems are comparatively cheaper than TAR systems.
- In dirt applications, use of GWR systems is usually preferred over TAR systems. However, TAR sensors can also be employed in dusty conditions though they do not offer self-cleaning.
However, the few disadvantages associated with GWR systems are:
- They are completely invasive sensing devices
- They come with high price tags as compared to other level detectors.
These level detection systems are capable of working with longer distance. However, in that case, the detecting system will involve additional purchase cost. These systems are mainly employed in complicated applications where other level sensing systems are not considered suitable to work.
These devices are usually located beneath the supporting base of the vessel for measurement purpose. Their accuracy percentage is around ± 0.2% which can get better too in some cases. They involve extremely high purchase, installation and calibration costs. They are extensively applied in food and pharmaceutical applications. “They are primarily used for “certified for trade” or internal accountability applications (where mass measurement is required), or on severe applications where other systems will not work.”4
These systems can be designed accordingly as per the requirement by the makers.
Strain Gauge Weight Systems
These level detection systems can be either coupled or bolted to the base of the process vessels. They offer inexpensive measurement solution with 1% to 5% of weight accuracy. After proper installation and calibration, these devices tend to provide almost stable and maintenance-free service.