24
Manual Supplement
00809-0200-4853, Rev AB
Rosemount 3051S Series Pressure Transmitter
November 2017
Rosemount 3051S Series Pressure Transmitter
Flow versus 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 bubblers.
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, and 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:
resonant frequency = fn
mode number = n
speed of sound in the fluid = C
impulse length (in meters) = L
A 10-meter impulse line filled with water could generate resonant noise at 37 Hz, above the frequency
response range of a typical Rosemount Pressure Transmitter. This same 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.
Additionally, 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:
= RC and =
1
/2 f
c
R = 8 L / r
4
C = Volume / Pressure
where:
Cut-off frequency = fc
Viscosity in centipoises =
Impulse line length in meters = L
Radius of the impulse line = r
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 3051S 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.
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