Background;

From my start over 40 years ago as a student, having worked for 7 years at way up through all the progressive steps of the basic Beater-Room operations, I’ve learned a lot regarding how pulp is made from recycled papers. I then enrolled in learning Instrumentation through an employer sponsored apprenticeship program. Upon completion of becoming a tradesperson, I became obsessed with trying to address all the issues that could effectively be overcome with use of automated controls. I found that the best any control strategy could deliver could never be better than the information received from the field transmitters. Through experience I also learned that the accuracies of all the much-needed information had also been biased by the actions of all the other controlled variables. Since at that time only pneumatic devices were available, it was very difficult to overcome these biasings, as only pneumatic relays were available for these tasks. Pneumatic recorders were also of no real help, since they were simply single lines without the possibility of superimposing common elements.

            Then with the emergence of PLC’s and DCS systems, the opportunity was available to better design the needed control strategies based upon the digitally enhanced field transmitters’ signals. It was at this time when our plant got its first DCS system, being an Accuray design with 3 scanners and about 2,000 points. I was responsible for setting up all the controls and strategies from the Beater-Room to the wet-end, where the new DCS took over all the needs of looking after the 8 multi-formers that made our final sheet. At that time, the Beater-Room’s controllers were all stand-alone single-loop electronic controllers without the capability to see each other. I got some electronic recorders but true process insight was very cumbersome as wirings were needed for all that was necessary. After about 8 years or so, the Beater-Room got a DCS of its own, being a Moore APACS system. It was at this time that an electrician with an extensive computer literacy and I moved up into a newly formed control department that was needed to design and maintain our new DCS systems. Here in our new capacity, we were given the responsibility to assure that our papermachine had correct and sufficient pulp for continuous operation.  

The need;

            In our new position we soon found that the greatest problem to overcome was to keep up with the volume of slurry needed for production. The basic purpose of the Beater-Room was to assure that the papermachine had all the necessary, properly prepared slurry available to assure fault-free run-ability.  On a large papermachine, it was very expensive to slow down or stop due to off-grade or lack of pulp. With the newly available tools that the DCS gave us, this is where I had the chance to better find the causes of our problems. To put simply, we were responsible to have whatever volume was needed for production always available. As well as having it available, it had to maintain the needed qualities that were necessary for on-grade continuous production. By usage of the superior capabilities of our DCS trends which I had used extensively, over a great period of time had confirmed that the only way we could be losing out on throughput is to be putting in too little. As long as the demand stays constant for the same product and the available volume drops off then the only missing item is the raw material. This simple finding was in line with my longtime belief that there was no guaranteed, fault-free, consistently accurate and repeatable consistency transmitter available. For great numbers of years, we’ve had to maintain a constantly stable consistency of our furnish being delivered to the wet-end. After having purchased and tuning every available consistency transmitter available to us, it clearly became conclusive that their accuracies had relied upon our abilities to keep other parameters to be within the manufacturers’ specs.

This finding was due to the fact that all the available consistency transmitters relied upon inferred methods of detection based upon other properties of the stock, such as the ability to flow through a pipe. This ability to flex a sensor was calibrated to reflect a consistency value within a calibrated span in an effort to control it. A major downfall of this means of measurement was the fact that it was also subject to flow variations, which were very prevalent in all delivery systems. we had found it extremely difficult to de-couple the flow upsets from the actual reading, even through countless tunings of interacting loops. We have also learned that tuning can only work as long as the process stays the same as when the tuning was carried out. As mechanical components wear or are changed, or any of the original conditions become different, the original tuning could even hurt the process unless again is retuned.  This is because controllers’ tunings are dedicated values that have been used to address the conditions present at that time. After a great deal of research into how all the available transmitters actually function, it became evident that the only way to assure repeatable accuracy was to take a hand sample of the slurry and lab test it. Of course, this method was not ideal for high-speed paper manufacturing, so another method had to be found that could replicate the same results as hand samplings. We tried to address this by assuring that all the different manufacturers’ specs had been in control but it proved to be an unsurmountable task. Another method had to be found that could address different conditions more effectively.

