Hot corrosion is basically vanadium corrosion. Vanadium is the undesirable impurity in the fuel which is naturally occurring in marine fuel in soluble form
When both vanadium and sodium are present in Na: Va of 1:3 ratio vanadium pentoxide is formed, a hard component whose melting point is 630deg.c.
This component eats into the metal surface, leaving the surface exposes to corrosion.
Practical ways to restrict
Vanadium cannot be separated by onboard centrifuges or purifier
By using special fuel additive like ash limiters
By maintaining exhaust temperature below a melting point less than 400 deg cels
Special Sterlite coating on exhaust valve seats
Regular inspection of the exhaust manifold and systems
Vanadium can be restricted by limiting the content in the fuel by checking the BDN and confirming with the help of lab analysis
What is cold corrosion
Sulphur is another element found in marine fuel
Cold corrosion is the abnormal corrosion that occurs, when there is a drop in engine temperature(due to low loads) and temperature fall below the dew point 120-160 deg celsius and sulphur products sulphur trioxide (So3)reacts with condense water to form H2So4
Sulphuric acid forms on the liner walls in an engine cylinder and corrodes the liner surface.
Effects of cold corrosion
This abnormal corrosion then creates excessive wear of the liner material.
Sticking of piston ring and piston grooves
Decrease in engine life
Cold corrosion is at its most serious in the newer engine designs. for part-load or low-load operation (also known ‘slow steaming.
Some modified engines become mildly corrosive whereas others may be more seriously affected.
Practical ways to restrict
Increasing scavenge temperature
Using modern lubrication methods like alpha lubricator system and pulse lubricating system
Using appropriate cylinder oil (TBN number)
Scape down analysis of the scavenge space oil and employing condition monitoring system onboard to in TBN and iron wear
Other modifications may include; turbocharger cut-out, variable turbocharger nozzle rings fitted, exhaust gas by-pass valve fitted, and engine tuning changes.
After 2020 Global sulphur cap
the IMO has decided that the global fuel sulphur limit of 0.50% will enter into force in 2020.
This requirement is in addition to the 0.10% sulphur limit in the North American, US Caribbean, North Sea and Baltic Sulphur Emission Control Areas (SECAs).
Vessels that have exhaust gas cleaning systems installed will be allowed to continue using high-sulphur fuel oil (HSFO).
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