Monitoring solids flow poses complexities not faced with fluids. However, optimizing plant performance often depends upon accurate measurements of solids flow, e.g., to control the amount of material going into a process or blending operation. Attaining better accuracy and speed as well as easier or reduced maintenance remain challenges at many sites. Fortunately, vendors such as Siemens, Global Technology Systems and Berthold Technologies are addressing these issues.
Munich-based Siemens offers two basic types of flow meters that continuously measure the impact force of material under gravity-feed conditions and convert the reading into a flow rate. The meters rely on either strain-gauge load cells or linear variable differential transformers. The range of instruments allows monitoring of powders, granulates and other dry bulk materials in sizes up to 25-mm diameter with flow rates from 200 kg/h to 900 mt/h.
Its Sitrans WF overcomes a number of the traditional challenges associated with the measurement of bulk flow, notes Matt Morrissey, Peterborough, Ontario-based product manager, weighing technology, process protection and continuous capacitance. “For example, continuous monitoring is carried out without interruption to the process. The technology remains unaffected by material buildup, so monitoring remains reliable. Both of these mean that maintenance and the need for recalibration are minimal.”
A recent project at a large potash mine in Alberta illustrates the benefits achievable. A Sitrans WF solids flow meter with stainless-steel sensing plate and flow guide replaced a nuclear densitometer used to measure prill flow on an inclined conveyor belt. It monitors flow rates of up to 15 t/h of the dusty and slightly corrosive material. As well as improving the reliability and accuracy of the process, the flow meter — together with a Siemens signal processor and sensing head — delivered what the customer describes as substantial long-term cost savings by eliminating the nuclear source from the plant.
Changes in specialty chemicals manufacturing are impacting the market for solids flow meters, notes Morrissey. “A lot of chemical companies have spun off divisions that focus on more niche products… These new companies are focusing on new, smaller volume but higher value products. So, instead of the 24/7, 365-d/y continuous processing that we traditionally saw in the market, there is a growing focus on the production of smaller batches. That’s where the market is going.”
This, in turn, has led to some device miniaturization, for example in mass flow meters, a move Morrissey believes could become increasingly important as chemical plants become more batch-oriented and modular.
“We really want to get down to a few products — across all technologies — that do a lot of things very well, rather than a larger number of more-industry-specific ones that we have at the moment,” explains Craig Allen, Peterborough-based manager, industry sales development, process instrumentation. “Ideally, an evolution into something less mechanical-based and more electronic-based would allow for greater use in a wider field,” he adds. However, Allen also believes that bigger is sometimes better in the world of solids where long life and robustness often are key requirements.
Metering By Microwave
Meanwhile, Global Technology Systems, Shalimar, Fla., is promoting SolidFlow, a microwave-based technology. The company has a background in traditional bulk-flow measuring technologies such as impact-plate flow meters, belt scales and weigh belt feeders. “These have been the most popular technologies for many years, but suffer a lot of wear and tear and so need a lot of maintenance,” notes Robert Harrison, president. In contrast, SolidFlow minimizes maintenance concerns, he says. “There are no moving parts, no maintenance and product buildup on the probe is not an issue because we look straight through it.” Moreover, it can successfully measure solids flow in both dilute- and dense-phase pneumatic conveying systems.
Installation is simple: a 5×8-cm socket is welded onto the pipe, which must be metallic; then a 20-mm-dia hole is drilled into the pipe opposite the socket and a sensor is inserted flush with the inside of the pipe.
The technology creates a microwave field inside the pipe. Solid particles reflect the microwave radiation, which a sensor reads. Evaluating the frequency and amplitude of the received signals enables assessing the quantity of flowing particles per unit of time. Using frequency as a factor ensures that only moving particles are measured and deposits are suppressed. The sensor is calibrated when installed by entering the reference quantity and pressing a button.
