Setting the standard

01 Jun 2004

Alfa Laval?s standardised test method ? a new five micron standard based on separation performance ? can enable buyers of fuel treatment systems to truly compare the different separators available.

Based on a reproducible test method approved by Det Norske Veritas (DNV) Alfa Laval?s Separation Performance Standard is now becoming a unified standard for independent verification of separation performance in centrifugal separators for marine fuel oils.
The standard is expressed as Certified Flow Rate (CFR) and DNV already offers a voluntary Type Approval of separators with CFR as part of its new Class Notation FUEL. The next step is the establishment of an International Standard on separation performance in cooperation with major players in the marine industry.
The reasons for cleaning fuel oil using centrifugal separation are easily understood.
Residual fuels may contain contaminants such as water, sediment and catalytic fines all of which have to be reduced to acceptable levels prior to entering the engine. Of these, the most damaging to the engine are catalytic fines, which are hard, abrasive, irregularly shaped particles of aluminium silicate. If catalytic fines are not removed before the fuel oil is injected into the engine, they can lodge in injection pumps, piston rings and cylinder liners and cause serious wear that can, in time, lead to breakdowns.
According to ISO 8217 standards, the maximum allowable catalytic fines in bunkered fuel is 80 ppm. The engine builder typically stipulates a reduction to a maximum of 15 ppm in the fuel oil when it is injected into the engine. To achieve satisfactory reduction of contaminants, minimise breakdowns, and prolong service intervals and engine lifetime, the oils are typically cleaned onboard ship by centrifugal separation before being fed to the engine.

Approved test method
The absence of an approved test method for separation performance, i.e. the stipulated reduction of cat fines from 80 ppm to 15 ppm, is however a grey area. To date, the shipping industry has placed no specific demands on makers of centrifugal separators to prove that the equipment they recommend can achieve this level of performance.
What Alfa Laval terms as "an important milestone in the history of centrifugal separation as a technology for cleaning marine fuel oils" has now been reached. A model test method for separation performance is now becoming a de facto standard. The company is, in cooperation with classification societies and other major players in the shipping industry, now initiating an International Standard on separation performance.
Selection of fuel oil separators is currently based on Maximum Recommended Capacity (MRC) tables provided by separator manufacturers. It is known that separation efficiency is a function of the separator's flow rate. The higher the flow rate, the more particles are left in the oil and therefore the lower the separation efficiency. As the flow rate is reduced, particle removal increases and cleaning efficiency improves. It is, however, essential to know at what capacity adequate separation efficiency is reached in the typical case.
There is no recognised reproducible method for measuring the relationship between capacity and separation performance so no one can be absolutely sure that separators chosen according to MRC actually provide safe removal of harmful catalytic fines from the HFO before it is injected into the engine.
Also, Alfa Laval explains that it is not possible to compensate for poor separation or to "back-up" separation performance by installing a fine filter downstream of the separator. The filter will mainly capture particles of 10 micron in size and upwards, while smaller catalytic fines will pass through if they have not been separated out earlier in the process. Centrifugal separators, on the other hand, can remove particles as small as two micron.
An alternative to MRC as a way of specifying separator performance is the new standard CFR. A separator's CFR is the throughput rate at which 85% of 5 micron Dyno particles (simulating harmful catalytic fines) are removed from the test oil (simulating a high viscosity fuel oil). The CFR represents a ?Safe? level for continuous, efficient separation.

Certfied flow rate
This gives the industry independent verification of separation performance and makes it possible to compare fuel oil separators based on separation rather than throughput capacity. With CFR measured for every separator, the customer can specify a unit of the correct size for the task.
The Dyno Test method uses 5 micron Dyno particles added into a particle free synthetic fuel oil of defined viscosity. The Dyno particles are identically sized, homogeneous, spherical plastic particles, normally used for the calibration of instruments. The selected Dyno particles size is in the critical size range of catalytic fines and their density is similar to catalytic fines. The Dyno test represents the most effective method to simulate the separation of catalyst fines from fuel oil.
A standard based on real catalytic Fines and real HFO is not possible because cat fines are not available in any standard size distribution. Fuel oils also vary in terms of both chemical characteristics, which affect the polarity, and in physical characteristics like density and viscosity. These wide variations make it impossible to obtain repeatable and comparable results from tests made on actual bunkers, hence a need for a standardized test method using a standard oil and well-defined particles.
The development of the Separation Performance Standard is positive for the shipping industry as a whole. For the ship-owner, the overall benefits are clear. Different separators can be compared on equal terms before deciding on purifiers for new-buildings. The ship-owner no longer needs to rely solely upon the claims of the supplier as there is an independent international performance standard.
The ship-owner may find the need to invest in a larger separator when making a selection according to CFR. But this can be compared with buying a separator based on MRC, plus paying an insurance fee against damage to the engine caused by inadequate separation. Analyses indicate that a reduction in engine wear of as little as 2% makes the selection of separators according to CFR a profitable investment.
Ship-owners have been quick to recognize the benefits and have already begun specifying HFO separators for new-buildings according to "the DNV certified flow rate method (CFR)". By doing so, they ensure a safe margin against wear and problems due to inadequate cleaning of the fuel. The margin is important in cases where fuel characteristics and operating parameters are substandard. Also, with separation performance specified, purifiers of different types and from different suppliers can be compared more easily in terms of price / flow rate ratio.
The general opinion among representatives of classification societies, ship owners, engine builders and oil companies seems to be that a standardised method for testing separation efficiency will benefit the shipping industry and contribute to enhanced reliability and safety in sea transports.
Based on the response received from the shipping industry, Alfa Laval says it believes that an International Standard on separation performance will be established soon.
Consequently, one given separator will have two alternative throughputs: MRC according to the separator maker?s assessment (separation performance unspecified), and CFR according to Class (specified separation performance), which is said to ensure a safe cleaning level of HFO with regard to removal of catalytic fines and other contaminants.

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