Pulsation study with the FiberScan
Paper machines are complicated mechanical systems which are susceptible to a large number of vibration related problems, some of which can affect the quality of the paper which is being produced.
Looked at from one perspective, a paper machine can be thought of as a gigantic multi-channel tape recorder. The effects of mechanical vibrations, pulsations, control loop faults, non-uniform consistency of the pulp, together with a number of smaller scale random variations all get written into the paper web. If not kept within bounds, they can affect the efficiency of the manufacturing process as well as the paper quality.
The technology partners of Feltest Equipment invested many years in the development of the FiberScan measuring instrument that by now completely replaced the well-known ‘gamma gauge’. The FiberScan has two important advantages on old ‘gamma gauge’. First it uses harmless microwave technology, compared to the radioactive backscattering technology with all its down-sides on safety, transport and accuracy.
Second, the FiberScan can measure with a very high frequency and this countless number of measuring values is used for a mathematical algorithm (FFT) to create an “amplitude-frequency-histogram”. By means of this method it is possible to find periodical variations (pulsations) in the forming section. With this instrument it is possible to make pulsation studies very quick and uncomplicated.
Case study: streaks and MD variations in the paper web
To introduce the possibilities of this measuring instrument, we turn to a case study. At one of the board machines of an Austrian mill an unknown phenomenon regularly occurs. It comes up suddenly and is uncontrollable. It leads to un-sellable paper quality and many sheet breaks, hence much production loss.
As shown in the picture the problem is unstable streaks, meaning variations of mass, moisture and caliper in the paper web. These streaks are fluctuations of high and low basis weight in machine direction and are completely parallel over the full sheet width.
At the darker parts there is a higher basis weight than at the lighter parts. This leads to different paper characteristics in relation to basis weight, thickness and moisture. The mass and moisture variations in machine direction point towards pulsations; the cause can be in periodically or continuously working machines (e.g., mixing pump: flapping of wings of the traversing wheel, pressure sorter, rotor frequency), raw turbulence, mechanical oscillations, aerial inclusions or foil and/or vacuum oscillations.
After several experiments like air in the approach flow system and in the de-foamer, attempts with the cleaner facility and its surroundings, the vertical sorter, head box, diffuser blocks and jet angles it was still unclear whether the variations were caused by the approach flow system or at the forming section. As there was no defined frequency which could be used to continue the search, the cause of the streaks remained unknown.
High speed measuring at the forming fabric
Now it was believed that the new high speed measuring technology of the FiberScan could shed some light on this problem. The task was to find out if the variations already existed in the approach flow system or if they are formed in the forming section, by means of undesirable foil angles and distances and /or the vacuum system. Also the question “do the stripes get amplified by the vacuum system” needed to be answered.
When the pulsations appeared, FiberScan measurements were immediately carried out. First a pulsation measurement (FFT) and immediately after that the standard drainage measurement, to assure that the conditions during the measurements are known for the pulsation study. It can also be investigated if the drainage behavior influences the pulsations.
As shown in machine sketch it was possible to measure at the positions 1-5. To be as accurate as possible, white-water consistency probes were taken from elements A-C which were analyzed in the wet lab of the mill.
Already at the first measuring point – immediately after the forming board – a dominant frequency of 26.27 Hz with a high amplitude appeared.
This reinforced itself due to the Varioline dewatering boxes up to an amplitude of more than 150 dB – please note the difference in the Y-axis scale.
After the divert roll the amplitude is substantially weaker but the peak is still clearly obvious.
With help of the FiberScan measurements it was possible to define the frequency and to limit the search area for the cause to the approach flow system. Clearly, the question if the vacuum system amplified the streaks could be answered with a yes.
In the past to find out pulsation frequencies required a large number of diagnostic and measurement tools, time and money. The FiberScan is a measuring instrument which allows quick and easy pulsation studies in the forming section. It gives an accurate frequency analysis (FFT) which helps to limit the search for these kinds of defects.
This is one of a series of articles on Paper Machine Clothing related topics which can be found in knowledgebase of the Feltest website.« Back