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Home : Pulp & Paper Solutions : Consistency Measurement Why Consistency Control?  Consistency is the foundation that papermaking is built on. Consistency measurement and control has a significant impact on both product variability and manufacturing costs. Even slight improvements in the measurement and control of consistency throughout the mill can result in cost savings of 100K USD annually. Although consistency is almost universally recognized as the most important variable in the manufacture of pulp & paper, it is often the most neglected. Good consistency control requires that the entire system be engineered and designed correctly. Each of the following components must be considered if the system is to provide the level of control that is required in order to achieve optimal performance:
Consistency MeasurementThe term “Consistency Transmitter” is in fact a misnomer, in actual practice all consistency transmitters measure a secondary property of the stock and infer consistency from that secondary property. For example, a fiber network strength device measure the force required to separate fibers in a stock slurry. The higher the consistency, the greater the force required. While this is an excellent way to measure consistency, other factors not related to consistency may affect the force. Understanding the affect and degree that parameters such as, temperature, pH, furnish, and fiberlength, have on the measurement go a long way to providing a robust consistency control system. Of the many technologies available for measuring consistency, each has its own set of advantages and limitations, specific to a given application.Dilution WaterProviding a stable source of dilution water is key to reducing consistency variability. Upsets in the dilution water pressure are translated directly into consistency variability. Because of the interactive design of most mill white water systems, variability generated in one location is often propagated throughout the system. How the white water header is designed and controlled can have a dramatic impact on overall system variability.Mixing & AgitationImproving agitation and mixing in stock chests is often the single most important improvement that can be made to reduce overall consistency variability. Stock chests behave as high frequency filters. The better the mixing, the better the removal of higher frequency variability. Determining the mixing time constant of the stock chest is imperative, so that the capability of the system is understood. Since the control system is only capable of attenuating low frequencies, it is important to maximize the mixing capabilities of the stock chest in order to achieve the lowest level of variability possible.Control ValveIncorrectly sized control valves are one of the largest sources of consistency variability. An oversized control valve limits control resolution and accentuates any mechanical non-linearities present in the valve. Undersized valves limit the controllability of the system. It is important that the valve have maximum rangeability. Because of the unique requirements of the pulp and paper industry this almost always means the use of an equal percentage ball valve. However, variable frequency drives are becoming the preferred option in applications where optimal control is required.Sampling & CalibrationCorrect sampling is the key to good calibration. The sampling port must be designed so that a representative stock sample can be obtained in a safe and consistent manner. The repeatability of the sampling and testing is highly dependent on the method and design of the sampling apparatus and technique. If correct sampling procedures are not followed, the entire process of sampling, testing, and calibration becomes worthless.Control Loop TuningThe tuning of the control system is the final piece of the consistency control puzzle. While tuning cannot make up for an improperly designed system, it can destroy a good system. Loop tuning is often approached in a haphazard “seat of the pants” fashion. While this approach can sometimes lead to adequate results when controlling other variables, this is not the case with consistency control. Consistency control is unique in that the dynamics change dramatically as operating conditions change. The process gain, process deadtime, and process time constant all change whenever operating conditions change. It is not unusual to see the process gain change by a factor of 3 to 5 and the deadtime to double or triple. This can cause instability and cycling if not taken into account. It is important to tune the loop for the worst case operating conditions and to use a non-oscillatory method of tuning. To find out more about our Consistency Application Audit services, click here, or call us at 800-528-8997 |
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