Furnish quality, total consistency, ash and retention controls help establish speed records for world-class SC-A+ machine

Instruments, Controls Add Stability To Stora Port Hawkesbury Machine

When scanning quality control systems were introduced, they significantly improved medium-term variability after a few cross direction sheet traverses and after a delay through the dryer section. But these first generation systems used only a handful of consistency transmitters or flowmeters for control. During breaks, the sheet composition often drifted off target, making it difficult to rethread and contributing to lengthy periods of lost production. With limited wet end instrumentation or controls, short-term variations were locked into the sheet before it reached the reel scanner. By then it was too late to control them.

To control furnish quality, retention, and sheet ash levels at the source, modern machines, such as the No. 2 paper machine at Stora Enso’s mill in Port Hawkesbury, N.S., are now being equipped with a multitude of online instruments and controls. This 9.4-m trim width Valmet machine, which started production in April 1998, produces Magnipress SC-A+ grades at speeds that are now consistently more than 1,600 mpm. World speed records for SC grades were set as early as September 1998. The machine is targeted to produce 350,000 tpy.

Mika Kämpe, manager, SC Paper, explains the mill’s philosophy with respect to online process and quality measurements: “Today, both good quality and high efficiency are necessary to pay back the investment in new papermaking capacity. The best available online measurements are needed to see the changes in the process at the wet end and to start corrective actions much earlier. With a high level of instrumentation, the furnish is stabilized when the machine is started up. This makes the startup period as short as possible.”

The combination of wet end stock preparation and short circulation measurements and a Damatic XDi distributed control system (DCS) and quality control system has helped the mill produce a stable, high-ash sheet that runs with few breaks and with quick recovery after a break. Measurements from a Paperlab automated paper testing laboratory are also used by the operators to optimize furnish quality. The automation equipment and the automated testing lab were supplied by Valmet Automation. (Valmet Automation merged with Neles Controls in July 1999 to become Neles Automation, part of the Metso Corp. formed when Valmet and Rauma merged.)

Stora Enso’s No. 2 paper machine at Port Hawkesbury is the world’s largest and fastest SC machine. The wet end is equipped with a high level of instrumentation and controls for stable operation.

TOTAL CONSISTENCY CONTROL. With sheet ash levels as high as 35%, fiber consistency control alone is not sufficient to supply a stable furnish to the machine. As a result, the mill decided to measure total furnish consistency—fiber plus ash—at all critical points in the stock blending system using Kajaani MCA microwave consistency transmitters (Kajaani products are supplied by Neles Automation).

Unlike blade or rotary transmitters that measure the shear force created by fibers, the MCA infers the total consistency by using microwave energy. The instrument calibration is the same for different furnishes. Kämpe describes the total consistency measurements as, “The heart of the process.”

The 13 total consistency measurement points on the No. 2 machine shown in Table 1. These include fiber furnish components, broke, recovered stock from the disk filters and the mixed stock. Blade consistency transmitters are used for other, less critical, applications.

FIGURE 1: A screen shot shows wet end consistency and retention measurement points. Both total retention and ash retention are calculated for each wire.

TOTAL CONSISTENCY CONTROL. With sheet ash levels as high as 35%, fiber consistency control alone is not sufficient to supply a stable furnish to the machine. As a result, the mill decided to measure total furnish consistency—fiber plus ash—at all critical points in the stock blending system using Kajaani MCA microwave consistency transmitters (Kajaani products are supplied by Neles Automation).

Unlike blade or rotary transmitters that measure the shear force created by fibers, the MCA infers the total consistency by using microwave energy. The instrument calibration is the same for different furnishes. Kämpe describes the total consistency measurements as, “The heart of the process.”

The 13 total consistency measurement points on the No. 2 machine shown in Table 1. These include fiber furnish components, broke, recovered stock from the disk filters and the mixed stock. Blade consistency transmitters are used for other, less critical, applications.

FLOW-COMPENSATED DILUTION CONTROL. Rod Corbett, manager of process control, says that the combination of total consistency measurement and dilution control strategies on the No. 2 machine is unique in the world. “First, there is a universal use of total consistency measurement,” Corbett says. “With such a high ash content, it is mandatory. Secondly, we have implemented flow-compensated dilution control on all dilution loops on the No. 2 machine and in the TMP mill.”

FIGURE 2: Ash levels in the short circulation system are controlled during sheet breaks.

In this scheme, each dilution flow is measured with a magnetic flowmeter. The consistency control loop supervises the dilution flow control loop. Flow, the final control element, has effectively a linear response, rather than the non-linear characteristics of valve position control.

The strategy has several advantages over traditional consistency-based dilution control. It minimizes the effects of varying dilution water pressure and varying pump suction pressures caused by tank level variations. The flow-compensated consistency control loops effectively handle flow changes, which would upset traditional consistency control strategies. Corbett concludes that the total consistency measurement and the flow-compensated dilution control have been successful so that, “on the No. 2 machine, consistency control is not an issue.”

ASH CONTROLLED AT ALL TIMES. Even if total consistency is well controlled, ash level variations in the furnish or in the short circulation system can lead to sheet quality or runability problems. For the best efficiency and quality, ash needs to be measured and well-controlled during startups and breaks, as well as during steady-state operation. For short circulation system stability, ash retention and total retention need to be continuously controlled as well.

As shown in Figure 1, percent ash and total consistencies are measured at five locations using a Kajaani RM-200 optical consistency measurement system. The thick stock ash measurement and associated feed-forward control of furnish ash content ensures that the ash level entering the short circulation system is on target despite changes in the flow or ash content of the broke or the recovered fiber from the disk filters.

