Managing the quality and performance of the felts in the press section can result in tremendous savings for the papermaker in the form of improved process efficiencies, reduced energy consumption, higher product quality, and reduced rejects. However, tools previously available to measure press felts were limited to unsafe, hand-held instruments. As a result, direct felt monitoring is typically performed infrequently and decisions on felt quality, shut downs for wash-ups, and damage are made indirectly or inferred.
Recently, Voith Paper Automation introduced a new scanning measurement and control system capable of withstanding the harsh environment of the press section. Using direct measurement of felt water weight, permeability, and temperature, it is possible to identify problematic felts, providing data for reducing downtime between wash-ups and extending felt lifetimes.
Installation on the top pickup felt of a NipcoFlex press on a 7.3 m-trim-width copy papers machine has shown that this scanning measurement and control system can also predict paper moisture streaks after the press section and before the size press. This allows the capability for press section optimization, reduced steam consumption, and enhanced line efficiency.
Real Time Scanning Measurement
The FeltView system provides real time, continuous measurement for up to four press felts. Each scanner carries a traversing, single-sided measurement head containing three sensors: microwave moisture, permeability, and infrared temperature (Figure 1). As the measurement head scans across the surface of the felt, consistent contact is maintained using pneumatically loaded ceramic guides, ensuring accurate and repeatable results. Scanners are constructed of stainless steel and are environmentally protected for continued reliability in the press section.
The FeltView scanner is optimally installed on the first top pickup felt after the showers and vacuum boxes. In this position, it provides an accurate picture of the felt condition after cleaning.
The single-sided microwave moisture sensor is well suited for the high water weight of a press felt. Changes in water weight are detected as changes in the felt's dielectric constant and are insensitive to felt construction, thickness, and imbedded minerals. The measuring element is imbedded in a contacting, wear-resistant ceramic disk that does not damage or tear the felt. Sensor response is linear over the entire measurement range of 200-1,800 g/m^sup 2^.
The permeability sensor measures the penetration of a water jet of fixed pressure (20 bars) into the surface of the felt. Permeability is a function of the pressure difference between the water supply, a flow constrictor, and the surface of the felt. Ongoing compensation is made for viscosity changes by measuring the temperature of the water supply. The contacting sensor components are also imbedded in an abrasion-resistant ceramic plate.
A non-contacting infrared temperature sensor provides fast cross machine (CD) and machine direction (MD) surface temperature. This can be used to monitor steam boxes, avoiding felt damage due of to overheating during grade or line speed changes.
Online Measurement Results
A series of tests were conducted to confirm the profile shape accuracy of the FeltView measurement system on a 7.3-m-wide copy papers machine operating at 1,300 mpm. The FeltView scanner was installed on a two-week old felt on the top pickup position of a NipcoFlex press, a single-nip shoe press with double felts. The rotational frequency of this felt is approximately 1.6 seconds.
Scanning water weight profiles were compared with those obtained using hand-held instrumentation. Since the handheld instrumentation is transported across the felt at walking speed, expectations were that any streaks detected would differ in width but not in magnitude.
Figure 2 shows the results of the tests on the felt entering the nip. There is excellent correlation between the water weight profiles obtained from the scanner and hand-held instruments. Average water weights compare favorably as well. For the handheld sensor, the average water weight is 850 g/m2 compared to 900 g/m^sup 2^ measured by the microwave moisture sensor.
Troubleshooting Moisture Profile Problems
The felt water weight profile can predict moisture features such as streaks, which occur downstream in the process. Figure 3 shows the FeltView water weight profile stacked on sheet profile measurements taken after the press section (with the Voith EnvironScan scanner) and before the SpeedFlo Jet size coater. These profiles have been overlaid without regard for the y-axis scaling. The water weight in the felt is approximately 900 g/m^sup 2^, while the moisture following the press section is 48.3% and before the size coater it is 1.9%.
The strong correlation between the moisture streaks on the felt and those on the sheet gives the papermaker a diagnostic tool for identifying and troubleshooting problematic felts. This increased process visibility provides further opportunity for enhancing machine efficiencies by improving runnability and reducing drying requirements after the pressing operation.
Reducing Energy Consumption
Another area of potential savings for an extended nip press is through the optimized use of the vacuum system. This translates into energy savings for both the vacuum system and drive load. There is also the possibility of extending the useful life of the felts by enhancing performance over their planned life.
To understand how the uhle box vacuum affects the pressing operation, two FeltView scanners were installed on the top and bottom pickup felts of the NipcoFlex press mentioned previously. Figure 4 shows the press section layout. A third, singlebeam scanner (the EnviroScan) with sheet moisture and temperature sensors was installed immediately after the press section. The uhle box vacuum on the top felt was switched off, and the process response was monitored.
Figure 5a-b shows the results of this test. As expected, the average water weight of the top pickup felt increased by 175 g/m^sup 2^ with the vacuum switched off. However, the profile shows a general overall improvement, especially with the elimination of the moisture streaks on the front (right side) edge.
The FeltView permeability profiles in Figure 6a-b are also improved, although not to the same degree as the water weight profiles. Note that the permeability profile is the inverse of the water weight profile.
