Capabilities now exist that enable paper, board, and tissue makers to scan the moisture level, permeability, and temperature of their felts without having to go onto the machine.
Obviously, safety is enhanced when individuals are not reaching into the machine with instruments to perform scans. These units can be programmed to scan on whatever frequency the mill wants, or even continuously.
Although these scanning units are a tremendous leap forward in technology, even newer instruments have been developed which do not scan, but are in a fixed position on the machine. These devices can provide much of the same information, but without the cost of beams mounted into the machine structure, and without the costly traversing mechanisms. These fixed units can be mounted in either the forming or press sections to provide data on the water content of the paper machine clothing, and/or the sheet. In this submission, only the press section instruments are discussed. These units operate on microwave technology, rather than gamma ray or beta particle principles, so only the water content is measured, not the mass of the paper machine clothing, the sheet, or the interference from the surrounding machine framing.
In this paper case studies are examined that show, not just improved process control, but true cost saving potential.
The first instrument described herein is the scanning unit for the press fabrics (Fig. 2). Already in Europe, and within certain US corporations, it is considered too dangerous to "break the plane" of the machine to take permeability and moisture content measurements with hand held instruments. This data, however, is still critical to the operation of the machine and to optimize energy usage and felt cleaning/change cycles. This unit can measure fabric permeability, water content, and temperature simultaneously and in the same place on the felt. Powerful onboard software is capable of producing 3D maps of the felt in perm or moisture modes, showing many felt aspects never before seen in 2D graphing. The units are also equipped with FFT capability, which will clearly show and analyze vibration issues for problem solving and predictive maintenance.
Felt Scanning Unit
Fig. 1. A felt scanning instrument with detail of the measuring sensor
The second type of instrument described (Fig. 2), is a fixed unit for measuring moisture content of felts or sheet and felt combined, but, as the name implies, is bracketed to the machine in a fixed position, inside the sheet run. These units can measure moisture only, but a great deal of information can be gained in this way, at a much lower initial investment, since no beam or traversing mechanisms are required. These devices are also equipped with software capable of performing FFT analysis for detecting periodic vibrations or pulsations.
Fig. 2. The press fabric head, shown here, has a deep microwave field for measuring thicker objects.
CASE STUDIES WITH PRESS SECTION SCANNING INSTRUMENTS
These scanning units can be configured to be mounted horizontally, vertically, or on an incline. As mentioned earlier, these scanners can take data on the permeability, water content, and temperature of the felts. While taking the data safely is important, the other critical benefit of these scanners is what they can do with the data after the data has been captured. The following case studies show how the high sampling rate of these units, allow 3D mapping of the felt structure as well as Fast Fourier Transform (FFT) Analysis of the data.
In this first study one can first see the 2D scans of felt permeability and water content (Fig.3). Whereas those familiar with studying these scans can see several "problem areas" in the scan, the causes must be guessed at because critical information cannot be seen in the graphs.
Even a casual glance, however, at the 3D graphs of the same felt shown in Fig. 9, shows an obvious pattern of shower streaks. Both the graph legends on the sides, and the color differentiation show the engineer or operator exactly what the nature of the problem is, as well as where they are located.
2D Scans of Felt Permeability and Water Content
Fig. 3. These are typical 2D scans of felt perm and moisture
3D Moisture Scan
Fig. 4. A 3D scan shows an obvious pattern of shower streaks
Vibration and Pulsation Analysis
These instruments come with onboard Fast Fourier Transformation (FFT) analysis. By running the units in FFT mode for several seconds, the high sampling rate (1024 Hz) can accumulate enough samples to give detailed data on the frequency or distance over which vibrations or pulsations occur. The most obvious type of vibration in the press section comes from the rotation of the press rolls themselves, although once per Yankee vibrations have also been identified on Tissue Machines with this equipment. It should be noted that the units are so sensitive that pulsations from the stock delivery system have also been picked up by FFT analysis in the press fabric. In the example noted in Fig. 4, a disturbance of high amplitude was noted at 12.56 Hz, which, at machine speed, calculated to be 1.48 M (Fig. 5 and 6).
FFT Trace Showing Predominant Frequency and Harmonics
Fig. 5. A predominant peak at 12.56 Hz which calculates to a wave length of 1.48 M
3D Moisture Scan in Same Felt
Fig. 6. The same felt viewed in 3D mode showing a pattern of water peaks coinciding with vibration
This 1.48 M wave length is the same as the 0.47 M diameter of the pressure roll on this machine. The cover was changed, as a result, before failure.
