March 1, 2007 By Pulp & Paper Canada
Cost reduction and cost containment are probably the most critical issues for the pulp and paper industry today. As a capital-intensive business, modern and up-to-date equipment is needed in order to …
Cost reduction and cost containment are probably the most critical issues for the pulp and paper industry today. As a capital-intensive business, modern and up-to-date equipment is needed in order to compete. However, many North American paper mills are facing tough challenges to improve the machine reliability and efficiency while reducing their costs.
Paper machine rolls and roll covers play key roles through new innovative solutions that help the mills become more reliable and cost competitive, thus offering the opportunities for operations to regain their competitive edge in the market place. Recent conferences organized presentations for ways to reduce costs for mills, with talks focusing on energy savings and one of the methods suggested included the correct use of roll covers to reduce steam cost and improve sheet quality in general.
“Although the covers and clothings are a very small part of the total paper production cost,” says Dr. Paul McCarten, Voith Paper’s vice president for roll cover applications, “by optimizing, you can really help yourself with overall costs.”
This is especially important in improving pressing performance and the effect that has on overall costs. “Typically in the dryer section, you might use 1.3 kilograms of steam for a kilogram of paper that you are producing,” says McCarten. “Just saving a little bit on steam usage can save significant quantities of money.”
A number of products have been developed to reduce steam usage and other energy costs. Voith, for instance, has developed a new generation of polyurethane press roll covers with superior abrasion and chemical resistance compared to the last generation.
“We have been able to get more open area design for suction rolls and press rolls, so we can improve de-watering in the press,” McCarten added. “The benefit of well-designed suction press roll covers is that they can reduce steam usage and this has been seen on many applications. Also, because of the toughness and chemical abrasion resistance of the covers, they can stay in the machine for good long runs as well.”
MACHINE EFFICIENCY IS ANOTHER IMPORTANT AREA.
“Changes in machine designs have reduced costs,” he adds. “A trend started a few years ago, where we have on-line coating and on-line calendaring. This was displayed in the most recent machine start up in Canada, making lightweight coated paper online. Cover quality is definitely very critical for these applications. The papermaker needs a cover with uniformity, durability and abrasion resistance.
“Having good cover wettability is critical for getting good paper quality, and being able to run at high speeds with minimal vibration is very important for film size press rolls,” says McCarten.
On the calender, developments have focused around new generations of epoxy covers. Mills are looking for abrasion resistance, which means longer running time between grinds, resistance to barring and resistance to orange peel.
“The challenge is getting good sheet quality over extended run times,” says McCarten. “Mills are looking for smoothness, printability, gloss, and brightness. Old technology, going back 50 to 60 years called for cotton-filled rolls on supercalenders. Cotton gives good gloss, but it doesn’t have the durability and it required more frequent roll changes.
More frequent roll change means extra labour costs, as well as lost machine time opportunities.
“All the suppliers have developed composite products, typically some type of fibre-reinforced epoxy,” he explained. “The challenge with these products is being able to produce the good sheet properties produced with cotton, and also get longer roll running intervals.”
THE POPULARITY OF SHOE PRESSES IS SPURRING RESEARCH AND DEVELOPMENTS FOR ROLL COVERS.
“Most of the new paper machines have a shoe press, so we are developing covers to meet the needs of the shoe press” said McCarten, “but on the other hand, the majority of the existing machines have suction rolls as a major press roll, so we are working on covers for suction and other soft press rolls.
“Suction roll is just one example,” he adds. “We are working on other roll positions as well, including tissue applications. One of the main targets is to develop covers that will improve the runnability of existing equipment in North America.”
INVESTMENT IN PAPERMAKING ROLLS IN NORTH AMERICA IS CRUCIAL.
“I would say lack of investment is a big concern,” says Jin Kim, Voith’s vice president of mechanical service and replacement rolls. “There is a lot of pressure on the paper companies to produce profits, so they are being pressured to keep the investment going. This is a vicious circle. If you don’t have modern equipment, you can’t be efficient, and then you fall behind.”
“The new paper machines are just amazing,” says McCarten. “The typical new machine width is 11 metres (433 inches) and has a running speed of more than 2,000 metres per-minute (6,560 feet per-minute). Compare that to a typical machine of the 1970s; this means, almost double the paper width and two to two-and-a-half times on the speed and that one new machine is equivalent to roughly four old machines.”
Kim says that newer machines have better automation controls and gauging systems.
“The mill can generally produce better sheet quality, and have more machine uptime,” he says. “On the paper machine roll side, there are quite a number of mechanical advancements taking place. If you compare the rolls from 30 years ago to today’s design, the machine builders have made a lot of progress.”
“We focus on reliability so they don’t break down as often,” says Kim. “Mills nowadays are looking for longer maintenance service intervals. In the old days, quite often mills had to take some of the major rolls out every six, nine or 12 months for a routine maintenance. That was typical industry practice. That’s not good enough, and then question becomes, can we double this time interval?
