Improvements to the No. 4 newsprint machine at Donohue’s Clermont, Que., result in higher reel moistures and improved machine speed and efficiency

Dryer Section Rebuild Boosts Production, Conserves Furnish at Donohue Clermont

Papermaking technology is much different. Today’s wide machines produce a stable, quality sheet at much higher production rates than machines built 20 or 30 years ago. However, unlike computers, upgrading an older machine with state-of-the-art technology has proven to be a viable strategy. A strategic investment plan to eliminate clearly-identified production or quality bottlenecks can get the best possible productivity and quality out of older machines at lower costs per ton. Investing in installed papermaking capacity can technically bridge the generation gap and, at the same time, improve cash flows in today’s cost-conscious business environment.

The Donohue Forest Products Inc. (Produits Forestière Donohue Inc or PFDI) newsprint mill in Clermont, Que., is a good example of a mill that has made strategic investments in machine im-provements that provided incremental improve-ments in quality, productivity, and costs. In Donohue case, the often-overlooked dryer section was one of the main targets for improvement.

VALUE ENGINEERING. Before devising a strategic plan, the Clermont mill contracted Valmet Canada Inc. of Montreal to conduct an audit of its No. 4 paper machine. The mill wished to improve the productivity and roll quality produced by the Dominion Engineering machine which, when it was installed in 1968, was considered “state of the art.” Since roll quality has become critical as many printers move to high-speed presses, the Clermont mill wanted to improve roll quality on the No. 4 paper machine so it approached that of the No. 5 machine—a Valmet machine installed in 1987.

Valmet’s audit team from Montreal and its Enerdry Division in Thunder Bay, Ont., conducted extensive in-mill diagnostic tests and confirmed the mill’s investigation that the dryer section was one of the limitations to improved quality and increased production rates. The existing steam system was not very flexible and was wasting steam. Valmet proposed an action plan that would involve re-engineering of the steam and condensate system to modern standards to produce bottom-line benefits and solve many operational problems.

In October 1995, a $3.7 million machine improvement program was approved. The scope included re-engineering of the steam and condensate system, new stationary siphons and turbulence bars in the dryer cans, modifications to machine approach piping, a new secondary screen, a new variable speed drive on the fan pump, and a new online quality control system.

AIR LEAKAGE AFFECTS PROFILES. The existing dryer section was causing operational problems and moisture profiles were difficult to control. Jean François Boivin, paper machine superintendent, says that there were “moisture streaks near the siphon shoe.” The shoes on the rotary siphons were positioned 3/4 of the way to the drive side. He reports that the existing siphons were difficult to adjust and to maintain. In addition, he reports that the moisture profiles were changing over time.

One of the profile instability problems was traced to air leaking into the system through poorly-sealed steam and condensate joints. The installation of bleed vents to purge the non-condensing gases was a temporary solution. However, the basic problems of siphon maintenance and uneven dryer temperature profiles needed a permanent solution.

Originally, the dryer section was divided into only two steam groups. The wet end group consisted of five heated top dryers in the unorun section. The bottom unorun dryers up to dryer No. 20 were not heated. The remaining 41 dryers in the section formed the second steam group or main group. This was a thermocompressor loop that cascaded back to the first group. This very large main group steam pressure was used for moisture control at the reel.

To avoid moisture profile problems, the main group was often run at a low pressure. But, to run at a differential pressure sufficient to remove condensate from the main group, some dryers were turned off. This destabilized the moisture profile. As a result of lowering the pressure in the main group, excess steam was vented to the condenser. The mill reports that, in winter months, this often resulted in wastage of about 8,000 lb of steam/hour.

Due to the high differential pressure required to drain the condensate through the rotary siphons, the first five top dryers could not be controlled to produce the gradual temperature in-crease required to minimize picking. The wet end dryers were also prone to flooding. This condition was indicated to the operators by severe machine vibrations. Up to an hour of downtime was required to drain them. Also, there was a large pressure difference and temperature difference between dryer No. 9, at the end of the first group, and dryer No. 11, at the beginning of the main group. Boivin says that “this large temperature difference caused picking and breaks.”

BETTER HEAT TRANSFER, AND CONTROLS. The dryer section steam and condensate system rebuild involved three components: new stationary siphons and steam joints, dryer turbulence bars, and a re-configuration of the steam and condensate grouping and controls.

The rotary siphons were replaced with stationary siphons and steam joints manufactured by Valmet. The new siphons eliminated the steam and air leaks. Due to a rigid support system, they are not sensitive to vibration. The siphon shoe is designed to limit condensate and steam hammer shock.

Orifice plates on the condensate lines are sized according to the condensate load and the amount of blow-through required in each section. All dryers could then be fitted with the same siphon vertical pipe. This design allows for more flexibility for future speed changes or different grades. Also, the lower steam and condensate velocities in the large siphon pipes reduce pipe erosion

Stationary siphons can operate at a much lower differential pressure than rotary siphons because the pressure required for good condensate removal does not have to overcome the centrifugal forces introduced by the dryer can rotation. At the pre-rebuild machine speed of 3,100 fpm to 3,200 fpm (944 mpm to 975 mpm) the differential pressure across the rotary siphons had to be maintained at a minimum of 9 psi to 11 psi (62 kPa to 75 kPa). The differential pressures would have to be increased as machine speeds increased.

New steam and condensate joints eliminated air leakage that contributed to moisture profile instability.

