Wear of vital machine components is a significant issue for the pulp and paper industry. It results in added costs for repairs, replacement and ongoing maintenance, as well as the cost of lost production. Martin Kirchgassner, Vice President and Chief Technology Officer of Castolin Eutectic, explains how preventive maintenance and repairs based on advanced wearfacing technology can extend the life of critical machinery.
Pulp and paper machinery is subject to various different types of wear such as abrasion, impact, erosion, fatigue, corrosion, heat and combinations of these. Since 1906, Castolin Eutectic has been developing and applying innovative wear solutions for all aspects of the process from the woodyard through to mill services. This approach is integral to its philosophy of Pioneering Industrial Sustainability, since when critical equipment functions optimally, it consumes less energy. Therefore, wear protection can not only help pulp and paper operators to boost productivity, it can also help improve energy efficiency and reduce their CO2 footprint.
This article outlines the practical advantages of wear protection in key processes throughout the pulp and paper production process.
Woodyard conveyor system
The conveyor chain sprocket is a key element in the woodyard conveyor system. The teeth on the sprocket are subjected to high levels of wear from loose bark, dirt, and sand. Conveyor downtime is very costly as chip production is stopped. Building up the teeth with two different layers of welding wire, a base layer selected for impact resistance while top layer provides abrasion resistance, will reduce wear significantly. However, it is important to ensure that the overlay is softer than the chain to avoid wear on the chain. This solution can extend the wear life of the chain sprocket by 200%.
The links of the chain itself wear smooth on top from abrasion which causes log slippage on the conveyor. It is much worse in the winter when there is frost and ice on the logs. This can be addressed by coating each link with a fusible alloy that blends coarsely-sized tungsten carbide and nickel-base alloy powder. The result is a hard, wear-resistant coating that is ideal when a gripping, non-skid surface is required. This solution is known to last for over five years with no sign of wear or lack of traction.
Log debarking is necessary to ensure that the wood chips will be free of bark and dirt. Several types of mechanical debarkers are used as well as hydraulic debarkers. The ring debarker is used on individual logs, usually in the woodyard , as it is portable. The drum debarker is the most common type of mechanical debarker and is available in a number of design variations.
The arms of the ring debarker suffer severe abrasive wear under high pressure. Previous solutions to improve wear life used carbide-brazed segments but proved unreliable, with lifetimes as short as only three days. A high resistance to cutting under pressure was achieved by applying two layers of protective Castolin Eutectic welding with a final hard coating. A reliable service life of three months is now possible.
The lumber and paper industries use the bark and wood waste from debarking in several ways. Lumber mills will utilize the material as a fuel to produce steam for operating kilns to dry green lumber. Others set up bark mulching plants to process and package the waste to be used for horticulture as well as a filler in building board. The paper mills process the bark and use it in bark- burning boilers for mill steam. Bark preparation uses hammermills, conveyors, feed screws, and fans. All of this equipment is subject to wear from the abrasive action of the bark and wood waste.
Some sites use hog shredders, where the hammer tips take most of the wear from shredding bark and wood. Foreign material like nails, wire etc. cause additional wear. Rebuilding the tips with welding and coating with powder can increase tip life by over 100%
Other installations with bark crushing machines experience rapid wear on the ejection strip installed at the exit. It wears quickly due to the high abrasion and erosion. In one case, replacing the original plate manufactured from wear-resistant steel with a CDP® (CastoDur Diamond Plate) solution based on composite bi-metallic materials has increased the service life by 200%.
After debarking the logs are reduced to chips suitable for the subsequent pulping operations. Several designs of chippers are used, the most common being the flywheel-type disc with a series of blades mounted radially along the face. Chippers operate at high duty cycles and are subject to high abrasive wear from the logs.
The primary disc chipper in the woodyard requires a very thorough and efficient maintenance program because of the extended operating cycles. Major wear is experienced on the chipper knives and the area immediately behind them. Sliding friction and abrasion from the logs wears down the bed knife face. This results in a larger wood chip size which requires rechipping and therefore higher energy consumption. Most mills use rebuilt chipper parts from the chipper manufacturer or an independent workshop.
