Is the failure of the air supply system on a stern tube air seal, critical for the stern tube system?
Air supply failure on stern tube air seal is of course critical since it risks vessel with sea water penetration at stern tube. Air supply failure is uncommon, however, if it happens it means that the system will be dependent on the lub oil pressure of tank unit, which supplies oil at fwd stern tube seals (engine room side). In case the pressure of seawater is higher (ie at higher drafts) than the tank unit pressure, then sea water will penetrate at stern tube, therefore air supply system should be made operational the soonest possible, which generally speaking is not very complicated to achieve.
@Philip.Argyropoulos is correct, nevertheless, in the event of there being a problem with the air seal flow regulating system or if the air control unit fails completely, lubrication of the stern tube is maintained
by circulating oil round the stern tube using the stern tube LO sump tank and stern tube gravity tank. The oil pressure unit pumps are still operational during this emergency operation and the same static head is continually maintained on the sealing arrangement.
In this case the system is not “optimized” as when the air supply is available. It is considered as an emergency mode allowing the vessel to sail safely until the air supply is restored.