(in lingua inglese)
Valve A had failed as a result of particles of material becoming trapped between the valve seat recess and the valve seat ball, leading to erosion of the valve seat recess, valve seat ball and the valve operating pin; this allowed hydraulic fluid to flow when the valve was nominally in the de-energised position. Analysis of particles within the valves showed that numbers of particles were up or greater than 100mm in size and this pointed to a failure to maintain the cleanliness of the hydraulic fluid system. The material of construction of the valve seats was not compliant with engineering drawings although the hardness values were close to those specified.