Analyses show entrained air as a contributing factor in sheet barring

Pulp & Paper, Sep 2003 by Taber, Philip L

FEATURE:

PAPERMAKING

Detailed analyses can identify air entrainment in stock approach systems that contributes to costly sheet barring and breaks, as a case study shows

Machine direction basis weight barring is not always induced solely by a mechanical defect in a rotating element. In some instances, normal pulsation amplitude from a rotating element can be greatly amplified by exciting the natural frequency in the stock approach piping, resulting in sheet barring.

In this article, a case study shows how entrained air contributed to sheet barring on a paper machine producing coated free-sheet papers. After detailed analyses that led to a way for reducing air entrainment, the mill has eliminated sheet barring, resulting in a reduction in sheet breaks from six or seven per day to just one.

ENTRAINED AIR IMPACTS NATURAL FREQUENCIES. The natural frequencies in a stock approach system can each be associated with one of the many standing wave patterns that can be generated.A standing wave is a non-traveling pressure wave of given wavelength and frequency that occurs under certain conditions and is determined by the speed of sound in the pipe and the physical characteristics of the piping system.

Physical properties of the stock, such as pressure, consistency, air content, and temperature, will determine the speed of sound in the fluid. While each of these variables affects the speed of sound, the amount of entrained air has the single-greatest impact in a stock approach system. Since wavelength and frequency are inversely proportional and the speed of sound in air is slower than water, increased entrained air in the system will cause a longer wavelength, thus reducing the natural frequency.

Conversely, decreasing the air content will cause a shorter wavelength and increase the natural frequency in the system. If the speed of the pulse generated by a rotating element matches the speed of sound in the pipe, the rotational frequency and related harmonics can be greatly amplified. A single medium, such as a stock approach system, will not support an infinite number of standing waves of continuously different wavelengths. Usually, only harmonically related frequencies can occur in a single medium. CASE STUDY. COATED FREE-SHEET MILL. A coated freesheet mill was plagued with extreme sheet barring that would begin approximately 24 hours after machine startup. Featuring a topwire former and a hydraulic headbox, this fourdrinier paper machine runs at speeds in the 2,500 fpm range.

The stock approach system consists of a single screen; primary, secondary; tertiary, and quaternary cleaners; and a single variable speed fan pump. The paper machine had no deaeration device. The header recirculation line was piped into the silo with dual control valves located near the silo wall, below the liquid level.

The source of the barring was identified as three times and six times the primary pressure screen rotational frequency.This screen utilized a segmented foil assembly consisting of seven separate rings with three foils per ring, mounted on a center shaft. The foils were distributed in a random pattern around the circumference of the foil assembly. Based on the foil geometry, two rows of three foils passed the outlet of the screen in one rotation, generating three times and six times the pressure screen rotational frequency. The actual cause of the increased pulsation amplitude could not be determined. A thorough inspection of the screen indicated the mechanical components were within manufacturer specifications.

Several modifications had been made to the screen to minimize barring, including the addition of a variable speed drive. When barring became a problem, the severity could be reduced by increasing or decreasing the screen speed. However, the barring would return over time, and the speed would need readjusting. The screen rotating assembly was also rebuilt by the manufacturer. However, just 24 hours after its installation, the barring returned.

TESTS SUGGEST STANDING WAVE PHENOMENON. Unable to impact the barring issue, the mill contacted the AstenJohnson Papermaking and Diagnostic Services Team (PDST) for assistance. This group is comprised of papermakers and diagnostic engineers charged with helping customers by troubleshooting and identifying solutions to machine productivity and product quality issues.

To properly identify this problem, a full set of pulsation/vibration instrumentation, including a multichannel data acquisition system, was used to characterize the variability in the process. Electronic machine performance data were taken by introducing pressure sensors into stock lines and triggers on stock delivery rotating elements, as well as placing vibration sensors on machine structural elements and electronic machine process signal monitoring, such as the basis weight signal. Simultaneous sampling of multiple signals is essential to determining the propagation of the variations through the system.

Figures 1a-b present the front- and the back-side machine direction basis weight spectra under normal or "as found" operating conditions.The averaged spectra represent 11 minutes of machine operation with the scanner head in single-point mode. A comparison of these two spectral graphs indicates the amplitude of the three times and six times periodic variations is much higher on the front side of the sheet than the back side.This same relationship was also evident in the headbox header pressure.