The device can handle powders, granulates, dusts, etc. with particle sizes between 1 nm and 10 mm in pneumatic transport systems or in freefall. It can provide online measurements from a few kilograms to approximately 20 t/h. The SolidFlow is suitable for use in potentially explosive atmospheres. The non-contact measurement system is easy to retrofit and suffers absolutely zero wear, boasts Harrison.
One of the most recent installations is at a lime plant in Alabama. Here, a SolidFlow unit installed on an 8-in. pipe for dilute-phase pneumatic conveying of lime measures flows of 8–15 t/y (Figure 1). The customer now can quantify daily production levels more accurately while also using the improved flow-rate information to maintain optimum process conditions, he says.
“We are finding many applications, particularly in dosing and in measuring the products from recycling processes.” An easy retrofit with GTS microwave sensor technology transforms small volumetric feeders, which remain very common at many plants, into mass flow meters, Harrison stresses.
The company has just released SolidFlow 2.0. “We have increased the processing power by over 100 times. Together with the addition of a new algorithm, we can tackle issues such as roping (where materials, especially in dilute-phase pneumatic conveying move like a rope, which is very hard to measure), which was an issue with the generation 1 technology,” he explains.
The third generation of SolidFlow is in the pipeline, too, and will encompass particle size analysis. “We expect that the market will be huge for this,” Harrison enthuses.
Berthold Technologies, Oak Ridge, Tenn., offers the LB442, a non-contacting, non-intrusive radiometric bulk flow meter that simply bolts onto where it’s needed. The device works by directing radiation through the material in question. Measuring the attenuation of this radiation and combining it with a velocity signal allows determination of a very accurate mass flow rate — not affected by dust, temperature, vibration, varying particle sizes or chemical properties of the measured material.
The nuclear sources are very small — typically emitting 3 mCi Co-60, 10 mCi Cs-137 or 50 mCi of Am-241, depending upon the application — and the lead shutter system makes the nuclear sources easy both to transport and maintain, says technical sales manager Chris Payne. “… [The LB442] is a very, very safe unit.”
Calibration is straightforward, too, he adds. This usually involves running a known load or using a truck weighing system, and typically results in accuracy exceeding ±3%. Some customers have achieved ±0.5-1%.
“Once a customer has started using a nuclear source, they do find it very, very easy to manage and maintain — and often then roll the technology out to other sites,” he adds. For example Syngenta in the U.S. has standardized on the LB442 for measuring the flow of corn products used in its bioethanol processing facilities (Figure 2).
“However, the customers who call us in are still typically those that have gone down the route of conveyors and load cells and then realized that the costs of recalibration and associated maintenance are very much higher than they expected. Remember also that we have no issues with drifting or with temperature — -40°C–+60°C — so the range is huge. With water-cooled versions, we can also go higher than 60°C as long as the detector is kept under 60°C,” Payne stresses.
Word now is spreading about the benefits of radiometrics because of its wider usage and a better understanding of how the technology works, he believes.
The new Shenhua coal-gasification plant in China exemplifies the advantages of the technology. Here, an LB442 measures the flow of pulverized coal that is pneumatically conveyed into the reactors. Every reactor has either three or four feed lines, each with an installed flow meter.
“This bulk flow measurement is highly critical as it is used to control the ratio between oxygen and coal. If there is too much coal fed into the reactor, the coal will start burning instead of gasifying and the temperatures can rise rapidly to many thousands of degrees centrigrade. That’s why our systems must be very reliable and quick reacting; the typical response time is 1–2 seconds,” explains Sonja Geenen, Berthold Technologies’ business development manager, process control, Bad Wildbad, Germany. “Our customer tried different technologies; however the radiometric technology is the only one that is able to provide the performance and long-term stability which is needed for this process,” she adds.
Meanwhile, batch chemical manufacturers are benefiting from the unit’s ability to store up to ten different product calibration points.
The LB442 can measure bulk flow on screw conveyors, bucket conveyors, plate conveyors and troughed chain conveyors, Geenen points out — unlike load cell technologies, which can’t be used on all of these.
Seán Ottewell is Chemical Processing's Editor at Large. You can email him at email@example.com.