Stora Enso’s Kvarnsveden mill has installed closed loop controls that coordinate primary and secondary arm movements during reel turn-ups, and provide even nip-loading and sheet tension during the reel building process. Previous winder break problems are eliminated.
Several centimeters of paper left on a parent reel spool is a common sight in many paper mills. These “left-on-spool” or “bottom waste” losses, which cannot be wound because of tension control problems and crepe wrinkles during the initial phases of reel building, may add up to several percent points of lost production. These chronic losses are common on older paper machines with imprecise pneumatic control of the loading, positioning, and coordination of the primary and secondary arms. But the Stora Enso mill in Kvarnsveden, Sweden encountered an acute problem when three of their newsprint machines started production of value-added grades containing fillers. In many cases, whole sets could not be wound because of density control problems in the first set near the spool.
To solve this problem, the mill installed three reel density optimization systems (ROS) provided by Vishay Nobel of Karlskoga, Sweden. Building on its expertise in hydraulic force and position control systems, ROS was developed in a Swedish mill in 1996. Since then, 22 systems have been installed.
The systems at the Kvarnsveden mill have solved the previous reel density control problems and have allowed the machines to produce jumbo reels with the right density profile from the spool up to their maximum diameter. The mill can now produce jumbo reels with a full complement of sets with a minimum of left-on-spool losses.
Losing whole sets
PM9 was the first to be equipped with ROS. PM9 was started up in 1956 as a standard newsprint machine. Kjell Sundin, Maintenance Technician, says that when the machine started producing improved newsprint with clay filler content up to 5%, the winding problems became very apparent. “The sheet was wrinkling and we were experiencing a lot of breaks in the winder,” he states. The problem was so severe that the machine jumbo reels were often limited to two sets of 125-mm diameter rolls instead of the normal three sets.
PM8 had similar problems producing well-built, full sized jumbos. The original newsprint 5.48-meter trim machine was started up in 1931 and rebuilt in 1983 when it was converted to SC production. The machine is designed to make 2.6- meter diameter 3-set jumbo reels for offline supercalending. But wrinkles and rope marks (caused by lateral paper wandering) caused similar winding problems and many breaks. “We could lose whole sets. And often we could make only two sets on the jumbo reel, “says Sundin.
The problems on these two lines resulted in a typical scenario seen in many mills. If the winders are having break problems and falling behind on their winding schedules, full parent reels build up in inventory until the reel spools run out. Then the paper machine has to be shut down. The whole line efficiency is affected.
To rectify these problems, the mill installed ROS systems on PM9 and PM8 in 2001 and 2002 respectively. Old pneumatic positioning and loading systems for primary and secondary arm control were replaced with hydraulic systems required for the precise position and force controls provided by the ROS system. The reel drums on PM8 and PM9 were carbide-coated at the same time to reduce paper slippage against the reel drum and to avoid the rope marking encountered on PM8.
The mill’s largest, newest (1998 startup) newsprint machine, the 8.68-meter trim width PM11, was also encountering similar winding problems when it started running filled grades. The machine is designed to produced 4-set jumbo reels, but the first sets were difficult to wind when filler was added to the furnish. Based on the success of the preceding two installations, a ROS was installed on PM11 in 2005.
Closed loop control
The old pneumatic pressure-controlled systems for the primary and secondary arms were based on open loop controls; there was no feedback measurement of the exact position of the cylinders nor was there any indication of nip pressure during the reel buildup. Friction and stickiness in the mechanical systems also caused imprecise control over the initial phases of reel building. Moreover, the all-important transfer from primary to secondary arms was not controlled precisely and sheet tension could change dramatically. Uneven nip loading and uncoordinated primary-to-secondary arm transfer often created irregular web tension, stressing the sheet and causing crepe wrinkles. Several centimeters of paper on the bottom set had to be discarded.
In contrast, Vishay Nobel’s ROS solution is based on precise feedback control of arm position, relief cylinder position, and nip loading during the primary and secondary arm phases. By using feedback measurements from load and position transducers, any mechanical friction, stickiness, or hysteresis is taken into account. In fact, the measurements are used to diagnose mechanical problems with the reel mechanics. With these measurements, time delays between hydraulic actuation commands and actual movements are measured and accommodated in the control strategy.
