A tutorial on the measurement of paper stock consistency

ISA Transactions 32 (1993) 277-282 Elsevier

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A tutorial on the measurement of paper stock consistency Michael H. Waller Paper Science and Engineering Department, Miami University, Oxford, OH 45056, USA

This paper is a general review of the instrumentation used for measuring paper stock consistency. An overall introduction of the problem is presented first, followed by a description of the leading types of sensors. The operating range, environmental concerns, and any special considerations for use of each gauge are discussed. A brief mention is made of gauges under development which employ special technology. Finally, typical examples of control loop arrangements are suggested.

Introduction The m e a s u r e m e n t of the consistency of p a p e r stock is a special concern peculiar to the pulp and p a p e r industry. More properly referred to as the concentration of p u l p - w a t e r mixtures, consistency is measured in the laboratory by using a gravimetric method described in T 240 om-88 [1]. This method purports to measure and report the percentage by weight of oven-dry fibrous material in the p u l p - w a t e r mixture. In fact, this method responds to anything caught on the filter paper. Thus, the percentage of total dry solids in aqueous suspension is measured. This may include various filler materials, pigments, coatings, starches, and the like. This contradiction has caused great confusion in the understanding of consistency m e a s u r e m e n t and the reporting of results. The issue is not settled. Non-laboratory methods of consistency measurement are inferential, in that consistency is not measured directly, but is secondarily induced, usually with either optical or rheological techniques. Optical methods rely on light scattering, transmission or polarization. Recent advances have extended the working range of optical devices to about 5% consistency [2]. The mechanical Correspondence to: Professor Michael H. Waller, Paper Sci-

ence and Engineering Department, Miami University, Oxford, OH 45056, USA.

pseudo-viscosity techniques all rely on the inherent viscosity or network shear stress that the stock suspension exhibits, and, unfortunately, they all respond to changes in stock flow rate. Thus, the m e a s u r e m e n t is not uniquely proportional to the fiber and filler concentration over the range of interest, typically above 1.5%. These gauges cannot be used at lower consistencies, since viscosity and network shear are independent of consistency [3]. In addition to flow rate, the measured consistency signal is often dependent on the stock temperature, pulp species, pulp history, freeness, and the presence of fillers and additives. Each separate set of conditions has mandated a new calibration and attendant operation over a very narrow range [4,5]. These problems have spawned a continuing stream of new approaches to the measurement of consistency. One new method employs a pattern recognition technique to measure the average fiber diameter and distance between the fibers, producing a volumetric consistency m e a s u r e m e n t which is dependent on the fiber density [6]. Another method relates dielectric constant to consistency by propagating radio waves through flowing p a p e r stock. This method is claimed to be as accurate as the gravimetric method [7]. Whatever the method and the sensor used, successful consistency regulation via dilution with water requires a systems engineering approach to

0019-0578/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

M.H. Waller / Measurement of paper stock consistency

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the 'problem. The measurement must be representative of the bulk of the stock, responding to consistency changes and not some extraneous variation. The dilutant water must be added in a suitable location, properly mixed, with the consistency control loop integrated into the mill's control strategy. Several techniques for consistency measurement below 5% will be discussed in detail, along with suggestions for implementation in a suitable control loop.

Measurement methods

Optical Present-day optical consistency devices operate on principles which are mainly empirical, with no simple theory to explain the interaction between light and pulp. Light passing into a pulp suspension may be reflected (or scattered), absorbed or transmitted, and the relative ratios of these quantities vary with the nature of the suspension. Since a typical stock suspension is a heterogeneous mixture of particles (both fiber and filler) of various sizes and characteristics, the ratios are not readily determined in advance. All optical devices for measuring consistency rely on the fiber's interaction with light in one or more of the ways mentioned above. One type of sensor, A in Fig. 1, uses linearly-polarized light from either a halogen bulb or a semiconductor laser which is passed through the measurement cell. The transmitted light is split into two beams,