At one site, issues were encountered with the chipper log-feeding spout that both guides and holds the logs during the cutting operation. The original had been factory-coated with cobalt alloy but had to be scrapped after one year of operation. Now with a preventive weld overlay coating, the service life has increased to three years. A “softer” alloy was selected because the operator was concerned that a hard brittle coating could break off and enter the chipper.
Chip transfer and cyclones
Within mills, chips are commonly transported either pneumatically within pipes or on conveyor belts. Other systems such as chain and screw conveyors are also used to move chips, but usually for relatively short distances.
In one case, strong erosion was found on a lifting tube situated on the exit of the chipper to the cyclone. Although wear-resistant steel had been used, a customer had found the unit failed prematurely. A replacement was manufactured from new using CDP wear plate. The outer side
edges were reinforced with an iron profile and welded with a specially selected wire. The internal weld seams were also hard surfaced with welding wire. The result was a 400% increase in service life.
Fan rotors also suffer from severe erosive wear due to the mixture of wood, sand and dust in the air stream. This results in costly down-time for each repair, as it has to be taken apart with a crane. For one repair, where the wear was found to be very localized, tailor-made CDP wearplate strips were cut and formed to suit. These were then quickly and easily welded to the fan. The joints were overlaid with protective welding. After two years, no signs of wear had occurred, confirming a very significant increase in the fan’s working life.
Considerable erosion wear also occurs in cyclones which separate air from the wood and sawdust. To address this at one site, a complete cyclone was manufactured from CDP wear plate with parts plasma-cut and formed to the small diameter. The joints were welded with specially- selected electrodes and the weld seam protected with a welded overlay coating. The result was a service life increased by five times over the original manufactured in wear-resistant steel.
Screening and storage
Chips of acceptable size must be isolated from undersized (fines) and oversized pieces by passing the chips over a series of screens. The oversized chips are rejected to a conveyor, which carries them to some type of rechipper for reduction to a smaller size. In recent years, chip thickness has come to be recognized as an important pulping variable and modern disc-type or roll-type screens, which segregate according to thickness, are now widely accepted as the industry standard. This means that wear must be minimized in order to maintain the accuracy of the screening operation.
A chip screen separator has disks with points that wear from chip abrasion. Excessive wear would allow large chips to pass through to the digester. To rectify one worn unit, the surfaces of the points were ground and powder applied by a spray fusing process. This solution allowed an in-situ repair to be carried out, with no need to take the unit apart, remove the screen disk and replace it. Major costs savings resulted since downtime was reduced and there was no need to purchase a replacement part.
Pulping refers to any process by which wood is reduced to a fibrous mass. It can be accomplished mechanically, thermally, chemically, or by combinations of these treatments.
The discharge elbow of batch digesters is subjected to severe corrosion, erosion, abrasion and thermal shock from cooking liquor, pulp and steam. In one example, worn areas in the elbow and piping were built up with welding electrodes developed for critically stressed areas. The faces of flanges were also overlaid in a similar way and machined to finish. This enabled wear life to be extended three times over that of the original elbow.
The top screw transports wood chips through the liquor, moving within a sieve with a clearance of 0.5 mm). Abrasion causes wear of the working edge of the screw. This can be repaired by rebuilding the base metal with a specially selected welding wire and machining it to the correct dimensions. A final protective powder coating is applied by the spray fusing process. The substantial increase in operating lifetime has resulted in this approach being adopted as the recommended repair procedure by the OEM.
The cooked pulp from the digesters must be washed to remove residual liquor that would contaminate the pulp during subsequent processing steps and recover the maximum amount of spent chemicals with minimum dilution.
Corrosion and metal fatigue cause the washer screens to break and crack. Previously, this component would have to be scrapped, with the associated dismantling and replacement costs. As an alternative, using a brazing rod with high silver content gives a quick and strong repair with its high tensile strength and low bonding temperature.
