The L.O. pump unit is arranged by oil pumps, oil filters, coolers and others. It has generally a automatic change-over system of the dual units to deal with the troubles of the unit. The pump supplies a greater part of the oil to the stern tube, while the rest of it is branched to the oil chamber by adjusting the valves in the oil circulation line at the time of installation of the system, and the oil flows from the oil chamber to the stern tube through under #3 seal ring like as air flow under #1 seal ring. In order to confirm the oil flow to the oil chamber, flow meter is set up on the way of the branch.
The pressure gauges are also provided to periodically measure each pressure in the air control unit, drain collection unit and L.O. tank, and the oil pressures in piping lines for the confirmation of normal operating.
Only #2 seal ring, which is lubricated by the oil, is directly rubbing on the liner. But #1 and #3 seal rings are always operated under the floating condition by the air and oil film respectively. Especially #3 seal ring is prepared as the spare of #2 seal ring, then it can be come to working condition by closing the valve to the oil chamber in the event of #2 seal ring damaged. So it will be expected that #1 and #3 seal rings are maintained comfortably, and the system increases the reliability and safety as the results.
Besides, the forward seal is enough to install the conventional type having two seal rings, because the running condition, which is hard to be affected by the sea water, is not so severe compared with the aft one.
2.4 Follow-up mechanism of the air pressure
A schematic drawing, which focuses to the follow-up mechanism of the air pressure on AIRGUARD sealing system, is shown in Fig.4. In the air control unit, the compressed air P1 from the air regulator flows into the air inlet of the flow controller, reduced to the secondary pressure P2 by the needle valve, reduced again by the changeable clearance C of the diaphragm in the flow controller, and streams toward the piping connected to the air chamber in the aft seal. The air pressure P3 in the air chamber blows into the sea through the clearance h under the lip of the aftermost seal ring.
The flow controller keeps the differtial pressure (P1-P2) constant by the needle valve and the diaphragm as its own function, and also maintains the air flow rate Q constant (about30-40 L/min ANR), due to the inlet air pressure P1 is preserved as constant by the air regulator in the air control unit.On the other hand, the aftermost seal ring keeps the differential pressure (P3-Pw) constant as the cracking pressure, about 15 kPa.
