A new technology for basis weight profile control allows productivity gains while saving energy
April 2007
By Charles Pound
As the gypsum linerboard industry seeks higher profitability for itself and its customers, much attention has centered on CD profile control improvements, and the benefits associated with better profiles. A recent innovation applied to various types of hydraulic formers, the trademarked Octopus stock approach system, has created a step-change in profiles thus helping to increase production capacity, improve sheet quality and market differentiation, as well as reducing fiber use and energy costs. Beyond the mill, a more uniform profile allows converters the ability to maximize production speed and reduce their need for process changes because of paper variation.
In contrast to traditional tapered inlets, known to be an obstacle to the consistency of cross-direction (CD) basis weight profiles, the Octopus system combines radial stock distribution with an integral non-contact pulse attenuator (Figure 1). This creates a uniform CD basis weight feed to the hydraulic former. The feed does not change with variations in machine speed or former feed flows. In addition, the Octopus reduces pressure pulses to the former coming from the fan pump or machine screen, which may cause barring (slice jet velocity changes).
When consistency profiling capabilities are added to the basic Octopus stock approach system, further decreases in CD basis weight profiles are possible.
Improvement Needed Now
In North America, approximately 80% of the gypsum linerboard machines still use multiple hydraulic formers to form the sheet.1 The wet ends of these machines are comprised of five to eight Horne, Bristol or Kobayashi units combined to make recycled sheets ranging from 40-55 lb/1,000 ft2 in basis weight. They account for 1,571,705 of the 2,359,855 tons/yr produced (66%) and are the oldest of the group of machines producing the grade.
Radial distributors have existed for a long time, but the recent application of this technology to hydraulic formers for gypsum linerboard has reduced the two-sigma CD basis weight values. As each ply in the multi-ply wet end has been modified with an Octopus feed, the result has been sheet variation decreases.
While upgrades of individual formers have taken place for either maintenance or process improvements, the basic technology used for CD basis weight profile control remained unchanged until recently, when the Octopus was introduced.
In one example, three Octopus stock approach systems were installed on a 112-in-wide machine configured with six hydraulic formers. Installations on plies 1, 2, and 3 reduced the two-sigma values from 3.14 to 1.91 lb/1,000 ft2, a decrease of 39%.
The basic Octopus stock approach system reduces the CD basis weight profile of each ply and contributes to a lower total sheet profile variation overall. The basic unit does not, however, offset problems caused downstream of the inlet or caused elsewhere in the paper machine.
Moving Beyond Hydraulic Formers
The purpose of a stock approach system is to change the round flow in the piping going to the forming device to a rectangular flow the width of the wet end. This rectangular flow will:
- Have even stock distribution across the full machine width
- Be cross current free
- Have minimal pressure pulsations that may affect the jet velocity in hydraulic forming units, including those used in the formation of gypsum linerboard
Hydraulic formers are fed by tapered inlet manifolds connected via a series of evenly spaced tubes to the former. Assuming the manifold is operating within the hydraulic flow range for which it was designed, the pressure at the entrance to each tube should be the same and deliver equal amounts of stock across the entire machine width. To accomplish this, the shape of the manifold — straight tapered or parabolic — has to be designed specifically for that application. Then, as flow rates are varied due to basis weight or speed changes, the recirculation valve at the end of the header is adjusted to maintain a uniform outlet pressure along the length of the header.
While this sounds straightforward, physical difficulties in accurately reading the pressure on each end make control of the recirculation valve unreliable in a closed loop control system. If the valve is not automated, human nature combined with the lack of reliable differential pressure information makes control even less accurate. Visual flow balancing tubes may assist the machine tender, but typically they get dirty and opaque and eventually become unusable.
If the flow pressure is not balanced across the width of the machine, the areas with higher pressure will be fed more paper stock and have an associated higher basis weight. As the flow is changed in the header, the pressure gradient will change and the CD basis weight will change. Additionally, the differences in flow volumes through the feed tubes across the width of the former cause cross machine currents. These may adversely affect the basis weight profile and change fiber orientation, leading to other problems. With unbalanced headers causing uneven basis weight profiles, the papermaker has to find other means to control the profile.
In most cases, this next level of control for the hydraulic formers used in gypsum linerboard manufacture is the use of hood tip or slice lip adjusters. These adjusters change the open area, redirecting stock to areas when increased, or away from locations where decreased. Unfortunately, this method to adjust the profile is not ideal. Relative differences in the hood clearance or slice opening create variations in the slice velocity and affect the localized rush-drag ratio of the sheet.
While the operator may reduce the basis weight profile variation, he can increase the fiber orientation variation, which may lead to other issues, such as cockles or curl.
Consistency Profiling Cancels Poor Profiles
Consistency profiling shapes the profile of one or two plies in a direction with such magnitude as to "cancel" the poor profile of the remaining plies when all plies are combined. In this method of control, no single ply has a flat profile. Instead, the combined variation of all plies is reduced.
Consistency profiling with an Octopus stock approach system is accomplished by injecting white water into the stock feed hoses from the Octopus body to the hydraulic former via engineered profiling taps. The dilution water prevents an equivalent amount of stock from entering the hose from the distributor section of the Octopus, by using valves with linear flow characteristics and controlled angle of injection as white water is introduced into each hose.
