For flow meters, why use two in operation with one as a spare instead of one in operation with one as a spare?
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1. Ultrasonic flowmeters: The LNG receiving station is equipped with one set of gas ultrasonic flowmeters (DN400, 4 channels, time-difference method) in each metering circuit of the external transmission bus; a total of three sets are installed, designed as two in use and one as backup.
This is a requirement during design, because the data from a single flow meter shows the exact value; any leakage or loss cannot be detected. Therefore, it is necessary to use two flow meters to measure the flow rate simultaneously in order to check whether the readings match. A spare one is there in case it’s needed to go to the relevant authorities for annual inspection or verification.