Sealing arrangements on ships
This is one of a number of articles on seals and bearings and ships. Because a propeller shaft passes through the hull of a ship, there is always the possibility of fluid leakage in either direction. Seawater leaking into the bearing or worse still into the ship is a big problem for the vessel but oil or lubricant leaking from the bearing into the sea is just as bad leaving the ship open to penalties for polluting and costing a fortune in lubricants.
Over time many different sealing arrangements on ships have been tried and examples of all types can still be found in service today. The oldest variant is the stuffing box which dates from the same era as the use of lignum vitae as a bearing material. In this type of seal, material is wound around the shaft inside a cavity that surrounds the shaft. At the inner end of the cavity there is a gland nut which is threaded inside the gland and which is tightened to compress the material making for a reasonably leak-proof joint.
The material used for packing has changed with organic materials such as hemp, flax, wool and cotton being traditional materials which have been replaced in more modern vessels such as Teflon and graphite. Whatever materials are used, they are usually impregnated with a waterproofing agent such as grease.
Stuffing boxes are simple constructions and maintenance consists of ensuring that the packing material worn away due to the friction caused by the shaft rotating is replenished regularly. The need to replenish will become obvious as the amount of leakage gradually increases therefore regular inspection is an essential for ships fitted with seals of this type.
If the shaft is damaged in anyway and is out of true and running eccentrically it will erode the packing material in a manner that will allow fluid to pass along the shaft. The shaft can suffer from corrosion and pitting as some of the sealing materials breakdown over time.
Another type of seal which is more sophisticated than the stuffing box and which is commonly found on ships is the lip seal. Lip seals can be simple or of the labyrinth type with the latter being the most usual. In its simplest form a lip seal consists of a rubber O ring compressed around the shaft and housed in a grooved collar. In many of the available versions there are more than one rubber ring with successive rings coming into play as the prior one fails.
While this arrangement can work, the pressure from either side of the seal can cause it to fail prematurely. To overcome this the labyrinth seal was developed comprising of a series of barriers that gradually reduce pressure such that the final lip seal is not subject to a large volume of sealing media at the higher pressures.
The labyrinth part of the seal comprises of two parts – one attached to the shaft and the other to the stern tube. The labyrinth channel can be either axial or radial depending upon the design.
Maintenance is much less than on the stuffing box type but the complexity of the sealing arrangement does mean that in the case of failure more drastic action is needed and the seal will need replacing rather than merely re-stuffing.
Because the final sealing material used in the simple and labyrinth types of lip seal is an elastomer it is subject to friction wear and in addition the heat caused by the friction can result in the material’s characteristics changing causing it to become less elastic over time and so reducing the sealing ability. With the more complex labyrinth type there is also an obvious increase in cost due to the engineering needed to produce the sealing mechanism.
The third type of seal found on ships and used particularly on ships with higher propeller speeds is the Mechanical Face Seal. In such a seal, two face plates one fixed to the aft bulkhead and the other to the propeller boss and therefore rotating are forced together by means of springs. Two metal plates in contact would obviously cause friction but the sealing media between the two plates cools and lubricates to some extent.
Using a phenomenon known as hydrodynamic lift, spinning faces allow a small, sometimes microscopic layer of sealing fluid to ride between the faces thus reducing, or in some cases, nearly eliminating friction. However, the system is not without its own issues because various speeds of rotating equipment create different hydrodynamic lift profiles meaning that under some circumstances the friction and heat produced are not fully dissipated.
The space between the faces is designed to be so small so as to prevent the ingress of foreign bodies that could cause wear to the components of the seal. No mechanical device can be said to be infallible and face seals do occasionally fail dramatically. Some designs of face seals have a secondary emergency seal often air activated that will contain the worst of the leakage until repairs can be carried out.
Similar to labyrinth seals, mechanical face seals require additional support materials such as flushing water, oil and grease to keep them running unless they are of the water lubricated type.
Some of the seals in use today combine the lip and mechanical face sealing systems and in many cases, modern design allows for seals to be repaired or replaced in water and also permit extended shaft withdrawal periods for classification purposes.