Stern tube

Oil sealing:

how oil loss due to seal failure can be restricted while on passage

Under normal condition, seal activated through #3 sealing ring.

            (valve V3 & V4 normally open).

     If oil leakages take place, shut V3 & V4 respectively and drain line – #3S will be activated.

     Pipeline connection to oil tank for chambers #2 & #3 seal – prevent oil loss into the sea.

     Sea water ingress into stern tube by draining into collection tank. If #3 & #3S sealing rings are both damaged, stern tube hydrostatic oil pressure to be lowered – using temporary oil tank & vinyl hose.

Sealing arrangement

(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, the elasticity of the sealing ring and

the 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 with the No.3 sealing ring.

Itemise the factors which adversely affect seal life.

• Rubber seal harden/deteriorate
• Damaged due to foreign objects-rope, fishing nets, sand
• Cooling failure
• Lube oil contamination
• Loading condition – axial & radial movement, vibrations
• Distortion/rupture of seals due to high pressure differential

Air Sealing:

Sealing arrangements:

.stern tube

→System operates by spouting air into the sea.

→Leakage of system by sea water or oil in system is drain down to a collection tank (leakage tank)

→Air pressure passing through #2/3 seal is about 0.2~0.4 bar higher than sea water

→The S/T LO tank is set about 2~3 meters above the propeller shaft centre line. This tank is air tight and connected to air control unit. The air pressure in the tank is same as air pressure through #2/3 chamber.

→LO pump when running passes through #3/3S seal. A needle valve in system is to be adjusted to adjust the LO pressure to about 0.3~0.5 bar higher than air pressure in #2/3 chamber.

Oil filling:

→Close air supply

→release air pressure

→meantime sea water pressure on seal no-01 will prevent water from entering to engine room

→ fill required amount of oil and afterwards normalize the system

Troubleshooting:

Stern tube liner: Chrome liner

.stern tube 

.chrome liner

Material:

High nickel chromium steel –

→  Excellent wear (abrasion) & corrosion resistance

→  Optional ceramic coating if applied on running surface, heavy duty rings are mandatory

Chrome liner fitting:

• Seal is gripping on to the chrome liner. Chrome liner is bolted on to propeller boss(aft). An o’ring is fitted between the chrome liner and propeller boss to prevent sea water ingress.
• Fwd chrome liner is fitted on to a clamp ring . Clamp rings are of 2 halves and have holes.
• Aft  sealing assembly consist of 2 main and 1 aux sealing rings. All sealing rings are spring loaded . Fwd sealing ring prevent oil leakage to sea .Lip seals hold oil within Stern tube and accepts small misalignment
• O ring fitted between boss and liner
• Chrome liner act as rubbing surfaces for lip seals. The chrome liner at the aft end protects the steel shaft from seawater and corrosion .Grooving will not happen on the shaft because of liner, so that the chrome liner can be machined or can be shifted axially by putting spacer. If grooving occurs leak will start.
• Seal no#2 and no#3 is connected to air supply system ,#3s is the spare seal 
• If water content found on the stern tube that means along with #1 and #2 , then #3 seal is also leaking. Then we have to put #3s seal in use.

Causes & Actions in case of seal failure?

• Causes:
  • Seal Ring damage.
  • Scored Chrome Liner
  • Seal Assembly loose & crack.
  • Oil line or airline leaking in aft peak tank
  • Fishing net caught in the Aft Seal assembly.
  • Bearing wear down will allow the shaft to drop, causing the seal to overload, clearance may develop between seal & liner.
  • Low LO pressure or air pressure
  • Misalignment & vibration
  • LO cooler SW side leaky

• Action:
  • AT SEA:
    • Inform master
    • Reduce engine RPM
    • Reduce Aft draft is possible
    • Check sea water cooled LO Cooler (If SW ingress)
    • Increase oil pressure/air pressure in seal by raising height of gravity tank or adjust air pressure or
    • increasing tank back pressure or using high viscous oil (Gear oil, mixing grease)
    • Isolate LO between #3 & #3S seal and activate #3S only
    • If SW ingress found in drain collection tank, then water is coming due to failure of #1 or #2 seal ring, in that case AIR PRESSURE need to be adjusted higher than SW pressure

 o    AT PORT:

• Inform office
• Arrange for a diving team for inspection
• If rope guard is damaged or propeller shaft entangled with fishing net then those
• needs to be removed
• Eventually replacing seal in-situ by shore team if possible