The total consistency and ash consistency measurements of the headbox flow and the white waters of the inner and outer wire are used to calculate total retention and ash retention for each wire. The consistency of the headbox dilution water is also monitored. The control of the dilution water consistency is necessary to ensure the stability of the cross direction profile control.

While the sheet is on the reel, the percent ash content in the sheet is controlled by the ash measurement sensor on the dry end scanner. This is the master feedback control that controls the amount of fresh filler added to the wire pit. For responsive control, about 1/3 of the total fresh filler is added here. The bulk of the fresh filler is added to the mixed stock.

If the ash level of the mixed stock changes from its target, the feed-forward ash control strategy will change the setpoint of the fresh filler flow to the wire pit. The sheet ash level will therefore remain constant even if the thick stock ash levels vary. Upsets will not be seen at the dry end.

If a break occurs, the control from the dry end scanner will be suspended, but a headbox ash control strategy will be activated (Figure 2). Since the ash levels in the short circulation system and in the sheet remain stable during the break period, the sheet is more easily threaded through the machine and the settling time for the dry end ash control is minimized.

RETENTION OPTIMIZED FOR EACH GRADE. Total retention and ash retention are measured for both the outer wire and the inner wire. Normally, the drainage and retention characteristics are equally distributed between the wires of the gap former so the control can be based on either measurement.

On the No. 2 machine, the control system uses the inner wire consistency. An imbalance between the inner wire and outer wire readings can indicate uneven, two-sided retention that can affect paper quality.

The mill concluded that the optimum retention is not necessarily the highest retention, and that the operating point for white water consistency control may change from grade to grade. In the DCS, the operators can access a table that shows the optimum white water consistency targets for each grade. Basis weights range from 40 to 60 gsm.

TABLE 1: Total consistency measurement points in stock preparation.

The two component retention aid system is supplied by Kemira Chemicals. The RM-200 measurement of white water consistency is controlled by the flocculant flow. In addition to stabilizing the sheet ash and fiber content, the control provides a stable consistency in the headbox dilution water. Cross direction profiles are reported to be excellent.

NO NEED FOR WET LAB. An observant visitor might note that there is no wet lab in the machine room. Indeed, there is no need for one since all furnish quality is either continuously measured or is automatically sampled and measured. Figure 3 shows how furnish quality is shown to the operators on the DCS video screens.

The thermomechanical pulp (TMP) and kraft post-refining operations are monitored by a PDA online pulp drainage analyzer with eight sample points and an FSA online fiber length analyzer with three sample points. The sample handling and measurement modules, located in the machine basement, are automatically supplied with pulp samples through either gravity fed or pressurized water supply lines. The Canadian standard freeness (Csf) freeness readings are taken after each series kraft post-refiner and after the parallel TMP post-refiners.

Fiber length measurements are taken on the TMP and kraft fiberlines after each storage tank, after each post-refiner, before each fiber stock enters the mixing tank, and after the stock has been mixed with broke. Using these quality indicators, the operators can adjust the post-refiner specific energies for the appropriate degree of fiber development and drainage.

Keith MacSween, process engineer, reports that the FSA and PDA have also proven useful in evaluating the trials of different refiner plates. He says that the online analysis, “makes trials very effective and meaningful.”

FURNISH CONTROL LINKED TO PAPER QUALITY. Stora Enso has provided the operators with an extra tool for linking the furnish quality to the final paper quality. By using a data link to the mill’s PI information system, tests from the Paperlab automated testing laboratory at the dry end are displayed on a terminal in the wet end control room.

Using the system’s Excel spreadsheet add-in function, the mill has constructed a “quality window” display, shown in Figure 4, which relates sheet porosity to cross direction tear strength. CD tear obviously affects pressroom runnability. Porosity relates to the printing characteristics of the sheet, particularly ink holdout.

The operator’s task is simple: aim to have both tests within the quality window. The results of the operators’ actions can be followed on the display as the more recent control actions are shown as progressively larger dots on the screen. The final porosity of the supercalendered sheet will be kept as low as possible if the porosity from the paper machine is minimized. Since much of the fiber quality is developed in the TMP mill, operators there can track the paper quality using the same display.

The Paperlab measures full-width CD strips from the No. 2 machine, the No. 1 newsprint machine, and from wound supercalendered rolls. On the No. 2 machine, measurements are available a few minutes after every reel turnup, which is about every 55 minutes. The measurement modules include basis weight, caliper, CD tear, MD and CD tensile, ultrasonic tensile stiffness orientation, burst, porosity, formation, BEKK smoothness, PPS smoothness, gloss, TAPPI brightness, and L*a*b* color. The online color sensor is linked to the Paperlab for calibration verification.

STARTUP CURVE BOOSTED. PM2 is one of the most highly instrumented and controlled machines in the world, with many wet end furnish measurements and controls, dilution flow measurements and controls all implemented in a DCS and quality control system. In addition, automated paper quality tests are an integral part of the furnish control strategy.

Kämpe summarizes the positive benefits of the wet end measurements and controls during the machine startup period. “Instrumentation has helped us to understand variations in the process and how to eliminate them. It speeded up our startup curve.” After setting a world SC monthly speed record in the fifth month after startup, the machine now runs consistently faster than 1,600 mpm with a low break frequency, according to Kämpe. *

FIGURE 4: The “quality window” display aids operators in optimizing both CD tear and porosity tests from the automated test lab.