“On the material side, we are using much more carbon fibres than have been used in the past,” he adds. “It is more expensive, but it has very good properties and for some applications, the benefits are well worth the extra cost. Carbon fibre’s lighter weight and vibration characteristics are pretty attractive for certain roll applications along the paper machine.”
Several years ago, Voith introduced a wire section roll with centre-supported design, and using carbon fibre roll shell. Many new large machines will have a couple of these rolls in the wire section.
Energy efficiency is a key area of research, especially since rolls consume quite a bit of energy, pointed out Kim. “We have new innovations to reduce power consumption. For example, take a suction roll. The sealing strips create a lot of drag. We have a new design that can reduce energy consumption by up to10%. Now take a deflection compensated roll. There is some internal resistance generated by oil inside of the roll. This is especially the case for rolls running at high speeds,” he adds. “We have a new design that reduces oil layer thickness, which lowers the drive power requirement, and that translates to energy savings. We are also trying to reduce noise and vibration. In terms of lower noise level, it is important for safety and for creating a good working environment. Less vibrations means less wear and tear on the machine.”
MILL OWNERS CANNOT IGNORE THE FACT THAT MACHINERY IN NORTH AMERICA IS NOT BEING REPLACED OR UPGRADED.
“The majority of the machines in North America are 20 years old or older,” said Kim. “Only a small portion of the machines have been rebuilt or modernized. However, even when rebuilt in part, older machines are no match for new equipment. It’s like going to a race where you have a 10-year-old car and the competitor shows up with the latest Ferrari.
“A good many of these advanced technologies are available when you buy a new machine,” he added. “They come as the new standard eq
uipment. But, a brutal reality is that most of the mills don’t have capital funding to purchase a brand new machine. Often, it is difficult to get funding to do a major machine sectional rebuild.
“We see the customer’s needs for technology advancements for better quality, efficiency, and reliability and we know there is only small and limited funding available,” Kim explains. “We are focusing more and more on what we call engineering upgrades to rolls and we look at our latest machine designs. We try to borrow and adapt the ideas and apply them to our customer’s unique situations.”
ENGINEERING UPGRADES COME IN MANY VARIATIONS.
“We see a lot of paper machines having wet end sheet breaks caused by wire trim related problems,” says Kim. “Many times what happens is that the suction pick-up roll picks up the trim and the trim goes into press section. This can cause sheet breaks. The solution is to re-engineer the suction roll’s end deckle area. This is an excellent solution where the investment payback could be a matter of few months.”
Kim outlined a possible payback calculation. “Let’s say this mill’s typical sheet break is about 10 minutes and the papermachine’s opportunity cost is $8,000 per hour,” he said. “If the mill spent $20,000 to make engineering upgrades on this pick up roll improvement project and that helps to reduce two sheets breaks per-week; that translates to about a less than two month payback. That’s money well spent.”
Kim says that another common situation that occurs is that in many of the older generation rolls, the deckle width adjustment nut can cause many aggravations.
“When paper grade is changed and sheet deckle width needs to be adjusted, the machine operator needs to turn this deckle adjustment nut,” he explained. “The nut is internally connected to end deckle holders. As you turn the nut, the deckle inside of the suction roll moves and you adjust the vacuum width to the desired sheet width.
“The problem is that there is not a good way to gage how much the end deckle inside of the suction roll moved,” he explains. “A lot of times machine operators have to struggle with a couple of tools — a flashlight and a long, thin steel wire. You have to poke through the suction shell holes with the wire and try to feel where the deckle is. Often mills end up wasting valuable time struggling with this adjustment process. In some cases the mills end up with a broken deckle rod while attempting to make this adjustment.
“During this adjustment time, you have lost an opportunity to produce paper and you lose money,” he continued. “We can redesign the external adjustment nut by replacing that with a visual indicator that has graduation markings like a steel ruler. It’s a kind of WYSWYG — what you see is what you get. As you turn the nut on this new arrangement, you get an instantaneous feedback of the axial movement of the internal parts.”
“Some of the paper machines have been sped up from their original design,” says Kim. “For example, a machine had an original design speed of 1,800-feet per-minute. Over the years, the mill may have increased the machine speed and now it’s running at 3,000-feet per-minute. The mill may want to convert from grease lubrication to oil lubrication. We call it a ‘grease to oil’ conversion. In that kind of situation, we would review the bearing housing design, and then modify some of the housing parts to be able to accommodate oil lubrication.
“Sometimes we see cases where mills are having water getting into bearing housing,” he adds. “The net result is that water will damage the bearings, which can be expensive. On the flip side, we see cases where rolls may have oil leaking out of the bearing housing. Usually this means a messy condition around the machine, and in some cases, oil on the floor could cause safety concerns for people walking near the area. For either of these situations, we can evaluate the bearing housing design and apply a fix to it.”
There are a variety of mechanical modifications and engineering upgrades that can be applied to older machines.
“For a small cost, mill owners can get quite an enhancement,” says Kim. “Many of these have very attractive ROI, which is what the mills really need.”
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