Exhaust restriction is a common final element problem. This is a very simple fault to detect yet many partial stroke testing systems do not test for it. Common causes for this fault are dust or sand, ice in cold environments, insect nests, or salt growth in offshore applications.
The black curve shows a slower decay than the previous red curve during the initial depressurisation. This is due to a decrease in the CV of the SOV caused by a restriction in the exhaust.
It is also possible for restrictions in air flow to be caused by damaged supply tubing but that is not the case in this example. This is determined by the fact that the re-opening portion of the curve is almost identical to that of the previous test, clearly indicating that there is no restriction of airflow into the actuator and therefore the problem is with the exhaust.