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.
3. The type of ball valve (V, regular, trunnion mount) is not specified in the description. In general, a globe valve is considered better at flow control than a ball valve.
4. Develop a titration curve. Identify color change points for the new indicators. If the color change is way before the equivalence point, that indicator will not help regulate the pH.
In the long run, consider other variables that could affect pH control: caustic addition location and mixing of the streams; and location of pH probes — it should minimize delay in adding caustic in response to changes in pH of incoming stream/streams.
GC Shah, senior HSE advisor
Forget About A Fast Fix
Obviously, there’s a lot corporate engineering doesn’t know. Apparently, production has been living with the illusion that what they didn’t know wouldn’t hurt anyone. Operators have been running this process in manual mode for a long time and only recently tripped themselves up because of changes in laboratory procedures. Clearly, somebody in quality control has been covering for them, too, to account for the occasional blips in poor quality. These circumstances are quite common in remote locations and ones with loose corporate controls.
I see the following problems with the current process: 1) deficient pH calibration procedures; 2) poor control element; 3) inadequate quality control checks; 4) lack of knowledge about why the probes failed; 5) no material balance to determine the range of additions; 6) inattention to system lags caused by fouling electrodes; 7) absence of insights on why manual control worked well for so long, e.g., the tricks the operators learned; and 8) potential inadequacy of the initial programming for the loop — did it cause difficulties in operator usability?
I don’t recommend anything but three-point calibration, although the third point is really only a check. In this situation, where the pH is so high, calibration standard solutions aren’t readily available for strong acids and bases because of Dept. of Transportation shipping limits. Instead, make the high pH standard yourself using NaOH pellets. If you’re calibrating more than one probe, do them all together with the same standards. With this type of instrument, it’s better to be consistent than accurate: pH ranges ±0.25 for individual probes; it’s unlikely the probes will agree within 0.1 points unless they’re all scaled up or otherwise not working. Note that temperature compensation for actual operating conditions is required for accurate measurement.
Although references such as the 4th edition of Shinskey’s “Process Control Systems” sometimes recommend ball valves, I don’t. Globe valves provide better control characteristics and should be used for fine-tuning. However, you may want to consider a V-port ball valve for startup as suggested by the plant instrument engineer.
If pH is a quality control parameter, how is it that the process pH probes aren’t working but the lab one is? Or, is pH being accurately measured at all in the lab? You’ll want to evaluate calibration procedures and testing to mirror what is done in the lab so results will be similar. This leads me to my next question: Why did the probes fail?
In a similar situation, we couldn’t really improve the probes but we could reduce calibration complexity and find the best probes that could run the longest. It turns out that making an instrument more user-friendly means it’s calibrated when it needs to be. In addition, we relocated the probes to a pump discharge line leg, reducing the line size to increase the flush velocity around the electrode.
Flushing helps avoid a lag from a fouled pH membrane. Take a critical look at the pH probe time lag. This is one of the most common reasons for operators giving up on automatic pH control. If you can’t solve this problem for most operating conditions, you’re stuck in manual control.
That brings up my last point: convincing the operators to accept automatic control. They may have learned a few tricks — like soaking the probe in a buffer solution — that you may want to incorporate in your new procedures. Yes, you’ll be writing new procedures. Part of those solutions may be learned from their experience.
Dirk Willard, consultant
At one of our pharmaceutical plants, an old powder weight-feeder system appears to be failing: the weights are correct most of the time but our rejects have increased significantly in the past few weeks. Some managers see this as a justification to get rid of the system, which doesn’t integrate with our new distributed control system, and install one that does. However, I’m opposed to ditching what has been a very reliable system.
This setup consists of a bin, a cyclone on a three-point scale, and a vacuum system. The powder is pelletized into pills in the next step. I’ve carefully compiled a list of changes made in the area recently. The ones that stand out in my mind are: 1) a larger blower was installed to meet additional air filtration requirements; 2) the seals were replaced on the bin rotary valve; 3) some timers were adjusted in the controls to compensate for the dust accumulation in the bin, to lengthen the time between filter changeout; 4) a tank dike next to the cyclone was dismantled — doing this required jack hammers and removal of old pipe and conduit; and 5) some terminals were re-arranged in an intrinsically safe barrier box also used by the feeder system.
The production manager says that caking in the cyclone has been a constant problem. The plant electrician claims that the vibration pads the manager installed interfere with the level switches used to control the blower. The safety director at corporate is concerned about the dust accumulation not only inside the cyclone but also around it when it’s cleaned, which apparently takes place more often than desired. What do you think should be done? Does this dust really pose a problem?
Send us your comments, suggestions or solutions for this question by November 16, 2018. We’ll include as many of them as possible in the December 2018 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.
And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.