Stern Tube Lubricant Absorbs Costs

In many industries and businesses, equipment failure or unexpected maintenance does not present a major problem. Standby systems can be brought online quickly, service engineers can repair machinery within a few hours, with spare parts that are readily available, or temporary equipment can be hired at a moment's notice.

For some industries, however, failure or serious malfunction of machinery can be very damaging, whether financially or commercially. Such industries include steel making, oil and gas processing, underground mining, and shipping.

Some systems aboard ships do not have standby backups, spare parts may be difficult or impossible to replace while at sea, even if they are carried on the vessel, and delays to voyages (whether through stoppage or slowdown) are likely to be costly. One item of equipment on ships that is not possible to replace or repair while at sea is a stern-tube bearing.

Benjn. R. Vickers & Sons Ltd., based in Leeds, England, offers the Hydrox range of specialized stern-tube oils. Hydrox stern-tube oils absorb water, which may enter the stern-tube bearing, and the resulting emulsions continue to provide a high standard of lubrication and corrosion protection to the shaft and bearings. All of the oils in the Hydrox range are approved under the Lloyds Product Verification scheme. They are the first lubricants to have been approved in this way, with benefits of the Hydrox oils acknowledged by other leading Classification Societies.

Oil lubricated bearings are more than likely white metal, but can be manufactured from specialized resin material. In either case the stern-tube is filled with oil, which is retained by means of a seal system designed not only to keep the oil in. but also to prevent seawater from entering. A header tank for the oil maintains a static head of pressure, which supports the sealing system in discouraging water entry.

Stern-tube bearings can operate satisfactorily — with few or no problems. Sometimes, however, the stern tube seals can become worn or damaged. Equally, conditions can arise whereby seawater bypasses the seal.

For example, the oil/sea-water pressure relationship can be radically affected if extreme pitching is experienced, or stern tube vibration can result in seawater being drawn in and/or oil being forced out. It is not at all uncommon for oil to leak out of the stern-tube, creating a pollution problem, or more often for seawater to leak in or indeed both — water can do enormous damage to plain bearings. Where a conventional oil is being used, the presence of water can cause rusting and corrosion, and it can seriously compromise the quality of lubrication offered to the bearings. Any of these factors is likely to lead eventually to bearing damage and failure. Obviously, a stern-tube should be fitted with high performance seals, but a specialist stern-tube lubricant, which is able to lubricate bearings even in the presence of significant quantities of seawater, can provide very real cost savings. The Hydrox range of stern-tube oils manufactured by Vickers are specifical- ly formulated to do just that.

Conventional engine oils that are often used to lubricate stern-tubes do not form stable emulsions with water, particularly seawater. Some "emulsifiable oils" may form emulsions, which are unstable, thereby allowing free water, leading to a breakdown in lubrication.

Water ingress into a stern-tube lubricated with a conventional oil or unsuitable "emulsifiable oil" may therefore lead to serious bearing failure and damage to the propeller shaft should there be any significant degree of contamination with seawater. Hydrox 550 has been developed especially for use in sterntubes.

It forms stable emulsions with water and these continue to provide a high standard of lubrication and corrosion protection. Consequently, unscheduled repairs are unnecessary.

Vickers calculates, for example, that an offshore supply vessel could achieve net savings of more than $21,538 where Hydrox 550 is in use and there is significant water entry into the system. These savings arise largely from the avoidance of unscheduled docking. Similarly, a large crude oil tanker could save more than $215,385 on a similar basis. The significance of the unit price premium is further diminished when the modest volumes of oil used in the stern-tube are taken into account.

For example, Hydrox 550 is currently being used on the Queen Elizabeth 2 (QE2), the Oriana and the Ocean Princess. It is also being used on Stolt- Nielsen ships and on Andros Maritime ships. Chris Zukowski, Atlantic Fleet Manager at Stolt-Nielsen Transportation Group, says, "Stolt-Nielsen vessels use Hydrox 550 as a stern-tube lubricant because experience has shown that seawater entry into the stern-tube can occur on occasions. Using Hydrox 550 greatly reduces the likelihood of interrupted sailing schedules due to emergency repairs."G. Foustanos, superintendent engineer at Andros Maritime adds, "Andros Maritime, the tanker fleet agent, is typical of the growing number of Hydrox 550 users who value the longterm benefits provided by the product." Hydrox 550 has been formulated so that it will form stable emulsions with up to 20 percent of seawater, which may enter the stern-tube. The oil is suitable for use in stern-tube systems fitted with circulatory oil-feed systems and is approved by leading seal manufacturers having been tested for compatibility with their seal materials. It may well be the case that a conventional oil is not compatible with the seal in which case excessive swelling or embrittlement of the seal can occur with consequent seal failure. Hydrox 550 is also used in cruise liner stabilisers, again to combat water ingress into the lubrication system.

The oil is approved by a number of stabiliser manufacturers, including Sperry Marine and Brown Brothers.

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Other stories from September 2002 issue


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