Validation of Noise Analysis Technique for Online Detection of Sensing Line Problems
The validity of the noise analysis technique for online detection of sensing line blockages has been established by numerous laboratory and in-plant demonstration tests involving a variety of pressure transmitters. (See H.M. Hashemian, "Long-Term Performance and Aging Characteristics of Nuclear Plant Pressure Transmitters," U.S. Nuclear Regulatory Commission, NUREG/CR-5851 [March 1993]). Table 2 shows representative results of such tests that involved a Barton pressure transmitter. The transmitter was tested alone, then with 30 meters of sensing line tubing, and finally with a snubber in the sensing line. In each case, the response time of the transmitter and the attached sensing line was measured by the conventional ramp method and by the noise analysis technique. The noise was generated for this experiment in a laboratory test loop that was designed to simulate process fluctuations for research purposes.
As shown by these results, the noise analysis technique identifies the response time of the transmitter and its sensing line with good accuracy and accounts for the effect of sensing line length and the blockage (simulated by the snubbers) on the response time.
Table 2. Representative results of validation of noise analysis technique for response time testing of pressure transmitters and associated sensing lines. Source: Analysis and Measurement Services Corp.
Solving Sensing-Line Problems
Remedies that remove voids and/or blockages in sensing lines are to periodically blow down, back fill, or drain the sensing lines.
Another solution is to test or monitor for the presence of voids or blockages in the sensing lines online. This can be done by using the noise analysis technique for in-situ testing of pressure transmitters’ response times described earlier. In fact, one of the main advantages of response time testing with the noise analysis technique is that its results will include the effects of sensing lines. That is, any response time result for pressure transmitters that is obtained by the noise analysis technique will inherently account for the length and diameter of sensing lines as well as for any blockages, voids, leaks, or freezing that may be present in the sensing lines.
—H.M. Hashemian (hash@ams-corp .com) is president and CEO of Analysis and Measurement Services Corp. Dr. Jin Jiang (jjiang@uwo.ca) is a professor of electrical and computer engineering at the University of Western Ontario.
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