An accurate measurement of the Rosemount 1152 differential pressure cell response time

The primary heat-transport (PHT) system of a CANDU reactor includes four quadrants of reactor coolant channels, each fed from its own inlet header through a large number of inlet feeders. As part of the safety shutdown system 1 (SDS1) and Reactor Regulating System (RRS) of CANDU reactors, differential-pressure (DP) cells are used to monitor the reactor coolant flows in each quadrant and to register changes with a prescribed response time. This paper describes an accurate in-situ measurement of the response time of two Rosemount 1152 DPA22PB DP cells, one from SDS1 and one from an RRS fully instrumented channel. The response time measurement was done using high-frequency pressure-measurement devises temporarily installed on the high- and low-pressure sides of the DP cells. The results suggest that the actual time constant of the Rosemount DP cell is much faster than indicated in the specification which is based on the traditional instrument-air-step-response measurement method. Furthermore, the actual time constant is much faster than that assumed in the safety analysis report. An examination of the instrument-air-step-response method indicates that is produces conservative estimates of time constants, especially for small time constants. If further work confirms this finding it suggests that the actual time constant may be increased considerably without exceeding the time constant assumed in the safety analysis. (author)