This one-to-one displacement of stock by water assures that the flow from every hose is the same, regardless of the body feed pressure. This allows the machine operator to vary the consistency and associated basis weight in each feed hose. As each hose has equal flow and pressure, there are no forces to move the flows laterally in the former and the control zones pass through the former at the same width as the feed hose spacing. In other words, if the feed hoses are on 3-in centers, the control zones are 3-in wide.
This was illustrated when an Octopus feeding a 171-in-wide hydraulic former with 3-in hose spacing had blue dye injected into one, then two and four adjacent hoses. As shown in Figure 2, the dye streaks are approximately 3-, 6-, and 12-in wide, respectively.
The linear valves used in the consistency profiling equipment can be either manual or automated. When automated, control of the valves may be "remote manual" or "closed loop." When done manually, the machine operator views the sheet profile and manually moves the valves from a remote location to correct the profile. The "closed loop" approach is where the DCS and process scanner work together to control profile changes. The best results have been shown to occur in the "closed loop" mode where a typical 2 lb/1,000 ft2 two-sigma cross machine basis weight profile is reduced by more than 80%.
With a former being fed by hoses having equal flows and pressures but differing consistencies, the basis weight may be controlled without introducing cross currents and eddies that would effect the control zone width and fiber orientation. This method of cross machine basis weight control offers advantages including:
- CD weight control without affecting fiber orientation
- Narrower control zone widths than are possible with mechanical slice deflection
- Low-cost control of each zone from a DCS system
- Reduced possibility of slice or hood damage as their adusters are set and not touched further
Project Justifications
While each mill calculates return on investment using criteria specific to its individual needs, benefits of the Octopus stock approach system can be achieved in the following areas:
Paper Mill Benefits: As with most paper grades, the CD moisture profile follows the bone-dry fiber cross machine basis weight profile of the sheet (heavy fiber weight areas cause high moisture areas). And while other machine components (e.g., misloaded presses) may affect the moisture profile, a flat bone-dry fiber profile is the prerequisite for a flat moisture profile. If the bone-dry weight profile variation can be reduced, then the moisture profile will be improved.
Grade moisture specifications are typically given as an average moisture target with a minimum and maximum allowable value. On machines with large two-sigma spreads, the target range is typically the value where the minimum and maximum are not exceeded, rather than the ideal moisture value of the final product.
By decreasing the bone-dry basis weight variation, and subsequently the moisture variation, the target average value can be raised, without exceeding the maximum allowable value. This target shift allows the papermaker to improve profitability in a number of ways.
Increased Production: If the average moisture target is raised, less energy is required to dry the sheet. On paper machines where the machine is steam or dryer limited rather than drive limited, higher average moisture will allow the machine to increase speed at the same steam consumption without exceeding the upper moisture limit. The increase in speed results in additional production without overhead cost increases. A generally applied rule of thumb is that for every 1% decrease in sheet moisture entering the dryer section, a 4% speed increase or steam consumption decrease is possible.
Reduced Fiber Costs: Gypsum linerboard grades have target basis weights, which have minimum and maximum allowable values around the target (like the moisture profiles mentioned previously). These basis weight targets are air-dry (fiber + water weight) rather than bone-dry (fiber weight only) values. If the average moisture target can be increased by 2% as the result of a flatter profile, the same air-dry weight sheet will contain 2% more water and 2% less fiber, resulting in a fiber cost savings.
Energy Savings: On drive-limited paper machines, an increase in the average moisture target resulting from a "flatter" bone-dry basis weight profile allows the papermaker to reduce the steam used to dry the sheet without exceeding the moisture specifications. The reduced steam load results in decreased energy costs.
Board Uniformity: When the linerboard produced on the machine is more uniform and the process is stable, papermakers can fine-tune the process to optimize the sheet characteristics and reduce costs. One example is the modification of furnish to the machine to decrease fiber costs. When the machine profile is uniform and stable, variations in strength properties decrease. Substitution of a lower strength fiber component in the sheet furnish, without going below minimum specifications, can have high returns. A more uniform sheet also winds with fewer defects, such as wrinkles and cockles, thereby increasing the yield of saleable board without increased costs.
Converting Board Plant Benefits: Gypsum linerboard is converted at wallboard plants by taking the bottom liner (grey back) and lifting the edges. Gypsum slurry is added into this trough and another liner (cream face) is laid down over the slurry. The edges are rolled over and the "sandwich" of paper and gypsum slurry is fed into an oven to evaporate the water from the slurry. The wallboard is removed from the oven and is cut to standard lengths.
In the drying process, the water from the gypsum slurry passes through the facing paper. Variations in basis weight, moisture, and caliper can all affect the speed at which the moisture passes and consequently the speed of the production line. A more uniform profile facing paper allows the maximum production speed and reduces the need to change the process as incoming paper changes. Both result in significant cost improvements.
Charles Pound is manager, capital forming sales, for Kadant AES, located in Queensbury, NY.
1. Fisher Pulp & Paper Worldwide Database, October 2006
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