Paper industry economics are forcing machine builders to take a bigger interest in rebuilds - and that includes the forming section

Mind the gap as forming develops even faster

Understandably, paper machine builders are beginning to make far greater efforts to develop rebuild solutions for their clients and potential customers than they have in the past. The same is true of the forming section, where all the major suppliers are casting around for potential rebuilds that will go some way to making up for the drop-off in new PMs coming on stream in the current global economic climate.

Johann Moser is the product manager for forming at Voith Sulzer's graphic papers division. "Of course we're thinking about rebuilds very carefully at the moment, especially since Asia is not going to recover as quick as we might like to think," he says. "Generally, the idea is to upgrade the forming and press sections to improve the quality, rather than to increase the production. There's so much paper out there in the market that quality is becoming the most important thing as people try to keep the customers they have, as well as finding new ones."

Figure 1: The drainage system in the forming area of the DuoFormer TQ

Voith Sulzer's DuoFormer TQ is the company's latest generation of forming technology and according to Moser, it has produced impressive results on an SC (supercalendered) machine rebuild at UPM-Kymmene's Jämsänkoski mill in Finland. The original CFD former design at the mill was recently converted to the new TQ design under an upgrade program developed in cooperation between the papermaker and the supplier.

Unfortunately for PPI, the mill is still in the process of debottlenecking the line ahead of the former, so there has been little opportunity to run under the operating conditions the new former has been designed for. Even so, the mill's technical director, Leo Lindross, said, "Particularly advantageous for us is the fact that we produce an excellent paper structure with the new former. This guarantees excellent printability with even less refined TMP (thermomechanical pulp) and permits significant energy savings."

Figure 2: The drainage principle on the DuoFormer TQ

"I'm very pleased with the installation, but I don't really want to talk too early," Moser says. "Right now, they're running at 1,300-1,320 m/min, but it's still limited in the drying section. The initial results do look very good though."

In the Duoformer TQ set-up, initial drainage starts at the suction forming roll, where the diameter of the roll has been extended. The roll is followed by the D-section, which consists of a top wire suction box, located in the top wire loop, with forming blades in the bottom wire loop. A wet suction box and flat suction box are placed after the D-section. Immediately beyond these boxes, the sheet travels over a couch roll with two vacuum chambers. The first provides additional drainage, while the second provides a safe transfer to the bottom conveying wire.

According to Voith Sulzer, the new former significantly expands the operating window of the CFD former design so that a range of SC-A and SC-B grades can now be produced without compromising on quality. Machine operation is more stable and grade changes smoother, which is particularly important for papermakers as they try to optimize runnability.

Business rebuilding

Beloit too, is hoping to boost profits in the rebuild business. According to the US supplier, there are a number of very cost-effective upgrades that can be made to older forming sections to improve performance and output. The simplest project would probably be if the mill's former was an early Bel Baie II or III. Simply converting to a Bel Baie IV would provide significant improvements.

As Beloit's technical team explains, such an upgrade places the forming shoe in the #1 fabric, resulting in sidedness uniformity not possible on previous Bel Baie configurations. Although a headbox change is not mandatory, it is recommended. With the forming shoe in the #1 fabric, the headbox jet trajectory must be reversed (two o'clock instead of ten o'clock, or vice versa) and even though the headbox can be rotated by 180 degrees, it is usually impractical (often impossible) to do so because of plumbing restrictions. If a headbox change is made, it should be a Concept IV-MH with consistency profiling (CP). This has the added benefits of a uniform sheet tending side to back side and provides good CD basis weight profile control, the company says.

For a generic gap former, the group recommends replacing the second couch with a plain roll and a high vacuum flatbox. Drive power increases slightly, but the significant difference is a large decrease in vacuum power. This results in a lower power requirement overall, while eliminating a suction roll and its maintenance requirements. Sheet dryness leaving the former will be the same or marginally improved.

Many Bel Bond formers can be converted to high capacity units (literally doubling the capacity) by replacing the dewatering elements. This leads to a wet end that is more productive and can have significant sheet quality gains if combined with a new headbox as per above.

Beloit's technical team also points out that if the mill wants to add a top wire high capacity unit to a fourdrinier operation, a High Capacity Bel Bond CB can be added within
2 m of a new Concept IV-MH headbox. Sheet quality results will be superior, according to the machine builder, and the addition of counter blades means that sheet formation can be optimized for all basis weights.

