Effects of Sodium Vanadium compounds on Marine Diesel engines

Fouling

A not uncommon failure of Marine diesel engines is a fouling of turbocharger nozzle rings and blades with compounds containing sodium and vanadium as well as more traditional carbon deposits.
This leads to increased exhaust gas temperatures, loss of plant efficiency and reduced reliability. The degree and type of fouling is dependent on the constituents and amounts of contaminants in the fuel oil.

Corrosion

More specifically high temperature corrosion directly caused by the presence of compounds of sodium and Vanadium at temperatures over 500'C.

Sodium and Vanadium are found in heavy fuels up to 200ppm and 600ppm respectively forming Vanadium Oxides ( chiefly V₂O₅), Sodium oxidises to Na₂O and with sulphur also contained, sulphates NaSO₄ which are able to react further with vanadium oxides.

The various compounds that may be formed from these have a wide variety of properties one of the most significant of which is the melting points

At the moment of solidification certain compounds can release molecular oxygen which can attack the metal surface. Oxygen may be re absorbed into the deposition thus forming an oxygen pump which aggressively attacks the surface of the metal during melting/solidification processes at around 530 to 600'C. The iron oxide (or Nickel oxide for Cr-Ni Steels) diffuses into the melting cake. A typical reaction is

Na₂O.6V₂O₅ <-> Na₂O.V₂O₄.5V₂O₅ + 1/2 O₂

The parts of most concern in marine diesels are Exhaust valves, piston crowns as well as components of the turbocharger such as the nozzle ring and blades.

Effects of ratio of Sodium and vanadium in fuel

From the table above it can be seen that the ratio of Sodium to Vanadium in the compounds greatly influences the melting point and thereby the corrosive and slagging effect.
The danger zone is taken to be Na/V ratio of 0.08 to 0.45 of which 0.15 to 0.30 is particularly destructive

Effects of Temperature and SO₂

The temperature of the components in the diesel engine will decisively influence the temperature at which corrosion takes place. In addition the presence of SO₂ causes the formation of sulphates in the melt

SO₂ + V₂O₅ -> SO₃ + V₂O₄

SO₃ + Na₂O -> Na₂SO₄

The sodium sulphate cannot exist in the melted Sodium Vanadates and is released to further attack the metal surfaces. The SO₃ may combine to form sulphurous deposits stripping protective oxide layers from the metal surfaces.

Bunker quality and the effects of Fuel Conditioning

A look at a general cross section of the fuel oils being supplied around the world reveals that a significant portion contain sodium and vanadium in ratios around that considered to be the most destructive.

Passing the fuel through a purifier was the effect of reducing the Sodium content significantly although there is little effect on the Vanadium content.

I have recently been in correspondence with an engineering manager of a large power generation plant. His concern was that the fuel being supplied to the engines had a water content greater or equal to 0.25%.
Water was being introduced into the fuel at an early stage of its conditioning as a method of washing the sodium from the bunkers being supplied in an attempt to reduce the effects of sodium vanadium corrosion.

Vanadium may be found in lubricating oil.

Alteration of the Na/V ratio

As mentioned careful purification can have a significant effect on the amount of Sodium in the Fuel. However Sodium can be re-introduced into the combustion process in the form of salt water spray laden air of due to leakage of sea water cooled air coolers. It should be noted that where ratios are equivalent corrosion processes were the sodium was already contained in the fuel are significantly higher.

Even when Na/V ratios are out of the danger zone it is possible for pockets or 'banks' of products to build up and be released to form these damaging ratios. Typically Sodium deposits may be found in the scavenge areas and Sodium and Vanadium deposits in the exhaust areas.

Fuel additivies

Magnesium salt based additives are available on the market. The effect of these is to increase the melting point of the compounds formed. deposits tend to be loose and easily removed and little corrosion may be evident

Recommendations

1. An exhaust gas temperature of 530 to 560'C and Na/V ratios of 0.15 to 0.30 are the danger zones. For reasons described it is very difficult to avoid these ratios, however the following recommendations are given which should significantly reduce corrosion and could possibly influence the degree of slagging.
2. Use fuels with sodium vanadium ratios outside the critical zone of 0.15 to 0.30 and preferably 0.08 to 0.45
3. Ensure efficient separation to reduce sodium content
4. Keep mean exhaust gas temperatures below 500'C and turbocharger inlet temperature below 530'C
5. Ensure that demisters are operating properly and cure sea water leaks in coolers.
6. Use Mg salt based fuel additives

Further Reading:
Mechanisms of High Temperature Corrosion in Turbochargers of Modern Four Stroke Marine Engines:Motoren und Energietechnik GMbh Rostock