How Yankee metallurgy plays a critical role in machine runnability

The Yankee dryer is the heart of any tissue machine. When Yankee chatter and surface variations become a continuing mystery, after checking all the usual items, grinding and runnability problems may very well be due to the basic metallurgy of the cast-iron dryer shell.

John Holton

On tissue machines, chatter damage on cast-iron Yankee dryers has become far more common in recent years, with chatter being a main reason for increasing grinding frequency of Yankees. The key contributor to the increased chatter is the trend towards operating at ever-lower reel moistures, a common means of increasing product softness. On chatter-prone Yankees, where previously regrinding might typically take place at intervals as long as 18-24 months, as the Yankee shell becomes thinner, the problems become more evident and grinding can become as frequent as every 6 months. We have recently carried out new studies into chatter wear patterns and surface variations, including detailed examination of the metal properties of the cast-iron Yankee surface. The investigation involved etching the Yankee surface by standard methods and taking photomicrographs to determine if there was any difference between the areas showing wear and chatter versus areas that were not wearing. This has given us an interesting new understanding of one chatter-initiation mechanism and ways to avoid it. At the same time it has confirmed the usefulness of homogeneous metallic coatings on the Yankee surface.

FIRST THINGS FIRST: THE CONDENSATE SYSTEM. When you have hot or cold areas on the Yankee shell the first thing you always look at is, of course, the condensate system since it is arguably the most common source of chatter initiation. But if you have completely ruled out the condensate system and other issues like crowning and you still have problems, they may be due to the metallurgy of the cast iron itself. Cast iron is by definition a non-homogeneous material. At ever increasing machine speeds and lower moisture targets, cast iron by its very nature can become production limiting.

GRAPHITE CONTENT AND TYPE IS KEY. The primary purpose of our study was to determine if there are significant differences in the graphite characteristics in the high-wear areas versus the low-wear areas. Thus we conducted an in-depth analysis of graphite concentration and shapes on the cast-iron surface. Graphite is necessary to reduce friction and allow the creping blade to slide over the surface in a smooth and uniform manner. Typically there are variations in the type and concentration of the graphite flakes inherent in cast iron. As can be seen in Figure 1, shapes can be either flakes (Type I and Type II) or more nodular as with Type V and Type VI. Ideally you want low and uniform graphite concentrations of less than 10% of the exposed surface area and all Type I and Type II graphite formations. This observable percentage is not related to the percent of carbon by weight. The type and concentration of the graphite depends on the metallurgy and the cooling rate of the cast-iron when it is formed. If areas of the Yankee cool too slowly, this will create more Type V and Type VI graphite, which have a tendency to be plucked out of the surface by the blade, as can be seen in the photo in Figure 2. When this occurs, this pocked area of the Yankee is rougher, the increased surface area creates a more tenacious organic coating, which is also a well-known contributor to chatter initiation.

GRAPHITE IS IMPORTANT FOR TWO REASONS: First, it has very low conductivity so concentrations of nodular Type V and Type VI will make the area run cooler (Figure 3). These areas show up as “ghosts” in high speed infrared analysis even directly after a Yankee grind. Cool areas create an immediate low area, the local nip pressure is then also low, which makes it difficult to build a robust and protective Yankee organic coating. In turn, this causes surface wear, deepening the low area, and the cycle continues. This will lead to either grinding for chatter or simply grinding for the created low area if chatter does not initiate. Secondly, graphite is also important because it provides a form of lubrication for the surface. The higher the graphite concentration, the lower the friction, which improves blade life and release characteristics. The relationship of graphite concentration and friction is linear, as can be seen in Figure 4. And the slope of that friction curve becomes steeper in wet environments, which papermaking certainly is.

WHEN YOU HAVE CHATTER THE BLADE IS BOUNCING ON THE SURFACE. That means you have high pressure followed by low pressure. As the blade bounces away from the shell, it has low pressure contact; as it snaps back onto the surface, it becomes high pressure contact. So a single low spot on dryer shell, which may be caused by graphite content variations, can lead to multiple low spots (ie. chatter) due to the continuous bouncing. Chatter is a self-propagating phenomenon; as the blade digs in at one spot then snaps over the next spot, and digs into the next area, and so on. It’s like a washboard effect on a dirt road: In the long run, one single pothole can cause miles of washboard effects.

TWO CASES STUDIES. As an example, on one Yankee we worked on recently there was a problem with ‘dishing’, which means a low area. The problem has been there from Day One. They did a condensate rebuild, but the cold areas were visible directly after grinding even though the condensate system has been triple checked by experts. When this occurs and you have gone through all the possible causes and do not find a suitable answer or solution, the chatter may be due to the metallurgy of the cast-iron shell itself and the graphite Distribution or Type. Three dished areas, which were historical, were analyzed, along with three adjacent areas and six random areas. All three dished areas had very high graphite concentrations (well over 5%) and all Type V and Type VI graphite flakes (see Figures 5 and 6). In this case the only reasonable solution was to apply a metallized surface on top of the cast-iron. This Yankee is now running well at over 2000 mpm and without the historical chatter problem.