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one passing through a second transverse-plane polarizing filter, the other passing through a third in-plane polarizing filter. The beams are detected by photodiodes and combined to produce a relative depolarization signal. The amount of depolarization is a function of the total fiber and filler [8]. The signal is insensitive to brightness, color, freeness or soluble additives, but because of the small optical gap, is restricted to consistencies below about 1.5%. A second type of optical sensor, B in Fig. 1, is based on the measurement of the transmittance of light. One version uses light from a high-intensity infrared pulsed LED which is then detected by a photodiode. Designed to be used on 50 mm bypass lines, this sensor can be used on pulp consistencies as high as 4%. Another version of this device uses an incandescent bulb as the light source, and thus is limited to slightly lower consistencies for good sensitivity. It again is designed for installation in 50 mm sample lines [9]. This type of sensor is relatively sensitive to changes in freeness, bleaching performance in terms of color and black liquor carry-over and exhibits non-linear behavior with changes in filler and dissolved solids. Sensor C in Fig. 1 uses forward- and backscattered light to produce a signal combined from the several detectors which is proportional to consistency. To prevent reflections from the back wall of the pipe, diameters of at least 100 mm must be used, with consistencies in the thick stock range up to 6%. In general, this type of sensor's sensitivity to variations in the content of non4ibrous substances lies between that of sensor A and B, except for filler, for which it is the most sensitive [10]. Attempts to improve the performance and extend the range of these optical devices is on-going. A recent U.S. Patent describes an improvement in the type C sensor by temperature-compensating the signals received from the detectors [11]. Another device under development uses scanning laser microscope technology to relate consistency to particle size analysis. Back-scattered light provides a measurement of the particle size distributions, and a knowledge of the densities for each population of particles allows a calculation of

M.H. Waller / Measurement of paper stock consistency

consistency. Data have been taken up to 1% consistency, with extension to about 3% possible [12].

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Mechanical All mechanical, or rheological consistency devices operate by placing an obstruction in the stock flow, the force on which is a function of consistency. Whether the design is that of a blade, a rotating element or a probe, the force is also a function of temperature, freeness, fiber morphology, filler content (to a minor extent), and sometimes flow rate. Blade sensors respond to changes in consistency by sensing the change in shear forces for consistencies higher than about 2%. These devices are subdivided into two categories: fixed and moving. The fixed blade, an example of which is shown in Fig. 2, measures shear forces along the side of the blade. Impact forces on the front are cancelled by either an uplifted tail section or by ribs along the side, so that the blade is insensitive to flow rate changes. Moving blade devices stroke the blade, cutting across the flow in the plane of the blade, measuring the time required to complete the stroke. Higher consistencies will

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require a longer time, and vice versa. Compensation for velocity is effected by a deflector mounted upstream [13]. Rotating devices, an example of which is shown in Fig. 3, measure the motor torque required to turn the rotor in the stock suspension. Stock consistencies as low as 1% can be sensed, with

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M.H. Waller / Measurement of paper stock consistency

280 0 transmitter

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flow insensitivity for velocities less than 2 m / s [5]. For certain models, different rotor designs are used for different consistency ranges and fiber types. The probe device, shown in Fig. 4, responds directly both to changes in consistency and flow rate. Performance is best above 2% consistency, with the requirement that flow rate is simultaneously measured and used to compensate the consistency signal. Force measurement is accomplished with strain gages built into the probe. A deflector in the stock pipe upstream of the probe is usually installed to prevent accumulation of strings and like material [14].

The control loop

The obtaining of an accurate measurement is only half of the story; using it is the rest. Understanding which spurious changes will affect which gauge in exactly what manner is a topic of continued investigation and sometimes hot debate. Selecting the best sensor for the particular installation will be dictated by the type of furnish, the present installed base, and mill experience [15]. The typical consistency control loop shown in Fig. 5 offers a very short time delay from the point at which dilution water is introduced to where consistency is sensed. This is ideal from a control loop standpoint, since any delay time causes tuning difficulties. This situation may be compromized somewhat by the installation requirements of the sensor, since a suitable length of pipe before (and after) the measuring device is required. Figure 6 shows a nomograph for sizing the proper length of pipe upstream and downstream of a particular sensor [16]. For a typical consistency of 4% and a pipe velocity of 1.85 m / s

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Fig. 5. General arrangement for consistency control loop.

(about 6 ft/s), the coefficient k is about 6.6. Thus the minimum length of upstream pipe, L1, must be about two feet for a 4" pipe. Assuming a closely-coupled pump and dilution water line entry point, the total length of pipe might well be only 6 ft, resulting in a transit delay time of only 1 s. This still would seem to be a very favorable control loop situation. One problem with the typical loop of Fig. 5 is that centrifugal pumps frequently do an inadequate job of mixing, presenting a time-varying consistency to be sensed. Installing a better mixing device will usually add to the delay time experienced by the stock, which may or may not be significant. The "conventional" approach to consistency control shown in Fig. 5, when used in a stock blending situation, often involved the use of high signal filtering to dampen variations in blendedStock Velocity = 12 ft/~

