This Month's Puzzler
We experienced a massive quality control failure in our batch process when a problem occurred in a high pH (12–12.5) control loop. We’re down and our sister plant is handling our customers until this problem is fixed. In our process, we neutralize a weak acid feed solution with a caustic solution. The automatic control for caustic dosing worked fine in the past but doesn’t anymore. So, operators have been doing lab tests and manually manipulating the caustic addition loop. Titration has changed recently because a particular indicator was out of stock. Because the company guideline doesn’t call for a specific indicator, the operators have used several but can’t seem to get the titration right. We also have tried pH probes from various vendors but all suffer fouling problems. Now we’re regularly facing situations where the pH probe slows down and then the control valve bangs up and down on its limits. In addition, I’m concerned about our use of a ball valve for dosing the caustic instead of a smaller globe valve. A corporate electrical engineer chose it — and asserts it works as well as a globe valve for caustic addition and, besides, it’s cheaper. That valve handles the addition of the large quantity of caustic that sometimes is necessary during startup as well as the small amount usually needed during normal operations.
I also question the accuracy of the pH probe because calibration relies on only two points, at 7 pH and 10 pH, at 25°C when the process operates at about 55°C.
The plant instrument engineer installed three pH meters a few years ago with the idea of a voting logic to average the pH signal for valve control. Currently, none of the probes and meters are in working order. He’s convinced we should opt for installing a pair of valves, one large and the other small, to control caustic flow.
What do you think we need to do to get the process running smoothly?
Review Your Controls
Measuring pH is hard under the best of circumstances. But this particular puzzler would likely qualify as a pH measuring nightmare. There are so many issues to address here that it would take pages to thoroughly cover them all, so I’ll just try to hit the high points.
1. Temperature will absolutely impact the pH reading. So, the pH of a hot sample will almost certainly not match the pH measured on that same sample that has cooled down. I would strongly advise, if you are using grab samples for control, that you keep them as close to the actual process temperature as possible during the measurement process. Clearly, having a working pH probe in the process would provide far superior accuracy.
2. Trying to control the pH with a large ball valve is an almost impossible task. You don’t actually say if the pH neutralization is batch or continuous but if it is continuous, then having two control valves (one big one and a second with a flow coefficient that’s about one-tenth that of the larger valve) can work. The pH controller adjusts the small valve and a “valve position controller” monitors the % opening of the small valve — when it gets >75% open or <25% open, the big valve is moved to drive the small valve back toward mid-scale. This allows the system to flow a lot of caustic when it needs it but still have fine control throughout. If the neutralization is a batch operation, then the control system can use small and long “squirts” of caustic to adjust the pH, provided the vessel is very well mixed and a long wait time occurs between each shot to obtain the true vessel pH before the next addition is made.
3. All pH probes age and their response slows with time. The typical life of a pH probe in clean service is about 12 months. However, high temperature and high pH age the probes much faster. Your process has both high temperature and high pH, so your probes may be burning out in a very short time. You might also have chemicals in your process that “poison” the electrode, ruining a typical probe even faster. It is worth contacting your pH probe manufacturer and determining if the probes you are using are the right type for your service. High-temperature and high-pH probes may help extend the time between replacement. If poisoning is an issue, there are probe designs which can slow this process as well.
4. Your three-pH-meter arrangement was the best — but you have to continuously maintain the probes to keep them operational. As soon as one probe reads differently than the other two or responds more slowly than the others, it should be serviced or possibly replaced. It will take significant labor and money to maintain the system but clearly you can ill afford to ignore it.
5. If fouling is an issue, various systems can wash the probe on a routine basis. Alternatively, it may be possible to rinse and then isolate the probe in water until the measurement is needed again. Don’t let a pH probe dry out as it can take hours to restore its functionality once the probe has dried.
Keeping a pH probe operational in a difficult process is an expensive time-consuming task. However, it would seem that the measurement is critical for your operation so you have little choice but to spend the time and effort to get it right.
P. Hunter Vegas, project engineering manager
Wunderlich Malec Process Automation Group
Take Some Key Steps
My experience suggests a few moves that you may want to consider:
1. If the plant is using high concentration caustic, it is possible the control valve may be sticking (or stuck in closed or open position), or the line may be plugging up. In the short run, flush out the line and the control valve. In the long run, you might consider using a lower concentration of caustic.
2. The pH probes also show fouling and need cleaning. Provide flush connections for cleaning the probes or, if problems persist, consider self-cleaning probes.