A total nip solution pays off

By Steve Hightower, Glenn Harvey and Eric Diamond, Xerium North America

BRUSSELS, March 3, 2009 (Viewpoint) - The benefits of properly engineered paper machine clothing and roll covers can be exponential when a total nip solution is successfully investigated and engineered into a solution. This document describes how proper clothing engineering combined with the design of the total nip dynamics while utilizing the proper roll cover technology can provide such results.

A large producer of board grades has been working closely with the Xerium team to optimize its cost structure by maximizing the mechanical removal of water for the purpose of reducing energy costs and yielding additional potential output. The current machine configurations have been unchanged in numerous years. The customer is looking for total press section solutions that will drive as much cost out of its operations as possible. Each time a roll change is required, a new project is created to determine the optimal combination of press section dewatering elements. Then the change is planned accordingly.

The position covered by this case study is a first press on a 75-lb/1,000 ft² board sheet. The first press is a very important press on any paper machine. The first press is responsible for the majority of the water removal in the presses and sets the tone for the remaining presses. Incremental increases in nip intensity are required to produce effective moisture removal in successive nips. In order to optimize pressing without creating forces detrimental to sheet bulk and other sheet qualities, removing as much water as possible at the lowest nip intensity is required.

In this case study, the current configuration of the press is a 15 P&J rubber covered roll without venting in the top and bottom position (utilizing a regular double layer seam press felt design). Over time, the machine operating parameters have been modified to produce a superior quality sheet. Similarly, the clothing designs have been modified to maximize water removal. Consequently, the position’s vacuum capability and press felt compressibility have reached a maximum ratio; where any additional water removal needs to be completed mechanically at the nip. However, the mechanical load of the press has been reduced to maintain the sheet properties. The challenge was to improve the total water removal and maintain the current sheet properties. A detailed press section engineered analysis was completed to understand the opportunity.

In this application, the first press loading has been reduced by 50% to maintain sheet properties. The bottom press felt is totally saturated at this point. Any attempt to improve water removal through mechanical pressure has previously failed as higher loads generate higher stresses that affect sheet properties. Furthermore, any additional felt void volume hinders dewatering as a result of the limited vacuum capabilities.

Project

The Xerium team investigated the engineered approach of a total dewatering solution. Modeling the current pressing conditions and comparing these with optimal conditions was completed by using Xerium’s Press Manager program. The results of this study showed that properly designed roll covers will allow optimal loading while creating ideal nip conditions favorable to dewatering the sheet in a mild environment. This in turn, allowed the press felt design to also be modified to maximize the water transfer in this position.

The following table shows the current and optimized situations for both nip intensities and dwell times.

Figure 1 - Proper incremental increase in nip intensity is established through the presses with reductions of 23%, 19% and 8% in the second, third, and fourth presses. The improved nip width conditions represent increases of 116%, 30%, 22% and 17% in all of the presses.

Based on this study, recommendations were made to change the roll cover configuration to modify the nip dynamics. Since the flow in the nip will be modified, this will allow for the clothing to also be optimized. The same seam felt design was utilized but additional weight and drainage characteristics were adjusted to maximize the dewatering from the nip intensity and dwell time changes.

The basic recommendations covered:
• First press cover modifications to 70 P&J, 1-in. Super wear over a 10 P&J grooved roll to reduce the nip intensity for optimum intensity and dwell time. This will further allow utilizing the available 285 pli press loading without excessive compressive forces that cause premature felt compaction and sheet damage. This will generate more consistent pressures and profiles at the desired low-end range throughout the life of the cover.
• Modify the first press felts with higher permeability and additional void volume to help remove more water and improve the cleaning showers capabilities.

The clothing and roll cover combination was installed on the machine and immediate improvements were documented. The moisture levels measured out of the fourth press decreased by 2% points. This was also measured through a step drop in steam pressure. The benefits continued throughout the life of the press felts.

Based on these changes, the Value Results were calculated and approved by the mill. Cost reduction value was based on a per-year of 2% point reduction in wet end moisture.

The total innovative pressing solution available through the press modeling capabilities of Xerium can readily provide the necessary information to determine the optimal mechanical requirements for both, the roll cover technology and press felt application. This will allow papermakers to maximize the efficiency of water removal through the pressing stages and have a direct impact on the overall energy utilization.

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