REGARDING METALIZED SURFACES, the most common applications are pseudo-stainless steels, which are both harder and more resistant to chatter than cast iron is. These sprayed-on metal coatings are also completely homogeneous compared with cast-iron which certainly is not homogeneous. In addition, thermal conductivity is only about 25% that of cast-iron, which is good as it has the effect of distributing the heat transfer more evenly for a better profile. Because of this loss of heat transfer, drying limited machines with remaining cast iron thickness before de-rating should consider the removal of additional cast iron to improve heat transfer whilst remaining above the de-rate point. It should be mentioned that steel Yankee dryers are metalized with the same type of materials prior to delivery because the steel itself is too soft for creping and the wear rates would be unacceptable. This is the key reason that steel Yankees are far less prone to developing chatter.

ANOTHER CASE WE WORKED ON WAS A DRYER WHICH NEVER SUFFERED FROM CHATTER but was experiencing a problem with a strange and random Yankee marking pattern. The first conclusion is always – ‘we’ve got to change the condensate system’. And most often it is the condensate system, especially as there are some poor designs which have been used in the past and are still in use in many dryers. But in this case the condensate system was fine. This strange marking pattern (example: area 5 in Figure 7) had appeared directly after a profile grind even before the mill put any paper on the dryer. As soon as they loaded the blade these patterns appeared. So we conducted our graphite analysis with standard 2% Nital etching and took photomicrographs of the odd patterns, as well as areas without the marking. We examined eight marked areas and compared them to eight adjacent areas. The results were identical.

WE CONCLUDED THAT THE ROOT CAUSE WAS A SIGNIFICANT VARIATION IN GRAPHITE CONTENT. In this case it was a very low level of graphite in the marked areas, which creates a high friction condition, which caused the marks as the blade would instantaneously “stick” because there was not enough lubrication for it to easily slide across the surface. Without explaining all the science here, the conclusion for both of these cases is that when the dryer gets thinner, when the reel moisture is low, the heat transfer rate goes up. So the problems manifest themselves more quickly with each successive grinding, as there is less insulation and the non-uniformity becomes more evident. We have studied many more cases but these two examples give an idea for how graphite concentration, uniformity and shape can cause very different Yankee dryer problems.

HIGHER DEMANDS IN RECENT YEARS. Those metallurgical variances in the cast-iron dryer shell have probably been present in Yankees for a very long time. But only in recent years, as the speeds and the drying demands on the Yankee have increased, has the nature of the cast-iron itself become a limiting factor. As a co-related factor, it is interesting to note that steel Yankee dryers have recently become more popular and are running well. One known advantage of steel is better heat transfer when compared to a standard new cast iron Yankee. This is not because steel has a higher heat transfer rate, it is simply because steel is a stronger material and can therefore provide the required operating pressure with a much thinner cylinder. Since these Yankees are metalized, and metalized Yankees have a homogeneous surface, the operating window for these surfaces is much wider, organic coatings build more uniformly, and blade life is improved. It is also the preferred surface for optimized use of ceramic blades, with many mills now running these blades for three to six days.

CONCLUSIONS. These graphite investigations have led us to conclude that, as machine speeds and drying demands go up, any non-homogeneous nature within the cast-iron becomes more pronounced. This increases the potential to create surface problems, such as chatter. The fact is that these inconsistencies are unavoidable in cast-iron because, for the most part, they are due to the casting process which cannot be made absolutely uniform. You can never get perfectly even cooling around the entire Yankee mould. And as you speed up and push tissue machines ever faster, the imperfections and problems become amplified.

OUR STUDIES HAVE ALSO CONFIRMED THE VALUE OF METAL COATINGS ON THE YANKEE SURFACE FOR GETTING CONSISTENTLY GOOD RUNNABILITY. Metallization simply provides a better surface on which to make paper. In the early days, metallization was used as a means of asset preservation, meaning that if you put a metal coating on you could continue to run without de-rating the Yankee, maintain production levels, and avoid an expensive replacement. But as people have learned how well these coatings improve operations, it has turned into a machine performance-enhancing investment, rather than simply for asset preservation. If you have mysterious problems with your Yankee surface, and have checked and eliminated the usual suspects, it may be time to check the metallurgy. If large graphite variations are present in the cast-iron shell, the best remedy may be to metalize the surface, thereby increasing the creping operating window, which is the major benefit of a homogeneous creping surface.

For more information, you can contact John Holton at john.holton@jaeger-inc.com


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