How to measure velocity of fragile solids

In this installment of The Answer to Your Problems, a reader receives tips on how to measure material velocity and flow rate at several key positions when pneumatically conveying very friable materials.

The Answer




WE DO control systems for pneumatic conveying systems. An upcoming project involves conveying very friable materials in a system designed to minimize particle denigration. We need to measure material velocity and flow rate at several key positions to ensure material isn’t building up. System pressure doesn’t tell us enough. I’m not sure we can put a probe inside the 2-in. stainless tube. Any suggestions for an internal or external solution?

-- From March 2006 CONTROL DESIGN


Several Possibilities Exist
I assume particle denigration is particle breakage or attrition or maybe degradation. However, there seem to be two questions here. One is how to avoid attrition, and the other is how to avoid particle build-up. A well-designed dense phase conveyor would have some build-up, which is good for reducing attrition. It could be that the material ages, and they don't want build-up. Let me take on each one separately:

1) Particle build-up can be measured by electrical capacitive tomography. There are several people using this technology in experimental systems. The most appropriate contact would be at The University of Manchester Institute of Science and Technology (UMIST). This is a rather new technology, since about 1994.

A second method would be to measure the density of the mixture of air and solids using gamma ray tomography. However, many people don't like using this method.

Another method would be to use RF tomography. These devices are relatively inexpensive, since they’re used to measure moisture content, but require that the RF field be immersed in the solids. By installing these on the bottom of a non-metallic pipe, the presence of the solids can be detected. The easiest way to measure particle build-up in a pneumatic conveyor is to use mass-flow feeders at each end. If you don't get the same mass of material out (allowing for the transit time), then there is a build-up. This won't work with dense-phase conveyors because the transit time varies.

2) Particle velocity might be cheaper to measure by installing a transducer at an elbow and measuring the impact. Devices like this have been used to measure solids flow rates.

Another method involves measuring the electrical signal that is generated when a particle strikes a metal surface. The more-sophisticated triboelectric sensors can measure particle velocity by the strength of that electrical signal. This method is not that precise because particle build-up will decrease the particle velocity, but will impart a stronger electrical signal to the sensor.

Sonic mass flow sensors could be used to give a relative particle velocity, but this won't work in dense-phase systems, or when the particle-size distribution is wide.

To speculate on additional methods would require more knowledge of the physical properties of the material. There are ways to measure the particle-size distribution in-situ, and there are ways to measure particle shape in-situ, as well as changes in moisture content due to breakage.

To get more to the heart of the problem, I disagree that pressure isn’t a good indicator. It’s an excellent indicator of attrition, if both gas and solids flow can be controlled, because it will detect system problems such as a leaking valve, a loose fitting, etc. With a good model and the right experimental data on the material being conveyed, both particle build-up and attrition can be avoided. Most of the time, gas flow varies in dilute-phase systems, or solids flow varies in dense-phase systems, leading to inappropriate pressure readings.

Tom Blackwood, Director of Ttechnology, Healthsite Associates, Ballwin, Mo.

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August's Question

How should we secure the operator panel?
A customer wants us to upgrade the security features that determine access and levels of access to various parts of the control and automation system. They have issues with passwords being shared and keycards being borrowed or duplicated. Should we be looking fingerprint or retina scanners or the like? Just how “industrial” could we expect them to be? Are they cost-effective compared to more traditional security measures?

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