Cryogenic bearings smooth path to greener maritime power

THE PRESSURE to reduce greenhouse gas emissions continues apace as does the search for ‘greener energy solutions’ and one of the consequences of this is a significant growth in the use of liquid nitrogen for marine applications. LNG has long been regarded as a sustainable fuel and has gained significant traction as a viable alternative for a wide range of commercial transport applications, including marine and shipping.

It is estimated that 3% of global greenhouse gas emissions such as carbon dioxide is generated by maritime traffic which is why the International Maritime Organisation (IMO) is increasingly looking to reduce CO2 emissions.

Whilst LNG is a much greener fuel than the highly viscous diesel fuels typically used to power container ships and cruise liners, it does present significant challenges to fuel systems and pumps, specifically dealing with extreme (cryogenic) temperatures.

Overcoming the challenges 

A vital component in LNG pumps is the bearings, which have to withstand the extreme temperatures of being fully submerged in Liquid Nitrogen at temperatures ranging from (-196ºC / -320ºF), or in Liquid Hydrogen (-253ºC / -423ºF).  

As fuel systems approach operating temperatures of absolute zero (0ºK) the ability to utilise liquid lubrication becomes increasingly challenging, which means dry running bearings are mandatory. Standard steel bearings (rated 52100) are simply ineffective, offering poor performance resulting in short service life and unexpected breakdowns. 

In addition to the challenges of lubrication the expansion and contraction of the different materials used in the bearing housings (as aligned with the coefficient of the required thermal expansion) becomes even more critical for bearing efficiency. 

Moreover, balancing the pump system, to account for the contraction of the materials, is essential. Therefore, choosing a bearing supplier who can offer the extensive specialised 'cryogenic' experience will help you to overcome these challenges. 

In addition to lubrication issues, special material selection and heat treatment processes is essential to help ensure that warping or non-linear contraction of the bearing components doesn’t occur. This includes additional cryogenic heat treatment processes and other proprietary con-siderations to help achieve the best possible results. 

Choosing the right cage materials

Bearing cage failure is a common occurrence in cryogenic applications so material selection, in addition to cage design, is critical to performance. Many common engineering plastics become too brittle at cryogenic temperatures, which is why the experts at Oxfordshire-based Carter Manufacturing recommend utilising the best bearing materials. These include lower cost options such as PEEK and PTFE with custom additives, or even Durafilm or PCTFE can be specified if nec-essary.

Cryogenic bearing testing

Depending on the specifics of each individual project being met and your confidence that all the right choices have been made, how do you know if your cryogenic bearing is going to be suitable to meet the demands of your application? At this stage of the manufacturing process testing is crucial which is why Carter has developed proprietary cryogenic testing procedures to mini-mise the risks - before your bearings even arrive on-site. All our cryogenic bearings undergo a strict testing procedure where they are fully submerged in Liquid Nitrogen (-196ºC / -320ºF) and performance torque and dimensional analysis, helping to ensure optimal performance for your application. 

Conclusions 

Switching to greener energy options and helping to reduce greenhouse gas emissions contin-ues to gather pace, as proven by Carter’s involvement in the significant expansion of LNG projects for the marine sector. LNG as a marine fuel meets all the current IMO and EU regulations and switching to it means complete removal of Sox and particles, along with a reduction of Nox emissions of up to 85%. The result is a reduction in CO2 emissions by at least 20%.

For more information: 

www.carterbearings.co.uk

Tel: 01865 821 720