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M.H. Waller / Measurement of paper stock consistency

stock consistency caused by feed upsets in either consistency or flow rate. Fiber stock ratios for each furnish would be set on a volumetric basis, which relied on uniform consistencies from each furnish. Filtering was necessary to calm the sometimes unstable behavior due to the interaction between furnish flow and consistency and the variable process gain caused by changes in furnish flow. One solution to this problem is to decouple the control loops by setting a ratio of dilution flow to furnish flow in order to accomplish blending control on a fiber weight basis. Thus, changes in furnish flow will not influence consistency [17]. This solution is only partially successful, however, because the flow ratio needs to be adjusted "on the run" for certain process variations. This consistency control scheme for stock blending still suffers from two major problems: changes in furnish flow will cause variable process dead times, and variations in storage chest consistency will cause variable process gain, Both of these problems are candidates for solution with an adaptive tuner, as recent advances in hardware have enabled advanced control strategies to be implemented. Getting the correct sensor installation and control loop strategy is still only part of the answer. The consistency measurements must be part of the DCS, providing mass balances and, in some instances, redundant information. Startups, grade changes and shutdowns can be made in a coordinated fashion at minimum cost and maximum quality [18].

Summary The issue of consistency measurement and control represents a continuing problem to today's papermaker. Sensor development, particularly for optical devices, continues in the attempt to find a more repeatable, reliable and predictable device, insensitive to those furnish variables which are of no concern. The issue appears to be far from resolved. Different strategies for incorporating consistency measurements in control loops and the mill DCS are making progress, including the

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use of advanced control schemes. All this activity will result in continued progress toward minimizing profiles on the papermachine, achieving greater runnability, fewer breaks and higher profits.

Acknowledgement The author appreciates the support of the Paper Science and Engineering Department of Miami University during the writing of this paper.

References [1] "Consistency (concentration) of pulp suspensions", T 240 om-88, TAPPI Test Methods 1, TAPPI Press, Atlanta, GA, 1988. [2] J.S. Jack, R.G. Bentley and R.L. Barron, "Optical pulp consistency sensors", Pulp Paper Can. 9•(2) (1990) 59-64. [3] M.H. Waller, Measurement and Control of Paper Stock Consistency, Instrument Society of America Monograph 5, ISA, Research Triangle Park, NC, 1983. [4] W.C. Chase, A.A. Donatelli and J.W. Walkinshaw, "Effects of freeness and consistency on the viscosity of hardwood and softwood pulp suspensions", TappiJ. 89(5) (1989) 199-204. [5] R. Torborg, "Selection of a consistency sensor requires knowledge of all options", Pulp Paper 6•(6) (1987) 109113. [6] J.S. Jack, "On-line sensor systems in the pulp and paper industry: From yesterday to tomorrow", Pulp Paper Can. 91(1) (1990) T13-T18. [7] A.K. Gaigalas, "A new approach to the measurement of pulp consistency", Tappi J. 86(7) (1986) 103. [8] K. Connolly, "On-line retention measurement using optical low consistency transmitters", 1986 Engineering Conf. Proc., pp. 751-755, TAPPI Press, Atlanta, GA, 1986, pp. 751-755. [9] "Monitek pulp consistency monitor", Bulletin CC-115F88, Monitek Co., Hayward, CA, 1988. [10] H.W. Reed and J.O. Corbett, "Optical consistency measurement", Instrum. Pulp Paper Ind. 21, Instrument Society of America, Research Triangle Park, NC, (1985) 25-35. [11] I.R. Breholdt, "Pulp consistency apparatus", U.S. Patent 4,838,692, 1990. [12] J. Hanseler and W. McKean, "Laser technology offers new way to measure furnish components", Pulp Paper Can. 89(9) (1988) 25-32. [13] L. Agneus, "Moving blade consistency transmitter allows interference-free measurements", Pulp Paper 65(2) (1991) 104-105.

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[14] H.A. Thompson, "Consistency control, medium and high range", 1986 Engineering Conf. Proc., TAPPI Press, Atlanta, GA, 1986, pp. 593-596. [15] R. Olander and L. Agneus, "Consistency measurement: The foundation for accurate dilution control", Tappi J. 9•(3) (1991) 97-107. [16] "Pulp-E1 operating, calibration and installation instruc-

tions", Bulletin BCsl5OVA, 1990-09-30, Valmet Automation, Westbrook, ME, 1990. [17] D.P. Dumdie, "A systems approach to consistency control and dry stock blend", Tappi J. 88(7) (1988) 135-139. [18] C. Collins and B. Yeager, "Domtar consistency control program ups production, eases grade change", Pulp Paper 63(10) (1989) 88-91.