The solution;

            Having a DCS at our disposal had made it available as our source to possibly overcome this major problem. After more than 6 years of trying to overcome issues that we had felt were occurring, the means was now available to allow seeing if they were true. The different trends that we had set up over great periods of time were conclusive to the fact that for the most part our delivery system was way too light, in spite of the fact that all looked normal from our properly tuned controllers. By difficult conviction to management after a number of years to remove a consistency transmitter that had been used as a final control to our wet-end, it was even possible to clearly see a major improvement in overall productivity. This had clearly shown to me how a skewed reading had adversely affected our best efforts to speed up. At looking back to our original delivery system, we had 9 consistency transmitters from our pulper to the wet-end and even after the removal of the last one, there were remaining problems. Long term trends were used to show that all the differences between all the consistency transmitters were different day-to-day even though the controllers were on automatic and were using the same set-points. After a great deal of tracing we found that the main disturbances had started at the pulpers, as that being the most difficult point at which to attain repeatably. After asking for a better transmitter at this location, the plant purchased a $32,000 microwave unit as a trial installation, but it had failed to work whenever discharge pump suction problems occurred. This new transmitter then again became the source of ongoing problems.

This is when we had seen the need to design a strategy that worked at getting the consistency of the actual pulper tub, before it was even needed to be taken out. It had been based upon what the DCS had seen being loaded on the conveyor and the actual work that has been done by the rotor in breaking down the pulp. It had been done in an unconventional manner, meaning that while it was an actual controller with a faceplate and set-point, it did not use dedicated P and ID settings. Rather it found its own gain and reset values from the differences between the set-point and the process variable, as well as for how long it had been away from the set-point. Being only software, it had worked so well that the actual signal was replicated by the plant’s microwave unit when it had been working. After having put this strategy online and in full control, the whole rest of the delivery system worked noticeably better overall. The major source of our upsets had been addressed and we had consistently more available stock. We still at times lost our major chests, but the causes had been traced to insufficient stock on the conveyors, which the plant tried to address with a second feed belt. It was better but not flawless as there were still times when there were still not enough bales on the conveyors to fill our needs.

It was found that whenever the conveyors were kept filled and all the systems were left on automatic that the actual variability on the stock to the wet-end had been consistently less than + or – ½ of 1% of full span consistency for better than 24 hours. All this was repeated time and time again, whenever all controllers were left on automatic and letting the DCS have total control. The longest I had been able to keep all on automatic continuously was about a week as all the operators felt they wouldn’t be needed if the system ran as expected. As this design had been done to get better overall control of consistency, an advantageous side-effect that had been clearly seen was about 20% less delivery system power consumption. I suppose that was to be expected, as we had conventional pressure-controlled loops where if the systems went lighter the loops would have ramped up to deliver more of the lighter stock to maintain the required chests’ set-points.

The Control;

We have also learned to have the greatest faith in the information passed back from the scanners, since that represents how the final product had been made. It is the true measurement of the pulp’s consistency accuracy having been used in the manufacturing. With a constant speed of production, when the actual consumption increases then the pulp must have been too light, regardless of its control. The scanners’ readings will make the necessary speed adjustments to keeping the tolerance of the caliper in acceptable range. With normal production, the speed of the machine is an excellent check on the final consistency’s control.

            This is why at our last plant we had used the feed chest with the greatest retention time as an accuracy check for the whole delivery system. The control premise had been that the feed chest with a constant level set-point should keep the same feed rate as the speed of the machine. To clarify, if the consumption was to increase for the same grade, then the chest’s consistency must have been too light and if the consumption for the same grade went down then it must have been too heavy. This is how the accuracy of the scanners was used in getting the delivery system’s consistency reading corrected. Since our pulpers now had a working consistency control feeding the cleaning system and all the remaining controllers were on automatic, then while production is being accurately made, the demand from the feed chest’s level controller should remain relatively constant. If instead it started to go to either extreme, whether too high or too low then this knowledge was used to skew our setpoint to the pulpers’ consistency controllers in a responsive direction to avert further upsets. Whenever the rate of production increased or decreased, the feed chests’ level was kept a constant. The use of the skewing was very slight as the whole system was affected and had been set up after maximum proper production rates had been found.

Into this whole scenario another necessary algorithm had to be incorporated. The broke usage had been a major thorn in our sides, as is for most mills. The basic premise was to keep our broke storage chest at minimum level for possible breaks while assuring our freeness wouldn’t be lost when using it. It was a challenge but had been overcome by the use of an in-line freeness transmitter before the wet-end. Since we knew our dead-time and with past experiences, the broke usage was worked into our pulper level control strategy quite well. That strategy was also adaptive to the point that the higher the broke storage chest rose, the more aggressive the usage became to prevent overflowing. It had been done by prioritizing the consistency control over the freeness reading when the level became acute. It became so aggressive at times that the conveyors were stopped while still assuring the proper weight needed with the broke usage alone. This was not a good condition for a multi-former machine with four different liners being used, but was preferred over flooding our basement higher than the motors.