A Containership experienced loss of propulsion 56 miles away from ECA zone. It was reported that main engine unit #3 fuel pump seized when changing over from HFO to LSMGO. The vessel had a spare fuel pump element where the ship staffs replaced the old pump element with the new one. The vessel resumed passage and no more problems were observed.
Fuel Changeover – a Different Ball Game:
There can be difficulties when switching from one fuel to another due to low load operations and leaking pumps caused by low fuel viscosity. This is because distillate fuels can ‘gas up’ at a too high temperatures causing vapour locking and fuel starvation. Low viscosity fuels will flow through fine clearances previously ‘sealed’ by higher viscosity fuel resulting in fuel leakage. Distillates do not require heating like HFO. If the distillate fuel temperature rises higher, the viscosity reduces deeply down to a point at which the systems function incorrectly. Most engine makers recommend maintaining viscosity at injection not less than 2 cSt.
During a fuel changeover process, the heavy fuel oil which is usually at a higher temperature (around 130 degree Celsius to maintain injection viscosity) will gradually be cooled to a safe temperature where the distillate fuel does not ‘gas up’.
The cool down process should adhere to a thumb rule to maintain a temperature gradient (drop) of not more than two degree Celsius per minute during the changeover process. The engine speed/load has to be altered to meet the above thumb rule. Failure to maintain the above thumb rule or to bring the temperature of HFO sufficiently down in order to avoid ‘gas up’ of distillates could possibly result in seizure of fuel pump or fuel system components (or) loss of power/propulsion.
Please note: We have seen that if a vessel changes over from HFO, when MGO is introduced into the fuel system it may act like a solvent, releasing any asphaltenes (sludge) which then collect in the fuel filters/strainers and clog them which could result in a blackout.
Key Factors to Consider:
The most important issues to consider when running on low sulphur distillate fuel are:
1. Viscosity of the fuel – at engine inlet
2. Lubricating oil choice
3. Lubricity of the fuel
4. Change-over procedures to and from residual fuel
Fuel pumps are designed for a minimum viscosity and fuel anti-wear performance. When the viscosity of the fuel in the pump is too low, hydrodynamic lubrication of the pump can be inadequate, causing wear and scuffing.
A decrease in fuel viscosity may cause an increase in fuel leakage between the pump plunger and barrel. The leakage can lead to hot start and low fuel setting start difficulties, especially in worn fuel pumps. It is advisable to make distillate hot start checks at regular intervals so that the limits of operating conditions for a particular engine are determined. Low viscosity fuels can also lead to the engine not delivering the fully designed power output as the design amount of fuel is not delivered by the pump. This has led most 2- and 4-stroke engine builders to request a minimum viscosity of the fuel before the fuel injection pump of about 2.0 mm²/s (cSt).
In the next technical write-up we will cover in detail on the major factors that needs to be addressed in order to execute a flawless fuel switching.
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