I’ve been troubled for a long time by discussions regarding flow meters and concentration meters (those in the instrumentation industry are welcome to provide answers)
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Once, I was dealing with a problem in a paper pulp metering system belonging to one of my colleagues, and I noticed that the instruments were not giving accurate readings. The measurement of the pulp volume is achieved by using a flow meter and a concentration meter, and the mass of the dry pulp is determined through calculation. After the system was put into operation, significant deviations from the laboratory values were observed. At that time, through on-site observation, I found that the outlet pipeline of the flow meter was too long and had an upward slope; as a result, the pulp in the pipeline might flow back due to the pump stopping. I recommended installing a check valve and setting the flow meter to not measure flow in reverse direction. But after running it for some time, problems continued to arise. After ruling out various factors such as interference at the installation site, my attention focused on the instruments, and I began to wonder if there was a problem with the instruments themselves – perhaps such a design was simply unreasonable from the start! In my subsequent work, this awareness became increasingly firmly established in my mind – conventional flow and concentration transmitters available on the market simply cannot provide accurate measurements! We know that most electromagnetic flowmeters are calibrated using water circulation before leaving the factory, so what happens next? Then go test the pulp, chemicals, and chemical black liquor! How should one take into account factors such as the uniformity of the material being tested and the complexity of its composition? The issue with concentration is that there is no standard at all; no one can say exactly what the measured concentration is, as such a concept simply does not exist. We can only observe its general trend and compare it with laboratory results, but it is not possible to determine it with precision. I work in control systems, and over the years I have also been responsible for guiding on-site installations. I encountered a variety of problems, but found that once on-site and selection-related issues were ruled out, problems with the instruments themselves accounted for a large proportion. I think the manufacturers should take all these factors into consideration and make further improvements in the design of the instruments. This problem has been bothering me for a long time. I’m posting it here in the hope that friends who have had similar experiences will join in the discussion; I truly welcome everyone
I have also encountered the same problem as LZ. Concentration measurement in paper mills works perfectly well for process control, but it results in significant errors when used for measuring the amount of completely dry pulp; as a result, the data is completely unreliable for monthly settlements. The pulp concentration in the conveying system is very low, and as measurement errors accumulate over time and are combined with flow rate calculations, the resulting errors become excessive.
Oh... yes. Sometimes it’s a problem with the instrument itself. But the boss still suspects that we’re unable to handle it on our own, as an excuse to cover up his own shortcomings.
Which type of flow meter does the original poster use for measurement? Could you provide some information on the process parameters and the flow meter used? It’s possible that the flow meter selected for this operating condition is not suitable; using a flow meter based on a different principle might yield better results.
The turbine flowmeters and Moeber density meters used in our factory to measure the mass flow rate of naphtha, in my opinion, are the result of a design flaw; it would be better to simply purchase mass flowmeters instead. But the measurement results are still quite accurate, with errors within one thousandth; in most cases, the error is within 0.5 thousandth. I’m not sure whether this can be considered accurate
It is recommended to purchase a mass flow meter, either an E+H straight-tube type or a Coriolis elbow-type
Most paper mills use Yokogawa flowmeters, as pulp is a non-uniform fluid, different from mixtures such as sludge. The absolute concentration ranges from 0.001 to 30%, but in terms of transportation or processing, it is mostly within the range of 0.03–5%. It is indeed very error-prone to determine the flow rate when converting it to absolute dry slurry. There is also a large discrepancy between the actual concentration measured in the pulp and the concentration detected by the instrument; generally, the error is around 25%, and in severe cases it can reach 50%.
From two aspects, the fluid interference caused by the pulp itself in electromagnetic flowmeters is very significant, making it impossible to ensure accuracy. Secondly, due to the large fluctuations in the real signal, filtering is necessary, and it has to be done intensively, which further causes deviations in accuracy. On the other hand, it’s not clear what principle the concentration meter operates on. As far as I know, the method of calculating concentration based on density requires knowing the density of the solute and the density of the solvent; the density of the solvent is easy to determine – it’s water – but the key is the density of the solute. Is that known? Is it stable? If the solute density changes frequently or if theoretical values are used without any knowledge of their accuracy, the precision will inevitably be affected.
Although there are many types of flow meters, it’s actually not easy to find a suitable one. Many manufacturers claim their products are excellent, but once they’re used in practice, they’re not actually that good.