Using the noise analysis technique to detect response time problems in the sensing lines of nuclear plant pressure transmitters

Blockages, voids, and leaks contribute to nearly 70 percent of the age-related problems in the sensing lines that connect pressure, level, and flow transmitters in nuclear plants to the processes they measure, reducing their speed of response. Only the noise analysis technique provides an effective means for testing response times when a nuclear plant is operating. It also makes possible the simultaneous testing of response time of multiple pressure transmitters and is applicable for both force-balance and motion-balance transmitter types. The technique involves three steps -- data acquisition, data qualification, and data analysis. Commercial signal-conditioning equipment is used to extract the noise from the transmitter output by removing the DC component of the signal and amplifying the AC component. The AC signal is then digitized; qualified for stationarity, linearity, and other abnormalities; and then analyzed in the frequency domain and/or time domain. Tests performed by the author has validated the accuracy of the noise analysis technique and yielded the following conclusions: Long sensing lines and blockages increase the response time of a pressure sensing system. Increases in response times caused by sensing-line length and blockages depend on the compliance of the transmitter. The response time of transmitters with larger compliances is more sensitive to sensing-line length and blockages than that of transmitters with small compliances. When noise analysis identifies a sensing-line blockage, the line must be purged (flushed) and noise tests repeated to ensure the problem is resolved.