Failure analysis of a let-down valve at a petrochemical plant

The causes of recurrent failures of the stem of a cone valve in a 2100 bar internal pressure polymer reactor were analyzed. The valve generates fast but short drops of pressure within the reactor, as the stem is axially moved by a hydraulic control system. Most of the time the valve modulates pressure by positioning the stem at the 10% of its maximal displacement. Cracks in the quenched and tempered DIN 1.2343 valve stems propagated symmetrically from the outside, by alternative bending fatigue. Crack initiation was due to plastic deformation of the stem as supported against the cage, from a combined stress state caused by stem vibrations and fluid pressure. The plastic deformation and the failure mode point to flow induced vibrations on the stem. Modifications in valve design are proposed to diminish flow induced vibration in the stem.