Material Flow Characteristics in Silos and How it Helped Me to Stop Believing in Magic
For much of my 30 years of non-contacting level measurement experience, I believed that there was an element of magic associated with it. When applications were successful it was good magic, and when they weren’t, it was black magic.
As time went on, and my experience and knowledge levels rose, I realized that there was not quite as much “magic” as I initially thought but “REASONS” why things acted the way they did. Well, most of the time…but, not all of the time.
My most baffling experience happened in an application involving solids materials. This application shouldn’t have presented any problems and was a routine fix. On this particular application, a site was experiencing erratic behavior from the level sensors – and they had the trends to prove it. Every time we would send a technician, and sometimes that technician would be me, we couldn’t figure it out. There was absolutely zero reason for this behavior to happen. The echo profiles looked fine and the erratic behavior captured on the trends were of such a frequency and randomness that the technician would have to actually sit by the unit (for who knows how long) waiting to capture the right moment. Miss a chance and poof! The clock starts all over again.
Thanks to a colleague, and the constant bizarre behavior, we were able to resolve the issue with a little patience and resolution. It turns out that silo discharge geometry can play a major role in how the material flows during discharge. My colleague explained the difference between mass flow and funnel flow, but until I actually saw the difference I was not convinced. Then it happened: I saw the difference.
This particular site had six silos that had a wedge-shaped lower discharge section, and the very bottom was slanted in a vertical orientation that allowed the material to dump into the lowest part of this section where there was the takeaway screw.
They also had two newer silos that had the more traditional cone discharge section. Before I scaled the silos, I asked the operators if they noticed a difference in behavior between the six older silos and the two newer silos. They said they have never had an issue with the newer silos so, now I knew what to look for: the differences between the six older silos and the two newer silos.
The difference was the bottom discharge section. The proof was literally staring me in the face when I looked inside the first old style silo. There was a very large section of the material surface that had fallen away, leaving sides that looked like an ice berg looks when a large piece falls off into the ocean. The hole must have been a good 10-15ft. in diameter and about 6-8ft. deep. Our level transmitter was mounted right above the lower portion of the screw and, obviously, right above this newly created hole.
It is easy to imagine that the material would compact as it tried to discharge through this relatively small discharge area, and depending on the make-up of the material this “bridging” could stand up for some period of time before collapsing. Until this collapse occurred, the top level of the material would be a nice smooth surface. The kind we always want to see in these solids applications.
So, what did I learn? Well, in conclusion, not all of the erratic behaviors and apparent unsuccessful applications are a result of the level instruments’ capabilities. A little more investigative research/observation and a little less magical thought is now in my approach to troubleshooting problem applications.
So if you find yourself frustrated and starting to question the capabilities of your instrumentation, do yourself a favor and make sure you have looked at every potential influence that might be keeping your technology from being successful in the application.
Add an inspection of the silo from top to bottom to your troubleshooting checklist. And, if you can correct your discharge issues, or even avoid installing a silo that won’t lend itself to steady flow conditions, you would be doing yourself a big favor. This would even allow you to start trusting the level readings your instrumentation generates.