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.