Stem Shear or Disconnected Valve
Stem shear can be a dangerous failure because most partial stroke techniques are unable to detect it. This is true for any technique that relies upon measuring the position of the actuator to determine valve performance. This is because the position indication will show that the actuator has moved and therefore report a “Pass” for the test.
Because the SVM system does not measure position, and also moves the valve at the designed operating speed, this failure is easily detected. In the shown graph, the section of the black graph relating to initial movement is at an increased pressure than in the original partial stroke. This is due to the fact that if the valve is not connected to the actuator, it exerts no load upon it. The actuator therefore starts to move sooner and faster. Since the actuator is moving faster, the volume in the cylinder decreases more rapidly and, due to the fixed CV or the SOV, the pressure increases.
This fault could be caused by simple human error - the valve was not connected/re-connected to the actuator. Or, much more seriously, the stem is sheared off.