 If you are in a market situation requiring more production of the current paper grades, it may be tempting to believe that simply speeding up the paper machine will be sufficient to accomplish the goal. After all, aren't all paper machines "over designed," meaning that they have a given amount of excess capacity built in them? This may be the case in some instances, but it does not mean that the machine will operate with the same efficiency in all speed ranges. Failure to understand this will lead to adverse surprises in the form of production upsets later.
Every process has bottlenecks which limit the total production capacity. If these have been identified, corrections can be implemented in advance to remove the ones that are likely to prevent a successful production increase. However, there are likely to be other bottlenecks that are not apparent at the existing production level. These are in potential problem areas which must be analyzed before making a major change.
The usual thought is that increasing paper machine production means increasing the machine speed, but this is not always the case. For example, it may be possible to increase production by improving operating efficiency (reducing downtime) or by implementing actions to decrease rejects. But when efficiency-related improvement ideas have been exhausted, attention will invariably return to making a speed increase.
Of course, it may be apparent that to achieve the desired speed increase will require rebuilding some area(s) of the paper machine or its supporting operations. In these situations, in my opinion, audits to identify the design changes needed are best made by the manufacturer. These cases will involve significant capital investment, and the OEM can assist in providing data to justify the project. The OEM will also provide start-up support and, often, contractual guarantees that the target results will be achieved. However, other suppliers are also able to conduct paper machine sectional or complete audits to help identify areas which may need new equipment or modernization.
Even if it is assumed that the speed increase can be accomplished without rebuilding the paper machine or any of the supporting functions, the next step is to identify as many potential problems as possible. This means determining accurately the current operating conditions and then predicting the outcome from the change. In essence, an in-depth observational study, or base line audit, is made. It involves collecting data without changing the operating conditions. In many cases, the suppliers will assist in this phase, since the speed increase planned may affect the performance of their products. The suppliers may also observe operating conditions which are not considered efficient based on their broad industry experience.
Let's assume the mill has decided to increase the speed of the paper machine significantly (>10%) after analyzing the base line data. In this hypothetical case, it has been decided that the increase can be achieved without a major rebuild or other significant capital investments. Here are just a few of the major issues or bottlenecks that may occur as a result of this decision.
• The stock cleaner system could begin to pass through more contaminants and the screening system may get overloaded. This would result in more defects in the finished sheet and higher rejects.
• The base sheet fiber distribution, formation, and bonding could change due to increased stock flow and more demand on the refining system. If strength properties decrease, the possibility exists for more sheet breaks later in the process.
• The base sheet CD basis weight and moisture profile may become more variable at the higher headbox stock delivery. Adjustments may be needed to stay within the desired control limits.
• The sheet entering the press section will increase in moisture unless the forming section can handle the increase in drainage demand. At this stage, there should already be base line moisture data, to include samples taken at various points on the wire (depending on former design). A need to change the void volume of the wire(s) or modify drainage elements may arise as a bottleneck. When the speed increase becomes a permanent part of the production process, it is likely that wire life will decrease to some degree. The wire(s) may also need more cleaning attention under the changed operating conditions.
• The sheet leaving the press section will contain more moisture. Unless a complete press analysis has been made prior to the speed increase, including changing press loadings and roll crowns, this will inevitably occur. Moisture removal in the press section is controlled primarily by the press impulse, which is simple the linear loading divided by the machine speed, usually expressed in psi.sec or kPa.sec. It stands to reason that if speed alone increases, the press impulse will decrease, resulting in a wetter web.
• The sheet-draw relationship will change, due to the change in sheet dryness, and stronger adherence to rolls/felts may be observed, depending on the press design.
• There may be an increase in sheet break frequency, related to a change in moisture profile and wetter edges. This may also become evident in more sheet wrinkles or calendar cuts at the dry end.
• Felt life is likely to decrease to some degree, and more emphasis will be needed on felt cleaning.
• The drying load will increase unless steps have already been taken related to the press impulse.
• More sheet flutter could be noted in the dryer section, due to a change in moisture profile.
• A change in moisture profile could carry through to the finished sheet at the reel.
• The higher production rate will directly affect all finishing areas functions, to include off-machine coating and calendering, if applicable.
Obviously, papermakers will take the corrective actions within their means to minimize the bottle necks which have been identified before increasing the production rate. The majority of these issues can be exposed using a complete potential problem analysis approach. Realizing that all bottlenecks cannot be identified, some mills may elect to increase the operating rate more gradually, using incremental speed increase steps. However, each level of speed brings its own set of problems, and it should not be expected that a gradual approach will necessarily make it altogether easier to attain the overall production increase objective.
Robert Moore is a retired chemical engineer, and is an experienced technical and fictional writer. His past work experience spanned the chemical, paper and equipment manufacturing industries, including holding management positions at Voith Paper, Scapa plc, and The Mead Paper Corporation. He is also the author of humorous short stories about life in southwest Virginia, circa 1940-1960.
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