Introduction
As early as 2016, studies were introduced that showed numerous process control benefits that can be realized on paper machines equipped with the EasyScan traversing permeability, water content, and temperature measurement capabilities.
These scanners provide detailed data analysis including 3D mapping of the press felts in several modes, and Fast Fourier Transformation (FFT) analysis of the incoming data to show pulsation and vibration issues before they become problematic on the felts, and provide a powerful tool to the maintenance department, negating the need for "run to failure" mode.
These scanning units can now be supplemented with lower cost fixed-point instruments, the FiberScan, PressScan and the SmartScan, which accurately measure water content of the fabric, the fabric and sheet; or, the sheet itself. This ability to measure consistency in press section applications, heretofore not possible with accuracy useful enough for control and monitoring. All these instruments contain planar microwave sensors which can measure the moisture of the sheet, the forming fabric, the felt, and the felt and sheet at any point on the machine where the small measuring head can be mounted.
One case study is presented in which a gap former machine is converted from news and LWC, to corrugated medium using data from these units which can monitor and control desired sheet consistencies and targeted sheet properties.
Another case study demonstrates grade change strategies using instrument readings to go from one grade and weight to another, thereby minimizing time and waste while controlling the process scientifically.
Final case study information shows data gathered on vacuum systems and plotted against felt moistures to minimize the amount of vacuum required for felt dewatering. This strategy resulted in reduced cost to run liquid ring pumps, while simultaneously reducing the drive load required to pull the felt around the run. These results are summarized in kW/hr. savings.
For more than 20 years instruments have been available to measure the moisture content of press felts. This technology utilized a microwave resonance chamber to produce an analog signal that could correlate to the gsm of water in a press fabric. These resonance chamber units were sensitive to thermal expansion and contraction and had relatively low sampling rates as two of several areas that could produce less than desirable results.
In recent years, however, a planar sensor has been developed to provide a digital signal at extremely high (1024 Hz) sampling rate. This allows for data manipulation to provide sophisticated analysis such as FFT (Fast Fourier Transform) and 3D fabric mapping. These tools will show sources of pulsation/vibration and fabric or process variations, that could not be seen in 2D line graphs.
The accuracy of these planar sensors now can provide sheet and fabric water content in the forming and press sections, and consistency of the sheet itself in open draws.
THE INSTRUMENTS
A. Principles of Operation
Older technology relied upon nuclear radiation from so-called "back-scatter" gauges. There were numerous issues with nuclear technology, not the least of which were inherent radiation risk, governmental regulation, having a dedicated radiation officer, and disposal of the radioactive source material after its useful life.
A new generation of instruments has been developed using microwave technology. These units use less radiation than a cell phone produces and are safe to the touch.
Fig. 1 New planar sensor operates without a resonance chamber
Figure 1 demonstrates that a fabric or a fabric/sheet passing over the head will cause a change in the flux of the microwave field set up between the incoming source (antenna) and the receiver. That microwaves cause resonance in water molecules above the sensor, means that all the sensor perceives is the water itself, not the fiber, the fabric, or the surrounding machinery.
B. The Instruments
In the forming section the FiberScanFIX can be mounted anywhere that the roughly 10 X 12 cm head can fit. Since the instrument does not see the machine framing in its measurements, tight fitting spaces can be accommodated between table elements.
Fig. 2 is the portable version of the forming unit.
As is shown in figures 2 and 3, the instrument is available in both portable and fixed versions. The fixed version comes with a control box that is usually mounted in the control room. It is equipped with 16 inputs for sensor heads at various locations. It also has 16 analog outputs with a 4-20 ma signal used for controlling whatever strategy is employed, and one digital output for interface with the machine's DCS.
The control box shown below is the same unit used for all machine-mounted instruments for the forming, press, and other applications.
Machine Mounted Forming Unit
Fig. 3 is the machine-mounted version. The control box is shown upper left with two views of a "before couch" installation.