Complex Problem Solving
The 2D moisture trace below (Fig. 7) shows an obvious sign that some periodic variation is taking place in the felt, but it is difficult, if not impossible, to determine exactly what.
MD Moisture Scan
Fig. 7. Periodic moisture variation in an MD moisture scan
A quick examination of Fig. 8, however, shows that two different frequencies are involved. There is a high frequency undulation caused, in this case, by a roll vibration, along with a lower frequency modulation (dark streaks in Fig. 9) caused by shower streaks.
3D Water Content Scan
Fig. 8. Note both the high frequency press vibration and the low frequency shower streaks
FIXED MOUNTED PRESS FABRIC SCANNERS
Case Study 1, Vacuum Pump Elimination
Studies have established that there is a wide range of felt moistures that will not affect sheet moisture, whereas, above a certain limit, it will. In daily mill operation, the "more is better" theory often applies to vacuum application at uhle boxes, frequently resulting in excess vacuum, higher press section drive loads, and premature felt wear. At this crescent former mill, we installed 3 fixed moisture measuring heads (before the uhle boxes, between the 2 uhle boxes, and after the uhle boxes). See Fig. 9. There was a liquid ring pump supplying the pick-up roll and the 2nd uhle box, while a blower system supplied all other vacuum to the machine.
In a series of stepwise reductions, vacuum was reduced until sheet dryness was affected. It also became obvious from our measurements, that the 2nd uhle box was ineffective. Through these trials, we were able to redirect vacuum from the blower to the pick-up roll, and totally eliminate the liquid ring pump and the 2nd uhle box.
Machine Configuration Showing Positions of Heads
Fig. 9. Side elevation
Fig. 10. Savings summary from reducing blower amps and eliminating liquid ring pump
Case Study 2, Energy Savings and Process Control Enhancement
In this example there were 3 of the fixed heads mounted on a crescent former, one on the felt after pick-up with the sheet on the felt, one on the felt after the pressure rolls, and the third on the felt after the uhle boxes as is shown in Fig. 11.
Machine Configuration Showing Position of Heads
Fig. 11. Side Elevation
In this study continuous measurements were taken with the 3 heads and compared to energy consumption on the machine. A chart of these measurements is shown in Fig. 12. The moisture content from the heads were plotted continuously along with the energy being used to dry the sheet. By correlating the amount of water removed (felt water content with sheet on, minus felt moisture after the uhle boxes, to the amount of energy consumed during the felt start-up phase, we established a bi-modal distribution (Fig. 13). By comparing the population of felts with high moisture removal after the uhle boxes (? 2500 l/min. vs those lower than 2500 l/min.), it can be shown that the population with the lower total water eliminated though the interaction among flooding showers, high pressure cleaning showers, and vacuum dewatering, also consumed 2.2% more energy (Fig. 14)!
Furthermore, when the FFT software was applied to the sensor data, it became obvious where the differences were coming from (Fig. 14 and 15). The peak amplitude from the data plotted on a time domain, was at 8 hours, which coincides with shift changes. The crews had been given the freedom, in lieu of having hard data, to manipulate the showers and uhle box vacuums as they saw fit during the start-up phase of the felt's life. With hard data available from the moisture sensors, felt moisture can be controlled to the levels that will minimize energy levels and thereby maximize production.
Water Content and Energy Consumption Over Time
Fig. 12. Plot of energy and moisture vs. time showing erratic measurements during felt start up
Change in Moisture Content (Before Nip – After Uhle Box vs. Energy)
Fig. 13. Energy Savings Possible by Controlling Felt Moisture
FFT Analysis of Energy Data
Fig. 14. The frequency graph of the data shows discrete periodicity at 8 hrs.
Data Plotted During Start-up Phase
Fig. 15. Moisture data from sensor shows that each shift has their own conditioning strategy
- New moisture measuring instrumentation is available for the press section that can be either traversing or fixed, providing flexibility to the user.
- Onboard software in these instruments can perform vibration and pulsation analysis for predictive maintenance.
- 3D felt views provide information not able to be seen in older 2D scans.
- Significant energy savings and improved process control result in short ROI.