Mill Optimization

Curiosity may have killed the cat, but it isn’t likely to do a sawmiller any harm. In fact, compared to many other process industries, sawmillers have traditionally lacked a healthy dose of curiosity when it comes to the products they make and the processes through which they are made. Yet modern technology, computing power, tight margins and new management philosophies are starting to change that.

A good place to witness many of these changes is Weldwood’s enormous HI-ATHA sawmill complex in Hinton, AB. The mill opened to much fanfare back in the fall of 1993 as a quality producer targeting value more than volume. Not that HI-ATHA falls short on the volume side – the two-line mill produced over 170 million bdft in its first full year of production, and is targeting over 240 million bdft in 2000. Much of this production is square edge premium quality lumber, with some 20% consistently sent to Japan.

HI-ATHA was originally conceived as a high-end commodity producer, combining production flexibility with state-of-the-art technology. But more than its technology and production goals were different from the outset. One of the original personalities behind the mill’s construction, and current vice-president Dennis Hawksworth, was convinced that the future of sawmilling had to include more precise and sophisticated process control tactics than had been the case in the past. As a result, HI-ATHA personnel have been aggressively pursuing emerging process control technology since Day One, according to current process control facilitator Neil Robson. Over the past few years, technology has co-operated with this spirit, landing the mill three leading-edge systems. These include:

• Saw Control system, a real-time bandsaw monitoring and control system implemented in 1998.

SPEED CONTROL

Also relatively new at HI-ATHA is the use of Saw Control to measure band saw performance in real time. The mill only added the system, with all the bells and whistles, back in 1998. But to hear sawfiling facilitator William Hall talk, it’s hard to imagine the mill running without it.

The system uses one non-contact sensor mounted just beneath the top guides on all of the band-saws on the mill’s two canter twin lines. These measure the deviation and vibration coming off the saws, sending a signal to the Saw Control’s main box, which naturally enough resides in HI-ATHA’s filing room.

“It’s telling us what the saws are actually doing in the mill, at the time,” explains Hall, “and we’re using that to control feed speed and maximize production on the line without sacrificing quality or risking downtime.”

Logs move into the two canter lines at a pre-determined base speed according to log size, and thus depth of cut. Saw Control is tied to the mill’s Porter PLC, allowing the system to adjust log infeed speed according to actual band mill performance (instead of being restricted to a pre-set speed dictated by the Porter controls).

As the saw enters the log, the sensor continually reports on actual vibration and deviation. If deviation stays within acceptable limits, for example, the system will ask Porter to speed up the infeed. If deviation is higher than certain pre-set limits, Saw Control will ask Porter to go into a holding pattern, do a ramp slowdown, or enact an emergency slowdown.

Saw Control installs the system with certain pre-set deviation limits and feed speeds depending on the mill, after which the mill can carefully monitor performance and tweak the limits a bit. At HI-ATHA, the line is asked to speed up to the next speed increment as long as the deviation is below 3 thou on either side (to or away from the log), or a total of 6 thou. If the system speeds up and deviation then goes beyond 3 thou up to 8 thou (6 to 16 total), Saw Control has Porter maintain that new speed. If deviation rises above 8 thou to 14 thou (16 to 28 total), Saw Control will instruct the PLC to drop down in speed until deviation is back within acceptable limits. Finally, above 14 thou (28 total) an emergency slowdown takes place.

This automated, immediate, and relentless speed control allows HI-ATHA to process each log as fast as the bands will allow, without trading speed for lower quality and increased downtime. It has maintained the balance very well, Hall says.

First off, downtime from things like snaking bandsaws has been cut by over 75%. In 1998, prior to running Saw Control full out, the mill had over 600 minutes of snake-related downtime. In 1999, that fell to 173 minutes. And all of this happened in the face of ever increasing feed speeds.

“We used to have a maximum speed in the winter of 345 ft/min on a 4.1-in cant, and 355 in summer. Now our entry speed on an 8-in depth of cut is 375 ft/min, all year round. We’ve picked up 20 to 30 feet, and that’s just the entry feed speed. We’re also finding that the system is speeding up most of the cants above that already higher entry speed, picking up even more production. At year’s end, that adds up to a lot of extra cants.”

That was indeed the initial justification for investing over $200 000 in Saw Control with all the bells and whistles. Hall says the original pay back was almost two years, although they expected to see other fileroom and maintenance-related benefits as well that were not included in the ROI calculations. Hall won’t reveal exact costs or payback numbers, but says the mill beat the original budget numbers “big time”.

Just as balancing lumber quality and throughput is crucial for HI-ATHA, so too is maintaining chip quality. The mill, along with another area high-quality sawmill, are the only two suppliers whose chips are used by Weldwood’s Hinton pulp mill for a specialty pulp, which in turn is destined for specialty coated paper made by one of parent company International Paper’s mills. To maintain chip quality regardless of log infeed speed, the mill has invested some $600 000 in VF drives from Arrow Speed Controls for the chipping heads, a move that Robson says has worked out well.

SPOTTING TROUBLE

The Saw Control system has also helped improve both filing room practices and maintenance troubleshooting. HI-ATHA’s system includes the optional chart recorder and trend recording, analysis and reporting software, providing information and stats on saw and machine centre performance that mill management uses to better direct their efforts. For example, the system tracks how many times Saw Control has directed a given line to speed up or slow down, and can in turn identify which saw was causing an undue number of slowdowns.

Such information can be used at shift end to decide which saws require the most attention, over a few shifts or a week to target machine centres that need maintenance, or at any time to see problems before they result in downtime or poor quality lumber.

It has also improved sawfiling practices by providing filers with immediate and detailed operating feedback on the saws they have just worked on.

“The really big things were guides – it really drew our attention to how important they are- as well as dish – even a little bit makes a big difference on saw performance. Overall it has helped with morale – the filers all watch the system on a constant basis, which helps them direct the filing work or prioritize it when we are tight with time. It has also increased the saw’s running time on the small line, and thus reduced the filers’ workload a bit.” All of which is a long way from how process control happens at a lot of mills. There’s little comparison between the typical end-of-shift historical size control tactics used at most mills and the more immediate control that can now happen at a mill like HI-ATHA.

“The size control system might pick up a lot of thick or thin cants off one line,” Hall explains, “and Neil [Robson] will come in here and mention it. Then we’ll look at the chart on the Saw Control, and immediately see what band it is and respond. And that’s if we hadn’t already seen the problem ourselves.”

If it all sounds a little too sophisticated for your average mill, rest assured it’s a scenario that most other process industries would recognize all too well. It’s likely to get much more familiar in our industry as well.