A technology common in Europe, online porosity measurement offers smoother sheets, fewer web breaks, and faster recovery after grade changes

Online Porosity Measurement System Offers Cost-Effective Quality Control

By Kari Nettamo

For NEARLY 20 years, online porosity measurement systems have been operating successfully in European paper mills. Beginning with specialty paper machines in the late 1970s, this technology has spread to high-quality coated and kraft paper machines throughout Europe. Though there have been some systems in operation in North America for a few years, the concept of measuring porosity online is still relatively new here.

An online porosity measurement system mechanically replicates the laboratory testing procedure. However, the actual sample size tested allows much more accurate results. By measuring several hundred feet of the paper web during one testing cycle, the online system averages the equivalent of thousands of sampling points over the single sample area to much more accurately reflect the true porosity level for the finished reel.

As trials have shown, the small investment for such a system can be extremely cost-effective. An online porosity measurement system can improve refining control, wet-end chemistry evaluation, and vacuum control to offer substantial production and quality control benefits. In addition, the system is especially beneficial from a quality standpoint in mills where a variety of grades are produced in relatively short production runs.

SYSTEM DESCRIPTION. In the late 1970s, the first online system for real-time measurement of paper porosity was installed on a paper machine in Scandinavia. Initially considered a novelty, the inventors of the system had only enough resources to market and sell the system locally. At that time, virtually all other online systems were marketed and sold by major scanner suppliers, who did not have an online porosity system to offer with their systems.

However, several major paper companies in Europe followed the results of the new online system. Ultimately, beginning with specialty paper mills, this technology was adopted by European paper companies. Now, these companies are installing online porosity systems on high-quality coated paper machines to measure base paper porosity before the coating section. They are also installing them on kraft paper machines as a tool to help meet requirements for converting customers.

The online porosity measurement system replicates the laboratory testing procedure for porosity. As Figure 1 shows, the system consists of a measuring head, a pneumatic cylinder for lifting the head to the sheet surface, air filters, a control cabinet, and the mini-terminal or operator interface. Using these components, the system pulls a fixed volume of air through the moving paper web at a constant vacuum. The time for this volume of air to pass through the sheet is then used in calculating the porosity/air permeance values.

Online porosity measurement presents a major challenge, because the TAPPI test method requires a constant vacuum in order to measure air resistance rather than a pressure drop variation over the paper sample. To provide a constant vacuum in an online system, detailed attention must be paid to the measuring head and the vacuum control design for the system.

The measuring head must be designed to resist the tendency of air leakage from the moving paper web into the measuring area. To address this tendency, a unique dual vacuum head was created. With this design, the measuring vacuum is centered in the measuring land of the measurement head while a second vacuum is pulled through a sealing groove on the outer circumference of the measuring head. This second “sealing vacuum” minimizes air leakage from the edge of head where it makes contact with the moving paper web. A calibration parameter is also used to compensate for any air traveling with the sheet into the measuring land.

Additionally, the vacuum control for the system must be fast and accurate. If the vacuum for the system is not properly controlled, the system begins to measure both air resistance and pressure difference variations, resulting in an incorrect porosity measurement.

ONLINE VS LABORATORY TESTING. Although the online porosity measurement system mechanically replicates the laboratory testing procedure, the actual sample size tested with the online system reflects much more accurate average porosity values for the resulting paper production. With certain grades, such as newsprint, pinholes cause false laboratory results during porosity measurement. With other grades, such as saturating kraft, porosity variation is very large across one sample or one set of samples.

In fact, based on actual mill results, porosity across a single sample may vary by more than 20% in laboratory tests, as Table 1 shows. These results were found on a sample of saturating kraft, which typically has a wide variation in porosity. In order to provide a reliable average porosity in the laboratory, several tests should be taken from the same sample. In most cases, simple time constraints do not allow this luxury.

The online system, on the other hand, measures several hundred feet of the paper web during one testing cycle. Through this procedure, the equivalent of thousands of sampling points are averaged over the single sample area. This averaging result is usually much more representative in reflecting the true porosity level for the finished reel. And, individual defects such as pinholes have no effect on the porosity reading due to the speed at which they pass over the measuring head.

MACHINE DIRECTION VS CROSS DIRECTION. When the online porosity measurement system was initially introduced, it was not designed as a traversing system. The designers looked at the traversing systems in operation at that time, and at the source of variations in the parameters that those traversing systems measured. The papermakers were looking for the paper machine components that were sources of basis weight and caliper variations. Since paper machine components are causing variations both in the cross-machine direction and the machine direction, identification of variations associated with these components also required cross-machine direction testing.

Most variations in porosity, however, are caused by sources located before the headbox in the papermaking process. Therefore, the variation is usually distributed relatively uniformly on the web in the cross-machine direction, so a traversing system is not required. As a benefit, the use of a fixed position system allows a lower investment cost and easier installation procedures.

