New quality control systems (QCS) in both paper machines at Bäckhammar paper mill have drastically improved grammage and moisture profiles while reducing profile variations.
The Bäckhammar Mill in Sweden is the biggest pulp and paper mill in the Nordic Paper Group and has a capacity of 230,000 tonnes of unbleached kraft pulp and 160,000 tonnes of unbleached kraft papers. The mill has two paper machines, PM 4 and PM 5. Their QCS systems were old and had to be replaced.
PM 5 is a Fourdrinier machine with a capacity of 110,000 tonnes of sack kraft paper in grammages between 58 and 130 g/m2. In order to give maximum stretch properties to the sack paper the press section consists of five drying cylinder groups, one expanda and an air dryer section. Since 2001 the machine has a Valmet dilution headbox to control the grammage profile in the cross direction.
“The existing quality control system was installed long ago and had come to the end of its lifetime so it was a necessity to replace it,” says Thomas Skoglund, Project Manager for the recent QCS upgrades at Bäckhammar Mill. “The system was old and we couldn’t get spare parts anymore. The cross direction profiles were not up to standard so we had to get a new quality control system on PM 5.”
“The installation of a new system had to be done during our annual maintenance stop in September 2016,” says Skoglund. “We therefore had meetings and discussions with potential suppliers in order to find a system that would suit us best. Apart from price and the support our target, and hence demand, was to achieve at least a 20 % reduction of the 2 sigma variations of the oven dry weight profiles. Having evaluated the offers from the different suppliers our choice was a QCS system from Valmet. This meant that a OPC link would take care of the communication between Valmet’s QCS system and the existing DCS system from another supplier. The OPC link later proved to be a substandard solution and is therefore scheduled to be replaced by a modbus link in the future.”
The project comprised of upgrading of the headbox’ dilution controls, an IQ2015 Scanner measuring grammage, moisture and porosity, a moisture sensor at a fixed point before the expanda unit, MD and grade change controls as well as communication links between IQ and the existing DCS system.
PM 4 is also a Fourdrinier machine and produces unbleached MG kraft paper in grammages between 38 and 120 g/m2 with a capacity of 50,000 tonnes. The MG kraft papers from Bäckhammar are e.g. used as bags and wrapping papers. The headbox has a slice control of the grammage weight profile. The drying section has a few drying cylinders as well as a Yankee cylinder.
“Apart from a new quality control system for PM 4, we also needed to replace the old slice controls of the headbox,” Skoglund continues. “Again we evaluated offers from different potential suppliers and also this time we decided to go for a QCS system from Valmet. The reasons for this decision was that we had chosen Valmet as QCS supplier for PM5 and equally important that they could install a new upper lip and polish the under lip of the headbox.”
“The installation of the slice control and the QCS system took place during our maintenance stop in September 2017. The whole project comprised an IQ with cross direction control, one IQ one side scanner in the dryer section before the Yankee cylinder, an IQ 2015 Scanner measuring grammage, moisture and porosity, MD and grade change controls as well as communication links (modbus) between IQ and the existing DCS system.”
Neither of the paper machines has a separate moisture profile control. The control of the grammage and moisture profiles is done by combining the measured values for grammage and moisture into control values for the grammage profile. Depending on quality the measured values for grammage and moisture profiles are given different weights in order to achieve the best possible profiles of both properties. The moisture level before the expanda in PM 5 and MG cylinder in PM 4 are achieved by the way the dryer section is run.
“The new slice control and QCS system resulted in substantially better cross-direction profiles.”
Porosity is an import sack paper property and measurement of the porosity profiles is therefore incorporated in both IQ Scanners to make sure that production is run to porosity targets. The control of the porosity level is done by letting the vacuum level in the couch roll decide how the LC refining is done.
"The new slice control and the QCS system from Valmet did also on PM 4 result in substantially better cross direction profiles,” says Skoglund. ”Our target was an improvement of at least 20 % reduction of the cross profile variations measured as 2 sigma, which was exceeded with margin on some qualities and missed barely on others. All in all, we now have a better quality situation for both machines.”
“Talking about how the QCS projects were run, the following can be said,” Skoglund continues. “Looking back, the time for preparation and purchasing of the PM 5 QCS system was too short, which caused problems during the start-up and the period thereafter. In our opinion, it took too long time to correct the problems, and Valmet could have been more responsive and on the ball in that process.”
“We were much better prepared for the PM 4 project and could be very clear with our demands and expectations to the potential suppliers. This contributed to a smoother project with fewer problems during start-up and the period thereafter.”
“Today we have a service agreement with Valmet allowing 80 hours per year to be used for both machines,” says Ulf Henriksson, Automation Manager at Bäckhammar Mill. “The cooperation works well between Valmet and our staff. Valmet can connect to our systems and solve most of the problems which saves a lot of unnecessary emergency travelling. We have had no significant problems with the hardware and the problems we have had have been on the software side.”
“On PM 5 it was decided not to have a one side scanner in the dryer section before the expanda but a fixed moisture measurement instead,” Skoglund continues. “On PM 4, on the other hand, we chose an IQ one sided scanner positioned right before the Yankee cylinder. It is a tough environment with high temperature and humidity which has put the scanner to the test. Occasionally the scanner doesn’t move evenly under the paper web but runs a bit erratically. A sealing belt will be changed which will solve the problem, as it is important to control the moisture level before the Yankee in order to achieve good runnability.”
“To sum up both QCS projects, my opinion is that Valmet was the best choice for us, even if they were not our previous supplier. It has been some problems during the projects but almost all of them are solved and solutions of the few remaining ones are soon in place. Regarding the PM 4 project it was definitely an advantage that Valmet both could do the necessary technical upgrading of the headbox as well as install the new QCS system”, Skoglund continues.
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Article published in Forward magazine 2/2019.