In the press section, there are two instruments used. The first, the PresScan, is used to measure felt moisture, and felt/sheet moisture combined. Standard applications involve determining the efficiency of uhle boxes and doing water balances to determine the relative contribution of each dewatering element. The most common application is in reducing vacuum consumption for uhle boxes and suction rolls.
These instruments are also available with traversing scanning capability. These units measure felt moisture, permeability and temperature. These EasyScan units are equipped with sophisticated 3D felt mapping capability and can be interfaced with the machine's DCS.
The Fixed Head for Measuring Felt and Felt/Sheet Moisture
Fig. 4. The microwave field generated by this sensor measures thicker moisture-containing media.
Felt Scanning for Moisture Content, Permeability, and Temperature
fig. 5 shows the traversing and fully programmable felt scanner
Sheet Consistency Measuring Sensor
Fig. 6. A new generation sensor that can measure sheet consistency in open draws with high accuracy
In the past, only heavy weight sheets could be measured in open draws because the sensor head had to be in contact with the sheet, and only special cases could tolerate this without the risk of sheet breaks. Our "new generation" technology now allows the sheet to be as far as 10 millimeters away from the sensor head, and still get high accuracy and precision.
All the instruments described will provide an analog output for controlling, and a digital output for interface with the papermachine's DCS.
CASE STUDY 1
In this case, assistance was requested to convert machine operations from newsprint/LWC production, to 100% OCC corrugating medium. The forming section was designed for completely different sheet formation and sheet properties, than would be required with the new paper grades. From this work a desired set of running conditions could be presented to mill management, that had the projected outcomes for sheet properties and/or energy savings, so that the people running the machine could make informed decisions about the set-up that would produce the best overall results. As a follow-up, recommendations were also made that would allow the instruments to control, rather than simply monitor, the water being removed, to eliminate day to day variables such as stock differences, refining, temperature, or any other incoming issues, can be addressed.
Forming Section with Zones Identified
Fig. 7 is a side elevation drawing of the forming section. To simplify analysis the section has been divided into 3 zones.
The drainage curved measured was typical of what is seen on gap formers, with much dewatering happening early in the process, and relatively less done in the "high vacuum" areas (Fig. 11).
Drainage Curve in "Standard" Mode
Fig. 8. Drainage curve before trials
Our experiment design consisted of changing practices to reflect lower vacuums in the early drainage elements and higher/lower vacuum in the last, or high vacuum, areas as seen in Fig. 9.
Trial Plan
Fig. 9. The plan shows lower vacuums in zone 1, lower vacuums in zone 2, and lower/higher vacuums in zone 3
It has been seen in numerous other installations that the strategy of lower vacuums early in the process can significantly reduce the drive load in the forming section without negatively impacting sheet properties or steam consumption in the dryer section, so this is the strategy that was being evaluated.
The actual tests were carried out over roughly one-hour spans, allowing jumbo rolls to be produced for testing. Simultaneously, drive loads in the forming section were being observed and recorded. Test results for each condition was summarized and put into spreadsheet form. These results will be discussed later in the paper, but all conditions produced "in spec" paper.
SHEET PROPERTY MEASUREMENTS
Rolls from each trial were tested per the mill's criteria. The results of these tests are shown in Fig. 10, as measured by the percentage change from the initial set-up.
Percentage Change in Test Values
Fig. 10. ? in test values from standard, as a percentage
Taking the analysis of the test data one step further, we summed the ? percentages for each test condition and came up with the chart shown as Fig. 13.
Fig. 11. The "Quality Index" is compiled by summing the percentage change in properties from the standard settings.
As can be seen in Fig. 11, the best sheet quality readings come from running zones a. and b. at lower vacuums, allowing the later dewatering elements to compensate.
DRIVE LOAD REDUCTION AND RUNNABLILITY
Zone a. has the potential of providing the greatest potential for drive load reduction, since 42% of the total drive is consumed in that area. The forming roll, being a rotating element, will have significantly less impact on the total load than does the shoe, which, of course, is stationary and results in high drag load and consequent forming fabric wear.
