Is Your Process Whispering To You?

Pressure transmitter noise can pinpoint process problems.

By Roger K. Pihlaja, Emerson Process Management

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Some smart pressure transmitters include built-in diagnostics that can monitor and alarm problems occurring in the process rather than just within the instrument. The best known application of such external-problem diagnostics is detecting impulse-line plugging via differences in the subsonic noise patterns generated between plugged, partially plugged and clear lines. The technique has become well accepted in process automation.

Recent work by Emerson shows that great opportunity exists for spotting many more process problems via acoustic signatures using the latest fast-sampling pressure transmitters. We have evaluated noise phenomena in a range of process equipment at customer sites, university R&D facilities and private laboratories to detect incipient and actual process problems or failures. Of particular interest is work in

1. Catalyst circulation in fluid catalytic crackers;
2. Pulsation-induced measurement error;
3. Coated or plugged multiport pitot-tube (Annubar) flow meters;
4. Coal-pulverizer fan wear (somewhat afield of chemical processes);
5. Distillation column flooding;
6. Furnace/boiler flame instability; and
7. Wet gas flow.


The first four have been proven and are being used in a few chemical plants and elsewhere.

For instance, an Exxon refinery relies on process noise detected by a fast-sampling pressure transmitter to quickly alarm the beginnings of stick-slip catalyst flow in a fluid catalytic cracker. In the past, the automation system's historian was about 30 minutes late at conventional regulatory sensor-sampling rates in noting the problem and alarming the tag. This resulted in shutdowns that cost up to $1 million/day in lost output for up to seven days, plus very expensive equipment repairs. More details on the application are at:
http://www.emersonprocess.com/rosemount/document/notes/00830-0200-4801.pdf

The other three applications are in various stages of advanced development. Less far along is problem detection in aerated liquid flow, agitation, bubbler tank level, process leaks and pump/valve cavitation. Theoretical applications include steam trap failure, turbine blade wear and coating, and wet steam flow.

Today we know what factors must be present for a reasonable chance of success in both detection and application of process noise to reveal and control process problems.

KEY COMPONENT
Vital to detecting problems or failures in process equipment are HART 4–20-mA and Foundation Fieldbus pressure transmitters having fast-sampled (22-Hz) sub-sonic sensor readings (Figure 1). These signals are used two ways: (1) conversion to filtered process variable for regulatory control, and (2) conversion of noise hash into useful acoustic signatures within an on-board advanced diagnostics module based on statistical process control (SPC). Data provided by this module are trailing mean, trailing standard deviation (SD) and calculated coefficient of variation (Cv). These three values and the relationships among them — often at particular sensor-reading frequencies — can expose incipient and actual problems before they otherwise would be noted.

The 22-Hz sampling rate is useful for evaluating about 80% of the noise frequencies commonly generated by the large and heavy equipment and relatively slow events found in processes. Unfortunately, per the Nyquist sampling theorem, frequencies from 0 to 11 Hz (22 Hz halved) are the only ones available for evaluation. Above 11 Hz, frequency aliasing leads to garbled information.

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