Answer EKG Question 13

Q13. Sketch a sealing arrangement for an oil lubricated stern tube. Identify the common form of seal failure; State how oil loss due to seal failure can be restricted whist on passage? What material is used for sealing ring and propeller shaft liner?

Answer:

(A) Sealing arrangement

The SEAL is composed of an AFT SEAL, which prevents stern tube lubricant oil leakage outside the ship as well as seawater from entering the stern tube, and a FORWARD SEAL which prevents stern tube lubricant oil leakage into the engine room.

AFT SEAL

The AFT SEAL can be broadly divided into the casing, which is fixed to the hull, and the chrome steel liner, which is fixed to the propeller boss and rotates with the propeller shaft.

The casing is composed of three kinds of metal rings; flange ring, intermediate ring and Cover ring, which are tightened to each other with bolts. Three or four sealing rings are assembled between the metal rings with their pointed ends (lip section) touching the chrome steel liner. The lips are pressed hard against the rotating liner and maintain a sealing effect through water pressure, oil pressure, elasticity of the sealing ring and tightening force of the springs.

The sealing rings are numbered 1, 2 and 3 in order from the seawater side. The No.1 and No.2 sealing rings close out seawater, while the No.1 sealing ring also has the function of protecting the inside of the stern tube from foreign matter in the seawater. Lubricant oil in the stern tube is sealed in with the No.3 sealing ring.

FORWARD SEAL

The FORWARD SEAL is of similar construction to the AFT SEAL except that it is composed of two sealing rings. The sealing rings are numbered 4 and 5 in order from the stern tube. The No.4 sealing ring seals the lubricant oil in the stern tube. The forward chrome steel liner is tightened with bolts to the split clamp ring mounted to the propeller shaft.

(B) seal failure, loss of oil and temporary actions.

The common form of seal failure is the garter spring compression on the seal reduces due to grooved wear on the chrome liner causing leakage of oil through the seal. Hence at every propeller shaft survey the seals must be renewed to avoid stoppage of the ship and extra urgent docking of the ship.

In the event of oil loss from the seal which can be a noticed by watch-keeper  during the rounds by comparing the oil level in the gravity tank. 

• if the oil is leaking from the forward seal into the engine room through the cover ring, then the oil can be collected in a clean tank and can be reuse as a temporary action. inspect and repair the forward seal as soon as the situation permits. 
• if the oil is leaking is leaking from the aft seals into the sea water. adjust the oil pressure equal to water pressure.In case the aft seal is fitted with four seals in which two seals 1 & 2 are for sea water sealing and 3 & 3S are for sealing lube oil, close the lubricating oil supply into oil chamber between rings 2 and 3.

other methods include changing of oil to a higher viscosity oil such as main engine cylinder oil leakage quantity can be reduced. But this is only a temporary correction and renewal of the seals is inevitable and has to be done at the earliest.

(c). The material of the liner is chromium steel since it is non corrosive and the liner is in contact with the sea water. The material of the seal is VITON (flororic rubber) or NBR (Nitrile-butadiene rubber) which is resistant to wear.