However, many customers are now concerned about both the average porosity on the reel and the specific porosity at the edge of the sheet. With the increase in the sophistication of wet-end controls, edge porosity can now be controlled and is a production parameter that mills would like to trend. This production requirement is now driving a re-evaluation of a traversing online porosity measurement system.

If a traversing system is used for porosity measurement, the system cannot be a continuously moving system. The measuring principle—measuring the length of time for the known volume of air to pass through the sheet—requires that the head be stationary during the measuring cycle. The system could only traverse in the sense that it would move from spot to spot between testing times in the cross-machine direction.

MEASURING HEAD CONTACT WITH THE SHEET. During the design of the online porosity system, the measuring head was considered the most important component in terms of the operation and reliability of the system since it made contact with the sheet. As previously described, the measuring head features both a sealing groove and a measuring land.

Measuring heads come in four different models, which differ from one another in the measuring area. The choice of measuring head type depends on the paper grades to be measured, according to Table 2. Although a single head will usually cover the porosity range for a paper machine, two heads can be used for extreme situations, such as specialty mills that must capture the porosity ranges of many different paper grades.

Because papermakers are generally wary of anything that comes into contact with the sheet on the paper machine, the measuring head was required to be extremely smooth so that it would not mark the sheet. It also had to be extremely resistant to the abrasive characteristics of the sheet.

With this “friction-less” coating and unique mechanical design, the measuring head had no impact on the sheet during production. It did not mark the sheet or cause web breaks, as many other new online measuring systems did. In fact, since its introduction, there has not been a paper machine with a production speed too high for use with this head.

USES OF ONLINE POROSITY MEASUREMENT. In addition to having no limitations associated with paper machine speed, the online porosity measurement system operates with papers in a basis weight range of 6 to 200 lb/3,300 ft². Also, it has specific quality and production benefits associated with improved refining and vacuum control, better wet-end chemistry evaluation, and faster grade changes.

Refining control. The most common use of online porosity measurement is in refining control. Paper machine personnel follow the porosity trend and give instructions to the wet-end or refining personnel to help keep the porosity within control parameters. This replaces the use of the data trend from the vacuum of the couch roll for the same control circumstances.

Online porosity measurement is the preferred trend to follow for refining control feedback, because changes in refining control directly affect the porosity. Also, the vacuum of the couch roll is affected by production processes other than refining. Mills have repeatedly reported fewer web breaks after implementing refining control based on online porosity, resulting in higher machine efficiencies.

Wet-end chemistry evaluation. Over a period of eight months, one mill performed extensive research and trials evaluating the relationship between online porosity measurements and its wet-end chemistry. The mill now controls selected aspects of its wet-end chemistry based on online porosity.

For example, mill personnel now monitor the reduction in porosity as they add the retention agent, allowing them to see when the saturation point is reached as the retention agent is added to the process. On this supercalendered (SC) grade, the mill has been able to produce a smoother sheet utilizing this new control parameter—a control technique that is especially beneficial for rotogravure grades of SC paper.

Figure 2 illustrates the results from a test run where an increase in the retention agent by 50% causes an immediate and distinctive change in the online porosity reading.

Vacuum control. The same mill which evaluated the relationship of its wet-end chemistry with porosity is also looking at the correlation between vacuum control of its suction roll with online porosity.

In trials, this mill discovered the threshold at which the vacuum on the suction roll generates such a high porosity level that pinholes began to form, causing quality problems. The mill also discovered the threshold at which the paper web became so weak as to continually break. Now, this mill uses online porosity feedback to maximize the vacuum without exceeding the porosity level causing such problems.

As this mill continues to increase the speed of its SC machine while maintaining or increasing paper quality, the online feedback becomes more and more important. Even the fastest laboratory results usually represent too significant of a time lag for the paper machine operators to control the machine based on that data. And, while the laboratory test results are necessary for online calibration and absolute test values, the relative numbers produced with online instrumentation are sufficient for online control of paper property trends in the manufacturing process.

Faster grade changes. Figure 3 illustrates how the porosity is affected during grade changes. Basis weight was changed by 10 g/m² and ash content by 15%. Although porosity is affected more by the change in ash content than by the change in basis weight, both changes produce a visible reaction in the porosity output.

The laboratory trend generated over the same time period provides no indication of the change in porosity that actually took place. This oversight provides an example of the inherent weakness in offline testing. The sampling procedure used in taking samples from the reel sometimes results in samples that do not show the representative porosity throughout most of the reel.

Most specialty paper mills run a wide variety of grades with relatively short production runs. It is especially important for these mills to achieve top quality paper as soon as possible. After a grade change, specialty paper mills using online porosity measurement have been able to bring the porosity of the sheet within operating parameters more quickly than before. In addition, online porosity measurement ensures that porosity will not move out of the operating parameters during the short production runs.

Kari Nettamo is president of Pulp & Paper Technology Inc., Duluth, Ga.