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Case
study
Monitoring
the Paper Machine Press Section
The
fundamental function of a paper machine press section is to remove water
from the paper sheet. Press sections come in several configurations,
however, a common denominator is the felt, which serves to soak moisture
from the sheet as the felt and sheet are both squeezed together between
heavily loaded rolls. When the paper web enters the press section, it
typically has water content of approximately 80%. Upon leaving the press
section the water content is usually reduced to roughly 50% to 55%.
Paper
machine press section analysis involves interpreting vibration signals
which may have multiple sources, due to the fact that press section is
comprised of many rolls, all supported and linked by the frame of the
paper machine. Some rolls are also coupled where they come into contact
with the felt, which passes between their surfaces thus creating a
transmission pathway for machine motion.
Determining
the source of a vibration signal can be a difficult and complex task
because the dynamic forces causing the vibration in any particular roll
can originate in any other roll, or rolls, which are mechanically coupled
to the roll under investigation, either through the paper machine frame or
via the interface between rolls, or both.
In
addition to the common causes of machinery vibration familiar to most
analysts (such as unbalance and misalignment), the press section of a
paper machine has its own distinct sources of vibration. The mechanical
conditions listed in Table 1 may cause dynamic forces which contribute to
press section vibration.
The
magnitude of the forcing function is one of two factors which contribute
to the magnitude of the press vibration. The other factor is the
combination of mass, stiffness and damping, referred to collectively as
press stiffness. This defines the response of the system to an input
force. If any force excites a press section component at its natural
frequency, the vibration amplitude will be amplified.
One
operator controlled variable parameter which defines the response of the
press to input forces is nip pressure. Since changing the nip pressure
changes the characteristic response of the press to input forces and could
shift the natural frequency away from the excitation frequency, it is
possible under certain conditions to reduce vibration by changing nip
pressure
Figures
1 through 3 show the effect of increasing nip pressure on vibration level
with paper machine speed held constant. In this example, one or several
press components is excited at its natural frequency (Figure I.)
In
Figure 2, the nip pressure is increased, and the vibration level is
reduced. The vibration level is reduced further in Figure 3, at maximum
nip pressure.
Varying
the nip pressure does not remove the source of the vibration. This is
apparent by examining Figures 1 through 3, because the vibration remains
the same in terms of frequency content. Therefore, although changing nip
pressure reduces vibration levels, it is not the solution to addressing
the root cause of the problem.
Another
variable which influences vibration levels is paper machine speed. If the
force causing the press section to vibrate at its natural frequency is a
function of machine speed, changing machine speed may shift the excitation
such that the vibration is no longer amplified by the characteristics of
press components.
Figures
4 through 6 show the effect of changing sheet speed on press vibration. As
with changing nip pressure, changing machine speed may be an unacceptable
solution to the root cause of a vibration problem if restrictions are
placed on machine operating speed, which reduce capacity utilization.
The
nature of the press section is such that the dynamic forces associated
with the listed mechanical conditions (see table 1
above) may alternate between constructive and destructive interference
with each other, resulting in amplitude modulation of the dominant
forcing function at the frequency of the interfering force. These
amplitude modulated forces result in amplitude modulated vibrations, which
can be measured and analyzed to determine their source.
For
example, a suction press roll with 15 bars, rotating at 100 rpm, which is
also unbalanced, would vibrate at a frequency of 1500 CPM. Examination of
a time waveform would show the 1500 CPM vibration always present, but its
magnitude would rise and fall some amount 100 times per minute.
Recognizing the frequency of amplitude modulation is one of the keys to
successfully analysing press section problems. Amplitude modulation is
characterized in the frequency domain by the presence of
"sidebands". In the example described in the preceding
paragraph, the main peak would be located at 1500 CPM, with sideband peaks
located at 1400 CPM, and 1600 CPM.
Amplitude
modulation is also apparent in the time domain, perhaps even more
recognisable than in the frequency domain, with the repetitive, rising and
falling amplitude clearly visible in the signal. Establishing the
frequency of the amplitude modulation in Hz from a time domain signal
simply involves calculating the inverse of the measured distance in
seconds. As an example, the frequency of the amplitude modulation in
Figure 7 is 1/ 0. 175, or 5.71 Hz, which, as discussed later, is equal to
the rotating speed of one of the rolls in the press section.
Paper
Sheet Barring
Paper
sheet barring is characterised by imperfections running either parallel or
perpendicular to the direction of paper movement through the machine.
These imperfections appear periodic if they are observed on a moving sheet
from a fixed reference point, and are typically detected by dynamically
measuring the moisture content of the paper.
Fluctuating
press section nip pressure is one potential source of sheet barring. Nip
pressure fluctuations typically result in vibration which can be detected
at the bearing housings of rolls in the press section. The mechanical
conditions listed in Table l can individually, or collectively lead to
fluctuations in nip pressure. As an example, the time waveform shown in
Figure 8 shows clearly that three events are occurring during each
revolution of the felt leading to speculation that three seam defects of
some type exist in the felt.
In
addition to causing sheet barring, left unattended, the nip pressure
fluctuations can transfer barring patterns into the felts and rolls of the
press section. Roll barring, also known as corrugation, is described as
surface profile variations running the length of a roll, parallel to its
center of rotation. Similarly, variations across the surface of a felt,
perpendicular to its direction of motion is also known as barring. The
surface variations on a press roll are typically spaced such that the roll
will develop between ten (10) and twenty (20) "bars" around its
circumference, depending on the diameter, mass and speed of the roll. Felt
"bars" can cover the entire surface of the felt, or only a
portion of the felt, and are spaced according to the forcing function
which created them.
Detecting
Barring
Many
paper companies now utilise portable vibration analysers for condition
monitoring. Such an instrument has the capability to process the signal
from an accelerometer, and provide the user with a high resolution
frequency spectrum in velocity units. Detecting barring requires the
resolution of the frequency spectrum provided by the analyser to be high
enough so that given the frequency range of interest, enough data is
available to distinguish individual peaks spaced at felt speed intervals.
Barring
is characterised in a frequency spectrum by families of peaks located at
frequencies equal to the rate at which bars are passing through the nip.
Typically, the family of peaks includes "side bands" spaced at
either the rotating speed of the felt, or at the rotating speed of one of
the rolls involved in the nip. As noted earlier, vibration migrates
through the frame and the nip from roll to roll. This migration means the
defective component is not easily identified because the same frequency
spectrum is collected from several, or possibly all of the rolls in the
press section.
Synchronous
Time Averaging (STA) is a popular analysis tool used to determine the
source of vibration in paper machine press section rolls. Briefly, with
STA, the vibration instrument averages the time records together before
the FFT calculation. By setting the beginning of each time record at
precisely the same time relative to some known event, the averaging
process eliminates signals which are not occurring at the same time.
Therefore, the source of vibration can be determined by moving the
triggering device, which sets the beginning of the time record to
different rolls in the press section, collecting vibration data at each
location.
Although
most modern, portable vibration data collectors are capable of performing
STA, one of the practical limitations of STA is that it requires a trigger
on any roll, or felt which is to be included in the analysis. Unless the
paper machine is equipped with permanently mounted roll triggers,
temporary trigger sources must be installed. The felt trade line is often
used as a trigger source, however, specialised optical or laser pickups
are required.
Paper
Machine Press > Examples
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