The hydraulic position and nip loading system operates in a closed-loop mode, providing even and repeatable nip loading against the reel drum, smooth primary arm lowering, and a bumpless web tension transition during the transfer from primary to secondary arms. The position and nip load are controlled on both the tending side and the drive side, ensuring parallel arm lowering and avoiding any skewing of the sheet tension. With the even tension provided by this system, the paper sheet is not subjected to changing stresses that might cause crepe wrinkles.
Force and position measurement
For primary arm control, force transducers are installed in the overhead reel spool hook, to measure its downward force, and in the relief cylinder underneath the reel spool to measure the pressure of the reel drum against the accumulating paper on the reel spool. To achieve an accurate measurement of nip pressure, these transducers are located as close to the spool as possible. Position transducers indicate the relief cylinder position and the angular location of the primary arms as they move downward. The position transducers are embedded in the cylinder to keep out papermaking dust and ensure reliability.
Before the reel turn-up, the relief cylinders start moving towards the spool. Once contact is made, the system changes to force control. As the winding of paper progresses, the ROS system automatically makes adjustments to the relief cylinder to compensate for the weight of the paper in the building reel. During the precisely controlled arm lowering period, the force of the paper on the reel drum is kept constant. In the lowered position, the system compensates for the force change when the hook is released.
The ROS system smoothly controls the movement of the secondary arms to achieve a bumpless transfer of sheet tension and nip force from the primary to secondary arms. The system then continues to control the nip force and the reel density with the secondary arms to the outside of the reel. The secondary arms are also equipped with force transducers (drive side and tending side) near the reel spool and positions transducers in the arm positioning cylinders.
The system builds the reel according to a target density profile. The paper thickness and density are calculated using the continuous readings of the reel drum rotation speed via a tachometer and the angular rotational speed of the reel spool via an optical or magnetic trigger sensor. The spool rotation speed changes as the reel builds up. To make this measurement, each reel spool is marked with an optical reference or a magnetic reference is embedded. The thickness and density are calculated by an equation that includes a basis weight factor. These measurements are updated after every three layers of paper.
High speed of response
To avoid uneven web tension, the speed of response of the ROS system measurements and controls is extremely important. Anders Olsson, Vishay Nobel’s Systems and Sales Engineer, notes that crepe wrinkles can occur if the web tension changes significantly within 100 milliseconds. Typically, the cycle times f o r force and position measurements are 2 to 5 milliseconds, providing the fast response time needed for responsive control.
Olsson reports that control can start very soon after the reel turn-up. Typically, the reel density is on control after 200 to 300 meters of paper have been wound. At 1500 m/min, that is only about 8 to 12 seconds of production time.
The real-time progress of the reel density optimization controls is shown to the operators on a control room video monitor. The reel density machine-direction profiles are selected from a recipe table according to the paper grade being run. The system also reports reel length and diameter so the operators can build the reel to the needed dimensions to suit the winders’ set production requirements.
Winding problems solved
Kjell Sundin reports that the bottom set winding problems were solved on all three machines after the installation of the ROS system. “On PM9, it was possible to make three good sets with filler levels ranging from 10 to 15%. We were not nervous about it,” he says. Previously, they could not produce three sets with filler levels at 5%. PM9 has since been shut down and the ROS is being re-installed on PM10.
On PM8, parent reels to the winder are run down to about 2 to 3 cm of paper left on the reel spool. According to Sundin, that is a minimum amount of waste, considering the start-up losses on the supercalenders. The winding problems with filled grades on PM 11 were similarly solved. The filled grades on PM11 have been discontinued. The ROS is still proving its worth on the lighter grades (40 g/m2), which were difficult to wind.
Excellent results on other newsprint machines
The Kvarnsveden mill solved the reeling density and winding problems related to value-added newsprint grades containing fillers. Producers of standard unfilled newsprint have also seen the benefits of more precise and repeatable reel density control. Recently, a newsprint mill equipped five newsprint machines with ROS systems. The bottom waste has been reduced significantly, adding to saleable production. At the same time, reel snap-off breaks have been reduced or eliminated and the consistency of winding density and wound roll sheet area yield has been improved. Reprinted with the permission of the original publisher, Pulp & Paper International magazine.