It was noted that the machine could also run well with vacuum lowered in zones a., b., and c. In fact, the press section could accommodate higher water loads when all 3 zones were lowered and the Hi-vac was completely turned off. When turned off, however, the sheet edges became unstable and wavered on the table, so a low (10 kPa) vacuum needed to be applied at the Hi-vac for runnability purposes.
Obviously, the highest drive load savings occurred when vacuum was lowered in all 3 zones. Total drive savings when vacuums were lowered in all 3 zones is 363 kWh. As can be noted in the results tables, the total consistency delivered to the press section decreased by 2.5%. Steam pressure decreased during the run with all 3 zones lowered, from 41.2 t/h, to 40 t/h. It is unclear whether changes in stock occurred during that time, or perhaps the increased porosity measured during that trial, allowed the sheet to dry more easily. Vacuum load requirements also dropped by 67 kWh, resulting in total savings of 430 kWh for the "best" setting for drives.
CASE STUDY 2
In this case study, the mill had two objectives, a) obtain energy savings from reduction of vacuum pump and drive load requirements in the forming section, and, b) establish a control loop before the top wire unit to control incoming consistency.
Machine Side Elevation of the Forming Section
Fig. 12 shows all the dewatering units
In this study the variables to be observed are a) the microwave water content readings (g/sqm and l/min.), b) vacuum levels (mbar), c) drive load, and d) paper properties. The study was conducted in 2 phases, a) varying vacuum on the suction boxes pre-Top Wire, and b) operation in closed control loop to the 3rd suction box, immediately before the top wire unit. It was anticipated that several grade changes would occur over the life of the study. In the benchmark mode, it was observed that the Couch roll had 22% loadshare, the wire turning roll had 48% loadshare, and the Top Wire drive roll had a 30% loadshare for a total of 426 kW.
Drainage Curve in Initial Mode
Fig. 13 plots water content in gsm and recorded vacuum levels.
These measurements indicate an incoming consistency to the Top Wire at 3.7%, the consistency of the bottom ply after the vacufoils being 7.1%, and consistency is estimated to be 10+% at the bonding point, a level that is considered to be quite high for good ply bond. The water removed by the Top Wire was 1330 l/min/m, vs. good industry practices of 1600l/min/m. Therefore, it was hypothesized that the Top Wire unit could do more work and help the z-direction uniformity of the sheet, which has shown to be a positive impact on sheet properties. In the initial settings, pre-couch consistency was 16.3%
In a series of steps, vacuum to the first 3 suction boxes was reduced from 140 to 80mbar in box 1, from 140 to 100mbar in box 2, and from140 to 100mbar in box 3. Consequently, the higher incoming consistency (to the Top Wire) forced the Top Wire to remove more water and resulted in a 17.6% pre-couch consistency, an 8% improvement in overall solids. Measured amperage was 383 kW, a 10% reduction is drive load. The load share percentages did not change.
It is theorized that the bottom ply was at least partially sealed in the initial measurement, and sending a wetter overall sheet into the secondary headbox, allowed greater overall water removal.
The 2nd phase of this trial involved putting suction box 3 in "control mode" by using the 4-20ma output from the microwave device. Water content measurements of the incoming sheet (to the Top Wire) were made first, allowing the operators to use their "normal" operating methods, and then putting the 3rd suction box into "control" mode, allowing an approach valve to open and close proportionate to the signal. Incoming variables such a temperature, refining, raw material, and pH are accommodated. Figure 9 shows a time domain during which the trial was run. Reduction in variability Is visually obvious on the run chart.
Sheet Water Content Over Time
Fig. 14 shows water content (blue) without control mode (left and center right) vs. in control mode (lower right). Variability is greatly reduced.
CONCLUSIONS
- New microwave instrumentation allows measurement of fabric and/or sheet water content in the press and forming sections, as well as sheet consistency in open draws.
- Process parameters can be benchmarked and controlled using sheet water content as a measurement target.
- Sheet properties can be benchmarked and controlled using water content measurements at critical points on the forming table.
- Cost savings in vacuum reduction and drive load reduction can be achieved by the scientific application of vacuum in the forming and press sections.
Frank Cunnane,
Product Manager,
Cristini NA
