EXHAUST VALVE FAILURES

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Three factors have been found to accelerate valve failure by gas leakage. These

are influenced by the fuel quality/combustion process:

1. Mechanical dents

•              These are formed by hard particles of catalyst or slag/ash particles. Softer faced valves will yield to remove the damage, but with the harder faced valves the number of indents increase with time and can produce gas blow-through

2. Spallation of salt layers

•              The molten salts from combustion flow onto the valve seat area and solidify on the colder seat area. The salt deposit forms a hard, brittle layer which builds up. If the layers are thick enough, then thermal stresses in this layer cause cracking and break up, which creates leakage paths.

3. Sealing face corrosion

•              The corrosion is caused by molten salt deposits. Once the liquid salt dissolves the protective oxides on the valve surfaces, aggressive corrosion and degradation will occur. When a channel has been formed in the surface, the local temperature is increased and corrosion accelerates.

Hence the problems with the valves are mainly dependant on the fuel quality or

impurities, and are greatly influenced by the `quality of combustion' which occurs.

These impurities include the catalyst fines, sulphur, vanadium, and sodium

compounds. The exact chemical composition of the oxides formed depends on the

oxygen excess and combustion temperature. Oxides found include Sodium oxide

(Na₂O), Sulphur oxides (SO₂, SO₃) and Vanadium Oxides (V₂O₅). These oxides

react with each other and the calcium from the lube oil to form low melting point

salts. These salts include Sodium sulphate (Na₂SO₄), Calcium sulphate (CaSO₄)

and sodium vanadates of varying composition.

The salts can combine to have a stiction temperature as low as 400^oC, but it is more

common for the salt compounds produced to give problems above 530^oC. The

actual combination of the salt varies according to valve alloy and fuel composition.

The inner layers have been found to be vanadium rich, with various type of sodium

vanadates, whereas the outer layers are sulphate rich with sodium and calcium

sulphates.