Emerson Rosemount 3051CF Series Reference Manual page 132

Advanced pressure diagnostics
July 2020
not affect these noise signatures significantly. These signatures provide the opportunity to
identify a plugged impulse line.
When the lines between the process and the transmitter start to plug through fouling and
build-up on the inner surfaces of the impulse tubing or loose particles in the main flow
getting trapped in the impulse lines, the time and frequency domain signatures of the
noise start to change from their normal states. In the simpler case of a pressure
measurement, the plug effectively disconnects the pressure transmitter from the process.
While the average value may remain the same, the transmitter no longer receives the
noise signal from the process, and the noise signal decreases significantly. The same is true
for a DP transmitter when both impulse lines are plugged.
The case of the differential pressure measurement in a flow application with a single line
plugged is more complicated, and the behavior of the transmitter may vary depending on
a number of factors. First the basics: a differential pressure transmitter in a flow
application is equipped with two impulse lines, one on the high pressure side (HP) and one
on the low pressure side (LP) of the primary element. Understanding the results of a single
plugged line requires understanding of what happens to the individual pressure signals on
the HP and LP sides of the primary element. Common mode noise is generated by the
primary element and the pumping system as depicted in
open, the differential pressure sensor subtracts the LP from the HP. When one of the lines
are plugged (either LP or HP), the common mode cancellation no longer occurs. Therefore
there is an increase in the noise of the DP signal. See
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Figure 7-9. When both lines are
Figure 7-10.
Rosemount 3051