Advanced ex-press sheet management for improved quality and paper machine performance, Solutions!, Online Exclusives, August 2004

The paper machine can be considered as a series of manufacturing units, each performing a discrete function as the sheet passes through it. The principal units are the head box, forming section, press section, dryers, sizing/coating and calendering. Despite the contribution that each of these units makes to production and quality, very little is either measured or controlled between the forming section and ex-press, especially when one considers that the majority of the sheet’s characteristics are born in this part of the process. For example, nearly half the water within the sheet is mechanically removed between these two points, with virtually no process visibility available to optimise this part of manufacture. In addition there are few universal and widely accepted measurements to help describe the condition of the forming fabrics, press felts and transport belts involved in the de-watering process. Yet most modern paper machines will change the press felts every fifteen to thirty days as their age and diminishing efficiency reduces productivity and quality. There is a clear requirement and justification to do two things: first, to examine the sheet at the ex-press position; second, to use the information to improve quality of the sheet and runnability of the machine through its pressing operations.

Wet end water removal: Measurement and control overview
The wet end of the paper machine has many mechanical devices that can be used to manipulate water removal or influence moisture distribution. Table 1 summarizes a cross section of the variables at the operator’s disposal that allow him to manage both water removal and moisture prior to the drying section.

Table 1. Wet End Water Removal: Process, Variables & Controls

Table 1 suggests that only twenty percent of the variables on this list are subject to closed loop control. This implies there is a significant opportunity for advanced measurements using supervisory or optimization controls. Currently, however, most are straightforward setpoint controls, subject to operator judgement based on process observations or quality measurements from either on-line or off-line instruments.

Many of the mechanical or manually-controlled variables are adjusted by skilled operators following routine foil, fabric and felt changes, again based on judgement or historical best practice. These adjustments are continually required in response to machine clothing and roll condition wear, again inviting an opportunity for advanced measurements and control. An example of this might be following a press felt change where the Uhle boxes are run at a reduced vacuum to encourage dewatering in the press nip to keep the new felt clean. Then later, as the felt begins to fill, the Uhle box vacuum is increased to maintain the overall press section water balance.

In order to enhance the likelihood of good performance for the felts and fabrics, automation systems provide chemical treatment, and routine skewing in order to eliminate or reduce press section vibration.

Clearly, from the stock approach through to the end of the press section, there are many potential variables that can be optimized, but to date there has been a shortage of measurements for control and optimization.

Ex-Press moisture measurement
Quality measurements (QCS) are traditionally positioned either prior to the size press or at the reel and their measurements are used to control various machine direction disturbances and cross direction profiles. This measurement and control strategy does not guarantee that an optimal sheet is presented to the dryers following the press section. Also, it cannot be used to either optimize press section operations or for press-felt management purposes. The limitation of this approach is that it can only measure and control a composite moisture profile that is the product of the ex-press, ex-main dryers and in some cases ex-size press conditions.

Moisture measurement following the press section, however, represents a sound strategic starting place for both measurement and control purposes (Figure 1) for the following reasons.

Figure 1. Ex-press moisture measurement location

1)Location: It is the first available location on the paper machine that provides measurement that focuses on the dewatering processes rather than downstream conventional drying.

2)Diagnostics: The measurement focuses on events upstream of the press section and excludes any downstream dynamics. This expanded level of process visibility allows operators to adjust the following items with confidence and eliminate guesswork:

• Press roll loads
• Control crowns
• Uhle box vacuums
• Press felt showers
• DuoCleaner activity
• Felt guide roll skewing

The measurement can also be combined with high-frequency vibration monitoring techniques in order to identify problematic felts and rolls in the press section.

3)Control: The location permits faster control response, as the measurement does not have the process time delay of the dryers built into it. As a consequence, profile optimizers using steam boxes and sectionalized zone control rolls can provide superior control performance.

4)Process optimization: Ex-press moisture averages can be minimized while the moisture profile shape entering the dryers can be optimized, for reduced steam consumption, shrinkage and dryer breaks.

5)Press clothing: By integrating advanced HMI (Human Machine Interface) displays and commercial databases, press clothing can be monitored and analyzed in order to achieve optimal dewatering over its lifetime. The measurement can also be used to reduce felt run-in times, such that quality and productivity are not compromised during this period.

