The combination use of a differential pressure transmitter, orifice and impulse line to produce a flowmeter is widely used in the process industries. Since the most frequent problem encountered in the usage of this flowmeter is impulse line blockage, an online blockage detection system is required. In previous works, the authors developed an indicator of impulse line blockage, called the blockage index, which is based on pressure fluctuations. Although the blockage index is able to detect the blockage effectively without being affected by the change in the flow rate and pressure, it can not detect the blockage as the rate of line pressure change increase. In addition, there are continuous and frequent line pressure changes in certain applications. Hence, the objective of this study is to detect impulse line blockages even when the line pressure change rate is so high that the blockage index is unable to detect the blockage. In this paper, the authors present details on the delay in the pressure transmission caused by the blockage, and evaluate the delay quantitatively by using the phase difference between the pressure upstream and downstream of the orifice as a new blockage index. In addition, by selecting frequencies with high power spectral densities, the new index becomes sensitive enough to detect the blockage during the more rapid line pressure changes under which the former blockage index failed.

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