More interesting changes occurred in the sheet moisture profiles measured after the press section. Figure 7 shows a moisture color map generated by the EnvironScan at the time the uhle box was shut off. Here, specific moisture levels are represented by colors ranging from blue (lower percentage moisture) to red (high percentage moisture). Using this two-dimensional representation, MD and CD moisture variations on the web over time are easily visualized.
Figure 7 clearly illustrates that there is a reduction in the overall moisture level of the sheet after the press section with the vacuum shut off. In addition, the CD moisture profile is considerably flatter. Essentially, more felt dewatering is being performed in the press nip without the top uhle box vacuum, resulting in a dryer sheet entering the main dryer section.
Color maps taken from downstream scanners positioned before the SpeedFlo coater and at the reel showed a similar picture of flatter moisture profiles and lower average moisture.
Figure 8 looks at the actual numerical profile data measured by the EnvironScan and Pre-SpeedFlo scanners. Results are summarized in the table within Figure 8. As a consequence of switching off the uhle box vacuum on the top pickup felt, the average moisture entering the dryers and prior to the SpeedFlo was reduced by approximately 0.5%. The moisture profile spread was improved by 19% prior to the dryers, and 37% prior to the SpeedFlo.
Simplifying Felt Management with Data Archiving
The FeltView system offers an information system (Voith InfoPac System) for archiving profile measurements for up to one year. This data is useful for troubleshooting press and felt problems; managing felt operations such as cleaning and replacing; comparing performance of various felts; and extending felt lifetimes.
For example, poor roll conditions are often manifested as a repeating pattern through subsequent press felts, e.g. a streak that appears in the same CD position. High frequency analysis reveals problems originating from the rotating elements, including the felts themselves. Figure 9 shows the results of such an analysis, where changes in the felt moisture and permeability over time are traced to the rotational frequency of the felt.
Data archiving provides the capability for monitoring the condition of the felt as it progresses through its life. The color map display of felt moisture and/or permeability offers a way to visualize the result. Figure 10 is an example of a color map that shows changes in the felt permeability over a period of nearly one month. Note that, as the felt wears, its permeability is reduced until a wad-burn in the nip causes permanent streaks on April 4 and the damaged felt is removed.
The trend data shown at the bottom of the color map display can be used to indicate when felt wash-ups are required. Interestingly, on this particular felt, the benefits of the wash-up (seen as small blips in the permeability) are short lived and last only 0.5 days. Armed with this data, the papermaker can make more sound decisions on extending the time between wash-ups, thereby reducing the downtime on the machine.
Color maps, as shown in Figure 10, offer the capability for side-byside comparison of different felts. They can also help in speeding up the running-in period after the installation of a new felt.
Predictive Models, System Configuration
The ability to continuously and reliably measure and time stamp felt properties enables the development of predictive models. Advanced neural network software, such as Voith Paper Automation's WebProflt product, is a tool for testing the correlation between felt condition and process variables. Once the correlation is established, "soft" (software) sensors may be constructed to predict, in real time, key process variables such as MD and CD variability and strength, events such as breaks or holes, and ultimately end-user paper quality. This offers the paper-maker a tangible economic benefit over "end-of-reel" sampling.
The FeltView system is part of a suite of press section products called "embedded solutions" from Voith Paper Automation. The products are application-specific measurement and control systems designed to operate either in a standalone mode or integrated into a single automation platform.
Embedded solutions are interfaced with the mill's distributed control system, quality control system, profiling system, or mill information systems using an Ethernet or TCP/IP interface. Network data conforms to the industry standard OLE for Process Control (OPC) communication protocol to ensure data transparency with respect to other third-party compliant systems.
Economic Benefits Summary
The installation of FeltView on the NipcoFlex press described in this article has shown that the system a variety of opportunities for improving press section operation. Improvements fall into the following categories:
* Quality. Flatter moisture profiles entering the dryer section translate into a reduction in moisture-related broke.
* Energy Utilization. Optimizing the use of the uhle boxes results in energy savings for the vacuum and drive systems. Also, there are steam savings associated with the production of a dryer, more uniform sheet after the presses.
* Felt Management and Application. Continuous, quantitative measurement of key felt properties enables more consistent performance while the felt is in operation. Archived data may be used to compare different felts to aid in selection of the best felts for each press location. The system's scanning temperature sensor prevents damage to the felt by alerting the operator to overheating by the steam boxes.
* Line Productivity. A stronger, flatter sheet entering the dryers reduces paper breaks. As more felt dewatering takes place in the nip, there should be a longer time between washups. There are less paper breaks in the dryers and in size and on-machine coalers with the reduction of moisture streaks.
* Operations. The system improves process visibility and provides the operators with a tool for running the press section.
Table 1 quantifies economic benefits resulting from the above improvements. Papermakers can use these forecasted savings to calculate the actual return on investment for the application of FeltView in their particular operations.
© 2005 Paperloop, Inc. Provided by ProQuest LLC. All Rights Reserved.
Source: Pulp & Paper