New technologies to deal with cat fines in HFO
Remains of cat fines in the fuel oil entering the engine account for a considerable part of the wear of the combustion chamber components in two stroke engines; GEA Westfalia Separator, MAN Diesel & Turbo and NanoNord have joined forces to offer a solution.
Recent attempts to lower the amount of cat fines in bunker fuels to the maximum 60ppm permitted under ISO 8217:2010 have, unfortunately, not been shown to reduce the average cat fines content. In practice, the opposite has happened: increased use of lower sulphur fuel in ECAs has led to a significant increase in the number of cat fines related engine wear situations.
Cat fines entering the engine create wear by means of so-called three-part abrasion. The sliding surfaces made of cast iron are the most sensitive, as the cat fines has a tendency to embed into natural porosities of the cast material structure and create wear on the counterpart. Thereby cylinder liners, piston ring grooves and piston rings become the most affected components in two-stroke engines. It is rare that cat-fine related damage is seen on the fuel equipment due to the high hardness of those components.
Recent statistics, involving a significant number of high cylinder and piston ring wear cases, where replica techniques have been used to detect cat fines particles embedded in the liner surface, showed that in nearly 90% of these cases, the wear resulted from cat fines. These investigations showed that even small cat fines particles below 10 micron contribute to the wear. Analysis results of HFO bunkered in most of the high wear cases showed that the vessels in question had bunkered fuel oil within the limits of the ISO 8217:2005 specification. Consequently, the cause of the high wear may be found in either too low separation efficiency onboard caused by insufficient separation temperature control and smallest particle sizes, by settling and accumulation of cat fines in the different tanks onboard, or a combination of both.
This highlights a need for an approved method of specifying separator size and efficiency, e.g. certified flow rate (CFR) or similar methods. It also calls for regular checks of the onboard separation efficiency, e.g. by participating in a fuel system check (FSC) programe. Commercial methods such as cat fines size distribution (CSD) screening are able to add an extra dimension, by evaluating the cat fines particle sizes.
A severe cat fines attack was monitored by measurements of cylinder liner and piston ring wear through online drain oil analysis. The results showed that within a few days of changing fuel and manually cleaning tanks, thus stopping supply of cat fines to the engine, wear dropped from an extremely high to normal level.
The conclusion is that cat fines damage over a long period of time is the result of a continuous flow of cat fines to the engine, and that the wear is not stopped until the contaminated system (including settling and day tanks) has been manually cleaned. Proper layout of tank and pipe connections, and of the onboard fuel oil cleaning systems, can prevent cat fines accumulation by ensuring the tank bottoms are continuously cleaned. This, in connection with optimised flow rate through the cleaning system, and recognizing the fact that today, marine engines are mostly operated at part load, should offer a significant improvement in cleaning efficiency. New systems including settling- and day tank lay-out, recirculation pipe connections with flow measuring device, dynamic control of the separator supply pumps and high temperature separation are measures that can further increase cleaning efficiency.
Technologies such as CatGuard, GEA Westfalia Separator’s system used in combination with the MAN COCOS engine diagnostic system are further steps towards optimizing fuel system treatment. By implementing such measures, risk of cat fines related wear can be significantly reduced.
SEPARATING CAT FINES
To counteract the problems caused by cat fines in the fuel, GEA Westfalia Separator Group’s solution concentrates on more effective onboard fuel treatment through separation of cat fines from fuels. The system consists of a separator as the core component, a frequency-controlled feed pump for altering the flow rate of the HFO, an optional measuring and analytical device, such as CatGuard from NanoNord, as well as the GEA Westfalia Separator IO control system.
In order to achieve optimum performance in separating cat fines of various sizes, the system is able to vary two essential process parameters. Firstly, the separating temperature can be changed with hot separation, whereby the viscosity of HFO decreases. So very small cat fines can be separated easily and effectively. The second variable is the flow rate, which can be adjusted by means of the frequency-controlled feed pump to suit actual fuel consumption in driving operation.
The GEA systemis claimed toworks at optimum energy efficiency to ensure a reduction of very small cat fines, of below 5µm. Full automation is provided through integrating the GEA Westfalia Separator unitrolplus to monitor the defined separation and clarifying processes, and react to deviation in compositionof the oils to be processed.
Until recently, measurement of cat fines in HFO was only possible by laboratory analysis – essentially a post-mortem analysis. NanoNord’s in-line analyser, known as CatGuard, enables analysis of cat fines as an integrated part of the fuel cleaning process. It is designed to:
- Be fully automated;
- Provide results in near real-time;
- Offer equivalent measurement accuracy to that achieved by accredited laboratories;
- Monitor separator efficiency by taking measurements at inlet and output; and
- Use measurement outputs to automatically control the separator, providing optimum separation and minimising risk of human error.
CatGuard has been trialled onboard ocean going vessels and in power plants for over two years, and has been shown to be robust and effective.
In a practical exercise to explore the effectiveness of using this technology in pro-actively managing the fuel oil cleaning process and cat fines removal, a CatGuard unit was installed on Hapag Lloyd vessel Chicago Express in June 2012. Participants in the project were the Chicago Express technical management and crew, engine supplier MAN Diesel & Turbo, separator supplier GEA Westfalia Separator Group and CatGuard supplier NanoNord. With CatGuard on board, the crew had, for the first time, a visible indication of cat fines entering and leaving the separators, while concurrently monitoring separator efficiency.
GEA Westfalia Separator developed new standard operating procedures (SOPs) based on the information from CatGuard. This primarily involved fuel oil heater temperatures and volume flow into the separators. Two of the ship’s cylinders were examined at quarterly intervals by MAN Diesel & Turbo engineers. After fine tuning and implementation of the revised SOPs, cat fines levels in HFO into the engine were reduced to between 3ppm and 10ppm. During the 10,000 running hours before the project, average liner wear in the two observed cylinders was 0.0875mm/1,000 hours and 0.0583mm/1,000 hours. In the 10,000 hours after implementation, average wear was reduced to 0.0366mm and 0.0288mm/1,000 running hours respectively.
The Chicago Express project demonstrates that integration of the fuel cleaning system, cleaning process efficiency monitoring and the engine management system will significantly reduce engine wear and the risk of serious engine damage.
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