Measuring the volume of a solid product that doesn’t naturally tend to settle is a persistent instrumentation challenge. Figuring out how much of a dry powder, pellet or granule feedstock one has on hand can be difficult because the material doesn’t present a uniform level like a liquid.
Lydia Miller digs into this pile in an article published by Powder & Bulk Solids. She examines a variety of technologies used to measure level, which most of the time is really a proxy for volume. A plant may be concerned with a tank filling up, but usually when a precise level measurement is needed, it is to determine how much of a given product is in a vessel. Whatever the motivation, it isn’t always an easy job.
Few solids flow readily enough to spread evenly, and their tendency to clump, pile, and stick can change with atmospheric conditions. With filling and emptying cycles, a multitude of peaks and troughs can form, and constantly change as product is added and removed. Depending on how much the material can pile before sliding and the width of the vessel, the difference between the level of a peak and a trough can be as much as 100 percent. Additionally, the particles of the materials can vary from very fine powders to effectively large rocks. These characteristics make level measurement much more difficult than with liquids.
Plants that haven’t gone down the costly path of installing a series of load cells to weigh what’s in a vessel usually resort to sending an operator to the top to open a hatch and see what’s inside, and some may even use a tape measure. It sounds simple enough, if not very accurate or repeatable, but there are safety-related downsides.
There are obvious potential safety concerns with someone climbing a vessel, and additional concerns with being exposed to the product. If the operator is opening the hatch after a recent product movement, he or she might be greeted with a blast of dust. If the product is benign, this might only be an annoyance, but for other products it could be irritating or even toxic.
The biggest challenge is compensating for peaks and valleys, which depending on the nature of the product, can be minimal or severe. If it’s not too bad, a single point measurement might suffice. Some research may turn up products like “yo-yo” mechanical level measuring devices, but these are only point level measurements, and often bring maintenance headaches of their own, especially in dusty environments.
Therefore, guided-wave radar (GWR) has replaced many manual and other level measurement methods for a variety of reasons.
GWR level transmitters provide continuous level measurements based on microwave technology. Low-energy microwave pulses are guided down a probe. When the microwaves are reflected from the material surface back to the transmitter, the position of the surface can be measured. The distance corresponds to the spot on the surface where the probe contacts the material, so there is no compensation for uneven surface contours. GWR level transmitters are especially well suited for smaller vessels with a diameter of less than 33 ft, that contain powders and small granular materials, and in installation areas where space is restricted.
GWR level transmitters can provide a very accurate point measurement, but where there needs to be compensation for uneven surface contours, a much different approach is called for.
Some level transmitters offer a configuration similar to non-contact radar, but use acoustic frequencies rather than microwave. Acoustic transmitters using phased-array antennas can perform sophisticated scans of the product surface in a vessel. After sending signals at three different frequencies, the antennas receive multiple echo signals from the walls and contents. Using these integral antennas, the scanners continuously measure the direction and distance of each echoed signal and generate a coordinate of the echo inside the vessel.
The system she’s talking about is the Rosemount 5708. It uses a sophisticated stereo effect, bouncing sound in different directions and using different frequencies. This brings some useful capabilities.
Digital signal processing within the instrument samples and analyzes the echoed signals, and produces accurate measurements of the level and volume across the entire surface within the much wider beam angle of the device. Matching the received data with known vessel dimensions allows the instrument to calculate product volume, providing continuously accurate inventory values.
As Lydia points out, there are many tools available to get as accurate a measurement as necessary for the application, regardless of its complexity. Ultimately, it’s a matter of choosing the right technology, and Emerson can help.
You can find more information like this, and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Level Group and other specialty areas for suggestions and answers.