One point that the Beloit, Valmet and Voith Sulzer experts all agree on is that clean wires are a prerequisite for good runnability and to eliminate dead spots and fiber contamination. Among the many systems on offer are Beloit's Air Wedge (this utilizes a soaking shower and a blast of air to cleanse the fabric of water, contaminants and sheet fines as it transfers the sheet) and Voith Sulzer's JetCleaner and DuoCleaner devices.

Such systems undoubtedly help improve the efficiency of the process and the quality of the paper, but as Beloit's team suggests, there are several other ways in which mills can improve the maintenance element in the forming section. The company points out that the most ignored item on today's paper machines are doctors and the most critical of these is the breast roll doctor.

"If the blade of that doctor doesn't contact the roll surface uniformly across the width of the machine, the whitewater extracted from the headbox jet will adhere to the roll surface, creating air and/or water spotting on the finished sheet as the extracted whitewater contacts the underside of the headbox jet on its return," Beloit's experts explain. "The breast roll doctor requires a conforming holder and a reliable oscillator to maintain consistent sheet quality and optimum performance between scheduled maintenance outages."

According to the group, another area for attention is showers. The team believes that the use of overly rich whitewater for forming section showers has created cleanliness conditions that can only lead to frequent and excessive downtime. Closing whitewater systems to reduce fresh water volumes without due regard to selecting the leanest whitewater streams to send to an efficient bank of cleaners has resulted in unacceptably high fines/dirt accumulation in many forming sections.

Optical view

Valmet has been very active developing its forming technology in recent years, especially in light of the company's development of its Opti concept paper machine. The OptiFormer has been created by applying a modular construction to the forming zone. To facilitate customization of the former, the OptiFormer has two different forming modules - a loadable blade and a multi-foil shoe module. The two are similar in several ways, including the fact that the initial drainage on both is carried out via the forming roll with constant drainage pressure. But the systems do vary significantly from a process point of view.

Figure 3: The OptiFormer with loadable blades

The paper grade determines the wrap angle used, but the main difference is to be found in the blade shoe modules. The loadable blade shoe module has a series of stationary blades on the outer fabric side, while on the opposite side of the sheet adjustable loaded blades are located between the stationary blades. This creates very strong drainage pulses with high shear forces, which creates a great potential for improving the formation, Valmet explains. The multi-foil module is a curved shoe with fixed ceramic blades divided into chambers with a vacuum applied. This configuration generates much softer drainage pulses.

In general, Valmet recommends the loadable blade model for woodfree and most newsprint grades, while the multi-foil would provide a better solution for the majority of SC grades and LWC (lightweight coated) base paper. But the furnish and the desired grade should be determined on a mill specific basis.

According to Valmet, one of the OptiFormer's main strength is that it can be customized, while the modular approach to the construction of the forming zone helps ensure that paper quality is maintained no matter what grade is being produced. The machine builder also says that the unit has more than enough drainage capacity to run at speeds in excess of 2,000 m/min. And not forgetting current business trends, the company also points out that the Valmet SpeedFormer can be rebuilt to a OptiFormer concept with a loadable blade module to improve formation and efficiency results.

Looking ahead

The challenges facing paper machine engineers over the coming years are formidable. Increasing PM speeds are putting a great deal of strain on the process in a number of areas, but according to many people, there is one particular issue that is attracting much interest in the R&D departments of the big machine builders. As Moser explains, "Of course, there are still some bottlenecks to be sorted out, but the main thing is the "free" jet from the jet of the nozzle going into the gap. If we can get rid of that we can solve a lot of formation problems and reduce cockling."

Beloit too, has been working towards the elimination of the free headbox jet. As the company points out, the jet has been the source of many operational and sheet quality problems as machine widths and speeds have increased. And that is the case whether the jet is impinging on a fourdrinier forming board, matching the wire wedge of a twin wire gap former or landing on the fabric/suction roll interface of the more forgiving roll/blade former. But extending the jet "into" the former will result in the same dilemma as the disappearance of the fourdrinier dry line with the advent of top wire forming. The visual assessment of headbox jet behavior (one of the last remaining on-machine papermaking tools available to the boss-machine tender) will be gone. Visual assessments will be replaced by computerized inputs, which will be manipulated to plot the headbox/jet forming zone relationship.

Given the very rapid development of roll and blade gap forming technology over the past decade or so since it was first introduced on newsprint PMs though, it is difficult to see this problem being an insurmountable one. So it looks as though PM speeds will be hitting ever heights in the near future.