Proper press maintenance, key operational checks and upgrades

Loading systems are an essential part of the press section. From single nip straight through presses to multi-nip presses, the loading system provides the mechanism to transmit force through the rolls into the nip. Because of its importance to the papermaking process, mills should provide regular inspection and maintenance service to the loading systems to maintain sheet dryness, high runnability, and essential sheet properties.

Depending on the type of press and felting arrangements, the press removes water from the sheet and can impact various sheet properties, such as caliper, strength, and surface finish. In order to minimize downtime due to poor press performance, it is imperative that mill personnel (operators and maintenance) pay close attention to the operating condition of the various press loading systems.

Potential problems

Process problems that can be caused by loading systems issues include:

• Sheet wrinkling
• Sheet bagging
• Poor machine direction sheet release
• Poor moisture profile

Possible machine problems that might occur are:

• Roll wear
• Vibration
• Decreased felt life

Key operational checks

To achieve good loading system operations, mill personnel should pay close attention to:

• Uneven sheet moisture out of the press. Check to see if loading is even on the tending side versus drive side. Also make sure that benchboard, DCS and/or on-machine devices are functioning and reading properly.
• Vibration of the nipped rolls. Machine speed increase, misalignment of the press, pivot pin/bushing wear or uneven loading are the probable causes of this problem.
• Even loading pressures, but variable cross machine sheet properties. If this occurs, inspect for mechanical interference, control component failure, airide set-up/type or pivot pin binding.

Changing design or components

Machine design changes also affect the loading system, such as when the fixed or pivoting roll types change, such as going from a granite roll to a steel/cast shell roll. When this happens the loading system parameters change, so the loading system setup must change correspondingly to obtain an even uniform nip.

Adding or removing components on the pivoting roll assembly changes the loading parameters. For example, installing or removing a doctor or changing roll covers may require the roll to have or not have water cooling. This in turn affects the press loading so that the load curves are no longer accurate.

Many machines have increased in speed which can result in machine and or loading system vibrations. A machine component could be vibrating causing the press loading system and or roll to vibrate negatively affecting the nip/sheet properties.

Upgrading technology

Pneumatic loading systems are essentially a "spring" loaded system which has natural frequencies and can vibrate caused from the felts and or external components which then can cause operating issues. Upgrades and new technology can improve the operation and possibly also increase the loading resulting in rigid running system with increased sheet dryness coming out of the press.

Calibrate the press

Mills are operating some presses at their original theoretical loading curves, which are based on design calculations. Many times, mills cannot accurately identify actual weights or friction factors.  To rectify this situation, mills should calibrate the press to verify and obtain the actual loading parameters. Then, mill personnel can revise the load curves to actual results so that operators know where to run the press to obtain a uniform and even nip.  From this point, they can then adjust the press loading to obtain required sheet properties.

Inspect regularly, consider investments

These are basic measures that mill personnel can check and track easily. If there is a need for more in-depth tests/inspections, mills can use various diagnostic tests to determine root causes and develop action plans to correct the problem.

Loading systems should be diagnostically checked, thoroughly inspected and properly calibrated every 4-5 years. This will provide the operators with the knowledge on how to operate the press, along with the confidence to make loading changes to obtain the best possible sheet profile and properties.

Loading systems should be reviewed for upgrades as small investment in the system can have dramatic payback. The improvements can be better run ability, increased sheet dryness, uniform sheet dryness, fewer roll changes, and better nip profile control.

For the most complete and accurate information on press upgrades and maintenance, contact Farhad Fereydoonzad, Senior Product Manager, Pressing at farhad.fereydoonzad@valmet.com.