A suction roll may be the single most expensive component found on a paper machine. Suction rolls vary in size from 1 foot to over 5 feet in diameter, with lengths ranging from 100 to 400 inches and more. In addition, speed and loading vary by application. Speeds range from 100 fpm to 7,000 fpm, and loading can be anywhere from 50 pli to 800 pli. Mill maintenance departments face a real challenge in keeping these rolls running at top efficiency.
To optimize the efficiency of and ensure the best return on a suction roll investment, mill maintenance departments should follow these basic guidelines.
The starting point for any optimizing program should be the operating and maintenance manuals provided by the original equipment manufacturer (OEM). A basic maintenance program includes the following steps:
- Lubricate suction rolls according to instruction plate installed by OEM. These instructions establish the type of lubrication and recommended time intervals.
- Thoroughly flush the suction roll whenever the machine is shut down or on crawl for more than a few minutes.
- During regular shutdowns, move the end deckles in and out (full length) to clean the surfaces on which they move. Also keep the deckle adjusting screws clean and lightly covered with a good grade of lubricating oil.
- Rotate the suction box in both directions, approximately 30 degrees, and return to its original position. This practice will keep the adjusting mechanism in operating condition.
- Once a week, check suction roll bearings when the roll is operating. In addition, maintenance personnel should purge grease lubricated bearings of water-laden grease whenever the machine is down.
- Remove the suction box from the shell and thoroughly clean both at least once a year. In addition, check packings and airload tubes and replace if needed.
- Grind the outside diameter and balance on a routine basis.
In addition to the basic maintenance guidelines, the key to optimizing a suction roll’s efficiency is a more comprehensive approach to roll repair. Because suction rolls are uniquely designed to match operating conditions, a customized maintenance schedule developed by the suction roll manufacturer and a roll specialist can be the key to successful optimization. It includes:
- Roll data - An effective maintenance program begins with accurate and current roll data.
- Nip impressions - To optimize roll and roll cover performance, mills should take nip impressions each time they reinstall a suction roll. This ensures a uniform and symmetrical pressure in the nip and allows mill personnel to check the amount of crown needed to compensate for the bending of the nipped rolls,
- Alignment - Mill personnel should routinely check rolls for proper alignment. Misaligned rolls can cause premature felt and cover failure.
- NDT - A non-destructive testing (NDT) program should be set up to monitor suction rolls. On a predetermined maintenance/overhaul schedule, key components of a suction roll should be non-destructive tested. An optimum time to inspect the shell is just prior to applying a new cover.
- PLI rating - A roll specialist should inspect a recovered suction roll and, if warranted, give a new PLI rating. Due to normal operating conditions, wear, corrosion and erosion, mills may need to replace a suction roll or re-rate and run it at a reduced loading pressure.
- Roll cover - Suction roll performance can be enhanced by using the proper roll cover. Mills must consider paper grade, felts, chemicals, sheet temperature and nip psi when selecting a roll cover. A roll repair specialist can help establish suction roll baselines to determine the optimum time to run between roll change-outs.
- Balance - Suction rolls require a precision dynamic balance to ensure roll stability and to minimize vibration and run-out at high speeds.
- Roll regrinding - Mills should base suction roll regrinds on run time to ensure maximum cover life. Crown adjustments should be recalculated according to press loads and recent nip impressions.
With the complexity of today’s mill operations, mills should expect suction rolls downtimes of $35,000 or more. Optimizing suction roll performance ensures the best return on one of the biggest component investments on the paper machine.
For more information on suction roll maintenance, contact your Valmet representative. Another good source for suction roll performance optimization tips is the "Modernizing Rolls - Suction rolls" white paper found in the list of ROLLS related links.