A multi-level assessment and correction method for Venturi tube flow measurements
Introduction
The principles for calculating differential pressure form the basis of measurements of the flow of the main feed water in nuclear power units (Hashemian and Jiang, 2009). Venturi tubes provide the means by which this flow can be throttled. In view of the medium- and high-speed fluids flowing through them, Venturi tubes inevitably become eroded over the course of long-term operation. This leads to changes in their pressure sensing capabilities, affecting the accuracy of their measurements of differential pressure and flow. The flow measurement signal for the main feed water is an important process parameter in nuclear power units, informing the water level regulation in the steam generator control systems (Bongulwar and Patre, 2017, Chen et al., 2019a, Zhao et al., 2000). It also plays an important role in triggering steam-water mismatch signals in a reactor's protection logic (Chen et al., 2019). Thus, the accuracy of the signal measurement directly affects the safe and stable operation of a nuclear power unit. A serious accident caused by Venturi tube perforation has already occurred in a nuclear power plant as a direct result of the long-term erosion of a Venturi tube. Cases of flow measurement degradation or failure caused by erosion are too numerous to mention. So, there is an urgent need to establish an effective way of assessing the condition of Venturi tubes and correcting main feed water flow measurements when necessary. However, the majority of research regarding Venturi tubes focuses on developing internal pressure-drop models (Pan et al., 2019, Wu et al., 2020, Wang et al., 2020) and measurement models for mixed media (Gupta et al., 2016, Liu et al., 2014, Nasseh et al., 2009). Very few studies examine the characteristics of Venturi tube erosion and the underlying mechanisms governing its occurrence. Venturi tubes have a wide range of applications, so establishing their erosion characteristics can help to ensure the accuracy and effectiveness of their measurements across a range of domains. In this paper, the erosion characteristics of Venturi tubes are examined and a multi-level assessment method for measuring main feed water flow is presented that is based on operational data (some of it gathered for more than 20 years) regarding main feed water flow measurements in multiple nuclear power units.
Section snippets
Basic principles
Venturi tubes are a standard component used for throttling. The principles of how they measure flow are detailed below. When a fluid filling a pipeline enters the throttling section of the pipe at the throat of the Venturi tube, there is a contraction in the flow of the fluid and the flow rate increases. Meanwhile, the static differential pressure drops, resulting in a differential pressure at the front and rear of the throat of the Venturi tube (Long et al., 2017). A fluid flow continuity
Multi-level assessment technology
A three-level assessment method for the measurement of the main feed water flow in nuclear power units was developed that covered the following concerns: degradation; failure; and replacement (see Fig. 4). To better characterize the erosion characteristics, the real flow of the main feed water under ideal conditions was used as a reference. As the actual flow was not obtainable, a higher-accuracy flow was adopted to approximate to the ideal. A high-precision orifice plate was installed at the
Venturi tube differential pressure correction
If the Venturi tube flow measurement is rated as degraded or has failed, differential pressure correction is required to restore the measurement accuracy. The correction technology proposed here can be described as full-power accurate differential pressure correction. It makes use of a full-power (100%PN) heat balance flow, , on the basis of which the new range of the main feed water differential pressure can be obtained.
The reference standard flow value, , is squared to provide the
Venturi tube replacement
Table 1 shows the differential pressure data for three Venturi tubes after several years of operation in a nuclear power unit. The differential pressure range when the tubes were first installed was 1900 mbar. Having been subjected to erosion, the differential pressure characteristics of the Venturi tubes had changed significantly, with a loss in performance of between 20% and 40%. The multi-level assessment method presented above was applied, with After cumulative
Conclusion
Having established the impact of erosion on the characteristics of Venturi tube main feed water flow measurements in nuclear power units, a multi-level method to assess the adequacy of measurements of the main feed water flow has been proposed. This covers three levels of concern: degradation; failure; and replacement. Depending on the nature of different assessment results, corresponding maintenance strategies have been suggested to ensure the measurement accuracy and safe and stable operation
CRediT authorship contribution statement
Yongwei Chen: Conceptualization, Methodology. Yonggang Li: Supervision. Xinxing Zhou: Writing - review & editing. Yongjing Xie: Data curation, Writing - original draft.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We gratefully acknowledge the support of this work by the by CAST (China Association for Science and Technology) Young Elite Scientists Sponsorship Program, Project Code: 2018QNRC001.
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