Monitoring Compound Concentration
How can we continuously monitor our mass finishing fluid?
Q. How can we continuously monitor our mass finishing fluid? We debur and finish stamped parts made from 22-gauge spring steel. Our finishing room has 10 bowl vibrators (20 cu ft each), two double-barrel centrifugal finishers, and a 5 cu ft centrifugal disc machine. The mass finishing operation is the final process before inspection and packaging.
All the machines use the same cleaning/inhibiting compound solution distributed by a central system. Compound is received in totes, and a venturi type proportioner draws compound from the tote and mixes it with tap water to be stored in a 30-gal reservoir with a pump for distribution to the finishing machines. The proportioner is set to provide a 2% concentration.
The vibratory time cycles are 16 -20 hours; it is possible for parts to corrode during processing if the concentration is not right. At any time we have about 0.5% of our annual sales in these machines, so a corrosion event is extremely expensive. That can occur if the concentration falls below 1%. We did have two such events in the past year. In one case, the operator did not notice that the tote of concentrate was empty; in the other case a leak developed on the vacuum side of the proportioning system due to a loose hose connection.
Management controls are now in place to make an operator error very unlikely—knock on wood! We are also considering an automatic pH monitor that can shut down the system if the pH falls below a preset level. Do you have any experience with such systems? What is your recommendation? P.L.
A. Many choices of pH monitors with high and low set points are offered in industrial catalogs. Expect to spend $800–$1,500 for a good instrument and a quality gel-type electrode. I am not an advocate of these instruments for controlling mass finishing systems, although I have seen several in the plants I have visited. If any reader has a good experience with this method, please let me know.
Allow me to explain my view on pH controllers before I continue with recommendations for your operation. My first objection is that pH meters require regular calibration using a lab grade pH solution. Some systems recommend daily calibration; that’s OK in a laboratory or high tech manufacturing operation, but not OK for the average deburring and finishing application. The pH electrodes are available in refillable fluid and gel styles. Both require considerable attention to maintain accuracy, and the gel style must be replaced when it is dried up. What happens is that pH controllers become unreliable over time. This is compensated by setting the high/low limits on the safe side, resulting in a much higher concentration than necessary.
The second problem is that good mass finishing compounds are highly buffered to maintain an effective pH range. For example, ferrous metal corrosion inhibitors work best above pH 8.5. The pH is challenged by many external conditions such as the available tap water, the volume and nature of contaminants on the workpieces, bacteria growth, ratio of parts to media, etc. That is why the compound is buffered, and this results in a solution that does not change pH in direct proportion to changes in concentration; a very large change in concentration only makes a small change in the pH. To illustrate this, I prepared several different concentrations of the compound you sent us and measured the pH at each concentration. Here is the result: at 100% the pH=9.5; at 5% the pH=9.2; at 2% the pH=9.1; and, at 1% the pH=9.0. You can see the problem with trying to control the concentration by continuously measuring the pH.
A more effective measurement is a refractometer reading. In the July and October 2008 issues of this column you will find useful information about refractometers. You can buy continuous monitoring refractometers, but they are very expensive—in the $10,000 range. I have two suggestions to solve this problem. First, use a good quality digital refractometer. Pocket models suitable for shop floor use sell for less than $500; a step up gets into portable models with more accuracy for less than $1,000; finally you can get into lab models with extreme accuracy for about $5,000. Issue a pocket model to the foreman or operator for daily checks—twice each shift in your operation, and invest in one of the more expensive models for your QC lab to use on a less frequent schedule.
My second suggestion is to install a float in the tote and attach it to a cut-off switch that will shut down the compound pump when the tote is nearly empty. This would be a semi-portable unit easily moved to the next tote.
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