The “black magic” of solids flow measurement
18 November 2016
Walking through your Portland cement plant, you see a huge amount of equip- ment every day. Conveyor belts, pressure and temperature measuring devices, filter systems, additive dosing machinery—the list goes on and on. Take solids flowmeters: you know what they do, how to fix them (or who you should call to fix them!), but have you ever wondered how they work?
Don’t be fooled by their apparent simplicity—material goes in, material comes out, you get a measurement. But what’s really going on inside a solids flowme- ter? The answer will change the way you think about this piece of equipment as you walk past it each day.
First, the process
After the journey from the quarry to grinders and the kiln, the final step in the process of making Portland cement is the finish mill. Clinker, along with a proportional amount of gypsum, is fed into the mill to be ground into finished cement ready for storage or transportation. The design of a finish mill generally consists of a very large diameter steel tube filled with a designated quantity of steel grinding balls. As the mill is rotated at an opti- mum speed, the grinding balls crush the clinker/gypsum mixture into a fine powder.
A high-efficiency cyclone separator controls the particle size. The drum is generally divided into two or three chambers with differently sized grinding media. As the clinker particles are ground down, smaller media are more efficient at further reducing the particle size.
In a closed-circuit system, coarse particles are separated from the finer product and returned to the start of the process for further grinding. This is called a recirculating load, and to ensure peak efficiency, the mill should run with an optimum load.
The most common application for a solids flowmeter here is the coarse returns downstream from the cyclone separator. It is important that this reading be instantaneous, since the load of the mill needs to be maintained at a level to achieve the most efficient grinding. With this rate feed- back, the system controller can vary the input of clinker and additive feeds to quickly adjust for the best grinding control. The flow rate in the coarse return process varies from system to system, but rates of up to 800 t/h (880 STPH) are possible.
The impact of impact-based fowmeters
The most reliable solids flowmeters available are impact based. These flowmeters have an impact plate mounted at an angle that the material strikes as it flows down, continuously from one point to the next. The flowmeter sensing element – either load cells or an LVDT-based mechanical assembly – measures the impact force generated by the flowing material and converts this data into an electronic signal. This signal is converted into a mass flowrate with a very repeatable accuracy.
Source: https://w3.siemens.com/mcms/sensor-systems/CaseStudies/Black-Magic_en.pdf