Emerson Rosemount 3051CF Series Reference Manual page 136

Advanced pressure diagnostics
July 2020
n is the mode number,
C is the speed of sound in the fluid, and
L is the impulse length in meters.
A 10-meter impulse line filled with water could generate resonant noise at 37 Hz, above
the frequency response range of a typical Rosemount
impulse line filled with air will have a resonance of 8.7 Hz, within the range. Proper support
of the impulse line effectively reduces the length, increasing the resonant frequency.
Second, long impulse lines can create a mechanical low pass filter that dampens the noise
signal received by the transmitter. The response time of an impulse line can be modeled as
a simple RC circuit with a cutoff frequency defined by:
t = RC and t = ½ p f
R = 8 y L /p r
C = ΔVolume/ΔPressure
where:
f is the cut-off frequency,
c
y is the viscosity in centipoises,
L is the impulse line length in meters, and
r is the radius of the impulse line.
The C formula shows the strong influence of air trapped in a liquid filled impulse line or an
impulse line with air only. Both potential issues indicate the value of short impulse lines.
One installation best practice for DP flow measurements is the use of the Rosemount 405
series of integrated compact orifice meters with the Rosemount 3051 Pressure
Transmitter. These integrated DP flow measurement systems provide perhaps the
shortest practical impulse line length possible while significantly reducing overall
installation cost and improved performance. They can be specified as a complete DP
flowmeter.
Note
The plugged impulse line diagnostic capability in the Rosemount 3051calculates and
detects significant changes in statistical parameters derived from the input process
variable. These statistical parameters relate to the variability of the noise signals present in
the process variable. It is difficult to predict specifically which noise sources may be
present in a given measurement or control application, the specific influence of those
noise sources on the statistical parameters, and the expected changes in the noise sources
at any time. Therefore, it is not absolutely warranted or guaranteed that the plugged
impulse line diagnostic will accurately detect each specific plugged impulse line condition
under all circumstances.
136
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Reference Manual
00809-0100-4774
™
Pressure Transmitter. This same
Rosemount 3051