Determining the operability of a fluid driven safety valve

A method for determining the operability of a fluid driven safety valve with a valve member (120) and a spring return,

• wherein the valve member is to be moved to a safety position by the spring return in case of a complete pressure drop of the driving fluid;
• wherein the safety valve has means for determining the pressure of the driving fluid;
• wherein the pressure of the driving fluid acts against the spring (160);
• wherein the safety valve has means (180) for determining the position of the valve member;
• wherein the method comprises the following steps:
• a partial stroke test is performed on the safety valve by varying the pressure in the driving fluid;
• the position of the valve member (230) in dependence of the pressure (220) in the driving fluid is recorded during the partial stroke test, whereby a stroke-pressure curve is obtained.
• the stroke-pressure curve is mathematically modeled beyond the measured range;
• the mathematically determined stroke-pressure curve is extrapolated up to the safety position of the valve member (340, 350);
• the pressure (320;
• 420;
• 520) corresponding to this safety position in the mathematically determined stroke-pressure curve is determined.