Absolute consistency: a cornerstone investment for paper machine stability

Mark Willliamson

Feedforward basis weight control was the first wet end control developed by QCS suppliers when digital systems were introduced in the late 1960s and early 1970s.

It was a simple concept which was intended to stabilize and manage the furnish flow to the paper machine so the dry basis weight at the reel was stable. With this "weight on the wire" control, feedback control could theoretically be limited to a few trimming actions. Figure 1 shows how the control works. The feedforward weight is calculated from measurements of thick stock flow via a magnetic flow meter, wire speed and thick stock consistency. Speed and flow measurements have been well established for some time but absolute consistency measurement was a problem for many years.

Back then, reliable, absolute thick stock consistency measurement was hard to achieve with shear force measurements which did not recognize ash. Fibers create drag forces but fine filler particles do not. Varying fiber types in the furnish also caused calibration shifts. With these uncertainties, feedforward control was purposely implemented in many cases with fixed consistency values, and the thick stock flow target was managed to respond to changes in machine speed or basis weight target only. That was good enough for control during production rate changes but the control did not recognize any problems caused by furnish instabilities until the effect on basis weight was measured at the reel.

Does your consistency transmitter measure up?

With today's advancements in absolute consistency measurements feedforward weight control has become a cornerstone for all subsequent wet end controls which have flourished over the last twenty years. Think of it as a wet end basis weight measurment; then it's importance is realized. This control has become more critical as papermaking furnishes have become increasingly complex, with ash showing up in many previously virgin fiber furnishes through DIP and RCF, or added intentionally as fresh filler in value-added grades. Adding more filler has a significant impact on the cost of the furnish, since it displaces more valuable fiber.

If the machine chest consistency is measured by a shear force device coated or filled broke added during long breaks can destabilize the consistency control since the effect on total consistency is not fully recognized. For machine stability from the wet end a trustworthy absolute total consistency measurement – ash and fiber together - after the machine chest is therefore a wise investment. But does your existing consistency transmitter measure up?

blog6MW-photo2Figure 1: Feedforward weight is calculated from measurements of thick stock flow via a magnetic flow meter, wire speed and thick stock consistency. Thick stock flow is controlled in response to variations in consistency measured at the machine chest. The disruption in dry fiber rate is not seen at the reel.

If the furnish contains ash, consistency measurement suppliers recommend replacing shear force transmitters with an absolute measurement of the total furnish. There are two types: optical and microwave. The microwave transmitter measures the time of flight of microwaves through the slurry and that relates to the total mass consistency including fibers and fillers. Today's optical consistency sensors, once used only for low consistency whitewater, can also measure the total consistency from 3 to 4%, and even higher than that. They do this by the differentiation of light transmission characteristics.

Stabilizing the entire machine

The importance of absolute consistency measurement is illustrated in figure 2 which shows the improvement in basis weight stability after long breaks on a copy paper machine when a shear force measurement was replaced with an absolute consistency measurement - a microwave unit in this case. The shear force measurement did not recognize ash variations from the broke flow , causing instabilities after the reel scanning resumed. The basis weight and moisture levels were well off-target so the feedback controls had to correct for large offsets and the paper quality took some time to settle down to normal levels. Absolute consistency measurement stabilized the machine considerably.

blog6MW-photo3Figure 2: On a copy paper machine, the basis weight deviation from target after a long break was reduced significantly by using an absolute thick stock consistency measurement rather than shear force. (Legend: WEM = wet end management control with shear force measurement. Y08 Micro = the same control with microwave measurement.)

This machine is equipped with multi-variable control of white water consistency, basis weight, moisture and ash level at the wet end and the reel. The improved feedforward weight control stabilized the whole machine so the wet end management controls peformed better and provided some convincing bottom line results described in the next article in this series.

If your machine's existing thick stock consistency measurement is antiquated, consider replacing it with a modern absolute measurementt. With this trustworthy measurement, coupled with multi-variable controls, the sheet's basis weight and related properties can be kept on course during breaks, production transitions and other upset periods. These absolute measurements have also proved their worth before the paper machine in furnish blending applications. The improved machine stability and quality have provided some important benefits.

In part two of this series, the results from wet end management control, including white water consistency, wet end ash levels and electrochemistry are described.

Data and photo courtesy of Metso