28-12-2012, 02:57 PM
Instrumentation – Can you be fooled by it?
1Instrumentation.pdf (Size: 128.1 KB / Downloads: 49)
INTRODUCTION
With the liquid completely covering the displacer, the instrument did
not indicate liquid level. Instead, the force on the displacer gave a
measure of the relative density of the displacer and the liquid in which
it was submerged. In other words, the instrument was not designed to
function properly if the level was high enough to completely submerge
the displacer. The column was heated during the startup. As the
temperature of the liquid increased (the green line in the graph above –
Line 2), the density of the liquid decreased (the purple line - 3). The
change in density of the liquid changed the force on the displacer,
resulting in a decrease in the “level” indication (fourth drawing, with
hot liquid), even though the column level was actually increasing. The
column overflowed, flammable material was released, and there was a
major explosion and fire.
What You Can Do
Know what can fool you. Review examples of
incidents where the instrumentation provided
information that did not represent the data that
was wanted (for example, density of the liquid,
not level). This is not always an easy concept to
grasp, so consult with the engineers and
technicians who know the system best.