Since the ultrasonic flowmeter is to measure the flow rate by measuring the propagation time difference of high-frequency sound pulses in the pipeline, noise interference has become the main factor affecting the accuracy of measurement. When the noise level reaches a certain level, the ultrasonic flow meter cannot be metered. There are mainly the following reasons that affect the measurement accuracy of ultrasonic flowmeter.
 

(1) The noise sources of the metering system using gas ultrasonic flowmeter mainly include pressure regulators, throttle valves, manifolds, compressors, etc., among which the throttling noise generated by throttling parts such as pressure regulators and throttle valves has the most common impact on ultrasonic flowmeters. Under normal circumstances, the pulse signal frequency emitted by the ultrasonic flowmeter exceeds 125kHz, and the throttling noise frequency generated by throttling parts such as pressure regulators and throttle valves is generally lower than this frequency.

However, with the change of valve structure, natural gas flow rate and pressure difference, the frequency range of noise gradually widens, and after the noise is received by the ultrasonic transducer, the signal-to-noise ratio of the flowmeter increases, and the measurement deviation occurs.
 

(2) Due to the non-standard installation, the noise increases, affecting the accuracy of measurement, such as not equipped with straight pipe sections, noise reduction devices or incorrect installation positions, etc., can increase the impact on the accuracy of measurement.
 

(3) According to GB/T 18604-2014 "Measurement of Natural Gas Flow with Gas Ultrasonic Flowmeter", due to the proximity of the station site to the railway and construction site, the vibration source or geological settlement leads to increased noise impact, which further affects the accuracy of measurement.
 

In order to fundamentally solve the influence of the pressure reduction noise of the pressure regulating skid on the normal measurement of the ultrasonic flowmeter and ensure the normal measurement of the ultrasonic flowmeter, after the field investigation and survey of the site, the noise generated by the pressure of the flowmeter and the pressure regulator skid and the structural length of the pressure regulator were recalculated.

The results show that the flow velocity at the outlet of the regulator skid is 92.5m/s. The normal flow rate of the ultrasonic flowmeter shall not be greater than 30m/s, and the signal-to-noise ratio shall not be less than 20dB. Therefore, according to the current operating conditions, the flowmeter cannot be accurately measured.