This facility also provides the user with a historical record of felt performance that can be used to support selection of appropriate felts for each position.

6)Quality and productivity: Flatter ex-press moisture profiles entering the dryer section reduce the total volume and zone-to-zone water application of downstream rewet actuators. As a consequence, steam consumption is also reduced in the main dryer section.

This type of measurement, control and diagnostics should not be viewed as a conventional Quality Control System (QCS). Rather, it represents an expanded definition as it provides application-specific embedded functionalities that are designed to optimize individual elements of paper manufacture, such as a press section. The measurement has a wider application than to simply measure the moisture in the sheet itself. The measurement and control system offers expanded capabilities for analysis of press section clothing, roll condition, identification of high-frequency disturbances and historical database archiving for quality, clothing and roll condition.

The system also includes an HMI with diagnostic displays that gives the operator advanced decision support to quickly identify process problems.

The ex-press measurement system
In response to the requirement for advanced measurement, control and diagnostic systems (or Embedded Solutions), Voith Automation has developed a unique moisture measurement system called EnviroScan (Figure 2). Because this measurement is required to perform in hostile, inaccessible environments, it is essential that its maintenance requirements are minimal. Measurement failure, for any reason, cannot be fixed until the paper machine is shut; therefore, detailed attention has been paid to designing a system that is accurate, repeatable and with a high mean time between failure (MTBF).

Figure 2. Ex-Press Moisture Measurement System

This measurement is designed to operate in demanding environments, particularly following the press section. The outer walls are insulated to avoid condensation problems while also serving to keep the scanner interior cool. The external skin is constructed from 316 stainless steel around an internal beam structure of 304 stainless steel. Due to its compact external dimensions of 483 x 280mm, it can be easily mounted in the most demanding locations. The beam is mounted at 50-100mm from the surface of the sheet and can be orientated any angle to allow integration into areas with limited space. The measurement system is designed for operation in 120 degrees Celsius 98% RH non-condensing environments. The scan speed is rated at 400mm/second, which provides fast scanning data acquisition for tight profile control at the wet end of the paper machine.

The measurement sensor is embedded within the scanner frame envelope for compactness and reliability and is transported across the sheet on linear bearings that help ensure head carriage rigidity and measurement accuracy. The sensor is cooled via a vortex cooling system and does not require water for its operation.

The sensor is based on infrared technology (Figure 3), which simultaneously processes four infrared wavelengths. This provides the ability to measure and diagnose process disturbances for moisture and temperature up to 200Hz using fast Fourier analysis.

Figure 3. EnviroScan™ Moisture Sensor

By using proprietary optics and modelling the infrared light path in the sheet, it renders the overall measurement insensitive to furnish composition, formation, and scattering coefficients. This also provides improved measurement accuracy and easier, more rationalized calibration. The sensor has an integral air curtain around the external measurement window, which is designed to eliminate the accumulation of contaminants and condensation from the optical path. Also included is an external optical standard, which ensures long-term measurement repeatability.

The moisture measurement range is from bone dry to 75% percent moisture, providing application opportunities for the wet end, size press, coaters and tissue machines.

Measurement application and results
To date, eight systems have been sold and installed on machines manufacturing the following grades: newsprint, supercalendered magazine, lightweight coated base, fine paper, and sack kraft. Seven of the systems are currently operating on either new paper machines or major machine rebuilds. The measurement is installed either in the press itself or directly following the press section. One system is installed prior to a Clupak unit on a sack kraft machine.

Papermakers typically use the system as a diagnostic tool. By measuring the quality of the sheet as it leaves the wet press, they are able to make valid decisions and take actions prior to the press section and quantitatively assess the results. Figure 4 illustrates a typical modern press section, including some examples of the potential problems that can be witnessed during normal operation. These problems could relate to machine clothing, roll condition, paper quality or overall press set-up. The results that follow show how some of these problems appeared and how they were solved.

