Emerson 3051S Dokumentacja Pobierz pdf (Strona 99)

Reference Manual

00809-0200-4801, Rev CA

July 2010

6-21

Rosemount 3051S

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 low noise condition is further enhanced by the presence of air in the

impulse lines and transmitter if a liquid application. The PIL diagnostic will

alert the user 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 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:

fn is the resonant frequency,

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 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.

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:

 = RC and  =

/2  f

R = 8  L /  r

C = ΔVolume / ΔPressure

where:

is the cut-off frequency

 is the viscosity in centipoises,

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