Modern bleaching is achieved through a continuous sequence of process stages utilizing different chemicals and conditions in each stage, usually with washing between stages. Washers and pumps in the bleaching operation must be made from high-alloy stainless steels, nickel alloys and titanium. Controlling chemical corrosion in the bleaching equipment is an ongoing problem.
A particular challenge is the refiner disk that suffers from stress corrosion cracking and wear at the transition area between the disk and cylinder. This can be repaired by sandblasting and then arc-spraying with a coating based on the nickel-chromium alloy 625. The coating can be machined and ground to the required dimensions and surface finish. The treatment is fast to apply and reduces the dangers of further stress corrosion cracking, without causing distortion to the disk.
In most pulp and paper processes, some type of stock screening operation is required to remove oversized, troublesome and unwanted particles from good papermaking fibers. The major types of stock screens are vibratory, gravity centrifugal, and pressure (centrifugal or centripetal). They all depend on some form of perforated barrier to pass acceptable fibers and reject the unwanted material.
Further preparation of the pulp stock is refining of the fibers. Two major types of continuous refiners are used for stock preparation: disc refiners and conical refiners.
Efficient movement of stock through the various processing steps is at the heart of a pulp and paper mill operation. No mill can operate successfully without reliable pumping units.
One challenge is found on the refiner shaft, where the stainless steel sleeve is worn by fretting from the collar bush that holds it in place, this wear is accelerated by corrosion. This can be solved by spraying the shaft with a torch-fused powder developed to prevent corrosion and reduce friction. The lifetime can be increased by 300% due to the superior hardness, corrosion resistance and sliding properties of the coating.
In another case, a pump housing suffered from numerous surface cracks and cavities from the erosion and corrosion environment. A cold repair with a composite polymer gave a quick, simple repair and a smooth, corrosion and wear resistant coating with no modifications to the base metal. This had the advantages of speed, low coating thickness and smoothness and requires less skill to apply than the previous brazing rod solution.
Scrap paper and cardboard
Many pulp and paper mills use recycled paper and cardboard as another source for papermaking pulp. Some mills operate with only recycled materials. The scrap is fed into a hydrapulper, which is a
vertical tub with a revolving rotor in the bottom. Paper and cardboard with water are shredded by the rotor into pulp and a perforated extraction plate under the rotor pulls the fibers out of the tub. Further cleaning and de-flaking of the pulp prepares it for papermaking. The rotor in the hydrapulper is subjected to high wear from the wastepaper as well as foreign materials such as stones and scrap metal.
In one case repair was required on the impeller at the bottom of the tank. The production capacity decreases dramatically when the impeller is worn out. For the repair, critical areas were protected by welding with electrodes developed to offer combined protection against abrasion, pressure and moderate impact. In addition to increasing the lifetime from 2 weeks to 5 months, the welding can be carried out three times to further reduce costs.
In the paper-making process the flowspreader takes the incoming pipeline stock flow and distributes it evenly across the machine. The pressurized headbox discharges a uniform jet onto an endless moving fabric that forms the fibers into a continuous matted web while the table drains the water by suction force.
The sheet is conveyed through a series of roll presses where additional water is removed and the web structure is consolidated. Most of the remaining water is evaporated and fiber to fiber bonds are developed as the paper contacts a series of steam-heated cylinders in the dryer section.
The sheet is calendered through a series of roll nips to reduce thickness and smooth the surface. The dried, calendered sheet is wound onto a reel. A properly designed and installed vacuum system is a vital component in a successful paper machine system.
An important area for repair is the rewinder roll that must have a gripping surface. Once wear occurs on the surface, it becomes smooth and the paper starts to slip. This affects the paper quality and accuracy. The worn, polished surface can be restored with thermal powder spraying. As well as producing a very good gripping surface, this coating increases the wear life by over 200%.
