Factors affecting the sealing function: Temperature
In previous blogs we mentioned that there are different factors affecting proper functioning of the lip seal. In this blog we highlight the factor temperature. Both the temperature of the lubricant under the lip, as well as the temperature in the tip of the lip, are important to the seal’s lifetime and reliability.
Figure 1. Factors affecting the functioning of the lip seal
What is a sealing gap?
As can be seen in figure 1, there are different factors affecting proper functioning of the lip seal. For a radial lip seal, the sealing function depends on the mechanism that is capable of bringing a hydrodynamic lubricating film between the lip seal tip and its counter surface on the liner. During running in, the lip seal tip will wear and the sealing edge will be formed and roughened. The area occupied with the lubricating film is called the sealing gap. Both the temperature of the lubricant under the lip as well as the temperature in the tip of the lip seal are important to the seal’s lifetime and reliability.
FAILURE MODES DUE TO EXCEEDING THE MAXIMUM PERMITTED TEMPERATURE
There are several failure modes that affect proper functioning of the lip seal, when the sealing gap temperature exceeds the maximum allowable operating temperature. These failure modes, resulting in heat cracks or blistering, often lead to leakage and unscheduled maintenance in dry dock.
When a lip seal operates in temperatures above the operating limit, it is likely that heat cracks will arise. Heat cracks are small axial cracks that are visible on the lip seal edge, caused by hardening of the compound. If the lip seal has hardened it is also possible for breakouts on the sealing edge to occur. Heat cracks and breakouts will inevitably lead to leakage.
With regard to the water-facing lip seals, another phenomenon can occur; water vapourises under heat. This vapour can enter the elastomer material and when the water condensates within the molecular structure of the elastomer blisters will be formed, affecting the sealing function of the lip seal.