Chemical Processing - August 2007 - (Page 33) Making sound use of noise Now embedded in the Rosemount 3051S series of pressure transmitters (Figure 1), ASP uses a basic principle known as Statistical Process Monitoring (SPM) and relies on the fact that virtually all dynamic processes have distinctive noise characteristics — and that any significant changes in that noise can indicate that something is happening in the process or equipment (Figure 2). The problem, however, has been in detecting and monitoring those changes. Typical process control systems generally filter out high frequency noise to improve the signal-to-noise ratio and only can read transmitter outputs every second or so. This is fast enough for most process control but it’s considerably slower than the speed at which many instrument sensors are actually receiving signals from the process. The 3051S, for instance, samples its process variable 22 times per second, says Schmeling. The high speed of the device together with the internal calculation of statistical parameters are key to the SPM technology. The patented software resides on a “diagnostics feature” board mounted in the transmitter head and computes the mean and standard deviation of the input signals. SPM features a “learning” module that establishes baseline values that are considered normal for the process and installation. These values are constantly compared to the current values of the mean and standard deviation and then, based on sensitivity and user-selected settings, the SPM diagnostic can generate alarms, alerts or other action when a significant change is detected in either value. For instance, if impulse lines to the pressure transmitter are plugged, the transmitter output may appear normal (because it’s effectively continuing to read the pressure at the time of the plug) and the mean will remain unchanged. However, Emerson’s research has shown that in a typical flow application the standard deviation at a given flow rate changes significantly if one or both impulse lines are plugged. The SPM diagnostics will pick this up, alerting the operator to the abnormal situation. All the diagnostic information generated from the SPMequipped 3051S now can be viewed through an enhanced EDDL-based graphical user interface — “much better than just showing numbers in a box,” notes Schmeling. ABB, Houston, also is keenly interested in improving the diagnostics of its instrumentation. The company is “in continuous discussion with customers and end user groups to determine new sensor technology requirements for each product line,” says Sean Keeping, vice president technology for ABB Instrumentation, St. Neots, U.K. The focus is on “transforming the sensor data into value added information.” As one example, he cites the FSM4000 electromagnetic flowmeter, which employs a diagnostic coil to actively adjust the zero and eliminate any line noise. It effectively measures 100% of the useful signal and is said to deliver a higher signal-to-noise ratio than other systems. Keeping says the www.chemicalprocessing.com company also has patented a concept for in-situ validation of magmeters. The CalMaster 2 and CheckMaster products connect to the flowmeters and run a series of tests to verify their accuracy and repeatability against the original factory calibrations. The validation is based on advanced fuzzy logic analysis of the measurement data from the sensors. ABB’s 2600T series of Foundation Fieldbus differential-pressure transmitters (Figure 3) now all feature built-in plugged-impulse-line detection diagnostics known as PILD. A board mounted in the transmitter runs the “self-adaptive” PILD algorithm, which is based on the spectral analysis of the differential pressure signal. It adapts to the specific process conditions at the time of installation and during normal operation compares the noise level to these conditions. If the noise level is statistically higher than the original value, it indicates a single line plug (and can detect which of the lines); while if the noise is substantially lower then the diagnosis is of two lines plugged. The algorithm has been proved to work equally well for both liquids and gases. “At the moment we can detect if an impulse line is open or closed, but we are doing more R&D to see if we can detect the onset of a line becoming blocked before it actually becomes plugged,” explains Eugenio Volonterio, R&D manager for pressure products with ABB SACE in Como, Italy. The range of Foundation Fieldbus pressure transmitters from Yokogawa, Newnan, Ga., already boast that capability. Because they feature two sensors on the one chip — one measuring differential pressure, the other static pressure — the transmitters can discriminate between >> Advanced diagnostics Figure 1. Statistical Process Monitoring software in this pressure transmitter enables detection of plugged impulse lines and provides other diagnostics. Source: Rosemount. August 2007 • 33 http://www.chemicalprocessing.com
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