Emerson Rosemount 3051CF Series Reference Manual page 135

Reference Manual
00809-0100-4774
accurately replicate a plug, you 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 PLD
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
require elimination of all the air to insure the most accurate measurement.
DP flow and low GP/AP vs. high GP/AP measurements
This is best described as a noise to signal ratio issue and is primarily an issue for detection
of plugged lines for high GP/AP measurements. Regardless of the line pressure, flow
generated noise tends to be about the same level. This is particularly true for liquid flows.
If the line pressure is high and the flow noise is very low by comparison, there may not be
enough noise in the measurement to detect the decrease brought on by a plugged
impulse line. The presence of air in the impulse lines and transmitter in liquid applications
further enhances the low noise condition. The PLD diagnostic alerts you to this condition
during the learning mode by indicating Insufficient Dynamics status.
Flow vs. level applications
As previously described, flow applications naturally generate noise. Level applications
without a source of agitation have very little or no noise, therefore making it difficult or
impossible to detect a reduction in noise from the plugged impulse line. Noise sources
include agitators, constant flow in and out of the tank maintaining a fairly consistent level,
or bubbler.
Impulse line length
Long impulse lines potentially create problems in two areas. First, they are more likely to
generate resonances that can create competing pressure noise signals with the process
generated noise. When plugging occurs, the resonant generated noise is still present, the
transmitter does not detect a significant change in noise level, and the plugged condition
is undetected. The formula that describes the resonant frequency is:
fn = (2n – 1) × C/4L (2)
where:
fn is the resonant frequency,
Reference Manual
Impulse line length for further explanation. Liquid DP flow applications
Advanced pressure diagnostics
July 2020
135