Is Your Process Whispering To You?

Pressure transmitter noise can pinpoint process problems.

By Roger K. Pihlaja, Emerson Process Management

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The transmitter, of course, should be located close to the expected diagnostic signal source and far from interfering signal sources. Its impulse lines should be short and purged often. Once the transmitter is in service, it's important to check whether new or intermittently operating process equipment generate noise in the 0–11 Hz range. If so, the transmitter may need retuning for proper operation.

Many more suggestions, tips, do's and don'ts, and the like gained from practical experience exist — more than can be covered in a short article.

COLUMN FLOODING
Let's now look at some of the thought processes in recent work to spot distillation column flooding.

A smart ΔP transmitter used to monitor and control upward vapor flow in the rectification section of a packed distillation column also can serve to detect and alarm, via its noise signature, incipient flooding in that section (Figure 3). The diagnostic technique was developed and proven on a pilot-plant column of the Separations Research Program at the University of Texas at Austin. (See: "Technology Targets Towers.") It hasn't yet been applied to a full-size tower.

When operating normally in the section's continuous phase, droplets of distilled product fall through the packing while vapor flows upward. Increasing vapor flow creates an aerodynamic drag on the droplets. If drag becomes excessive, the drops can't fall and flooding begins.

Eventually a complete phase inversion occurs; the void space is flooded entirely with a combination of liquid and entrained vapor bubbles being forced up through the liquid. In due course the transmitter's regulatory function would sense a substantial increase in ΔP due to flooding throughout and trigger an alarm — but much too late.

The transmitter's diagnostics are configured to detect a change in the emitted noise pattern from the flooded packing caused by the sound of the bubbles randomly nucleating, growing and breaking — not unlike the noise of a carbonated drink.

Figure 4 shows a SD curve of the transmitter's bubble noise signal, modified by an SPC differencing filter, that indicates a relatively sharp increase before settling at a higher level. The one-minute trailing trace is the most graphic indication of the beginning of flooding and the ideal parameter to alarm.

A power spectral density analysis can refine the sensing technique and check possible interference with the transmitter's regulatory function. Figure 5 shows bubble noise during normal and flooded conditions. Note that from 4 to 11 Hz the normal trace runs as much as 10 db quieter than the flooded one, making that range best for flooding detection. Also note that the two traces are quite flat and unspiked, indicating they are "weak white noise," i.e., noise that minimally affects the accuracy, reliability and repeatability of the regulatory signal.

 Power Spectral Density Analysis
 Figure 5. Biggest difference between flooding and normal noise occurs at 4–11 Hz.

LISTEN TO YOUR PROCESS

Smart fast-sampling pressure transmitters can break apart what appears to be random process noise. What's discovered within that noise can identify process problems. While I highlighted early detection of distillation column flooding, a variety of potential applications exist. Each differs in its own way. Additional statistical analysis methods can manipulate process signals to tease out otherwise unknown phenomena and also to establish trailing-mean learn/monitor periods to give the most consistent results.


ROGER K. PIHLAJA is principal engineer — process diagnostics for Rosemount, Emerson Process Management, Chanhassen, MN. E-mail him at Roger.Pihlaja@Emerson.com.

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