On one vacuum pump rotor, wear was found on the impeller, shaft sleeves and bearings, combined with erosion and cavitation wear to the cast iron body. The overhaul involved a combination of welding electrodes to repair the cracked cast iron body, followed by the application of a composite polymer coating to the exposed corrosion surfaces. The bronze impeller diameters were reclaimed using bronze welding wire. Finally, the soft packing sleeves and bearings were thermal sprayed with an alloy powder. This in-house solution was a low-cost alternative to replacement or external refurbishment.
Mill services provide the support processes for the pulp and paper mill. The major areas are chemical recovery, lime kiln, power generation, and water and waste treatment. Each area is essential to the mill operation and requires a very active maintenance program. The chemical recovery boiler and lime kiln are high maintenance areas and require joining and wear protection solutions. Thermal cycle-induced cracking in the lime kiln and corrosion of tubes in the chemical recovery boiler are typical problems.
Reliable electrical power requires maintenance of steam and water turbines as well as the generators. Process control valves and pumps in power generation are constantly being rebuilt.
Water and waste processing involves large centrifugal pumps, intake screens, vacuum filters, aeration pumps and piping. All must be maintained for continuous operation 24 hours a day.
One of the major challenges in the manufacturing of pulp has always been the maintenance and operating efficiency of the lime kilns in the causticizing plant. These are large, round, fabricated steel tubes, of 4-6 meters in diameter, and between 45-150 meters long. Since they are lined with fire-resistant refractory brick, constantly rotating on rollers (spaced approximately every 5 meters), and heated internally to 1450°C, they are susceptible to serious operating and maintenance problems. The longer the kiln, the greater these problems can be, especially with respect to cracking and misalignment of the rollers.
The kiln shell, however, does not run directly on the rollers. Around the kiln at each set of rollers, there is a large cast-steel tire mounted around the kiln shell. The tire is not actually attached to the kiln shell. It is mounted on pads that are located around the shell and fixed in position by steel blocks, which are welded to the kiln shell. The tire rotates with the kiln on the rollers. The kiln is rotated by a drive pinion that drives the kiln girth wheel. The lime mud, as it travels slowly down the kiln, is converted to pellets in the hot zone and continues to travel to the end of the kiln, where it eventually falls out over the nose casting into the lime coolers.
In one case, a year after five cracks in a kiln shell had been repaired using a non-Castolin Eutectic procedure, a crack of 4.7 m occurred, emanating from the heat affected zone (HAZ) of the previous weld. After determining the exact geometry and full extent of the crack, the repair was made with a low-heat input, high nickel-based alloy electrode, following a well-defined procedure. This repair held for six years before the shell was eventually replaced.
On a kiln tire, high pressures and contact stresses caused surface cracking and the detachment of a large splinter. The excessive cost of a replacement tire and a lengthy downtime period resulted in the need for the splinter to be welded back on to the tire. The traditional pre-heating was impractical due to the mass. A successful repair was made possible using low-heat welding electrodes and a tightly controlled procedure. Significant cost savings were achieved as no new part was required and the downtime was cut from 10 weeks to 2 weeks.
Because the kiln rollers support the weight of the kiln they are subjected to large and cyclic compressive stresses. Over time, this can give rise to loss of material from the rolling surface in the form of “pitting” or splinters. Welding repair is difficult to perform in place due to the high stresses from the kiln weight and the need to maintain the roller’s exact circular dimensions. Welding with low-heat electrodes enables a successful repair to be performed in-situ, with the associated substantial saving in downtime costs.
Repair and refurbishment often achieves better results than replacement
The first instinct of maintenance teams in the pulp and paper industry is often to replace worn parts with brand new items. However, in this industry, continuity of operation is usually the primary concern. Therefore, repair and refurbishment can prove superior in many cases. As well as ensuring a faster return to action, using the latest wearfacing technology can also significantly improve operational life, often several times over. The result is greater reliability, improved sustainability and the best possible total cost of ownership.