Simplex sterntube seals, made by SKF Marine GmbH (formerly Blohm + Voss Industries, have been used since 1948, setting the standard for all other manufacturers to follow. The SC2 and SC3, also called the Simplex-Compact 2000, are the latest developments of the oil-lubricated sterntube seal. Demand shows that this is still the preferred technology for ocean-going vessels.
The SC3 features include:
The SC2 features include:
circumferential liner speeds of up to 10m/s The SC2 and SC3 seals can be supplied as a completely split seal, so that the sealing rings and the liner can be changed without pulling the shaft. The AFT housing is also axially split. The split components allow easy and quick service and the arrangement is therefore ideal for reducing drydocking costs and time off-hire.
In addition liners with special coatings for severe operating conditions are available.
The SC2 and SC3 seal system is available in a number of basic configurations, but the seal can also be adapted to the particular application.
The AFT seal is equipped with an oil pipe through the sterntube for pressure control by means of a 30 litre tank located in the engine room. The pressure control is designed for medium and large shaft diameters and is provided for seal sizes 400mm and upwards.
With the “Leakproof” system, the sterntube gravity tank is arranged in such a way that the pressure of lubrication oil in the sterntube is slightly lower than the seawater pressure. The oil chamber of the seal is connected to a 50 litre settling tank by means of two pipes leading through the sterntube. With the shaft rotating, oil is circulated and fed by the circulator back to the settling tank. Any water entering the oil is diverted through a pipe to the settling tank and can then be drained off.
The design for the AFT seal with four sealing rings is the result of specific market demands. The seal includes an additional sealing ring which faces the sterntube and serves as a spare ring. Using appropriate bypasses it is possible either to let this spare ring run without load on the liner or to activate the ring if required.
This is a four-ring sealing system with a dedicated air chamber allowing for a perfect separation of sterntube oil and sea water. Any liquid which passes into this airchamber is blown back into a holding tank using compressed air, which operates automatically according to timings which can be adjusted by the crew. The life of the sterntub bearing is expanded by eliminating water ingress into the sterntube system. Likewise, no oil can escape to the sea. The Simplex Airspace seal is recognised as an environmentally friendly, non-pollution seal which can continue to use mineral oils under the terms of the Vessel General Permit 2013.
Diagram of an airspace non pollution seal
The choice of Simplex sterntube seal sealing ring material is based mainly on the vessels operating conditions (circumferential speed, oil pressure in sterntube, size of seal, distance from shaft centre to load waterline, shaft revolutions).
The SC3 sealing ring material selection is at present:
The SC2 sealing ring material selection is:
