Shaft Seal Maitnance

Global effects of CoVid-19 may result in significant changes to the operation and maintenance of commercial vessels. With a reduced opportunity for planned dockings and restricted travel for visiting service technicians, this shaft seal maintenance guide will help ensure the ongoing reliability of your main propulsion shaft line seals.

Proper maintenance of the shaft seals has a direct effect on the operational reliability of your Sterntube bearings too.

Oil-lubricated Stern Tubes

These typically consist of aft and forward seals, normally 3 lip seals aft and 2 lips seals forward.

The following monitoring and maintenance should be carried out.

Aft Seal

  • When in port or during manoeuvring check for any oil leakage, visible on the seawater surface
  • If significant oil leakage found, plan a dive inspection to:
    • Perform wear down measurement
    • Check security of all mounting screws and plugs
    • Remove entangled lines and significant debris
    • Perform closer inspection of the oil leak source
  • Perform water content analysis of Sterntube oil and aft seal header tank oil
  • If significant water content found, and with reference to makers instructions, consider utilisation of high header tank position to minimise water ingress and consequential damage to bearings
  • Drain water from Sterntube where possible and replace with new oil.

If all the above checks are carried out and aft seal oil leakage is still an issue, use a stop leak additive in the Sterntube oil to minimise the leakage. Ensure a full service of the shaft seals is carried out at the next planned dry dock

Foward Seal

  • Check oil leakage rates are within makers limits
  • Ensure seal is suitably vented and lip seals are not overheating (Overheating can be identified by sight, touch and smell)
  • Ensure the header tank is at the correct level
  • Check alignment of the seal assembly, according to makers instructions.

Consider reducing rpm if:

  • Excessive oil leakage outboard not remedied by dive inspection/maintenance or use of high header tank
  • Sterntube water content cannot be managed and/or bearing temperatures spike or are consistently above normal
  • Forward seal leakages cannot be kept within limits after ensuring the above checks are made.

Water Lubricated Sterntubes

water lubricated seal

These typically consist of a single fwd. seal, normally single barrier with an emergency inflatable seal.

The following monitoring and maintenance should be carried out:

Foward Seal

  • Check water leakage rates are within makers limits.
  • Ensure seal is suitably vented and sealing interface is not overheating. Overheating can be identified by sight, touch and smell.
  • Check seawater flushing flow rate to seal (and Sterntube bearings) is to makers recommendations
  • Check seawater flushing pressure does not exceed seal limits, according to makers instructions
  • Check seawater flushing filtration standards are met, according to makers instructions.
  • Check alignment of seal assembly, according to makers instructions.

Consider reducing rpm if:

  • Forward seal leakages cannot be kept within allowable limits executing the maintenance above.
The oil level in the sterntube gravity tank is dropping. What is the issue?

If the seal header tank oil level is rising, then the lip seals either side of the Sterntube volume are leaking. This should be monitored, and plans made for repair.

If the seal header tank oil levels are not rising, then either there is a system pipework leak (trace and rectify) or oil is leaking outboard of the vessel. This should be investigated immediately with a visual check at the eft end of the vessel and a dive inspection if required.

My forward water lubricated seal is hot to the touch, is this normal?

There is a contacting sliding surface so friction and consequently heat is created. If extremely hot then it is likely that either lubrication is insufficient (check flow rate and venting of the seal), the pressure is too high (check flush supply pressure) or the load on the seal is too high (check compression settings with consideration of axial shaft movement).

Your oil and water lubricated seal spares are available at ATZ .

Sterntube oil ‘stop leak’ is available ex-works for emergency oil leakage situations.

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