plays the most important role in the system. Since the seal ring consist of not only a rubber body but also a garter spring, an attentive consideration should be paid to the garter spring. If the garter spring is broken down, the difined differential pressure (abt. 15 kPa constant) can not be achieved because of reducing the total radial load, and especially in condition of bigger shaft vibration, the possibility to infiltration of the sea water into the air chamber become considerably higher.
Then, to get the highest reliability for garter spring, the material have been revised from the rubber coated stainless steel to Hastelloy(nickel alloy), which has one of the strongest corrosion resistance.
2) Flushing by the fresh water
The air, which always secures discharging from the needle valve of the drain collection unit to inboard, is essential to operate AIRGUARD as reliable pollution free sealing system. Because, if the air does not flow inboard, the sea water or oil leaking into the air chamber can not be collected with the drain collection unit.
To cope with some experience that the piping line from the air chamber to the drain collection unit is clogged by the salt, which is made from the sea water slightly spilt into the air chamber through the aftermost seal ring for a along time, a flushing line of the fresh water have been provided downstream of the air control unit. As the harden salt is easily dissolved by flushing of the fresh water, a small amount of the flushing water is perodically introduced to the drain collection line through the air chamber. While dissolving the salt on its way, it will be soon collected inboard.
3.4 Investigation of the used seal
Kinds of ships which installed or ordered AIRGUARD system is shown in Fig.10. Although more than 170 ships installed this system are navigating now, the seals which had been overhauled are not so many and only 31 sets were reported as shown in Table 1.
All of the removed seal ring which were collected by us after using have been very carefully inspected, then no failure was recognized in them. Since the aftermost seal ring is the most important on system, some typical conditions after using of these seal rings are illustrated in Fig.11. All of the used seal rings generated some wear on the tip of lip part, but the maximum rate of wear height is estimated approximately 0.24 mm/year and it must be equal or smaller than that of the conventional seal. On the other hand, all of the contact width areexcellently good as compared to that of the conventional seal.
