What are the challenges of measuring level in cement?
The number one problem with solids is dust. In contrast, most of the time, liquids in bulk storage offer a relatively easy environment for level measurement. The liquid surface is large and flat, offering excellent reflection properties for non-contacting technologies. For contacting devices, liquids present no abrasion issues or strong tensile forces that might be cause for concern. The environment above the liquid surface is typically placid.
On the other hand, solids materials can present an aggressive environment with a range of unique challenges including various surface dynamics, tensile forces, and silo obstructions.
The surface of solids can vary enormously. The angle of repose may be flat and smooth. More often, however, solids have a sharply sloped and irregular surface as is typically the case with aggregate type materials. Particle size ranges from very fine powders, such as finished cement, to coarse materials, like clinker or coal.
Tensile forces further complicate solids measurement. Tensile forces in tall solids silos can reach several tons. As material is drawn from the vessel, the intense force can break off cables or moving parts within the silo. This makes contacting cable or mechanical systems problematic. Vacuum dust collectors, filling streams, aeration devices, static electricity, acoustic and electromagnetic noise are also significant factors in solids silos. This is the reason why many cement producers worldwide used to rely on manual measurements taken from the top of the silo using a rope/tape and weight.
The evolution from contacting to non-contacting level measurement
For many years, the industry installed mechanical level devices. These devices, often known as “Yo-Yos,” used a weight suspended by cable. This weight was lowered on a timed basis automatically into a silo to detect the surface of the material. The device computed the level by measuring the specific length of the cable required to contact the material. The technology was simple and easy to understand but it had several drawbacks. It required frequent maintenance because material got carried into the spooling mechanism for the cable. Cables would break on occasion and damage material handling components downstream from the vessel.
How do radar sensors fit into the cement industry?
As a result of their relatively lower frequency, traditional radar devices are well-suited for liquid applications. However, to use them effectively on solids applications requires a large horn up to 10” (250 mm) diameter or even a parabolic dish antenna to capture sufficient signal. This large antenna is just not practical on most vessels. Where process connections are available at all, they are normally too small to accommodate a large antenna without costly modifications. If a process connection must be created, it is often less costly to create a small opening, particularly if the vessel roof is concrete. With its smaller antenna sizes and ease of installation, high frequency radar offers significant advantages for solids applications.
Vessels vary in shape and size, and may contain various internal challenges. Silos assembled in sections have seams that may create false signals. Internal ladders, man-way access hatches, point level switches and, even, fill streams are potential false echo signals for level measurement equipment. Silos containing finished cement are often tall and narrow, some over 150’ (50 m) high and occasionally with material buildup on the sides. For all of these situations, narrow conical beam angle sensor transmissions are preferable to reduce sidewall path interference and reduce false signals from internal obstructions.
What are the advantages of using high frequency radar?
High frequency radar provides a narrower beam angle than low frequency radar. For example, a 78 GHz radar instrument has a narrow 4° conical beam angle compared to a 36° angle for a 6 GHz instrument with a 4” (100 mm) diameter horn antenna. This makes high frequency radar instruments more effective on solids. Smaller antenna, easier installation and a narrow beam angle are important advantages of the higher frequency radar instruments. Another substantial benefit of higher frequencies is the reflection property from sloped solids surfaces as it relates to wavelength and “skip” effect. A wave striking a sloped surface may reflect directly back or it may skip away from the sloped surface, or both. This causes the signal to diverge into two or more paths so that the receiver sees noteworthy multiple signals instead of one dominant reception. Severe “skipping” may result in the second echo being higher than the first one. This is a common problem for low frequency systems on solids and notably degrades the signal to noise ratio. This was a key issue that had prevented radar being suitable for solids level measurement for some time. Using high frequency ensures that the largest amount of signal reflects directly from the sloped surface.
As you can see, there’s a variety of level measurement technologies that apply to the cement industry. Whether you are updating your system process, learning about the differences between the technologies or unsure of the evolution concerning level instrumentation and the cement industry, this is a good place to start!
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