Having a good press felt monitoring and conditioning system is paramount in order to avoid paper machine runnability and quality problems. This article presents some best practices regarding press felt monitoring and cleaning.
There is no single approach for keeping press felts performing well. Press design, grade, furnish, speed, and wet end additives differ by machine, so each machine line requires a tailored program to keep its press section operation optimized.
Before discussing what the cleaning schedule should be, we must identify what type of contaminants are in the press felts that affects water carrying capacity of the felts. This can be done by performing a used felt analysis. From the test results, a chemical cleaning program can be developed.
Chemical programs may include caustic, acid, or detergent type chemicals. These washes are carried out either by batch on the fly, batch during sheet breaks, continuously, or a combination of these. Each machine should be optimized using one or more of these cleaning methods. Many mills will perform a scheduled batch on-the-fly felt cleaning, usually every 1-2 days. Others will opt for only doing batch washes during sheet break situations or when sheet defects get too high or machine runnability too low. In either case, a good wash-up of the press section should be done periodically to eliminate debris which collects on the press section felt rolls, doctors, uhle boxes, saveall pans and machine frames.
For sheet side felt rolls with doctor blades, remember to wash behind and under the doctor blades, especially if the blades are pulled out for replacement – in order to eliminate deposits that could dislodge and transfer directly to the wet felt and thereby to the sheet. Deposits on top of the doctor blades don’t usually cause sheet defects; it’s what's under the blade that will cause the most harm as uhle boxes don’t do a very good job at removing fiber clumps from the press felt surface.
While used felt analyses are important, there are other tests that can be performed while the felt is running which can help explain machine runnability, drying demand, moisture profiles, sheet defects, etc. These include press felt moisture profile scans and water permeability scans.
Moisture scans can identify wide and narrow felt moisture profile irregularities and give an idea of press nip pressure profiles on young wet felts, before the felt becomes too compacted. Moisture scans are normally taken before and after the uhle boxes when possible. This provides information on the amount of water that the felt is carrying into and out of the press nip and the efficiency of the uhle box at removing water from the felt.
In the early life of a wet felt, moisture/permeability scans tend to be higher as most wet felts start out as being felt, or uhle box, dewatering – a condition where most of the water in the press nip is carried out of the nip with the felt. As the felt ages and is compacted and filled, its water carrying capacity decreases and more water is removed at the press nip. By trending these measurements, along with uhle box vacuums, one can determine how a felt is running compared to historical wet felts.
These trends also provide insight into the effectiveness of the felt conditioning program. For example, the uhle box vacuums will increase as the felt fills with fiber, fines and wet end additives. After a batch cleaning, vacuums will drop and the press felt will be able to carry more water out of the nip once again. At the end of the press felt’s life, batch washes will have limited ability to reopen the felt structure.
One final comment on nip dewatering - as more water is removed at the nip, there is more water spray and thus more fiber buildup on the savealls and machine framework. These areas will tend to build up deposits quicker and cause issues with sheet defects and machine breaks. They should be periodically be cleaned during wet end sheet breaks.
If the moisture/permeability scans indicate irregular or streaky profiles, the source of the streaks must be determined. Often the source of the problem can be traced back to plugged or partially plugged shower nozzles – either low pressure fan type conditioning and lubrication nozzles or high pressure needle shower nozzles. Low pressure shower nozzles that are used for chemical application need to cover 100% of the felt, overlapping if possible, and in a perfect world they would oscillate.
Partially plugged high pressure needle showers can do more harm than plugged nozzles if the spray pattern breaks up and becomes turbulent before contacting the felt. This condition increases the felt’s wear rate. For these needle showers, mills have been having good luck using ruby tipped nozzles and a clean filtered source of water for long nozzle service life. High pressure showers are normally set in the 150-200 psi range depending on the machine. In summary, all shower nozzles and oscillating systems need to be functioning properly in order to main flat moisture and permeability profiles and extend fabric life.
Interpreting press felt moisture scans and water permeability requires good understanding of the press section and wet felt design. Moisture scans and water perms change over the life of the wet felt. Comparisons between wet felts should be made at similar ages.
Early in their lives, press felt moisture content after the press nip will be higher as there is more void volume in the felt. At this stage, the felts will also accept more water as seen in the permeability scans. Looking at the moisture and permeability scan profiles during this stage of dewatering can be quite useful in understanding what the roll nip load profiles look like. On most uhle box dewatering felts, the felt moisture will tend to be higher where there is more nip loading as more water is forced out of the sheet and into the wet felt (i.e. on an undercrowned nip, felt moisture will be higher on the ends than in the center of new wet felts). As the felt compacts over the course of a week or two, the moisture in the felt after the nip decreases and more water is removed at the nip. On applications that start out nip dewatering, the opposite is seen – lower felt moisture where the nip load is higher. In either case, as the felts age, they compact and become more nip dewatering. Determining relationships between moisture profiles and nip load conditions becomes more difficult as the felt ages and deficiencies in felt cleaning systems affect felt moisture/perm profiles.
Another effective wet felt monitoring tool is an infrared thermal camera. Some fabric supplier representatives have handheld thermal cameras which are used to identify thermal banding in the wet felts and in the sheet coming over the center roll. Hot edges on pickup felts with steamboxes on the suction roll will be seen when the suction roll deckle seals are set out too far. A hot streak should be no more than 1/2” wide and less than 5-10 °F higher than the rest of the felt. If more than this, the felt strands and batting will oxidize and can cause edge pickup issues later in the felt life.
In summary, monitoring wet felt and press performance is a critical means to explain machine operating performance. Valmet has knowledgeable press felt service representatives that can help your mill understand how your press felts are performing. They also gather a lot of relevant machine information and conduct thorough inspections of the press section equipment as part of their service to the mill. From their observations and experience, they can make recommendations and suggestions for future wet felt design changes and equipment/process upgrades.
For more information on improving your press fabric monitoring, contact Simo Metsäranta, Technology Manager (firstname.lastname@example.org), or your Valmet representative.