Patterns of repetition of problems during production of paper can provide important clues as to their root causes. Both the time periods and the locations (especially if the problems are visible in the product) can be important.
Questions to ask about a recurring problem include (a) is there a fixed time period between occurrences, and (b) does the problem occur after some other event takes place, as in the switch-over to a new batch of wet-end starch or retention aid.
In cases where a problem occurs at a well-defined frequency, the next step is to consider the cycle times of processes in the system that might possibly be related. A lot of imagination is needed for this kind of exercise, since the root causes may not be obvious. For instance, the filling of starch preparation tanks with water for an adjacent paper machine may be changing the water pressure and adversely affecting retention aid flows on your own paper machine. The cycling on and off of an intermittent biocide program may be changing the level of surface-active chemicals in the system. There may be an imperfection or filled area on a wet-press felt or dryer can. There may be "harmonics" in the hydraulic system of the machine. There may be cycling of the amounts or types of broke introduced into the furnish. Sometimes there is a process control loop that needs to be tuned so that it doesn't overshoot and cause cycling. Each paper machine is different, so one needs to make one's own list.
PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (firstname.lastname@example.org). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.