Reference Manual
00809-0200-4801, Rev CA
July 2010
Rosemount 3051S
6-20
However, there is a combination of factors that may affect the output of the
DP transmitter under single plugged line conditions. If the impulse line is filled
with an incompressible fluid, no air is present in the impulse line or the
transmitter body, and the plug is formed by rigid material, the noise or
fluctuation will decrease. This is because the combination of the above
effectively “stiffens” the hydraulic system formed by the DP sensor and the
plugged impulse line. The PIL diagnostic can detect these changes in the
noise levels through the operation described previously.
Plugged Line Detection
Factors
The factors that may play a significant role in a successful or unsuccessful
detection of a plugged impulse line can be separated into positive factors and
negative factors, with the former increasing the chances of success and the
latter decreasing the chances of success. Within each list, some factors are
more important than others as indicated by the relative position on the list. If
an application has some negative factors that does not mean that it is not a
good candidate for the diagnostic. The diagnostic may require more time and
effort to set up and test and the chances of success may be reduced. Each
factor pair will be discussed.
Ability to Test Installed Transmitter
The single most important positive factor is the ability to test the diagnostic
after the transmitter is installed, and while the process is operating. Virtually
all DP flow and most pressure measurement installations include a root or
manifold valve for maintenance purposes. By closing the valve, preferable the
one(s) closest to the process to most accurately replicate a plug, the user can
note the response of the diagnostic and the change in the standard deviation
value and adjust the sensitivity or operation accordingly.
Stable, In-Control Process
A process that is not stable or in no or poor control may be a poor candidate
for the PIL diagnostic. The diagnostic baselines the process under conditions
considered to be normal. If the process is unstable, the diagnostic will be
unable to develop a representative baseline value. The diagnostic may
remain in the learning/verifying mode. If the process is stable long enough to
establish a baseline, an unstable process may result in frequent
relearning/verifications and/or false trips of the diagnostic.
Well Vented Installation
This is an issue for liquid applications. Testing indicates that even small
amounts of air trapped in the impulse line of the pressure transmitter can have
a significant effect on the operation of the diagnostic. The small amount of air
can dampen the pressure noise signal as received by the transmitter. This is
particularly true for DP devices in single line plugging situations and GP/AP
devices in high pressure/low noise applications. See the next paragraph and
“Impulse Line Length” on page 6-21 for further explanation. Liquid DP flow
applications require elimination of all the air to insure the most accurate
measurement.
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