TECHNICAL BULLETIN

n. 26    

TYPICAL ANOMALIES OCCURRING ON THE EXHAUST VALVES
OF DIESEL ENGINES AT GENERAL OVERHAULS.

BLOWOUTS, CORROSION, THERMAL FATIGUE, COMPACTING, PITTING.
Photographs of exhaust valves showing these anomalies

At general overhauls on exhaust valves in Diesel engines running on fuel oil in Cogeneration Plants, the principal typical anomalies displayed by the valves are as follows:

Blowouts:

This is the total perforation of either one of the valve seats (or of both, i.e., the cylinder head, or valve basket, and the valve cone). This necessitates the complete replacement of the valve or the component affected (the valve cone or the valve basket) or the mechanical rectification of the valve seat of the cylinder head. It is localised corrosion at one point, and is not necessarily accompanied by generalised corrosion of the valve seat.

Corrosion:

This is generalised corrosion, to greater or lesser depths, of the valve seats, causing the need to rectify the surfaces of the seat to a depth proportional to the severity of the corrosion. This is mainly caused by sodium vanadates produced by combustion, when the fuel oil is not suitably treated to prevent it. The greater the depth of corrosion, the greater the depth of valve seat rectification required, which in turn shortens the operational life of the valves, because they are able to withstand fewer subsequent rectifications.

Thermal Fatigue:

This is the failure, due to heat, of the mechanical resistance of the valve and is detectable by the appearance of circular cracks on the flat surface of the valve cones, following thousands of hours in operation. The extent of the fatigue should be evaluated before deciding on replacement.

Compacting:

This appears in the form of marks and deformation of the surface of the valve seats and is caused by the great quantity of solid residues generated by combustion of fuel oil, which can become trapped in the seats when the valves close. This is remedied by slight mechanical rectification of the valve seats.

Pitting:

This refers to marks on the metal left by compacting, when the solid residues are not retained on the seats. This is similarly remedied by slight mechanical rectification of the seats.

Throughout our many years of activity, the technical department has participated in numerous engine overhauls, gathering information about all these anomalies. We must emphasise that the anomaly of "corrosion" can easily be minimised by treating fuel oil with the additive "rb bertomeu" beco F1/ASF, whereby at overhauls a superficial rectification of the valve seats is sufficient (instead of deep rectification) and the operational life of the valves tends to be prolonged up to the limits determined by the appearance of "thermal fatigue of the metal" because the valve is able to withstand a greater number of rectifications. In our experience, it is possible at least to double the operational life of the valves if the fuel oil is treated with the additive "rb bertomeu" beco F1/ASF.

For further information on this subject, please consult the following documents from rb bertomeu (click on the chosen title).

• Photographs of valves showing "blowouts" and "the early stages of blowouts"
• Photographs of valves showing "corrosion"
• Photographs of valves showing "thermal fatigue"
• Photographs of valves showing "compacting" and "pitting"