Clamp-on ultrasonic flowmeter field installation

1. Clamp-on ultrasonic flowmeter

The selection of ultrasonic flowmeter depends on the process conditions, the adjustment range of the system and the properties of the fluid. Factors such as the flow rate of the metering pipeline, fluctuations, corrosion, cleanliness, operation and economic rationality should be considered. A flowmeter with good performance should cover a wide flow range and have good linearity.

Ultrasonic flowmeters with different installation methods have different application scopes. Under normal circumstances, the tube-type ultrasonic flowmeter is suitable for situations with high accuracy requirements and a small diameter; on the contrary, the clamp-on type and other types of ultrasonic flowmeters are suitable for low measurement accuracy requirements and large diameters.

In addition to the factors mentioned above, there are some unique factors to consider when sizing an ultrasonic flowmeter correctly.

2. The installation of the external clamp ultrasonic flowmeter

In the design of natural gas stations, the design flow rate of natural gas is usually the volume flow rate under standard conditions, which needs to be converted into the flow rate under working conditions according to the gas composition. Therefore, gas composition plays a role in identifying the flow rate range.

The ultrasonic flowmeter has a wide range, and the flow rate can usually be from 1 m/s to 30 m/s, or a larger flow rate under the condition of ensuring high measurement accuracy. Therefore, the selection of its diameter can generally be smaller than the size of the main pipeline, which can reduce the overall investment in the straight pipe section and the front and rear valves in the metering station. However, at the same time, the requirements of the process for the allowable pressure drop of the pipeline should be considered, and comprehensive consideration of the selection should be made.

In the "Code for Design of Natural Gas Engineering", based on safety considerations, the upper limit of gas flow velocity in natural gas pipelines is clearly specified. In the actual selection, the selection of the flow rate should be lower than the specified upper limit in most cases, so as to avoid the components such as throttling parts and elbows from vibrating when the high-speed gas passes through, causing high-frequency noise and affecting the ultrasonic measurement.

3. Instructions for use of the external clamp ultrasonic flowmeter

Liquid ultrasonic flowmeters have been recognized by the market in recent years and have been used to a certain extent, but most of them are used for water detection and leak detection. Since the promulgation of JJG 1030-2007 Ultrasonic Flowmeter Verification Regulations, the commercial measurement application of other crude oil and petroleum products has been promoted. Although the ultrasonic flowmeter has a strong flow capacity and no moving parts, it is suitable for a wide range of media, but because its technology is relatively new, the composition of the metering scheme needs to be carefully considered.

When selecting the type, it is necessary to first consider the water content, gas content, impurities, etc. in the oil. If the above composition exceeds the allowable range of the supplier, the accuracy is difficult to guarantee. In addition, it is very important for accurate metering whether the fluid medium is fully flowing.

Third, it is necessary to select the type according to the viscosity of the fluid and the Reynolds coefficient to ensure that the flowmeter can achieve the required accuracy in the entire operating range.

Similarly, the calibration of liquid ultrasonic flowmeter also has the problem of matching the capacity of existing calibration facilities and the caliber of the flowmeter. Therefore, in the measurement of petroleum and its products, for large-diameter liquid ultrasonic flowmeters, the on-site calibration method should be considered when selecting models. At present, more than one main table is used as the standard table for value transfer. In the field of water measurement, calibration methods including dry calibration are all under consideration.

4.The working principle of the clamp-on ultrasonic flowmeter

When the ultrasonic wave propagates in the fluid, its propagation speed is affected by the fluid velocity. By measuring the propagation velocity of the ultrasonic wave in the fluid, the fluid velocity can be detected and the flow rate can be converted. Taking the most widely used transit-time ultrasonic flowmeter as an example, when the ultrasonic wave propagates in the fluid, the propagation speed of the ultrasonic wave will increase in the downstream direction and decrease in the countercurrent direction, that is, the same propagation distance has different propagation times. The relationship between the difference of the propagation velocity and the flow velocity of the fluid to be measured is obtained by calculating the flow velocity and converting the flow rate. That is, when the ultrasonic beam propagates between the "upstream sensor" and the "downstream sensor" in the flow direction of the water medium in the pipeline, the flow of water will cause a small change in the propagation time of the ultrasonic beam relative to the static propagation, and this propagation time The change is proportional to the flow rate of the water, which is the measurement principle of the time difference ultrasonic flowmeter. The theoretical expression of its relationship is as follows: V=MD/sin2θ×△T/TupTdown

In the formula, M is the number of linear propagation of the ultrasonic beam in the water.

θ—is the angle between the ultrasonic beam and the water flow direction

Tup - is the propagation time of the ultrasonic beam in the positive direction (the propagation time from the upstream sensor to the downstream sensor)

Tdown - is the propagation time of the ultrasonic beam in the reverse direction (the propagation time from the downstream sensor to the upstream sensor)