REFINING AND IMPROVED PAPER MACHINE RUNNABILITY
Poor runnability is an expensive problem and the further on in the process chain, the greater the losses and the higher the cost in raw material and production losses. Unfortunately there is a low degree of "real knowledge" sometimes based on faith and belief rather than a clear definition and quantification. One definition of runnability is the production speed of a job through the process to an agreed specification; another is the expected or mean frequency of web breaks for a given material under a specified loading condition. These definitions are concentrated on effect not cause.
Further observations are that web breaks do not occur in areas of average strength but in areas of local weakness. Therefore a well formed sheet with low strength could have better runnability than a poorly formed one with high strength. In other words strength or any other single measurement does not imply runnability.
There are two ways to define runnability:
1.1. In the first case good runnability gives longer runs (through manufacture and conversion ), higher production and more efficient raw material usage. Also of importance is the production of a consistent product with the correct specification for good runnability in further processes. A paper with good runnability performance enables the use of cheaper raw materials without loss of product performance, increased process speeds and greater tolerance of abuse. These benefits will be of relevance to paper and board manufacturers, printers and converters.
1.2. In the second case poor runnability may be defined as excessive downtime due to breakdown because of wear or damage. It is this latter case which is considered here with particular reference to failures in refiners due to incorrect operation and damage. The talk covers refiner parameters and materials used in refiner filling manufacture.
2. Refiner Parameters
2.1. No load power
Because of its contribution to the efficiency of a refiner the no load has a major influence on the strength development of papermaking stock(1). The no load power of a refiner can be changed by the condition of the refiner, stock throughput and condition of the fillings. No load is a function of the motor, turbulence due to rotation speed and filling design and pumping. In many installations backing off refiners does not change the power reading at all. No loads are only occasionally measured and this is not observed and the refiner and the refiner cannot develop the fibre to give good runnability. High no loads are indicative of bearing wear or failure and component wear. An example of no loads before and after maintenance is given in table 1.
Table 1. Correction of high no load (1)
The no load power of a refiner is dependent on groove depth which affects the hydraulic capacity or pumping action of the refiner. When a filling is worn the grooves disappear and pumping ability is lost. The resulting low no load gives an appearance of efficiency (fig. 1) but in fact operation of the refiner has ceased to be effective and runnability will be affected.
Figure 1 No Load with wear (2)
2.2. Stock throughput with double disc refiners
Refiner fillings for a double disc refiner are designed for a specific stock throughput, normally specified as volumetric flow in litres/minute. The range of flow capacity, which is determined by the bar angle; can be between 50% and 110% of that specified. The central rotor is allowed to float to maintain an equal gap between discs in each zone.
However with incorrect flow the equalisation of gap is affected. This is particularly obvious in double disc refiners in monoflo (series) operation but also happens in duoflo (parallel) mode. When flow is below the capacity of the plates, number 1 and 2 discs are pulled together giving the equivalent of a single disc refiner plus a high no load due to friction between the plates. If flow is too high then numbers 3 and 4 discs are pulled together with the same effect. The occurrence is obvious by the noise at no load, but as many refiner systems are located in remote places it is not always noted. Where the refiners are in good condition the problem is self rectifying but where bearings are worn and sliding mechanisms rusted together the plates stay together giving a result which is not as expected.(3)
The means of prevention is to measure no load at a regular interval, e.g. once per shift, and to maintain the refiners regularly. Where there have been significant changes in flow then the manufacturer should be contacted to see if changes in pattern are needed.
2.3. Residence time/throughput
The residence time in a refiner is determined by a number of factors, i.e. throughput, direction of angle and whether fillings are dammed, preventing flow across the grooves. Dams in fillings are a part of 'refining lore' but there is little evidence to support their efficiency.
Where a refiner is oversized and there is cavitation, the fibre mat between plates will be thin and at some times non existent. Although in most systems refiners may be initially correctly sized, problems can occur where different grades, of different deckles, are made on one machine. In this case the throughput can vary beyond the range of filling and refiner capacity.
Correct fillings can resolve some of the problem but the best solution is to keep flow through the refiner constant by means of recirculation loops. With a recirculation loop stock is returned to the inlet of the pump. As stock flow requirements are changed the control valve will alter the degree of recirculation and flow through the refiner remains constant. The loop can be pressure controlled which avoids the possibility of fillings clashing because of low flow condition. However a disadvantage of this system is that, as demand for refined stock varies and the recirculation flow alters appropriately the treatment of the stock varies.
3. Filling parameters
Basic fillings available remain cast, fabricated and machined. Whilst the principal objective of a filling remains the same, with a basic design to refine paper fibres efficiently, the methods of production have become increasingly sophisticated to take advantage of improved material metallurgy, modem casting techniques, heat treatments and coating methods (4). Refiner fillings are a means to paper production, subject to the problems of a continuous process, including stock failure, overloading and machinery defects. As such refiner fillings must be regarded as wear items.
The initial choice of material also imposes certain constraints. A brittle material is not suited to be used in a design calling for long or thin cross sections, so whilst offering greater wear resistance it may be subject to premature failure in use. Each filling manufacturer has a range of materials best suited to various applications. This is an area that is constantly being developed in order to provide fillings with a better life and efficiency.
Refiner fillings materials can be put into four broad families: Ni-Hard, High Chrome Iron, High Carbon Stainless Steel, and the Low Carbon Stainless Steel groups. The basic chemistries of the families are shown in table 2.
