Solids Flowmeters Can you go with the Flow? Solids Flowmeters for Industrial Applications
Solids flowmeters are an interesting solution to indicate flow rates in pipes and chutes. Matt Morrissey, Product Manager Weighing Technology at Siemens, says 'interesting' because “they are only moderately accurate, costly, require a lot of tuning after installation and are usually only as good as the whole process around them.”
Solids flowmeters are an interesting solution to indicate flow rates in pipes and chutes. I say “interesting” because they are only moderately accurate, costly, require a lot of tuning after installation and are usually only as good as the whole process around them. That being said, one of the reasons the technology exists is because there isn't anything else that can do the job.
Just like my dad used to say when I would show up with a rubber mallet when he asked for a hammer, “You need the right tool for the right job.” And flowmeters are exactly the right instrument for measuring the flow rate of many solid materials. This simple fact has led to the development of several different versions of solids flowmeter. In this article we will discuss and evaluate these, so that you too will have the right tool for your industrial flow application.
There are a handful of technologies used for metering the flow of solids:
1. Impact Flowmeter – the most popular form of solids flowmeter, impact meters, as they are often called, guide the material through an infeed pipe or chute and create a specific trajectory for the material to strike a flat sensing plate. The amount of force the impact creates is measured by means of load cells or an LVDT (linear variable differential transformer). As the plate is deflected by the force of the material, the load cell or LVDT deflects and generates a signal, which is converted into a flow rate by an integrator (Fig. 1).
2. Centripetal Flowmeter – this is a variation on the impact design. A centripetal solids flowmeter guides the material through a curved sensing plate, which is connected to one or more load cells. The material must be guided in parallel to the sensing plate as it enters the curve, and the tangential force exerted on the load cell(s) is transmitted to the integrator and then converted into a flow rate (Fig. 2).
3. Coriolis Flowmeter – the solids coriolis flowmeter does not use the same principle as a liquid coriolis meter. In a solids application, material enters the flowmeter and is directed onto rotating vanes driven by a motor. The motor is connected to a torque arm, which is mounted to a load cell. As the amount of material fed into the coriolis meter increases, the torque on the motor increases. The load cell detects this increase and sends a signal to an integrator, which translates it into a flow rate (Fig. 3).
4. Microwave Flowmeter – one of the lesser-used technologies, microwave or radar flowmeters, emits a 24 or 125 GHz microwave into the material flow in a pipe or chute. Based on the Doppler principle, the change in microwaves reflected back to the sensor is measured and transmitted as a 4 to 20 mA signal for scaling in a PLC system to become a flow rate. Microwave-based products can be used in pneumatically-fed systems, as the extra force of the material flow does not affect the measurement as is the case with the three technologies discussed above (Fig. 4).
5. Capacitive Flowmeter – solids flow sensors using capacitance are based on two independent measurements. One is the change in capacitance from an empty pipe to a full pipe, which is proportional to the concentration of the material. The other is a velocity measurement, which uses two sensors to indicate the time it takes for the material to move from the first sensor to the second. The signals from these measurements are then fed into an integrator, which outputs a flow rate. Capacitive measurement can also be used with pneumatic systems (Fig. 5).
Of course, each technology has its own advantages and disadvantages. Fig. 6 shows some of the more critical aspects to consider when trying to identify the best product fit for your particular application.