Damaged Valve Seat or Internal Cylinder Corrosion
In this example, in addition to the red partial stroke, we have also shown a full stroke in blue to further exemplify the fault. It is evident that there is a problem with this valve since it should start to close at approximately 3 bar (point A). However, at point A, the curve flattens before returning to exponential decay. This flattening indicates a small amount of movement as the actuator moves off its end stop. It then encounters the resistance of the valve and then briefly stops moving. This indicates the problem is likely with the valve and not the actuator. Internal actuator corrosion would result in a similar graph but without the flat section at point A.
At point E, the valve begins to move but it jumps from the seat rather than moving smoothly. Further examination of the partial stroke reveals that the actuator doesn’t start to reopen until pressure reaches 8 bar (point B) and that it takes in excess of 10 bar (point C) to fully reopen the valve. Examination of the full stroke after point E shows the valve juddering as it continues to close (point D), further documenting the problem.