Paper Machine Process Troubleshooting — Diagnostic Techniques
By Pulp & Paper Canada
By Pulp & Paper Canada
The first technique reviewed in this article is a technique which can be used and implemented by operating staff and crews through documentation of past experiences and process knowledge itself without external resources. The technique has been used by Omni for over 30 years in consulting and training work. Many mills have formalized this themselves and use it not only for mill troubleshooting but also to capture existing operator knowledge in a manner useful to crews moving into new positions on the machine. This is not an insignificant consideration in many mills in these times of a workforce facing large turnover due to retirement.
The basic technique is well known.
* Obtain the client’s definition of the problem.
* Get physical evidence that the problem as described to you is truly the correct problem
* Be alert to a divergent point of view
Having defined the problem, some action is chosen as a result of taking in which one of two consequences may be expected. From these consequences, other actions are taken and results observed. Finally, after working through the diagram, the cause of the problem becomes obvious. One note of caution is appropriate: often there are several problems co-existing and the troubleshooter must really know the background technology and be on his toes to sort out the situation that exists.
We have developed over 50 logic diagrams in our work in the industry, some of which have been customized by our clients to fit their exact processes and grade sensitivities. We have also read and been exposed to a number of other excellent examples generated by mill staff and suppliers.
Two examples from our work follow:
Paper chemistry-related problems are often a result of loss of delivery, quality of the incoming chemical or interaction of other chemicals. Figure 1 gives operating staff a picture of how to troubleshoot a loss of sizing situation methodically.
Twin Wire paper machines often have specific sheet structure defects particular to their design – blade versus roll versus roll blade etc.
This diagram, taken from a collection of Twin Wire troubleshooting diagrams highlights some of these specific areas of interest and concern.
High Frequency Variability Analysis
The next technique reviewed requires not only process expertise but also diagnostic equipment. Variability which occurs in paper machine systems is spread across a wide frequency spectrum as shown in Figure 3. Process variability at any frequency will compromise paper quality and machine runnability. Yet, the vast majority of variability in the sheet is not even visible at the operator console. This is due to the limited ability of the Quality Control Systems (QCS) systems and the Distributed Control Systems (DCS) to report/display process data. Excessive filtering applied to the field sensor or at the DCS can further mask the variability. Machine Direction basis weight cycles that are faster than two minutes in period are often impossible to identify and troubleshoot from the operator console.
A key variability troubleshooting concept is that product variability cycles are caused by process cycles occurring at the same frequency. The first troubleshooting step is to characterize the spectral content of the product variability. The process sources are identified by matching process and product cycles. A high speed data acquisition system with analysis software is often an indispensable tool in the troubleshooting effort. These systems can sample a process signal, such as Basis Weight, as fast as 200 times per second. Analyzing the collected data using time series techniques such as the power spectrum and cross correlation analysis provides an important window into the nature and source of the variability. High speed data acquisition systems are not commonly found in paper mills but they are very affordable – in the $10,000 to $12,000 range.
The case studies below illustrate the capabilities of these systems.
Case 1 – Stock consistency variability causes long term BW variability
The Scan Average BW on this newsprint machine cycles at a period of 2000 seconds (Figure 4). This slow cycle is visible on the BW scan average trend plot. The variability spectrum suggests that the thick stock system (Figure 5) or poor BW / Moisture control will be the likely source.
There are many potential sources of this cycle and an analytical approach is required. Is the BW cycle induced by the BW controller? Does the machine chest consistency contain the cycle? If it does, what is the ultimate source of the machine chest consistency cycle?
In this case, the 2000 second cycle was visible in the Machine chest consistency and was tracked all the way back to a consistency loop upstream of the Blend Chest (Figure 6). Poor stock chest mixing and unnecessarily slow tuning in the Blend Chest consistency controller allowed the cycle to pass all the way through to the reel. This problem was corrected by retuning the consistency control loops and improving consistency setpoint management.
Case 2 – Headbox consistency cycles
High frequency data collection equipment revealed a strong 5 to 10 second MD BW cycles on this tissue machine. These fast cycles were not visible in the scan average trend plots, but were compromising machine runnability and hurting converting efficiency nonetheless. The cycles did not show up in the thick stock flow or consistency or any of the lean stock pressure and level signals. However, the dewatering box vacuum and BW signals were strongly positively correlated (Figure 7), indicating that the variability source was at or upstream of the headbox. The findings pointed to the Headbox consistency as the culprit. A portable consistency sensor was used to diagnose the ultimate source. The strong, positive correlation between the BW and Fan pump discharge consistency (Figure 8) confirmed that poor thick stock/lean stock mixing at the Fan pump suction was responsible. The mill is designing a mixing wedge to correct this problem.
Infrared analysis can be used for ‘visualizing’ sheet non-uniformities. Once characterized, these irregularities can often be traced to their origins in the process.
Thermography is the technique of making this invisible thermal energy visible for analysis. All objects constantly emit thermal energy in the form of infrared radiation. As an object heats up, it radiates more thermal energy from it’s surface. We may be able to feel this energy, but we can not see it with our eyes.
Thermography has been used in the paper industry for over 30 years. Its widespread use as a process troubleshooting tool however had been, until recently, limited due to the cost and complexity of the camera and the related software. Over the past 5-7 years camera costs have dropped dramatically in price and they have become more user-friendly with a growing place in many mill preventive maintenance groups for on the run inspections for areas such as bearing, couplings, lime kilns etc. In many mills it is now not uncommon to find one to two cameras dedicated to process troubleshooting. IR cameras used for this are in the range of $15,000 CDN.
Following are a number of process examples (courtesy of Asten Johnson).
If you are looking for a camera primarily for troubleshooting the paper machine/process, we recommend keeping it fairly simple. An example is the FLIR Thermacam E series, which is very compact and has intuitive controls. Your camera should be easy to use one-handed, and can be aimed much like a flashlight. Another important feature to consider is the ability to change lenses. Some cameras have a fixed lens, which limits the field of view. You should be able to choose a lens that fits your particular needs: you may want a wider angle view, for example, in order to look at the full width of the reel from a reasonable distance.< /p>
Probably one of the most important features to consider is the software for importing images/analyzing images/generating reports. It should import the images as fully radiometric, meaning that you can make all of the same adjustments to the image in the software that you can make on the camera (excluding focus of course). This is very useful for analysis.
These cameras require some training which is readily available from the camera suppliers and clothing service staff experienced in this area are an excellent resource and in my experience are usually keen to assist in developing in-house mill staff.
The techniques outlined in this article, together with many other diagnostic techniques, should be part of the arsenal of the mill troubleshooter.