Yankee dryer operation and maintenance
The Yankee dryer is one of the most essential parts of the tissue machine. This pressure vessel serves as a base for creping, pressing and drying of the paper sheet. Because of its importance to the tissue-making operation, mills should provide regular maintenance services to the Yankee dryer to maintain high runnability and ensure top-notch quality. On the Yankee dryer, the paper goes from approximately 40% dryness to just over 95% dryness, and is then doctored off into the sheet transfer area. In terms of paper quality, there are a lot of parameters that depend on the Yankee dryer’s performance, such as dryness, dryness profile, sheet quality, and so forth. In order to minimize downtime due to poor Yankee dryer performance, it is imperative that mill personnel pay close attention to the operating condition of the Yankee dryer.
Review Yankee dryer condition and performance
In order to maintain good Yankee dryer operation, machine operators should follow some basic steps. Some points to pay close attention to are:
- An uneven moisture profile can cause insufficient coating. Mills should check to see if the doctor blades are properly adjusted and/or if any Yankee wear can be spotted.
- Periodic inspections are required to determine if the complete Yankee surface is coated, otherwise the doctor blades will wear the Yankee surface where coating is missing.
- Uneven wear on the doctor blades indicates that there probably is something wrong with the coating or unnecessary Yankee wear.
- The profile of the doctor blades should be aligned after the Yankee surface to achieve even wear.
- Although Yankee surface edge wear does not directly affect production, it leads to an unnecessary amount of material that must be grinded off the Yankee the next time it is reground, in order to achieve the correct crown curve.
Additional Yankee tests
These are basic measures that machine operators and mill maintenance people can track themselves. If there is a need for additional tests, Valmet provides three types of safety service packages: routine-periodic inspection, fitness-for-service inspection, and non-routine inspection.
The routine-periodic inspection should be carried out annually. This inspection includes both an internal and external inspection in order to ensure safe performance and optimum efficiency. Experienced engineers use the latest technologies to inspect the pressure vessel, internal condensate removal system and all papermaking surfaces to ensure the safe operating performance of the Yankee dryer.
Fitness-for-service inspections are in-depth inspections of the structure of the Yankee dryer, and are normally performed after running the Dryer for several years (10-15 years). This is a non-destructive examination (NDE) of the Yankee dryer. It includes dye penetrant testing, wet and dry magnetic particle testing, ultrasonic testing and in special cases also radiography testing. This test will determine remaining vessel life and the presence of any large-scale anomalies that might adversely affect the Yankee dryer.
Non-routine inspections should be performed in the event of unexpected incidents. Valmet can provide prompt service and will use non-destructive examination equipment to make recommendations to rectify the damage. The goal of this service is to provide the mill with a prompt risk assessment and repair strategy to restore the Yankee dryer to safe operation.
These three inspections deal with the condition of the vessel and its mechanical status, but Valmet also offers a number of other services for the Yankee section:
- Press Loading Test to verify the existing load curve
- Crown Optimization defines the correct Yankee and Press Roll crown curves for the actual operating condition
- Yankee Doctor Test verifies the wear of the used doctor blade and tells if the correct parameters are being used
- Yankee Regrinds provides the Yankee dryer with an exact crown curve
- O-T-R (On The Run) Profiling verifies the Yankee profile during operation
- Internal Condensate Removal System Reviews verify the condition of the internal equipment inside the Yankee dryer Valmet has extensive experience and is well-qualified in the inspection and maintenance of Yankee dryers.
For more information on Yankee Dryers, contact your Valmet service representative.