Weigh The Cause Of Feeder Problems

Dec. 7, 2018
A variety of recent changes complicates troubleshooting

This Month’s Puzzler
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?

Answer Some Key Questions

This is an interesting presentation from the perspective of someone whose position in the hierarchy is unknown. It also gives that person’s take on the perspectives of three other people at the company. However, I wonder if those other people agree with the problem description presented.

Questions I have to ask are:
1. Has the problem become a recurring situation since the changes were made or did some parts exist before the changes were made?
2. Does the production manager’s comment that caking of the cyclone has been a constant problem refer to before the changes were made or after? If the problem already was there, what did he do to produce quality product? If this was a known issue, how was it addressed?
3. What is important to the company: product quality or personnel disagreeing on a positive solution and their ability to come to consensus to solve a problem that, from the description, is going to become more acute with time?
4.Who is in charge or the decision-maker? Do we have a correct perspective of the problem?
5. Was dust accumulation an issue at any point before the recent problem started?

(By the way, if the rejects have increased and various attempts at fixing the issue haven’t worked, then the suggestion that this is “a very reliable system” doesn’t hold water.)

I would suggest doing an immediate calibration of the system to ensure that it’s working properly as designed with all the changes that have been made. This could give them a road map. I hope all the people (problem writer, production manager, plant electrician) work together to draw up a corrective action plan to be checked out in defined time. They are entrusted to solve a problem and have a pharmaceutical operation producing repeated quality product. Airing dirty laundry, as this case seems, isn’t a pleasant situation.
Girish Malhotra, president
EPCOT International, Pepper Pike, Ohio

Blame The Dike Repairs

Whenever something that has worked well in the past stops working, it’s wise to look at recent changes for clues to the culprit.

Disturbing the weigh cells with jack-hammering possibly damaged one of the sensors or perhaps a wire. A wire seems likely because the problem is quality control and not outright failure. Look at the wiring, especially the grounding. Another possible contributor is the wiring in the marshalling panel. If this was close to the concrete repair work, a wire could be loose there as well.

It’s also possible that the larger blower has changed the particle distribution of the powder being conducted. A higher velocity will draw heavier particles. Larger particles may not work as well in the pelletizer.

I don’t see how changing the timers could affect the weigh cells but it’s possible. What if the timers don’t allow sufficient time for the weight to stabilize?
Dirk Willard, consultant
Wooster, Ohio

February’s Puzzler

The 5-ft-diameter caustic scrubber at our refinery is supposed to capture H2S. We sell the sodium bisulfide produced; this decreases the disposal costs of caustic soda. In our last turnaround, we replaced 30 sieve trays with two beds of high-efficiency glass-filled random packing. This design should improve efficiency because the caustic now goes through a plate-and-frame chiller that allows us a much larger reservoir in the tower. However, since the turnaround, our bisulfide product has been contaminated with sodium carbonate, making it a struggle to meet our bisulfide sales obligations.

Inspection of the liquid distributors — which were retained from the old internals at the insistence of operations — shows they are level and not too badly plugged. A model of the tower indicates that the packing is underperforming. The H2S going to our vent system exceeds our permit.

The sales staff knows that customers worry about potential contaminants in the bisulfide and blames the current problem on upstream CO2 membranes, which were replaced during routine turnaround maintenance. One engineer now working in sales warns that the sodium carbonate soon will prove that opting for a plate-and-frame chiller was a mistake because such units foul easily, adding that selecting a spiral heat exchanger, which resists fouling better, would have been more sensible. I’m not sure about fouling in the exchanger because it lacks taps for pressure gauges and thermowells.

Operations personnel want their sieve trays back. They never were fans of packing and were against the change, asserting “if it isn’t broken, don’t mess with it.” They repeatedly argued for a larger pump to move the bisulfide; the plant control engineer always hated the level control in the scrubber.

I think something must be wrong with the packing. I’ve seen this problem following three or four years of operation but never just a few weeks after addition of new packing.

Do you think sales is right? If so, how do we prove it? Is there something wrong with the packing? Did we miss something with the liquid or vapor distribution? Should we go back to the sieve trays?

Send us your comments, suggestions or solutions for this question by January 11, 2019. We’ll include as many of them as possible in the February 2019 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at [email protected] 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.