Bearing and seal performance improved through use of composite materials
Wärtsilä claims to be the only current supplier able to provide composite technology for both seals and bearings, and as a result can offer longer life, easier maintenance and added customer value in marine applications.
Wärtsilä says its seals and bearings business has always been focused on providing the appropriate product for the relevant application requirement. With the ever increasing demand for higher performance, high reliability and rapid availability, the emphasis today is on further development of products and solutions through the use of the latest technologies. New and improved material technology developments can be tailored to meet the demanding conditions found within the modern marine propulsion industry, but compliance with classification requirements is also critical.
The company’s global marine technology plays a key role in the development and improvement of its seals and bearings related products, services and solutions. The strategic goal is to ‘be recognised as the leading seals and bearings solutions and services provider in the marine industry’.
Wärtsilä has made several major acquisitions to aid its seals and bearings business. These include Deep Sea Seals and Japan Marine Technologies Ltd in 2003, Railko Marine in 2007, and most recently in 2011, Cedervall & Sons. These acquisitions have added experience and comprehensive composite test equipment, along with dedicated material scientists.
Wärtsilä has crelocated its composites operations to a dedicated new facility in Slough, England, and set about the development of new, high-performance materials for use in its evolving seals and bearings portfolio.
The company has launched new sealing solutions in 2012. The first of these is the Jetguard Seal, specifically for waterjets. This aims to improve on other waterjet sealing systems on the market through longer time between overhauls and increased overall in-service operational life, thus providing greater customer value.
The Wärtsilä Jetguard Seal design is based on the successful Maneguard seal range. In-house testing has operation in the arduous conditions that may be encountered by water jets on shaft sizes up to 410mm. To comply with high rpm and possible negative water pressure differentials, the water flush on the Jetguard Seal is directed effectively towards the seal interfaces. The interfaces are made of Silicon Carbide (SiC) to provide extended life, even in abrasive-laden waters. The seal has been specifically designed for composite components and offers simplicity, light weight, no corrosion and cost-effectiveness. Being a partially split design with an inflatable seal, it enables the wearing parts to be maintained without shaft removal.
The Jetguard Seal is now specified as the first choice for the Wärtsilä midsize waterjet range, which the company expects to account for some 300 new installations over the next four years. The Sealift Inc HSV-2 Swift, a wave-piercing, aluminium-hulled, commercial catamaran, is the most recent test bed for the Jetguard Seal, with trials running successfully under extreme operating conditions.
Another recent introduction is the Wärtsilä Enviroguard Seal, for large size water-lubricated sterntube applications. This has been developed as a high-end product for the commercial propulsion market. Primarily aimed at larger shaft sizes (460mm to 820mm diameter), it is suitable for both open and closed water sterntube systems. As with the Jetguard Seal, the composite material provides long life and reduced corrosion, and is easy to install due to its light weight.
In addition to the introduction of composite housings, the new sealing interface combination of advanced composite running against phosphor bronze has proven to be highly resistant to abrasive conditions during in-house tests. Highly abrasive water conditions were tested for 1400h, using maximum pressure and a maximum seal compression state. Results confirmed zero interface wear, the ability to run up to 150rpm (depending on shaft size), and increased axial capability for large shaft movements or the thermal expansion of interfacing equipment. Like the Jetguard, the composite Enviroguard Seal is equipped with an inflatable seal to enable overhaul while afloat.
General features and benefits of Wärtsilä composite seal technology include:
- ‘Hard’ sealing contact materials (face and seat) are selected for their suitability for a water lubricated application where abrasives are frequently present
- Closing force at the face is generated by an elastomeric element or helical springs.
- Can accommodate large and repeated shaft movements
- The seal is ‘pressure balanced’ so that changing draught will automatically adjust the closing force to an ideal value
- Static Inflatable seal - activated to allow maintenance of the seal without dry docking
- Composite material prevents galvanic corrosion
- Simple lightweight installation
- Negligible shaft or sleeve wear
- Proven capability in aggressive environments
- 15 years-plus in-service life
- In-house production enables quick turnaround.
Wärtsilä composite bearings cover both oil and water lubricated applications. Since 2006, the company has provided more than 1,300 composite bearing installations. The seals and bearings development team is constantly aiming to improve its products and materials via ongoing testing and validation trials. One result of this development process is a new composite bearing material, Envirosafe, launched for trial water-lubricated sterntube applications in 2011.
According to Wärtsilä, installing a water lubricated composite bearing seal system not only eliminates the environmental impact from the propulsion shaft line, it also brings other benefits. Composite materials are non-metallic and have been designed to cope with extreme operating conditions, such as high loads, speeds, temperature fluctuations and dirty conditions. Depending on the application and grade, composite materials can operate dry, partially lubricated, or fully lubricated in oil or water (even emulsified).
Wärtsilä’s new water lubricated propeller shaft bearing solution fits both new build and retrofit applications, and is suited to naval and other ocean going vessels, as well as offshore installations. In the Envirosafe water lubricated bearing, thermosetting cresylic resin together with filament winding produce a material that runs on a water equivalent to oil/white metal combinations. Compared to existing alternatives, the new material has improved the stability and hydrodynamic running capabilities. It has class approval for ABS, LR and DNV, and is available in various split and non-split configurations based on in-situ serviceability requirements. Scopes of supply vary from raw billet through to fully machined assemblies, including housings.
Development, validation, and testing within Wärtsilä’s bearing test facility continues to improve the Envirosafe material, and to certify its performance advantages over currently available bearings. Each trial carried out has demonstrated good hydrodynamic operation and extremely low wear achieved at very low shaft speeds. During trials, four separate tests were carried out starting from a maximum 200rpm, and reducing for each test.
In order to compare the wear (and therefore establish the life) of the new material against those in current use, performance of various bearing materials was analysed. The tests were undertaken in highly abrasive (silica) loaded sea water conditions against stainless steel. Initially, the rubber material performance was similar to that of the polyester/nylon and Envirosafe composite. However, over time the wear rate increased rapidly resulting in the test on the rubber material having to be terminated at around 850h.
The Envirosafe material exhibited a more linear (steady state) wear result. The test in this case was continued to 2,000h. The bearing was still capable of further operation. It is worth noting that the new bearing took almost four times as long to reach the same wear down level as the rubber bearing. The shaft sleeve was not heavily worn; as the new material is relatively more hard than rubber, the material does not allow abrasive particles to embed in its surface, therefore maintaining a hydrodynamic water film and increasing bearing and shaft sleeve life.
Based upon the test work completed, Envirosafe has demonstrated:
- Consistent and predictable hydrodynamic performance, which improves over time.
- Potentially increased bearing service life and longer docking cycles.
- Improved wear in abrasive water conditions when compared to competitive materials
- Reduced shaft removal requirements.
- Potential reduction in total ownership costs.
- Operational capability on multiple shaft liner materials.
- Lower thermal expansion than previous material (reduced by three times).
- Good thermal stability (up to 150°C).
- Lower swell factor (up to five times lower).
- Low risk (may be run dry).
- Classification Society approvals from major societies
Wärtsilä adds that composite technology does not only exist in water lubricated applications, the company’s portfolio includes Sternsafe and Steersafe, developed for oil lubricated sterntubes and rudder bearing systems with various lubricated requirements.
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