Pil Physics - Emerson Rosemount 3051S Series Reference Manual

Reference Manual
00809-0200-4801, Rev DA
Figure 6-16. PIL Basics
Testing at Emerson and other sites indicates that SPM technology can detect plugged impulse lines.
Plugging effectively disconnects the transmitter from the process, changing the noise pattern received
by the transmitter. As the diagnostic detects changes in noise patterns, and there are multiple sources of
noise in a given process, many factors can come into play. These factors play a large role in determining
the success of diagnosing a plugged impulse line. This section of the product manual will acquaint users
with the basics of the plugged impulse lines and the PIL diagnostic, the positive and negative factors for
successful plugged line detection, and the do's and don'ts of installing pressure transmitters and
configuring and operating the PIL diagnostic.
6.4.2

PIL physics

The physics of PIL detection begins with the fluctuations or noise present in most pressure and
Differential Pressure (DP) signals. In the case of DP flow measurements, these fluctuations are produced
by the flowing fluid and are a function of the geometric and physical properties of the system. The noise
can also be produced by the pump or control system. This is also true for pressure measurements in flow
applications, though the noise produced by the flow is generally less in relation to the average pressure
value. Pressure level measurements may have noise if the tank or vessel has a source of agitation. The
noise signatures do not change as long as the system is unchanged. In addition, these noise signatures
are not affected significantly by small changes in the average value of the flow rate or pressure. 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
pressure sensor subtracts the LP from the HP. When one of the lines are plugged (either LP or HP), the
Advanced Pressure Diagnostics
Advanced Pressure Diagnostics
Figure 6-17. When both lines are open, the differential
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