Figure 4. Ex-Press Moisture Measurement as a Diagnostic Tool

Figure 5 shows two moisture profiles taken following the ex-press location. The top one indicates there is room for improvement on two counts. First, the moisture 2-sigma spread is +/- 0.94%; second, the average moisture at this position is 48.8%. By adjusting the Uhle box vacuum and the high-pressure showers, the profile spread was reduced to +/- 0.73%, for an improvement of 20%, while the average moisture was reduced by 1.6% to 47.2%. These actions resulted in a drier, stronger sheet with less shrinkage entering the main section dryers, which also reduced the steam consumption.

Figure 5. High Pressure Showers & Uhle Box Vacuum

Considering there is between a three- or five-to-one relationship between the moisture profile spread at the wet end compared to the reel, these adjustments would have considerably reduced the volume of water applied by the downstream rewet system and provided further incremental steam savings, improved runnability and better quality.

Figure 6 demonstrates the manual control capability of press section zone control rolls at the edge of the sheet. The system provides a new level of process visibility and permits operators to make considered adjustments to their zone control rolls in order to reduce the longer wavelength profile disturbances. Typically though, moisture problems at the edge of the sheet receive prompt attention as they cause tight edges, edge cracks and breaks. In this example, the edge control zones, shown highlighted in yellow, have been adjusted to help flatten the moisture profile. Experience with this adjustment indicates that by adjusting the local nip output load by 30%, it has a 1.5% effect on the local moisture content. It is appreciated that the ‘gain’ of this device is non-linear.

Figure 6. Second Press Edge Load Adjustment

The next step would be to operate the zone-controlled rolls using the ex-press measurement for moisture profile control. The control strategy could also include a steambox where the zone control roll could be used to attenuate the longer wavelength disturbances, leaving the steambox to concentrate on the narrow streaks. This approach would provide fast profile control without the process time delay using a downstream measurement. It also offers the benefit of being able to present the dryers with an “ideal profile” for best runnability and final quality.

The final example, shown in Figures 7 and 8, illustrates a moisture profile following a press section that has a transfer belt instead of a conventional press felt operating on the lower second nip.

Figures 7 and 8. Worn Transfer Belt

This example clearly shows a moisture streak highlighted in yellow, towards the middle backside of the machine (Fig. 7). This streak was traced to a worn transfer belt, which can also be seen on the color quality map (Fig. 8) as a permanent red moisture streak running across the display.

Conclusions
Production personnel have quickly accepted the measurement as a diagnostic tool to help improve the runnability of their machines. It has also contributed toward improved quality and reduced energy costs (Table 2).

Table 2. Summary of Potential Benefits

Measurement at the ex-press position, by definition, provides exclusive upstream information, while conventional scanning systems will include the effect of the dryer section into their profiles. The key benefit of the system to the operating personnel is the ability to distinguish the source of a potential quality or productivity problem. It has provided them with a new level of process visibility such that they can effectively manage more press section variables with a greater level of confidence. This has resulted in reduced ex-press moisture averages with flatter moisture profiles entering the dryer section.

Examples shown in this paper indicate that ex-press moisture can be reduced in the region of 1.5-2.0% using the process visibility afforded by the system. This equals a 5-10% saving in either steam consumption or speed increase, depending on process operational constraints. Also, as the sheet will be entering the dryer section at a higher dryness, it will be stronger and suffer less shrinkage, translating into 10-20 percent less dryer breaks.

From a control standpoint, the system offers the benefit of providing fast-feedback response without the measurement time delay endured through the drying section. As mentioned earlier, the dryer section can be presented with idealized moisture profiles for better quality and machine efficiency. For example, the best moisture profile leaving the press section would have deliberately wet edges in order to compensate for more rapid edge drying in the dryer section. Moisture profile optimization following the press section has the added benefit of reducing downstream rewet application with automatic re-moisturizers, coupled with a reduction in main dryer section steam consumption.

The system has contributed toward improved felt run-in times and total felt performance over the useful life of the felt. The measurement includes valuable information regarding the overall condition of the press and its clothing. Operators use the moisture information to adjust and optimize their Uhle box vacuums, felt showers, press crowns and zone control rolls in order to achieve flatter profiles with new felts.

About the author:
Raymond P. Shead is business director, Voith Paper Automation, Heidenheim, Germany. To learn more, email info.voithpaper@voith.com or visit www.voith.com.