Prepared by Anzar Hasan (Chief Inspector)
The purpose of this technical bulletin is to advise inspectors engaged in the inspection of Yankee Dryers to be aware of a head/shell corrosion condition. This condition – and corrective action taken by the mill – should be a topic of discussion during the Yankee Dryer inspection and evaluation process.
There have been catastrophic failures of Yankee Dryers since the tissue paper manufacturing industry started utilizing Yankee Dryers. Due to the increasing number of failures that often require replacement of the dryers, the issue was taken up by TAPP in 1985. Studies such as finite element and root cause analysis were completed and several papers were submitted and published in TAPPI.
This crevice corrosion is a serious condition, and if not detected, could result in cracking of the head and shell. Depending on the size of the dryer, the replacement cost could exceed $3M, with a delivery time ranging from 9 to 14 months. If detected in the early stages, repairs and modifications can be satisfactorily completed.
In the majority of cases, the crevice corrosion started from the head spigot corner and clamp fit of the head and shell flange. The investigative studies concluded the problem was caused by chemical showers during the tissue paper drying process. The chemical showers are used to improve creeping characteristics of the paper. Uneven temperature condition in the head and shell caused a gap between the head and shell flanges at the outside radius, reducing the efficiency of the clamping force. The chemicals seep into the gap, resulting in corrosion to form in the crevice.
The growth of crevice corrosion is slow and difficult to detect as the shell deformation is masked by natural shell wear and the shell grinding process.
Special gauges are used to measure head tilt, head to shell spigot loss and establish crevice corrosion severity.
Nondestructive examinations techniques are also utilized to track corrosion growth and verify Yankee dryer structural integrity. These tests included acoustic emission and ultrasound.
The head/shell joint on new dryers has been modified to reduce and/or stop the crevice corrosion. If the corrosion is detected in the early stages, older dryers can be modified with successful results.
The methods utilized include:
- Removal of crevice corrosion by machining substantial cutback of the head outside diameter to remove corrosion-jacking force that concentrate stresses at the head spigot corner.
- Bore out the existing bolt holes and upgrade the bolt size to increase the clamping forces necessary to overcome permanent deformation caused by crevice corrosion jacking.
- Remaining dryer life should be calculated as recommended by engineering and/or Jurisdiction requirements.