A balance is needed between compliance and engine longevity, writes Ronald Oyer
Today’s environmental regulations present many challenges to vessels, especially those equipped with two-stroke diesel engines.
To burn regulation-compliant low sulphur marine fuels while maintaining optimum lubrication, acceptable cylinder wear rates and power component cleanliness, we must understand the impact of fuel sulphur on the development and maintenance of a well-conditioned cylinder running surface.
Sulphur in marine fuels, while environmentally objectionable, serves to etch the surface of the cylinder liner chemically when the fuel is burned at a properly proportioned rate and in a mechanically sound engine.
This etching provides minute porosity in the liner surface, a characteristic essential for proper lubrication and sealing.
The extent of the etching is controlled by a balanced relationship between the lubricant’s alkalinity, measured by Base Number or BN, and the sulphur content of the fuel being consumed.
Ships with two-stroke engines tend to find cylinder lubricants with 70 BN adequate when burning relatively high sulphur fuels (3–5 per cent). Their engines experience an acceptable level of acidity for controlled corrosion – etching – as well as depletion of the additive package, which reduces the build-up of harmful deposits.
The arrival of low sulphur fuels, generally regarded as bunkers with less than
1.5 per cent sulphur content, has added a new factor to the equation.
Studies have shown that extended periods of operation on such fuels, coupled with traditional 70 BN cylinder lubricants, may cause the controlled corrosion to diminish and deposits to accumulate in the ring pack areas as well as on the piston crown.
Hard deposits on the piston crown cause increased wear and scuffing of the cylinder liner, while ring groove deposits restrict movement of the piston rings.
A quick fix appears to be in reducing the cylinder oil rate based on the sulphur content of the fuel. This procedure seems to have merit, but it opens the door to a very subjective application of theory.
On the other hand, with the advent of sulphur oxide emission control areas, lubricant manufacturers have stepped up to the plate with lubes in the 40–50 BN range, viewed by some as relief in the form of a ‘mechanic in a can’.
Discussions on fleet-wide operations on distillate fuels will pose yet another challenge to engine builders and lube suppliers alike.
Shifting to distillates is not as simple as it seems, particularly in plants employing large two-stroke engines. We have to investigate the effects of long-term distillate use on cylinder running conditions.
With this in mind, changes in lubricant characteristics, cylinder liner and ring pack metallurgy and surface profile may be required.
An old cliché among ship engineers is that “the fun begins at 50,000 hours”. This refers to the average life of a well run in and maintained cylinder liner and the resulting replacement work involved.
Many cylinder liners see a life far below – and some way above – the 50,000-hour average.
Experts agree that cylinder conditions and the overall combustion space characteristics of a two-stroke engine over its lifetime are determined by the initial running in of the power system components such as pistons, rings and cylinder liners.
That said, a conscientious plan of initial inspections, adjustments to cylinder oil rates and follow-up inspections, as well as further adjustments, often leads to a plant delivering maximum availability with minimum downtime. Four-stroke plants, while not faced with the complexities of independent cylinder lubrication, must still deal with fuel sulphur and its impact on cylinder condition as well as corrosive effects on other areas of the engine, for example bearings and their respective journals.
While ring pack accumulations are not common due to the flushing effects of splash lubrication, close lubricant condition monitoring should be conducted to assess the suitability for continued use, particularly when burning fuels with elevated sulphur levels.
How do we attain the balance that ensures environmental compliance and component longevity required for profitable operations?
To make informed decisions on burning low sulphur fuels we must have first-hand knowledge of the physical and chemical characteristics of the fuel as bunkered.
This is achieved through conscientious fuel procurement, followed by timely analysis based on internationally recognised standards such as ISO 8217:2005. Such analysis yields a number of quality indications, including sulphur content and the presence of abrasive contaminants in the fuel.
While abrasives in marine fuels have always been a prime concern, it becomes more so in two-stroke plants running on low sulphur fuel and high BN cylinder oils at reduced feed rates. This applies to operation for brief as well as extended periods.
In this regard, DNV Petroleum Services encourages all operators to work closely with their respective engine manufacturers, including subscribing to service bulletins and taking advantage of factory-sponsored training.
The websites of many prime engine builders also contain up-to-date service and environmentally sensitive information.
In all instances ship operators should follow the recommendations of their engine builders strictly for low sulphur fuel operation.
Developments in cylinder lubricators, including placement of injection points and feed rate based on engine load, go a long way towards easier calculations and adjustments of lube oil feed rates.
Yet, nothing to date can replace the factual indications obtained by visually examining the cylinder running surfaces, ring pack areas and scavenge spaces.
Above all, we can never over-emphasise the importance of proper fuel injection and combustion, especially when faced with reduced lube oil feed rates.
The same preface applies to lubricant and fuel treatment plant manufacturers.
Armed with the knowledge of fuel characteristics, reliable decisions on lubricant selection and fuel treatment plans will significantly ease the concerns of low sulphur fuel operation.
From the shoreside operations, until supplies of both low sulphur fuels and compatible lubricants are stabilised, ship operators should pay more attention to voyage planning and bunkering to ease the burden on the ship staff.
They should never lose sight of their vessels’ limitations in terms of multigrade fuel and lube use.
As shipping is a complex endeavour, and considering the possible consequences of fuel-related problems, involving more parties in the fuel and lube equation may see some traditionally price-driven factors viewed differently.
The entire ship management team and operating staff should work towards a well defined system of total fuel management from conceptual design, delivery and subsequent operations.
Be it two-stroke or four, a well-commissioned and properly maintained plant under the care of trained and knowledgeable staff can deal effectively with issues related to low sulphur fuels.
Finally, as increasing numbers of jurisdictions reach out to develop low sulphur legislation, fine-tuning individual plants for safe, reliable and profitable operation on low sulphur fuels will also require the full co-operation of all concerned.
Ronald Oyer is technical adviser with
DNV Petroleum Services’ Houston office.
Email: Ronald.Oyer@dnvps.com
For information on marine fuel management solutions: singapore@dnvps.com
Date: 2007-10-09