There is a wide diversity of FPR target values on different types of furnish and types of paper or board machines. A higher value of FPR captures more fine particles (fines) in the sheet and results in less two-sidedness, especially on a Fourdrinier machine. A low value of FPR results in recirculation of not only fines but also pitch and wet end additives, and can lead to problems such as agglomeration of deposits, filling of wet-press felts, and poor drainage.
Perhaps more important than the absolute value of FPR is maintaining a steady value of FPR. If the FPR varies, it results in variability in drainage rate and sheet moisture leaving the forming section, which can lead to web breaks. It can also result in basis weight variability.
Recently, a fine specialty paper manufacturer located in the USA was able to reduce costs and increase efficiency by installing a complete retention measurement system. Domtar’s paper mill in Hawesville, Kentucky, chose ABB’s KPM KRA Retention Measurement System
, including the KPM KC9-A optical consistency sensor and sampling systems. It had been determined that their previous retention and ash control system was not providing consistent and reliable measurements, and an upgrade was needed.
These systems were installed on the headbox and white water silo on the mill’s H-2 paper machine in early 2018 and calibrated to ensure accurate measurement and control. For the headbox unit, the sample is taken from the recirculation line immediately after the headbox. The sample moves through the KPM KRA unit and discharges into a sample funnel, where stock samples can be collected for use in initial and ongoing calibration. The sample is then returned to the wire pit of the paper machine.
For the white water unit, the sample is taken from the bottom of the tray going to the silo and uses gravity to feed the sample to a KPM KRA unit in the basement. In order to obtain an accurate measurement, the sample goes through a deaeration vessel to remove entrained air. The sample then flows through the KPM KC9-A sensor into a collection funnel and is returned to the paper machine wire pit using a sample return pump.
Initial calibration produced positive results almost immediately. When bump tests and changes in ash addition rates were performed with the new ABB system, the results tracked closely with expected outcomes. Ash and consistency samples were collected over the course of a month to cover the full range of grades produced on the H-2 paper machine. Calibration values were fine-tuned based on the results of lab ash and consistency data.
The mill has come to rely on the KPM KRA Retention Measurement System. Having accurate information has allowed operators to not only get back on grade faster after a sheet break, but also achieve cost savings by being able to monitor addition of broke going back into the system more reliably. The operators now rely on the information provided by the KPM KRA Measurement System to ensure the highest quality at the lowest operational costs.
Phil Hinchcliffe, Process Controls Manager at the mill, indicated that they were so pleased with the performance of the system on the H-2 machine that they purchased another KPM KRA Retention Measurement System for the Hawesville mill’s H-1 paper machine. This newest system was put into service in mid-2019. Installation went smoothly, and the unit was very responsive to changes in ash content and both headbox and tray silo consistencies. The success of the implementation has also led to other opportunities in the areas of sheet break detectors and consistency transmitters.
In summary, the use of a retention measurement and control system can lead to good machine efficiency and consistent product quality while optimizing furnish costs. The key to getting a payback on this type of investment is having accurate consistency sensors designed for continuous measurement at the headbox and the wire pit. Building on these measurements to manage the addition of furnish components and wet end additives can lead to reduced variability, reduced downtime from sheet breaks, faster grade changes and ultimately decreased chemical usage due to more predictable performance.