In contrast, the stationary siphons are designed to operate between 2 psi to 4 psi (14 kPa to 28 kPa) differential pressure at these machine speeds. The new steam and condensate joints eliminated air leakage that contributed to profile instability. With lower differential pressure and lower blowthrough rates, the existing condensate headers could be re-used, thereby reducing the installation costs.

Dryer turbulence bars were added to all dryer cans except the first. The Valmet-designed bars are easy to install as they are fixed on the dryer’s inner shell by special spring loaded straps that allow for thermal expansion. No mechanical attachments to the shell are required. The U-shaped steel bars improve heat transfer from the steam to the dryer shell by introducing turbulence into the condensate layer. This improves overall drying efficiency and improves heat transfer uniformity across the shell.

The dryer steam groupings were reconfigured to provide better control over the gradual temperature approach at the wet end and to allow better moisture control by the quality control system. The new steam group configuration, with five individually controlled wet end dryers plus three independent steam groups, is shown in Figure 1.

Figure 1: Steam and condensate system grouping, before and after rebuild.

New differential pressure control valves were added to dryers No. 1, No. 3, No. 5, No. 7 and No. 9. Dryers No. 11 though No. 22 and dryer No. 24 are now controlled as a separate group that cascades to the first section. The steam pressures in the third thermocompressor group, dryers No. 23 and No. 25 through No. 35, are used for the master control of the reel moisture by the online quality control system. The fourth thermocompressor group is comprised of dryers No. 36 through No. 51. Groups 3 and 4 are also equipped with booster thermocompressors. New equipment also included two skid-mounted separator stations, complete with pumps, transmitters, and gauges. Project costs were kept low by piping and testing these complete pre-assembled units.

BETTER DRYING EFFICIENCY, SIMPLE OPERATION. The improvements in drying rates have been documented in Figure 2. Even at the higher machine speeds, the steam temperature that is required to dry the paper is much lower, indicating better heat transfer to the dryer shell. The shell temperatures of the first four dryers are reported to be 8ºC to 10ºC higher than before.

Figure 2: Drying rates on the No. 4 paper machine have been improved considerably by the dryer section rebuild. The blue squares and the blue regression line indicate machines measured by Valmet. Other data is from TAPPI’s drying rate survey.

The moisture control at the reel using the third group steam pressure as the master is reported to be more stable. The second and fourth steam group pressures are controlled automatically as a selectable difference from the third section target.

The differential pressure control was design to operate via a selector switch, using a differential pressure or blowthrough control scheme. Blowthrough control will maintain a fixed ratio between the condensate load and blowthrough steam. It automatically adjusts the amount of blowthrough steam required to evacuate the dryers. During sheet breaks, when the condensing load is reduced, the control will lower the amount of blowthrough steam accordingly, reducing the opening of the thermocompressor. This will prevent steam from venting to the condenser during breaks and avoiding high dryer surface temperatures and picking when the sheet is re-threaded. The same control scenario can be achieved using differential pressure control.

HIGHER PRODUCTION RATES. Moisture profiles were improved so that the average moisture in the sheet was raised from 7.96% to 8.13% during the project evaluation period from 1996 to 1997. Recent average moisture levels are now over 8.3%. This moisture increase results in furnish savings. Drying steam is being used more efficiently and is not being vented to the steam condenser. The mill reports that the total steam consumption of the machine was reduced by approximately 15%.

The overall project, which involved changes to the wet end stock approach, dryer section, and a new quality control system, has resulted in significant speed and efficiency increases. Boivin reports that, before the project, machine speeds ranged between 3,100 fpm and 3,225 fpm (944 mpm to 975 mpm). Now, the 45 gsm grade runs at 3,250 fpm (990 mpm) and the 48.8 gsm sheet runs at 3,300 fpm (1,006 mpm). The mill’s rebuild evaluation study concluded that the average machine speed was increased by 15 mpm from 1996 to 1997. At the same time, the total machine draws have been reduced by about 5%. The current efficiency of the No. 4 paper machine is near the top of the list of newsprint machines surveyed by Valmet.

Regarding the dryer section rebuild on number 4 machine, Boivin has a concluding remark: “The dryer section on the No. 4 machine is not an issue now.”

Figures 3a, 3b, 3c, 3d: The dryer section rebuild has resulted in an increase in machine speed and efficiency, higher reel moistures, and reduced steam consumption.

Figure 3A: Machine speed.

Figure 3b: Machine efficiency.

Figure 3c: Reel moisture levels.

Figure 3d: Steam consumption.

No. 5 PAPER MACHINE IMPROVEMENTS. The mill is continuing its strategic investment program with improvements to the No. 5 newsprint machine, supplied by Valmet in 1987. The first-generation stationary siphons supplied with the machine have been replaced with the same new models that were installed on the No. 4 machine. Dryer turbulence bars have been installed throughout. Bottom unorun dryers in groups 1 and 2 have been converted to vacuum rolls.

Other modifications aimed at runnability improvements include rope run optimization, reduced open draws between drive groups, a dry end tail shooter, and felt drive gear clustering. The latter modification involves the removal of some of the original gears and reconfiguration of the felt drives so that the surface speeds of heated dryers are not mismatched with those of the unheated dryers in the unorun sections.

Other machine upgrades, which have been recently completed, include an extended unorun felt in the third felt group, vacuum roll conversion in the third felt group, HS blow boxes, a Symformer RMB upgrade, and a new slitter positioning system on the winder.

For the Clermont mill, upgrading to the latest state-of-the-art papermaking technology is a continuing process.

Mark Williamson, Freelance Writer, Thornhill, Ontario.