Vibration analysis system helps Repap fine-tune its LWC basestock machines
Williamson, Mark
THE RUNABILITY AND VIBRAtion monitoring systems provide us with a new window into the process. As an operations tool, it allows us to minimize downtime by minimizing the damage: says Tom Paisley, A2 paper mill general superintendent at Repap New Brunswick Inc. The new systems Paisley refers to provide Repap papermakers and maintenance staff with an early warning. Indeed, if vibration problems develop to the point where they are physically noticeable and adversely affecting runability, unscheduled downtime must be taken.
Worse still, mechanical damage may have progressed to the point where major remedial maintenance is needed. “With the Sensodec system we’ve installed, we can discern problems with machine vibration before we can feel it on the floor,” adds Rob Hanna, assistant superintendent of the A2 machine.
The staff of both A1 and A2 LWC basestock machines at the Miramichi, N.B., mill can see the operational benefits of the Sensodec advisor systems. These computer-based systems continuously monitor the stability of stock delivery, vibration levels in rotating elements, press roll nips, bearings, and the variability of final product quality. The A1 machine is a Valmet Sym-Former machine with a reel trim of 300 in., running basis weights from 21 to 28 lb/3,300 ft sup 2 at an average speed of 4,220 fpm. The A2 machine is a Voith Duoformer machine with a 311-in. reel trim, running basis weights from 27 to 44 lb/3,300 ft sup 2 at an average speed of 4,000 fpm.
AN OUNCE OF PREVENTION. Using advanced vibration analysis techniques, the user can infer and diagnose problems with uneven roll wear, felt condition, bearing defects, and persistent quality problems. The systems are configured by Repap to advise operators of instabilities which, if not promptly corrected, might lead to poor runability, poor quality, or more serious mechanical damage. “An ounce of prevention” aptly describes this predictive maintenance philosophy.
The development of the predictive maintenance program at Repap dates to 1988 when the mill began installing vibration monitoring equipment that could be surveyed on a regular basis using hand-held data loggers and off-line vibration analysis programs. This network of “walk-around points” has expanded to more than 18,000 points, surveyed on a three-week cycle.
In 1993, Repap evaluated online runability and predictive maintenance systems to monitor and analyze the vibration of rotating elements that were critical for machine runability. Some elements–such as dryer felt roll bearings–were inaccessible for manual data acquisition. The felt roll bearings could fail without advance warning and cause major machine downtime.
ANALYSIS TOOLS. Charlie Fives, predictive maintenance superintendent, describes the evaluation process for the online systems. “We looked at the analysis tools available and the operator interface. The Sensodec tools were superior. The advanced analysis capability lets us compare the synchronous time averages of any four machine inputs. To diagnose cause-and-effect relationships, we can play ‘what-if’ scenarios.”
Synchronous time averaging is a technique used to uncover low-level vibration signals, filtering out the influence of other, sometimes larger, interfering signals. For instance, the contribution of an individual press roll or a press felt to press nip vibration may be defined. This is done by averaging multiple time records of the vibration signal. Each time record is started at exactly the same point in the roll or felt rotation.
The analysis is synchronized by a triggering device, a magnetic trigger sensing roll rotation, or an optical trigger looking at the trade line of a felt or forming fabric. Fives adds, “Synchronous time averaging allows more precise interpretation of complex vibration waveforms. We have used this technique to diagnose press vibration problems that originated with the press felt. This diagnostic information allowed us to work together with the felt manufacture
Both A1 and A2 machines are equipped with accelerometer-type vibration sensors on all key rotating elements in the forming and pressing sections. The dryer felt rolls on A1 are also instrumented with accelerometers. Stock approach pressure and headbox pressure are monitored; and screen rotation and fan pump rotation are triggered magnetically. Online paper quality sensors on the reel scanner also are interfaced to the system. Their readings can be cross-correlated with wet-end measurements.
The operator interface for all real-time readings plus data history is through an X-windows terminal in each wet-end control room. The system is programmed to generate an on-screen alarm if vibration levels are excessive With a few clicks of the mouse, the problem area is identified, and the operators can alert the maintenance group.
As an operations tool, the systems have been used to detect stability problems caused by premature granite roll wear. The uneven wear across the surface resulted in excessive nip vibration, a vibration that was seen by the Sensodec system. To accommodate the problem while still maintaining runability, the operators recognize when vibration levels are starting to increase to unacceptable levels. A newly ground roll could then be substituted at the right time, before runability was compromised. This problem has since been solved by installing Valrok rolls at three of the four granite roll locations. A fourth Valrok is on order.
EARLY WARNING OF BEARING PROBLEMS. In addition to providing Repap operators with runability monitoring, the Sensodec system analysis techniques also provide predictive maintenance staff with valuable insight into bearing problems through its Maintenance Advisor (bearing vibration analysis) package. Fives comments, “The enveloping technique included in the system detects high frequency bearing problems. If you recognize the signs, enveloping gives you an advance warning of bearing failures. We can see bearing problems up to three months in advance.”
The enveloping technique filters out low-frequency vibration signals originating from roll imbalance or misalignment. Small repetitive impulses, such as those from bearings, are thus isolated. Recognizing the signs of bearing defects is just as important as the computer’s mathematical analysis. Maintenance and operations staff have received Sensadec-supplied training in the science of vibration diagnostic techniques.
Machine operators are particularly pleased that this technique gives them advance warning of a mechanical failure or excess vibration. In one case, the Sensodec envelope analysis suggested that a couch bearing problem was developing on the A2 machine. After inspecting the bearing during a shutdown, the wearing was evident. Major machine downtime and perhaps fabric damage had been avoided.
On the A1 machine, operators have seen similar results from the predictive maintenance diagnostics. “Every time we opened a suspect bearing, there was something wrong,” reports Timo Suutarla, A1 general superintendent. He also notes that the Sensodec system has aided the mill in minimizing nip vibration levels in the triple nip and fourth presses. They found that the nip vibration levels were strongly influenced by the tightness of the triple nip press roll bolts.
An engineering degree is not necessary, however, to see that vibration levels are getting too high. Barry Madden, assistant superintendent of the A1 machine adds, “The Sensodec system is an every day tool for the operators. It provides them with an instant feedback to changes they make. For example, vibration induced by the press felts may be minimized by making changes in tension and orientation, thus promoting increased felt life. Also, monitoring of the dryer felt bearings has avoided major machine downtime
SYSTEM EXPANSION, MACHINE SPEEDUP. With the predictive maintenance and operational benefits established, Repap is expanding the A1 Sensodec system’s capabilities. In Fall 1995, a press dewatering monitor plus a lubricating oil monitoring capability is being added.
Fives adds, ‘We are planning to increase machine speed from the current 4,220 fpm. The speed increase will be done in several steps, and the Sensodec system will be used to evaluate the results. It will be an integral part of the machine speedup.”
MARK WILLIAMSON is a freelance writer based Thornhill, Ont.
Copyright Miller Freeman Inc. Feb 1996
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