CUSTOM CHEMICALS of TEXAS

P. O. BOX 815
MANSFIELD, TEXAS
76063
(817) 453-0704

TECHNICAL MEMO #17: Automated Testing and Control

The theory behind most automated control systems is sound, and when they are properly designed, installed, and well maintained, they are capable of working efficiently. However, it is surprising to see the number of paint line washers that have either the remnants of an abandoned automatic control system, an installed system that isn't being used, or one that doesn't work. This usually results from unrealistic expectations as to what the systems are capable of doing, failure to maintain the system, or poor design.

Automatic testing and control systems are usually set up to monitor one (occasionally two) of the necessary control parameters for a given bath on a continuous basis. Appropriate sensing devices are used, and the necessary electronic equipment is normally present so that a low and high set point can be used to turn chemical feed pumps or water flow on and off as necessary. There may also be a recorder that will maintain a record of the readings obtained. Parameters that can be automatically monitored and potentially controlled include liquid level, conductivity, Ph, oxidation/reduction potential, and the concentration of a variety of specific ions.

The hardware and electronics used in most systems is usually reliable, with electric solenoids used to control water flow producing the most common mechanical failure. Most of the other problems are caused by the sensing devices such as conductivity probes or cells, Ph probes, O. R. P. probes, and specific ion probes. Although new designs are much improved, these are fairly delicate devices, difficult to keep clean and calibrated under line conditions. If not kept clean and calibrated, the sensing devices will be the source of most of the system's trouble.

These systems are sold partially on the basis that they will allow operators to spend less time monitoring the system. Problems often occur due to inattention. While it is true that the frequency of chemical tests can usually be reduced, if too much reliance is placed on the automatic control system, baths can go out of control without detection. It is also true that the automatic control system may not be tracking the only control parameter that is critical to proper bath function. There may be additional testing and correction that must be done by the line operator. These systems usually maintain the overall concentration, but only the more expensive and complicated systems are capable of adjusting bath balance. It is necessary that the line operator check the system and test the baths often enough to know whether everything is working normally. It is debatable whether an automatic control system, if operated correctly, will save the operator much time, especially if the time required to maintain the control system is considered. It may only give him a false sense of security.

Another selling point for these systems is that they will save chemicals. This is rarely, if ever, true. The actual process of cleaning and treating the metal will expend the same amount of chemical whether automated controls are in use or not. Chemicals wasted through mechanical problems or the inherent inefficiency of the line account for a surprising proportion of total chemical consumption, and are minimally affected by bath control. An automated feed system may, in fact, mask abnormal usage for a time.

A third selling point for automated control systems is that they will improve quality through more consistent operation, and that is a valid point. When the system is operating properly, the baths can be controlled within a much narrower range, which can produce more consistent quality. It should be noted that similar systems of control are frequently used on wastewater treatment systems, and can provide valuable added security in this application.

The attached table lists some common problems, their cause, and potential result. There are some alternatives to a fully automated control system that can help avoid some of the pitfalls mentioned. If production is fairly consistent (line load and speed relatively constant), the use of a chemical feed pump with adjustable stroke length and frequency (and possibly a percent-o-cycle timer) might be appropriate. Pumps used this way should only have power when the production line and the washer pump are running. The operator can vary feed rates almost infinitely, adjusting them as necessary after testing the washer. This method can produce relatively stable control. It is also possible to use the sensing and control portion of an automatic control system to turn on an out-of-specification alarm, rather than a chemical feed pump. This is very effective when used with the continuous-feed system just described. The alarm would cause the operator to test the bath and adjust the pumps or bath as necessary.

In any case, plant personnel must realize that the installation of an automated control system is not a turn-it-on-and-forget-it solution to bath control. If the washer and control system is not carefully monitored and maintained, you might be better off without the automated controls.