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Diverse grades demand stringent quality control

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Diverse grades demand stringent quality control

February 26, 2012 - 14:00

BRUSSELS, Feb. 27, 2012 (RISI) -In Part I of a two part story, journalist and engineer Mark Williamson looks at Port Townsend Paper in Washington State, US and the way it manages quality control issues across a diverse range of products.

Versatility is a word that comes to mind when describing the product portfolio of Port Townsend Paper, located on the shores of Puget Sound in Washington State. The 75-year-old company has carved out a specialized niche market for its kraft paper converting products in addition to the containerboard grades it produces for its sister company, Crown Packaging in British Columbia.

All of these diverse grades with many end users are produced on PM 2, a 240-in. trim single-ply fourdrinier machine. But versatility also means a lot of grade changes and transition periods, so it is vital that customer quality is spot-on every time a specific grade is run and the recurrent changes in grade specs are made as quickly as possible. Flat CD profiles and excellent roll structure are near the top of the quality list.

PM 2's quality and profile control capability had not been updated since the early to mid-1990s when slice control was incorporated in the then-new Valmet headbox and CD moisturizing and induction calender control profilers were added. The Valmet QCS and DCS dated from that time period as well. So the mill management realized that improvements had to be made to maintain a competitive edge and meet today's more exacting quality control standards.

Port Townsend Paper can produce 220,000 tons/yr of containerboard and specialty kraft papers

Consistent product, top priority

Dave Harke, paper mill production manager, describes the goals with renewed quality control equipment: "We have to make sure we are delivering a consistent product to our customers who have diverse needs and variable roll widths, some of which are very narrow. For instance, our gumming tape applications are coated and cut to 2.5- to 3-in. widths, and that demands a flat profile. Also, our containerboard end users need a sheet which is free from twist-warp. We had to resolve our profile issues and invest in the equipment needed to make our quality better."

During 2010, the mill installed new Metso quality control upgrades in several steps: The older slice control equipment was upgraded with new electronic controllers with faster and more precise control capabilities, a new higher resolution Paper IQ Select QCS system replaced the old Valmet unit, and new multi-variable MD and CD controls and closed-loop automatic grade change controls were implemented. The final addition - and one that draws the most attention - was a new IQSteamPro moisture profiler which replaces an older steam shower. The profiles are now so flat that the previous moisturizing profiler has been turned off.

Now that the quality control renewal project has been completed, Harke sums up the results: "The improvement in profiles has been dramatic. The reels are beautiful and you can see there are no ridges. With better profiles the average moisture level has been raised and we meet today's industry standards across the board. Machine productivity is up as well, and sometimes PM 2 outruns the pulp mill."

Seeing is believing. The reel structure on PM 2 is remarkably better than before reports Dave Harke (left). “There are no more ridges.” Tim Reandeau (right) says the new steam profiler has impressed him

Upgraded measurements and controls

To base new controls on more precise measurements and to make system servicing more effective, the old QCS system was replaced with a Metso PaperIQ Select system with basis weight, moisture and caliper sensors. The old scanner was coming to the end of its lifecycle and spare parts were becoming scarce. The new system features higher profile resolution required for better control, and there has been a significant increase in scan speed to improve control response. All machine direction controls are now based on a multi-variable model which coordinates wet end and drying controls to achieve better machine stability especially during upset conditions. In addition, a new closed-loop automatic grade change control was implemented.

The new PaperIQ Select system features higher profile resolution required for better control. There has been a significant increase in scan speed to improve control response

The first task for better profile control was to establish a flat basis weight profile. Fast and precise positioning of the slice lip is extremely important on multi-grade machines like PM 2 where grade changes are made frequently and the settling time required to get on quality must be as short as possible. The older motorized actuators and gearboxes were kept in place, however new IQSlice electronic controllers replaced the Jetmatic MT controllers. The new controllers use a continuous, variable speed and adaptive control strategy which approaches the target position quickly while simultaneously moving and measuring slice position without overshooting the target position. The positioning resolution has been improved to 0.01%. The new electronic controllers feature much better reliability and on-line diagnostics to detect and correct a problem quickly.

Metso-gathered statistics indicate the CD oven dry weight profile 2-sigma variability was reduced to less than 1%, representing a dramatic improvement from the older slice control system. As an indication of operator acceptance, the control usage is near 100%.

CD caliper control was also re-commissioned. The existing induction profiler controls nip pressure in a two-roll calender.

The steam profiler (circled) is located on the suction roll of a triple-nip press

Part II of this story can be readhere.

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