92 Field Instrumentation GEN Rev) operating pressure but low hydrostatic pressure
due to weight of the fluid, this is easy to occur. This is particularly important for closed tank systems. With an incorrectly connected DP cell the indicated level would go down while the true tank level increases.
Over-Pressuring: Three valve manifolds are provided on DP cells to prevent over-pressuring and aid in the removal of cells for maintenance. Incorrect procedures can inadvertently over-pressure the differential pressure cell. If the cell does not fail immediately the internal diaphragm may become distorted. The measurements could read either high or low depending on the mode of failure. Note that if the equalizing valve on the three-valve manifold
is inadvertently opened, the level indication will of course drop to a very low level as the pressure across the DP cell equalizes.
Sensing lines The sensing lines are the umbilical cord to the DP cell and must be functioning correctly. Some of
the errors that can occur are Obstructed sensing lines The small diameter lines can become
clogged with particulate, with resulting inaccurate readings. Sometimes the problem is first noted as an unusually sluggish response to a predicted change in level. Periodic draining and flushing of sensing lines is a must.
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