Table 2. Chemistries of Basic Alloy Families Used in Refiner Fillings (1)
There are many other elements added to those listed to give the individual alloys their specific properties. In addition, virtually all modern refiner filling alloys are heat treated to further enhance desirable properties. These basic families have vastly different resistance to the most common failure modes found in low consistency refining. These failure modes include mechanical damage, corrosion, bar wear and bar edge wear.
3.1. Mechanical Damage
The nature of the refining environment in the majority of production sites, although improving, is still relatively unstable. The inclusion of tramp materials in the stock which may cause mechanical damage to the fillings, baling wire may cause the bars of the refiner filling to fail or simply act as a trap for other contaminants in the system to fill the grooves to an extent where the throughput is reduced below a critical level.
Breakage resistance is very important as low consistency refiners operate with very narrow plate gaps and plate contact, together with tramp material, can cause catastrophic wrecks. The low carbon stainless steels are considered unbreakable in that they will smear or bend before they crack and break. All the other alloys are shown as a percentage of the resistance to breakage that these unbreakable alloys have (Figure 1).
Fig. 2. Breakage Resistance Range for Various Alloy Families (5)
Corrosion resistance in refiner filling materials is a function of the amount of free Chromium left in the material structure after formation and heat treatment. Higher amounts of free Chromium result in better corrosion resistance. Low carbon stainless steels have the highest levels of corrosion resistance because of their high Chrome/Carbon ratio. This high ratio means little Chromium is utilized to make Carbides within the structure resulting in more free Chromium. The low carbon stainless steels are considered risk free for corrosion in low consistency refining applications. Figure 2 shows a percentage comparison of corrosion resistance.
Figure 3. Corrosion resistance range for various alloy families (5)
3.3. Bar Wear
Resistance to bar wear, which is measured as the loss in bar height, is a function of chemistry and more importantly heat treatment. Figure 3 shows the range of bar wear resistance for the various families. A higher number means more resistance to bar wear.
Fig. 4. Wear Resistance Factor Ranges (5))
Wear resistance, however, does not necessarily mean longer life because the way the plate wears also has an impact. Bar edge retention is critical to pulp quality maintenance in a refiner as the majority of the work occurs on the leading edges of the bars as they pass each other.
The resistance to bar edge rounding is even more important than the resistance to bar wear when maintenance of fibre quality properties is the goal. The various plate material families have significantly different resistance to bar edge wear. Many wear with severe rounding or sloped typed wear. Figure 4 shows the loss in bar area when comparing used plates from various alloys. A higher number means more of the original shape is missing. From this figure you can see that the low carbon stainless steels, while having the lowest bar wear resistance, resist bar edge rounding the best.
Fig. 5. Bar Edge Rounding Factor for Various Alloy Families (4)
To further illustrate this point the following photomicrographs show the differences in bar edge wear for the various refiner filling materials. The straight lines illustrate the shape of the bar when the plates are new.
Fig. 6. Ni-Hard Bar Edge Rounding
Fig. 7. High Chrome Iron Bar Edge Rounding
Fig. 8. High Carbon Stainless Steel Bar Edge Rounding
Fig. 9. Low Carbon Stainless Steel Bar Edge Rounding
There is a vast difference in the amount of material lost on the leading bar edge for these materials. This can have a significant impact on pulp quality over time as the refiner fillings wear.
The results from laboratory trials utilizing a 24" diameter double disk refiner are shown in Figures 10-13. Unworn plates and plates with the bar edges purposely rounded to match the types of wear seen in used plates. In all cases, the rounded plates produced pulp of inferior quality and took more energy to do so.
Fig. 10. Rounded vs. Sharp Edges, impact on energy.
Rounded edges required more energy to attain a given freeness and as energy input increased the rounded edge trial levelled off with no further increase in refining.
Fig. 11. Rounded vs. Sharp Edges, impact on Tensile.
Rounded edges resulted in poorer tensile strength development and again the development curve flattened out rapidly as the refining efficiency dropped.
Fig. 12. Rounded vs. Sharp Edges, impact on Burst.
Impact on Burst was more pronounced with the rounded edges producing an inferior pulp quality.
Fig. 13. Rounded vs. Sharp Bar Edges, impact on Tear.
Runnability is not only related to the ability of the product to run through a process but also to the extent of downtime due to wear and damage. Refiner parameters such as no load can be used to measure wear and component damage. The correct flow through a refiner can influence wear and consequent downtime. Wear due to inclusion of tramp material such as baling wire and other contraries, even plastic can lead to bar damage and plugging.
The importance of material selection in maintaining pulp quality is often overlooked when choosing the alloy to be run. Too often, filling life is the sole criteria and the harder, more abrasion resistant alloys are selected despite their negative impact on pulp quality from bar edge rounding. Poor development of paper properties especially strength because of wear will lead to poor runnability in papermaking and subsequent conversion.
1. "Refining Technology" Pira International Ed. C.F.Baker page 122.
2. (TAPPI Stock preparation short course Atlanta April 1999)
3. Energy Efficient Refining of Papermaking Stock ETSU best practice guide 114 C.F.Baker
4. (P.Dean, "Improved refiner filling design" Pira International refining conference, Fiuggi Italy, March 1997)
5. (Tom Berger, "The impact of plate material" Pira International refining conference, Fiuggi Italy, March 1997)