With the expansion of the natural gas market in China, the measurement problem of natural gas is also becoming more and more important, a precise measurement for natural gas trade has very important influence, ultrasonic flow meter is a new type of gas flow statistics, wide measuring range, high accuracy, not easily fixed pressure loss, good repeatability, components, suitable for large diameter gas pipeline measurement, so it is widely used in the measuring work in natural gas transmission pipeline.

working principle

Ultrasonic flowmeter is a kind of measuring instrument that USES sound wave to measure the flow velocity of the fluid in the convection tube. The main equipment in the field of gas measurement is ultrasonic sensor, which is generally installed on the wall of the tube and arranged in geometric form between sensors.At present, there are many different forms of ultrasonic flowmeters used, and this difference is mainly caused by different geometric arrangements.The ultrasonic flowmeter USES the time difference method. In the specific operation, a pair of bevel ultrasonic flow is calculated and installed on one side of the early pipe, and a pair of transducers is installed on the other side of the pipe, so that when the gas passes through, the ultrasonic signal of the other side can be obtained at the same time or at a set time.

Noise impact

Various pipe fittings in the pipeline may appear resonance phenomenon, or be affected by air vibration, and emit noise, which will cause a certain degree of impact on the ultrasonic flow meter.The working principle of ultrasonic flow meter is to judge the flow rate of the fluid according to the velocity difference between sound wave and the fluid. Therefore, if the frequency of noise is the same as the working frequency of the flow meter, the flow meter will receive interference.According to literature records, the effects of noise on flowmeter has the repeatability, and obviously, up more than 2%, therefore, in the process of measuring pipeline design, should as far as possible by those who do not bend, not choke, in the process of flow meter installed at the same time, a regulating valve should be installed in its downstream, to ensure that the stability of fluid flow.In the selection of flowmeter, select the product with strong performance, so as to enhance its anti-interference and filtering ability to noise.

Pipeline configuration

1.Straight pipe length requirements

We are on a valve or a manifold flowmeter upstream and downstream of the resistance in the process of measurement, will affect the physical form of the measured medium, such as vortex or to cause airflow torsion distortion, etc., thus increasing the margin of error measurement, the more serious the situation to the distortion of the velocity distribution, the more serious the error, as a result, the flow meter installed at the same time, should increase a certain length in the upstream and downstream straight pipe, which have the effect of stable medium form, to ensure the accuracy of the final test.From the practical effect, the longer the length of the upstream and downstream straight pipe, the higher the final accuracy of the flowmeter.According to the GB/T18604-2001 standard, the length of the straight pipe upstream should be not less than 10D, in the downstream of the length of the shortest not less than 5D, however, because of the pipeline is more complex, therefore, the upstream line 10D is unable to meet the practical requirements, best can increase to 20D, if increase the rectifier in the pipe, you should ask manufacturers to provide relevant technical standards, as well as the construction location and the corresponding standard straight pipe length, and after experimental verification, before construction.

2. Quality requirements of the straight pipe section

Within the length range of the straight pipe section, any step or other form of protrusion will lead to the change of medium fluid shape, which will affect the ultrasonic measurement.According to the literature, if there are 0.01D steps in the vicinity of the upstream and downstream of the ultrasonic flowmeter, the maximum error caused by it can reach 0.05%. Therefore, in the assembly process of straight pipe segment, the appropriate pipe type should be selected to limit the number of steps and other protrusions.

Maintenance and overhaul in use

In the process of use, the accuracy of the measurement is affected by the following factors, such as the vibration of the pipeline at the probe position of the transducer installed on site, the change of temperature, the change of scale formation in the pipeline, and the drift of the working point of the circuit of the ultrasonic flowmeter, etc..These will directly affect the accuracy of measurement.Therefore, it is necessary to check and maintain the ultrasonic flow meter regularly.Routine work mainly includes:

1. Check the signal strength and signal goodness

Signal strength refers to the signal strength of the upstream and downstream probes.Signal goodness mainly refers to the signal peak value in the upper and lower transmission directions.If there are errors in the two signals, it is necessary to check whether the installation of the probe is loose or whether the coupling agent hardening is invalid. In general, in order to ensure the correctness of the above signals, it is necessary to reinstall the probe.

2. Check the transmission time and transmission time difference

The transmission time represents the average transmission time of ultrasonic wave.Transmission time difference is used to express the upstream and downstream transmission time difference.The measurement of flow velocity by ultrasonic flowmeter mainly depends on the above two signals, especially whether the transmission time difference can reflect the stability of the flowmeter.If these two signals are unstable, you need to check the installation point and determine whether there is a problem with its setting data.

From the practical situation, gas ultrasonic flow meter is a kind of completely suitable for natural gas trade flow statistics and equipment to improve the natural gas measurement level plays a very significant role, gas ultrasonic flowmeter has the range than wide, sturdy and durable, no pressure loss, high accuracy, low maintenance workload, etc, and less susceptible to the installation environment, noise and clutter, dirt, etc, therefore has the very good adaptability, will be in the field of natural gas flow measurement has very broad application prospects.